6.5: Neurons and Synapses

State the functions of the nervous system.



Understanding: Neurons transmit electrical impulses.

The nervous system is involved in receiving information about the internal and external environments (sensation) and generating responses to that information (response).

Outline how information about the internal and external environments is sensed by the nervous system.



Understanding: Neurons transmit electrical impulses.

The nervous system receives information about the internal and external environments through a chemical or physical stimulus.

External environments are sensed by taste, touch, hearing, sight and smell. Stimuli for taste and smell are both chemical substances (molecules, compounds, ions, etc.), touch and hearing are physical stimuli, sight is light stimuli.

Internal environment can also be sensed by the nervous system. For example, the stretch of an organ wall or the concentration of certain ions in the blood.

Outline how the nervous system responds to information about the internal and external environments.



Understanding: Neurons transmit electrical impulses.

The nervous system produces a response based on the chemical and/or physical stimuli perceived. The nervous system can activate contraction of all three types of muscle tissue (skeletal, smooth and cardiac). The nervous system can also stimulate glands (exocrine and endocrine).

List the three major types of neurons.



Understanding: Neurons transmit electrical impulses.

There are three major types of neurons:

- Sensory neurons
- Interneurons (AKA relay neurons)
- Motor neurons

Outline the function of sensory neurons.



Understanding: Neurons transmit electrical impulses.

Sensory neurons are neurons responsible for converting external stimuli from the environment into corresponding internal stimuli. They are activated by sensory input (such as visible light, sound, heat, physical contact, etc.) or by chemical signals (such as smell and taste).

Outline the function of motor neurons.



Understanding: Neurons transmit electrical impulses.

Motor neurons are neurons that transmit impulses from the central nervous system to muscles and glands throughout the body.

Outline the function of interneurons.



Understanding: Neurons transmit electrical impulses.

Interneurons (AKA relay neurons) form connections between the sensory and motor neurons. These neurons complete the circuit between sensory neuron input and motor neuron response.

Draw the structure of a motor neuron.



Understanding: Neurons transmit electrical impulses.

Neurons contain unique structures for receiving and sending the electrical and chemical signals that make neuronal communication possible.

-Cell body – shown with a nucleus.

-Dendrites – shown as thin extensions from the cell body.

-Axon – shown as double line longer than the longest dendrite.

-Motor end plates – at end of axon, not covered by myelin sheath and ending with buttons/dots.

-Myelin sheath– surrounding the axon.

-Schwann cells– surrounding the axon.

-Nodes of Ranvier – shown in axon between Schwann cells.

State the function of neuron dendrites.



Understanding: Neurons transmit electrical impulses.

Dendrites are branching extensions of the neuron cell body. Dendrites receive electrochemical stimulation from other neurons and propagate the message received to the cell body of the neuron.

State the function of neuron axon.



Understanding: Neurons transmit electrical impulses.

The axon is a long, slender projection from the neuron cell body that carries an electrical impulse away from the cell body to the axon terminals, which can then pass the impulse to another neuron or to an effector cell (muscle or gland).

State the relationship between neuron axon diameter and speed of electrical impulse.



Understanding: Neurons transmit electrical impulses.

Larger diameter axons conduct neural impulse faster than smaller diameter axons because the local current diffusion of Na+ spreads faster down a wide axon than down a narrow one. A larger diameter axon offers less resistance to the movement of ions down the axon, causing the impulse to be conducted faster.

State the function of neuron cell body.



Understanding: Neurons transmit electrical impulses.

The neuron cell body contains the nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular compartments. All the proteins for the dendrites, axons and synaptic terminals are produced in the cell body (including channels, pumps and neurotransmitters).

State the function of the neuron motor end plate.


Understanding: Neurons transmit electrical impulses.

Motor end plates are located at the end of a motor neuron axon, opposite the cell body. Motor end plates are the presynaptic terminal where a motor neuron connects to a target muscle cell. It is from the motor end plant that a motor neuron is able to transmit a chemical signal to the muscle cell, causing muscle contraction.

Outline the structure and function of myelin.



Understanding: The myelination of nerve fibres allows for saltatory conduction.

Some axons are covered with myelin, a fatty material that wraps around the axon to form the myelin sheath. This external coating functions as insulation of the electrical signal as it travels down the axon. Additionally, myelin greatly increases the speed of conduction of the electrical signal.

State the role of Schwann cells in formation of myelin.



Understanding: The myelination of nerve fibres allows for saltatory conduction.

Myelin around motor neuron axons is produced by Schwann cells. Schwann cells hold the neurons in place, supply them with nutrients and provide insulation.

Define "saltatory conduction."



Understanding: The myelination of nerve fibres allows for saltatory conduction.

Saltatory conduction is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node. The action potential “jumps” from one node to the next.

Outline the mechanism and benefit of saltatory conduction.



Understanding: The myelination of nerve fibres allows for saltatory conduction.

In myelinated axons, the voltage-gated channels are only found at the nodes of Ranvier, and the action potential “jumps” from one node to the next. This "saltatory conduction" accelerates the rate at which an action potential travels down an axon.

