Topic Four: Nervous System

Axolemma

Cell membrane of an axon

What is the difference between the central nervous system (CNS) and the peripheral nervous system (PNS)?

• - The central nervous system (CNS) consists of the brain and spinal cord. It processes information and coordinates activity throughout the body.

• - The peripheral nervous system (PNS) includes all the nerves outside the CNS. It connects the CNS to limbs and organs, serving as a communication network between the brain/spinal cord and the rest of the body.

Distinguish the sensory (afferent) from the motor (efferent) division of the PNS

• - The sensory (afferent) division carries information from sensory receptors toward the central nervous system (CNS). It detects stimuli such as touch, pain, temperature, and sends signals to the brain and spinal cord.

• - The motor (efferent) division transmits signals from the CNS to effectors such as muscles and glands, causing a response or action (like muscle contraction or gland secretion).

Distinguish the somatic (voluntary) from the autonomic (involuntary) motor division.

• - The somatic (voluntary) motor division controls skeletal muscles and is responsible for conscious movements, such as walking or picking up objects.

• - The autonomic (involuntary) motor division regulates involuntary functions by controlling smooth muscle, cardiac muscle, and glands. It manages automatic activities like heart rate, digestion, and breathing.

Describe the general flow of information in the nervous system: sensory input → integration → motor output.

• - Sensory input: Sensory receptors detect changes in the environment (stimuli) and send this information to the central nervous system (CNS) via sensory (afferent) neurons.

• - Integration: The CNS processes and interprets the sensory information to make decisions about an appropriate response.

• - Motor output: The CNS sends signals through motor (efferent) neurons to effectors (muscles or glands), producing a response or action.

Place the sympathetic and parasympathetic division within the autonomic branch…

• - The sympathetic division prepares the body for "fight or flight" responses (increases heart rate, dilates pupils, etc.).

• - The parasympathetic division promotes "rest and digest" activities (slows heart rate, stimulates digestion, etc.).

What is the direction a nerve signal travels through a neuron?

• 1. Dendrites receive incoming signals from other neurons or sensory receptors.

• 2. The signal is transmitted to the soma (cell body), where it is processed.

• 3. The signal then moves down the axon, a long fiber that carries impulses away from the cell body.

• 4. Finally, the signal reaches the axon terminals, which pass the signal to the next neuron or to an effector (muscle or gland).

What is the general job of neuroglia (glial cells) in the nervous system?

Neuroglia (glial cells) support and protect neurons. Their general functions include:

• - Providing structural support

• - Supplying nutrients

• - Insulating nerve fibers (e.g., forming myelin)

• - Maintaining the environment around neurons

• - Assisting in repair and defense against pathogens

What is the function of the myelin sheath and which glial cells form it?

The myelin sheath is a fatty insulating layer that surrounds the axons of many neurons. Its functions are:

• - Increases the speed of nerve impulse conduction

• - Protects and insulates axons

The glial cells that form the myelin sheath are:

• - Oligodendrocytes in the central nervous system (CNS)

• - Schwann cells in the peripheral nervous system (PNS)

What do astrocytes do?

Provide structural and metabolic support, maintain the blood-brain barrier, and regulate the environment around neurons in the CNS.

What do microglia do?

Act as immune defense cells in the CNS, removing waste and pathogens by phagocytosis.

(Modified macrophages)

What do ependymal cells do?

Line the cavities (ventricles) of the CNS and help produce and circulate cerebrospinal fluid (CSF).

What is the function of a sensory (afferent) neuron?

Sensory neurons carry information from sensory receptors (like skin, eyes, or ears) toward the central nervous system (CNS).

What is the function of a motor (efferent) neuron?

Motor neurons transmit signals from the central nervous system (CNS) to muscles or glands (effectors), causing a response such as movement or secretion.

What is the function of an interneuron (association neuron)?

Interneurons are found within the central nervous system (CNS) and connect sensory and motor neurons. They process and integrate information.

What is resting membrane potential and what is its typical value in neurons?