Compare the speed of nerve impulse conduction myelinated and unmyelinated neurons.​



Understanding: The myelination of nerve fibres allows for saltatory conduction.

By acting as an electrical insulator, myelin greatly speeds up action potential conduction. For example, whereas unmyelinated axon conduction speed range from about 0.5 to 10 m/s, myelinated axons can conduct at speeds up to 150 m/s.

Define "resting potential."



Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential.

”Resting potential” is the name for the difference in electrical charge across a neuron cell membrane when it is not actively sending an impulse. A neuron at rest is negatively charged, meaning there is a higher concentration of cations outside the cell relative to inside the cell.

Outline three mechanisms that together create the resting potential in a neuron.



Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential.

The resting membrane potential is a result of different concentrations of ions inside and outside the cell.

Sodium-potassium pump
A membrane bound protein pump that actively transports three sodium (Na+) out of the cell for every two potassium (K+) into the cell. As more cations are expelled from the cell than taken in, there is a net loss of 1 positive charge with each action of the sodium-potassium pump. As a result, inside of the cell becomes negatively charged relative to the outside of the cell.

Anions within the cell
Anions are ions with negative charge. The inside of the neuron contains anions such as Cl- ions, negatively charged proteins, inorganic phosphate groups and DNA.

"Leaky" K+ channels
K+ can move passively through channel towards the outside the cell, taking it's positive charge with it and leaving the inside of the cell relatively more negative.

State the voltage of the resting potential.



Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential.

The inside of a cell is approximately 70 millivolts more negative than the outside (−70 mV). This voltage is called the resting membrane potential.

Summarize the action of the sodium-potassium pump.



Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential.

The sodium-potassium pump is located in the cell membrane and actively moves both Na+ and K+ against their respective concentration gradients.

1. Three Na+ ions bind to the pump from the inside of the cell.

2. ATP binds to the pump.

3. ATP is hydrolyzed, leading to phosphorylation of the pump and release of ADP. As a result of phosphorylation, the pump undergoes a conformational change. The phosphorylated form of the pump has a low affinity for Na+ ions, so they are released to the outside of the cell.

4. Two K+ ions bind to the pump from the outside of the cell.

5. Binding of the K+ ions causes the dephosphorylation of the pump, reverting it to its original conformation.

6. The two bound K+ ions are released to the inside of the cell.

Explain how the sodium-potassium pump maintains a negative resting potential.



Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential.

The sodium-potassium pump actively transports three sodium (Na+) out of the cell for every two potassium (K+) into the cell, both against their concentration gradients. As more cations are expelled from the cell than taken in, there is a net export of a single positive charge per cycle of the sodium-potassium pump. As a result, inside of the cell becomes negatively charged relative to the outside of the cell.

Define "action potential."



Understanding: An action potential consists of depolarization and repolarization of the neuron.

An action potential is the temporary change in electrical potential with the passage of an impulse along the membrane of a muscle cell or nerve cell. In other words, an action potential is the "flip-flop" in charge (from negative, to positive and back to negative) that occurs in the neuron or muscle cell membrane.

Define "depolarization."



Understanding: An action potential consists of depolarization and repolarization of the neuron.

Depolarization is a change in the local charge distribution, with the inside of the cell becoming temporarily more positive relative to the resting potential.

Define "repolarization."



Understanding: An action potential consists of depolarization and repolarization of the neuron.

Repolarization is the restoration of the localized negative membrane potential of the cell, bringing it back to its normal voltage after depolarization has occurred.

Summarize the action of a voltage-gated ion channel.



Understanding: An action potential consists of depolarization and repolarization of the neuron.

A voltage-gated ion channel is an ion channel that opens and closes in response to changes in the membrane potential of a cell.

Channel = transport protein embedded within the cell membrane through which molecules move passively, with their concentration gradient.

Ion channel = an atom or molecule with a net electric charge is moved across the cell membrane through the channel. Cell membranes are generally not permeable to ions, thus they must diffuse through the membrane through protein channels. Channels are usually ion-specific, such as to sodium(Na+), potassium (K+), or calcium (Ca2+).

Gated = the channel protein changes shape to "open and close," regulating the movement of a molecule through the membrane.

Voltage-gated = The opening and closing of the channel is triggered by ion concentration between the sides of the cell membrane.

Outline the mechanism of neuron depolarization.



Understanding: An action potential consists of depolarization and repolarization of the neuron.

A stimulus triggers the opening of Na+ channels in the membrane. The stimulus may be a neurotransmitter binding to its receptor protein or physical stimulus of a sensory neuron.

Because the concentration of Na+ is higher outside the cell than inside the cell, Na+ ions will rush into the cell when Na+ channels open. Sodium is a positively charged ion, so the sodium cation entering the cell will cause the local charge near the channel to become positive. As the charge rises and the threshold voltage is reached, additional voltage-gated Na+ channels open and even more Na+ ions will enter the cell. The membrane potential will reach +40 mV. This is known as depolarization.

Outline the mechanism of neuron repolarization.



Understanding: An action potential consists of depolarization and repolarization of the neuron.

At +40 mV, the Na+ channels close, and voltage-gated K+ channels open. A concentration gradient acts on K+ and K+ will leave the cell, taking a positive charge with it. As a result, the localized membrane potential begins to move back toward its resting voltage of -70 mV. This is called repolarization.

Outline the mechanism of neuron hyperpolarization.



Understanding: An action potential consists of depolarization and repolarization of the neuron.

Repolarization actually overshoots the -70 mV value that indicates the resting potential. A period of hyperpolarization occurs while the K+channels are open and K+ is leaving the cell. These K+ channels are slightly delayed in closing, accounting for the short overshoot.

Define nerve impulse.



Understanding: Nerve impulses are action potentials propagated along the axons of neurons.

A nerve impulse is a wave of electrical depolarization that reverses the voltage across the neuron membrane. Individually, neurons send the impulse from the dendrites to the axon terminal. Within the body, neurons are organized in long chains, allowing them to pass the impulse very quickly from one to the other. One neuron’s axon will connect to another neuron’s dendrite at the synapse between them.

Describe how nerve impulses are propagated along the neuron axon.



Understanding: Nerve impulses are action potentials propagated along the axons of neurons.

A nerve impulse is the movement of the action potential along the axon of the neuron. The action potential is the temporary reversal of charge from the resting potential. A resting neuron has a more negative charge inside the membrane and a more negative charge outside of the cell membrane (–70 mV) and a greater concentration of Na+ ions outside than K+ ions inside the axon.

When a action potential is stimulated, volted gated Na+ channels open and Na ions diffuse into the cell causing depolarization of the membrane (from –70 mV to +40 mV).

Depolarization is followed by repolarization of the neuron. When the charge increases to +40 mV, voltage gated K+ channels open and K ions diffuse out of the cell. This repolarizes the membrane. The sodium-potassium pump then restores the ion balance and
–70 mV resting potential.

Some of the Na+ that moves into the cell during depolarization diffuses within the cell, increasing the charge of adjacent regions of the cell. This is called the local current. The increase of charge with the local current affects adjacent channels, causing action potential depolarization in the next region of the cell membrane. This sequence of adjacent action potentials is the "impulse" that propagates along the neuron.

Define "refractory period."



Understanding: Nerve impulses are action potentials propagated along the axons of neurons.

The refractory period is a period of time during which the neuron cell membrane is incapable of repeating an action potential. It is the amount of time it takes for the channels in the cell membrane to be ready for a second stimulus once it returns to its resting state following an action potential.

Outline the cause and consequence of the refractory period after depolarization.



Understanding: Nerve impulses are action potentials propagated along the axons of neurons.

Voltage-gated Na+ channels are inactivated at the peak of the depolarization (+40mV) and they cannot be opened again for a brief time—the refractory period. Because of this, the local current of Na+ ions diffusing back toward previously opened channels has no effect and the action potential will move in one direction, toward the axon terminal.

Define "local current."



Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

Local current is the diffusion of Na+ ions within the cell following depolarization. Local current results in the subsequent depolarization of the adjacent membrane and if this area reaches threshold potential, further action potentials are generated.

Explain movement of sodium ions in a local current.



Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

Going down the length of the neuron axon, the action potential is propagated because more voltage-gated Na+ channels are opened as the depolarization spreads. This spreading occurs because Na+ enters through the channel and diffuses along the inside of the cell membrane. As the Na+ moves, its positive charge depolarizes a little more of the cell membrane. As that depolarization spreads, new voltage-gated Na+ channels open and more ions rush into the cell, spreading the depolarization a little farther.

Explain how the movement of sodium ions propagates an action potential along an axon.



Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

Action potentials are generated locally on patches of neuron cell membrane. As Na+ diffuses into the local region of the cell membrane during depolarization, the ions will diffuse within the cell and can trigger an action potential on the neighboring stretches of membrane, precipitating a domino-like propagation. The areas of membrane that have recently depolarised will not depolarize again due to the refractory period – therefore the action potential will only travel in one direction.

Define "threshold potential."



Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

The threshold potential is the critical voltage which a neuron membrane potential must reach to initiate an action potential. The threshold potential is at -55 mV for most human neurons in humans. Changes in the membrane potential that don’t reach -55 mV will not stimulate an action potential.

Describe that cause of and effect of membrane potential reaching the threshold potential.



Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

The channels that start depolarizing the membrane because of a stimulus help the cell to depolarize from -70 mV to -55 mV. Once the membrane reaches that threshold voltage, additional voltage-gated Na+ channels open. Any depolarization that does not change the membrane potential to -55 mV or higher will not reach threshold and thus will not result in an action potential (failed initiations).

Define "synapse."



Understanding: Synapses are junctions between neurons and between neurons and receptors or effector cells.

The synapse is the gap between two cells: the presynaptic cell (neuron sending the the signal) and the postsynaptic cell (receiving the signal). The presynaptic cell is a neuron. The postsynaptic cell can be either another neuron or an effector cell (muscle or gland).

Define "synaptic cleft."



Understanding: Synapses are junctions between neurons and between neurons and receptors or effector cells.

The synaptic cleft (also known as the synaptic gap) is the space between the presynaptic cell (neuron sending the signal) and the postsynaptic cell (receiving the signal) across which a nerve impulse is transmitted by a neurotransmitter.