• - Resting membrane potential is the electrical charge difference across the plasma membrane of a resting neuron. It is created by differences in ion concentrations inside and outside the cell, and by the selective permeability of the membrane.

• - The typical resting membrane potential in neurons is about -70 millivolts (mV), meaning the inside of the cell is negatively charged compared to the outside.

What is the electrical charge relationship between the inside and outside of a resting neuron?

• - The inside of a resting neuron is negative. This negative charge is due to the distribution of ions across the neuron's membrane, creating a resting membrane potential (about -70 mV).

• - The outside of the cell membrane is positive.

What is the Na⁺/K⁺ pump?

The Na⁺/K⁺ pump (sodium-potassium pump) is a membrane protein found in all animal cells, including neurons. It uses energy from ATP to actively transport 3 sodium ions (Na⁺) out of the cell and 2 potassium ions (K⁺) into the cell. This process helps maintain the resting membrane potential and the proper concentration of ions inside and outside the cell.

What maintains the resting membrane potential in a neuron?

• - The Na⁺/K⁺ pump (sodium-potassium pump), which actively transports 3 sodium ions (Na⁺) out of the neuron and 2 potassium ions (K⁺) into the neuron, using ATP.

• - The uneven distribution of ions across the membrane, with more positive ions outside and more negative ions (and proteins) inside, keeps the inside of the cell negative relative to the outside.

How do unequal Na⁺ and K⁺ distribution and membrane selective permeability create the neuron's resting membrane potential?

• - Unequal ion distribution: There is more Na⁺ outside the cell and more K⁺ inside the cell, due to the action of the Na⁺/K⁺ pump.

• - Selective membrane permeability: The cell membrane is much more permeable to K⁺ than to Na⁺, so K⁺ tends to diffuse out more easily, leaving behind negatively charged proteins and ions inside.

What is the difference between leak channels and the Na⁺/K⁺ pump?

• Leak channels are like tiny holes that let certain ions (mostly potassium) drift in or out of the neuron naturally.

• The Na⁺/K⁺ pump is like a machine that uses energy to push sodium out and pull potassium in, keeping their levels different on each side of the cell.

Why is the resting membrane potential considered stored energy for a neuron?

The resting membrane potential is like stored energy because it creates a difference in electrical charge across the neuron's membrane. This electrical difference can be quickly used to generate a nerve signal (action potential) when the neuron is stimulated. It’s similar to a battery being charged and ready to power a device when needed.

What is a graded potential?

A small, temporary change in the electrical charge of a neuron's membrane at a specific spot. Graded potentials can vary in size (they are not all-or-none) and usually occur in the dendrites or cell body. They can add up (summate) to trigger an action potential if they reach a certain threshold.

What are two important features of graded potentials?

• - Vary in size depending on the strength of the stimulus (they are not all-or-none)

• - Fade or decrease in strength as they travel over short distances along the neuron's membrane

What is an action potential?

An action potential is a rapid, temporary change in the electrical charge across a neuron's membrane. It is an all-or-none electrical signal that travels along the axon, allowing the neuron to send information over long distances. Action potentials are essential for nerve impulse transmission.

What are two important characteristics of action potentials?

• - Are all-or-none: once triggered, they always occur at the same size and strength

• - Travel long distances along the axon without fading or decreasing in strength

Where do graded potentials typically occur in a neuron?

Usually occur at the dendrites and cell body (soma) of a neuron.

Where do action potentials typically occur in a neuron?

Typically start at the axon hillock and travel along the axon.

What does "all-or-none" mean for action potentials and what is the role of the threshold?

"All-or-none" means that once a neuron's membrane reaches a certain critical level (the threshold), an action potential will occur fully and always with the same size and strength. If the stimulus does not reach the threshold, no action potential is triggered. The threshold acts as the point that must be reached to start the action potential.

What is summation in neurons?