Define "effector cell."



Understanding: Synapses are junctions between neurons and between neurons and receptors or effector cells.

An effector cell is any cell that actively responds to a stimulus (the stimulus affects the effector cell). When a motor neuron releases a neurotransmitter at the synapse, the effector cell is the muscle or gland cell that responds.

State the role of neurotransmitters.​



Understanding: Synapses are junctions between neurons and between neurons and receptors or effector cells.

Neurotransmitters are chemicals that transmit signals from a neuron to an effector cell across a synapse. They are released from vesicles in the presynaptic cell, diffuse across the synaptic cleft, and bind to receptors in the membrane on the postsynaptic (receiving) side.

Outline the mechanism of synaptic transmission.



Understanding: When presynaptic neurons are depolarized they release a neurotransmitter into the synapse.

The action potential travels along the membrane of the presynaptic cell until it reaches the synaptic terminus. The depolarization of the membrane at the synaptic terminus causes voltage-gated calcium channels in membrane to open. Calcium diffuses into the presynaptic neuron which triggers vesicles containing neurotransmitter move to the and fuse with presynaptic cell membrane. When the vesicle fuses with the cell membrane, the neurotransmitters are released by exocytosis into synaptic cleft.

The neurotransmitter molecules diffuse across the synaptic cleft and binds to chemical receptor molecules located on the membrane of the postsynaptic cell. Binding of the neurotransmitter to the effector cell receptor stimulates sodium channels to open. Sodium diffuses into the postsynaptic cell, depolarizing that region of the cell membrane. If the threshold potential is met, a new action potential will be activated in the postsynaptic cell.

The neurotransmitter does not physically enter the postsynaptic cell. Neurotransmitters will bind to a receptor molecule on the postsynaptic cell membrane, but will eventually break loose from the receptors and drift away. The neurotransmitter is then enzymatically broken down and/or reabsorbed into the presynaptic neuron.

Explain why some synaptic transmissions will not lead to an action potential in a postsynaptic cell.



Understanding: A nerve impulse is only initiated if the threshold potential is reached.​

Neurotransmitters can increase (excitatory) or decrease (inhibitory) the probability that the cell with which it comes in contact will produce an action potential.

Inhibitory neurotransmitters cause hyperpolarization of the postsynaptic cell (which means, decreasing the voltage gradient of the cell, thus bringing it further away from an action potential).

Outline the functions of the acetylcholine neurotransmitter.



Application: Secretion and reabsorption of acetylcholine by neurons at synapses.

Acetylcholine (ACh) is the neurotransmitter used at the neuromuscular junction; it is the chemical that motor neurons release in order to activate muscles.

Acetylcholine also plays important roles in cognitive function, most notably in the neural mechanisms of memory, alertness, attention, and learning.

Outline the mechanism of secretion of the neurotransmitter acetylcholine.



Application: Secretion and reabsorption of acetylcholine by neurons at synapses.

Acetylcholine (ACh) is synthesized in the presynaptic neuron and then stored in secretory vesicles within the synaptic terminus.

Calcium is a key ion involved in the release of neurotransmitters from the presynaptic neuron. The Ca2+ channel is normally closed, but if there is a depolarization of the membrane (caused by a presynaptic action potential), the channel opens. The opening of the Ca2+ channel allows for calcium to flow down its concentration gradient from the outside to the inside of the presynaptic terminal. This influx leads to an increase in the concentration of the Ca2+ in the presynaptic terminal, which by interacting with proteins associated with synaptic vesicles leads to the release of the neurotransmitters into the synaptic cleft by exocytosis.

Outline the mechanism of action of the neurotransmitter acetylcholine.



Application: Secretion and reabsorption of acetylcholine by neurons at synapses.

Nicotinic receptors are receptor proteins on postsynaptic cells that bind to the neurotransmitter acetylcholine. Nicotinic receptors are found on skeletal muscle that receive acetylcholine released from neurons to signal the muscular contraction.

The nicotinic receptor is a ligand-gated channel, which means that the receptor is also a channel protein. Binding of the neurotransmitter (the ligand) opens the gate of the channel.
When acetylcholine binds to the nicotinic receptor, the receptor protein undergoes a conformational change that causes the opening of the channel formed by the receptor. This increases the Na+ movement into the target cell, leading to depolarization and generation of the action potential in the postsynaptic cell.

State the reason why neurotransmitter molecules must be inactivated after secretion.



Application: Secretion and reabsorption of acetylcholine by neurons at synapses.

After a neurotransmitter molecule has been recognized by a postsynaptic receptor, it is released back into the synaptic cleft where it must be quickly removed or chemically inactivated in order to prevent constant stimulation of the postsynaptic cell and an excessive firing of action potentials (or inhibition of action potentials for inhibitory neurotransmitters).

Outline the mechanism of reabsorption of the neurotransmitter acetylcholine.



Application: Secretion and reabsorption of acetylcholine by neurons at synapses.

Following dissociation from the receptor, acetylcholine is rapidly hydrolyzed by the enzyme acetylcholinesterase. The enzyme converts acetylcholine into its inactive component parts choline and acetate.

After hydrolysis, acetate quickly diffuses into the surrounding medium, while choline gets taken back into the presynaptic cell. Choline is then recycled by the presynaptic cell for use in the synthesis of more acetylcholine.

Define "pesticide."



Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

Pesticides are chemicals that are meant to protect plants (usually crops) from weeds, fungi, or insects. Insecticides are a class of pesticides that target insects.

Summarize the use of neonicotinoids as a pesticide.



Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

Neonicotinoids are a class of neurotoxic insecticides that act on the nicotinic acetylcholine receptor. Neonic insecticides transfuse into all parts of treated plants, including pollen, nectar, and fluids, and the foods grown by those plants. They are used for pest management in agriculture, horticulture, forestry and in household pest control products.

Outline the mechanism of action of neonicotinoids use as a pesticide.



Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

Neonicotinoids are pesticides that irreversibly bind to nicotinic receptors (the same receptors that acetylcholine binds). The binding of the neonicotinoid blocks the acetylcholine neurotransmitter from binding to the receptor, thereby preventing the signal from the neuron from spreading to the postsynaptic cell. When the postsynaptic cell is a muscle, the muscle contraction is blocked, causing paralysis of the insect.

Additionally, the acetylcholinesterase enzyme is unable to chemically digest neonicotinoids within the synaptic cleft. So the insecticide molecules remain active, able to continually bind to the nicotinic receptors.

Define "cholinergic synapse."



Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

Cholinergic synapses are chemical synapses that that use acetylcholine molecules as the neurotransmitter.

Outline the effect of neonicotinoids on pollinating insects.



Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

The impact of neonicotinoids on pollinating insects such as bees is a cause for concern. Because they are systemic chemicals absorbed into the plant, neonicotinoids can be present in pollen and nectar which are consumed by flower-visiting insects such as bees. Bees exposed to neonicotinoids can experience problems with flight and navigation, reduced taste sensitivity, and slower learning of new tasks, all of which impact foraging ability and hive productivity.

Outline the use of oscilloscopes in measuring membrane potential.


Skill: Analysis of oscilloscope traces showing resting potentials and action potentials.

An oscilloscope is a instrument that graphically displays varying signal voltages over time. A microelectrode is inserted into the cell and a reference electrode is placed outside the cell. The oscilloscope measures the difference in voltage between the inside and outside of the cell.

Annotate an oscilloscope trace.



Skill: Analysis of oscilloscope traces showing resting potentials and action potentials.

From the resting potential of -70mV (1), the oscilloscope depicts an increasing voltage, which is the inward flow of sodium ions during depolarization (2). Since sodium ions carry a positive charge, the inside of the neuron at that site becomes positive when compared to the outside of the neuron. At a threshold voltage of about -50mV, even more sodium channel will open, increasing the charge to about +40mV. The sodium channels close at about the peak of the action potential, and potassium channels open. Potassium ions (which are inside the neuron and also carry a positive charge) leave the neuron. This repolarization (3) results in the downward voltage as the inside of the neuron in the vicinity of electrode becomes negative again. Potassium channels are slow to close, resulting in a refractory period as hyperpolarization reduces the charge to below that of the resting potential. The action of the sodium-potassium pump returns the charge to the -70mV resting potential (4)

Define "gas exchange."



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

Gas exchange is the diffusion of gases from an area of higher concentration to an area of lower concentration across an organism's membranes.

Define "ventilation."



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

Ventilation is the moving air into and out of lungs via inhalation and exhalation (breathing).

Distinguish between ventilation, gas exchange and cell respiration.



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

Cell respiration depends on gas exchange and gas exchange depends on ventilation.

Ventilation is the movement of air into and out of lungs via inhalation and exhalation. Ventilation involves muscle movement.

Gas exchange is the movement of carbon dioxide and oxygen between the alveoli and blood and between blood and tissue cells.

Cell respiration is the release of energy through the oxidation of glucose. Aerobic cell respiration occurs in mitochondria.

State the location of gas exchange in humans.



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

In humans, gas exchange occurs in the lungs with the exchange of oxygen and carbon dioxide between the air of the external environment and the body fluids of the internal environment.

Gas exchange also occurs in the body tissues, where oxygen is taken up by the tissues and the CO2 that the tissues have created is diffused back into the blood for transport back to the lungs or gills to be released.

Draw a diagram showing the structure of an alveolus and an adjacent capillary.



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

Alveolus drawn as an oval with scalloped edges.

Alveolus wall drawn as a single line (representing 1 cell thick).

Alveolus lumen empty.

Thin layer of surfactant drawn inside alveolus wall.

Alveolar duct ending at alveolus.

Capillary drawn as a tube surrounding outside of the alveolus.

Capillary wall drawn as a single line (representing 1 cell thick).

Capillary lumen narrow.

Red blood cell(s) within capillary lumen.

Arrow to indicate direction of blood flow through the capillary.

Arrow to indicate diffusion of O2 from alveolus lumen into capillary red blood cell.

Arrow to indicate diffusion of CO2 from capillary into alveolar lumen.

Outline the role of the parts of an alveolus in a human lung.



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

Alveolus as an oval with scalloped edges- maximizes surface area for gas exchange.

Alveolus wall is a single layer of Type 1 pneumocytes- minimizes distance gases have to travel between the blood in the capillary and the air in the alveolus.

Lumen of alveolus- volume of air for gas exchange.

Surfactant produced by Type II pneumocytes- reduces surface tension and prevents collapse of alveolus when air is exhaled.

Bronchial tube ending at alveolus- tube for transport of air into and out of the alveolus.

Capillary surrounding outside of the alveolus- minimizes distance gases have to travel between the blood in the capillary and the air in the alveolus.

Capillary wall is a single cell thick- minimizes distance gases have to travel between the blood in the capillary and the air in the alveolus.

Outline the mechanism of gas exchange in humans.



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

Gas exchange occurs through diffusion. Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in concentration and is a passive process (no energy input). In humans, oxygen diffuses into capillaries at the lungs and into tissue cells throughout the body. Carbon dioxide diffusion out of the tissue cells throughout the body and then out of the blood in the lungs.

Outline the purpose of gas exchange in humans.



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

Gas exchange must occur so that cells have oxygen for performing aerobic respiration. Oxygen is the final electron acceptor in the oxidation of glucose during cellular respiration. Without oxygen, aerobic respiration will stop.

Additionally, the carbon dioxide waste product of the respiration must leave the cells. It is very dangerous if carbon dioxide builds up in the body, so blood carries the carbon dioxide to the lungs where it is released into the air with exhalation.

Outline the purpose of ventilation in humans.



Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air and alveoli and blood flowing in adjacent capillaries.

Ventilation (moving air into and out of lungs) maintains a steep concentration gradient of gases in alveoli of the lungs. New air is continually cycled into and out of the lungs from the atmosphere, ensuring O2 levels stay high in alveoli (and diffuse into the blood) and CO2 levels stay low (and diffuse from the blood).

Ventilation maintains the concentration gradient required for gas exchange.

Describe how the structure of the lung increases surface area for gas exchange.



Understanding: Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange.

Gas exchange occurs more quickly with larger surface areas. The lungs have a large surface area from having many alveoli. The alveoli themselves have a large surface area because the cells that make up their wall have a flattened, thin shape. A typical pair of human lungs contain about 300 million alveoli, producing 70m2 of surface area.

Outline the structure and function of Type 1 pneumocytes.



Understanding: Type I pneumocytes are extremely thin alveolar cells that are adapted to carry out gas exchange.

Type I pneumocytes are thin, flat cells that form the structure of the alveoli. Their shape increases the surface area of each cell individually and the surface area of the alveoli collectively. Type I pneumocytes line more than 95% of the alveolar surface.

Type I pneumocytes are the location of gas exchange between the alveoli and blood. Their thin shape enables a fast diffusion of gases between the air in the alveoli lumen and the blood in the surrounding capillaries.

Outline the structure and function of Type II pneumocytes.



Understanding: Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension.

Type II pneumocytes are larger, cuboidal cells in the alveolar wall that occur less frequently than Type I cells.

Type II pneumocytes produce a pulmonary surfactant that is continuously released by exocytosis.

Describe two functions of the fluid secreted by Type II pneumocytes.



Understanding: Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension.

Type II pneumocytes produce a pulmonary surfactant that is continuously released by exocytosis. Reinflation of the alveoli following exhalation is made easier by the surfactant, which reduces surface tension in the thin fluid coating of the alveoli.

Additionally, fluid secreted by Type II pneumocytes facilitates the transfer of gases between blood and alveolar air. The gases dissolve in the moist fluid, helping them to pass across the alveoli surface.

Draw a labelled diagram to show the human ventilation system.



Understanding: Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.

Nasal cavity
Trachea
Bronchi
Bronchioles
Lungs
Alveoli (enlarged as inset)
Diaphragm
Intercostal muscles

Outline the flow of air into the lungs.



Understanding: Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.

When air enters the lungs during inhalation it passes through:
Nostrils →
Nasal cavity →
Pharynx →
Larynx →
Trachea →
Bronchi (with cartilaginous rings) →
Bronchioles (without cartilage) →
Alveoli.

State the role of cartilage in the trachea and bronchi.



Understanding: Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.

Cartilage is a strong but flexible tissue. The cartilage in the trachea and bronchi form incomplete rings that support the structures while still allowing them to move and flex during breathing. If cartilage was not present then the trachea and bronchi would collapse inward during exhalation.

State the role of smooth muscle fibres in the bronchioles.



Understanding: Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.

The trachea divides into two bronchi (one for each lung) which continue to subdivide before becoming bronchioles. Whereas the bronchi have rings of cartilage that serve to keep them open, the bronchioles are lined with smooth muscle tissue and do not have cartilage. The muscle contracts and expands, effectively controlling the flow of air as it moves to the alveoli.

State the relationship between gas pressure and volume.



Understanding: Muscle contraction causes the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.

Boyle's Law describes the relationship between the pressure and the volume of a gas. The law states that as volume increases, pressure decreases and vice versa.

The mechanics of ventilation follow Boyle’s Law. When the volume of the lungs changes, the pressure of the air in the lungs changes in accordance with Boyle's Law. If the pressure is greater in the lungs than outside the lungs, then air rushes out. If the pressure is lower in the lungs than outside the lungs, then air rushes in.

Outline the pressure and volume changes that occur in the lungs during normal inspiration.​



Understanding: Muscle contraction causes the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.

Inspiration occurs when the external intercostal muscles and the diaphragm contract, causing an increase in size of the thoracic cavity and expansion of the lungs. With expansion of the lungs, the volume of the alveoli sacs increases, reducing internal air pressure in accordance with Boyle's Law. The air pressure inside the lungs decreases below that of air outside the body. Because gases move from regions of high pressure to low pressure, air rushes into the lungs.

Outline the pressure and volume changes that occur in the lungs during normal expiration.​



Understanding: Muscle contraction causes the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.

Expiration occurs when the external intercostal muscles and the diaphragm relax, causing a decrease in size of the thoracic cavity and recoil of the lungs. The volume of the alveoli sacs decreases, increasing internal air pressure in accordance with Boyle's Law. The air pressure inside the lungs increases above that of air outside the body. Because gases move from regions of high pressure to low pressure, air rushes out of the lungs.

Define "thorax."



Understanding: Muscle contraction causes the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.

The thorax is a part of the anatomy of humans and other animals located between the neck and the abdomen (the chest). The thorax includes the thoracic cavity (contains organs including the heart and lungs) and the thoracic wall (ribs and intercostal muscles).

Summarize the muscle contractions required to ventilate the lungs.



Understanding: Muscle contraction causes the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.

Inspiration (inhalation):
-External intercostal muscles contract moving rib cage up and out.

-Diaphragm contracts becoming lower and flatter.

-Additional muscles can be used if a bigger breath is required.

Expiration (exhalation):
-External intercostal muscles relax and move the rib cage down and in.

-Diaphragm relaxes, moving higher and becoming more dome shaped.

-Internal intercostal muscles and abdominals contract with forced exhalation.

Define "inspiration" as related to lung ventilation.



Understanding: Different muscles are required for inspiration and expiration because muscles only do work when they contract.

Inspiration = inhalation = breathing in.

The process that causes air to enter the lungs.

Define "expiration" as related to lung ventilation.



Understanding: Different muscles are required for inspiration and expiration because muscles only do work when they contract.

Expiration = exhalation = breathing out.

The process that causes air to leave the lungs.

Outline how the diaphragm and abdominal muscles work as an antagonistic pair during ventilation.



Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

Skeletal muscles work in antagonistic pairs, meaning as one muscle contracts, the other relaxes.

Ventilation includes the movement of the following antagonistic muscle pair:

Diaphragm (moves down with contraction to increase thorax volume during inspiration)
...is antagonistic with...
Abdominal oblique (contracts to push the diaphragm back up towards the thorax during expiration).

Outline how the external and internal intercostal muscles work as an antagonistic pair during ventilation.



Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

Skeletal muscles work in antagonistic pairs, meaning as one muscle contracts, the other relaxes.

Ventilation includes the movement of the following antagonistic muscle pair:

External intercostal muscles (contract moving rib cage up and out during inspiration)
...is antagonistic with...
Internal intercostal muscles (contract moving rib cage down and in during forceful exhalation such as coughing or during exercise).

Outline the direction of movement of the diaphragm and rib-cage during inspiration.



Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

During inspiration (inhalation) the external intercostal muscles contract and the rib-cage moves up and out. The diaphragm also contracts, moving down and flattening. Together, these motions increase the volume of the thorax.

Outline the direction of movement of the diaphragm and rib-cage during expiration.



Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

During expiration (exhalation) the external intercostal muscles relax and the the internal intercostal muscles contract, moving the rib-cage down and in. The diaphragm relaxes and abdominal oblique muscles contract, pushing the diaphragm up and into a domed position. Together, these motions decrease the volume of the thorax.

Outline the structure and function of external intercostal muscles.



Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

Each rib is connected to the rib below it by both external and internal intercostal muscles.

The external intercostal muscles are located on the outer surface of the ribs and are positioned at a diagonal in between each rib.

The external intercostal muscles are responsible for forced and quiet inhalation. Contraction of the external intercostal muscles elevates the ribs and spreads them apart, resulting in the inhalation of air from the atmosphere.

Outline the structure and function of internal intercostal muscles.



Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

Each rib is connected to the rib below it by both external and internal intercostal muscles.

The internal intercostal muscles are located on the inner surface of the ribs (deeper than the external intercostal muscles) and are positioned at a diagonal in between each rib.

The internal intercostal muscles are responsible for forced exhalation. Contraction of the internal intercostal muscles depresses the ribs and pulls them closer together, resulting in the forced exhalation of air from the lungs.

Outline the structure and function of the diaphragm.



Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

The diaphragm is a dome-shaped sheet of muscle located just below the lungs.

During inspiration (inhalation), the diaphragm contracts and is drawn inferiorly into the abdominal cavity until it is flat. The thoracic cavity becomes larger, drawing in air from the atmosphere.

During expiration (exhalation), the diaphragm relaxes and elevates to its dome-shaped position in the thorax. Air within the lungs is forced out of the body when the size of the thoracic cavity decreases.

Outline the causes of lung cancer.



Application: Causes and consequences of lung cancer.

Lung cancer occurs when cells in the lung mutate and divide uncontrollably to form tumors.

Smoking is the number one cause of lung cancer. Tobacco smoke contains many chemicals that are known to mutate DNA. The risk of lung cancer increases with the length of time and number of cigarettes smoked.

Other causes of lung cancer include:
-Particle pollution (very tiny solid and liquid particles that are in the air)
-Genetic predisposition
-Radon exposure
-Other hazardous chemicals (such as asbestos)

List symptoms of lung cancer.​



Application: Causes and consequences of lung cancer.

Lung cancer typically doesn't cause signs and symptoms in its early stages. Signs and symptoms of lung cancer typically occur only when the disease has advanced.

Signs and symptoms of lung cancer may include:
-A persistent cough
-Coughing up blood
-Shortness of breath
-Chest pain
-Voice hoarseness
-Unintentional weight loss

Outline the causes of emphysema.



Application: Causes and consequences of emphysema.

Emphysema is a lung disease caused by the weakening and rupturing of alveoli. As a result there are larger air spaces instead of many small ones. Having fewer and larger damaged sacs means there is a reduced surface area for the exchange of oxygen into the blood and carbon dioxide out of it.

Smoking is the leading cause of emphysema.

State the symptoms of emphysema.



Application: Causes and consequences of emphysema.

Shortness of breath and cough are the main symptoms of emphysema. As the disease progresses, other symptoms include:

-Fatigue
-Weezing
-Chest tightness
-Anxiety

Outline the reason why gas exchange is less effective in people with emphysema.



Application: Causes and consequences of emphysema.

Emphysema damages alveoli, reducing the surface area available for gas exchange. With less surface area, with each breath less oxygen is able to diffuse into the blood from the air and less carbon dioxide is able to diffuse from the blood into the air.

List treatment options for people with emphysema.



Application: Causes and consequences of emphysema.

The damage from emphysema is permanent. The ability to breathe properly cannot be fully recovered. Treatment of emphysema aims to ease symptoms and stabilize the condition.

Treatments include:
-supplemental oxygen
-inhaled bronchodilators
-inhaled steroids
-smoking cessation
-lung surgery to remove damaged tissue
-lung transplant

Define "tidal volume."



Skill: Monitoring of ventilation in humans at rest and after mild and vigorous exercise.

Tidal volume is the normal volume of air displaced between normal inhalation and exhalation when extra effort is not applied. Tidal volume includes the volume of air that fills the alveoli in the lungs and the volume of air that fills the airways. In a healthy, young human adult, tidal volume is 7 mL/kg of body mass.

Outline techniques for measuring lung tidal volume.



Skill: Monitoring of ventilation in humans at rest and after mild and vigorous exercise.

Tidal volume can be determined by measuring the volume of air inhaled and/or exhaled. There are multiple techniques for measuring tidal volume:

1. A spirometer is a device that measuring the volume of air inspired and expired by the lungs. It operates by measuring the velocity and/or pressure of the airflow as it moves past a sensor.

2. Air can be exhaled into a lung volume bag. The bag will trap the exhaled air inside and the volume on the bag’s scale can be measured.

3. Air can be exhaled through a tube that ends in a inverted flask of water. The exhaled air will displace a measurable volume of water.

Define "ventilation rate."



Skill: Monitoring of ventilation in humans at rest and after mild and vigorous exercise.

Ventilation rate is the number of breaths per minute . Under non-exertion conditions, the human respiratory rate averages around 12–15 breaths/minute.

Outline techniques for measuring ventilation rate.



Skill: Monitoring of ventilation in humans at rest and after mild and vigorous exercise.

There are multiple techniques for measuring ventilation rate:

1. A spirometer is a device that measuring the rate of air inspired and expired by the lungs. It operates by measuring the velocity and/or pressure of the airflow as it moves past a sensor.

2. Simple observation and counting number of breaths per minute.

3. Chest belt and pressure sensor that records the rise and fall of the thorax.

Outline the effects of mild and vigorous exercise on ventilation rate.



Skill: Monitoring of ventilation in humans at rest and after mild and vigorous exercise.

Both ventilation rate and tidal volume increase with increased intensity of exercise. During exercise the rate of cellular respiration increases and as a result more carbon dioxide is produced by the cells. The carbon dioxide production in the tissues exceeds the rate of breathing it out, which will lead to a drop in the pH of the blood. Chemoreceptors detect the change in blood pH and send nerve impulses to the breathing center of the brain. The brain responds by sending nerve impulses to the diaphragm and intercostal muscles which will contract more frequently (increasing ventilation rate) and with more force (increasing tidal volume).

Define "epidemiology."



Nature of Science: Obtain evidence for theories- epidemiological studies have contributed to our understanding of the causes of the lung cancer.

Epidemiology is the study and analysis of the distribution, patterns and determinants of health and disease conditions in defined populations.