Summation is the process by which multiple graded potentials combine at the neuron's cell body. If enough graded potentials add together, they can bring the membrane potential to the threshold and trigger an action potential. Summation can occur over time (temporal summation) or from different locations on the neuron (spatial summation).

What is depolarization?

A change in a neuron's membrane potential, making the inside of the cell less negative (more positive) than at rest. It usually happens when sodium ions (Na⁺) rush into the cell during the early phase of an action potential.

What is hyperpolarization?

A change in a neuron's membrane potential that makes the inside of the cell more negative than its usual resting state. This usually occurs when potassium ions (K⁺) leave the cell or chloride ions (Cl⁻) enter, making it less likely for the neuron to fire an action potential.

What are the main phases of an action potential, in order?

• 1. Resting state: The neuron is at its resting membrane potential (about –70 mV), ready to respond to a stimulus.

• 2. Depolarization: The membrane potential becomes less negative as sodium ions (Na⁺) enter the cell, making the inside more positive.

• 3. Repolarization: The membrane potential returns toward resting as potassium ions (K⁺) leave the cell, making the inside more negative again.

• 4. Return to rest (after-hyperpolarization): The membrane may briefly become more negative than resting before stabilizing back at the resting potential.

Which ions move during the depolarization and repolarization phases of an action potential?

• - Depolarization: Sodium ions (Na⁺) move into the neuron, making the inside more positive.

• - Repolarization: Potassium ions (K⁺) move out of the neuron, making the inside more negative again.

How does an action potential travel along a neuron's axon?

An action potential moves down the axon as a self-propagating wave. When one section of the axon membrane depolarizes, it triggers the next section to depolarize, causing the signal to travel all the way to the axon terminals without losing strength.

How do voltage-gated channels and the refractory period ensure action potentials move in only one direction along the axon?

Voltage-gated channels open to let ions flow during an action potential. After a section of the axon fires, it enters a refractory period during which the channels cannot open again right away. This prevents the action potential from traveling backward, ensuring it moves only forward along the axon.

What is saltatory conduction?

The way action potentials travel along myelinated axons. The nerve impulse "jumps" from one node of Ranvier (gaps in the myelin sheath) to the next, instead of moving continuously along the axon. This makes nerve signal transmission much faster and more efficient.

How does myelin speed up the nerve signal along an axon?

Myelin insulates the axon, preventing ion leakage and forcing the action potential to jump between the nodes of Ranvier (gaps in the myelin). This jumping (saltatory conduction) allows the nerve signal to travel much faster than it would along an unmyelinated axon.

How does axon diameter affect conduction speed?

Increases conduction speed because electrical signals face less resistance as they move through the axon.

How does myelination affect conduction speed?

Greatly increases conduction speed by allowing the action potential to jump between nodes of Ranvier (saltatory conduction), instead of traveling continuously along the entire axon.

What is the presynaptic terminal?

The end of the axon of the sending neuron, where neurotransmitters are released.

What is the synaptic cleft?

The small gap between the neurons.

What is the postsynaptic membrane?

The surface of the receiving cell (neuron, muscle, or gland) that has receptors for the neurotransmitter.

What happens when an action potential reaches the presynaptic terminal?

The arrival of the action potential triggers the release of neurotransmitter from the presynaptic terminal into the synaptic cleft.

What is released into the synaptic cleft when an action potential arrives at the synapse?

A neurotransmitter is released from the presynaptic terminal into the synaptic cleft.

What happens after the neurotransmitter is released into the synaptic cleft?

The neurotransmitter binds to specific receptors on the postsynaptic membrane of the receiving cell.

What is the effect when the neurotransmitter binds to receptors on the postsynaptic membrane?

The binding of the neurotransmitter to receptors causes a specific effect on the postsynaptic cell, such as generating a new nerve impulse or causing a muscle to contract.

What is the role of neurotransmitters in the nervous system? Give an example.

Neurotransmitters are chemical messengers that carry signals across the synaptic cleft from one neuron to another or to a muscle/gland cell. For example, acetylcholine is a neurotransmitter that stimulates muscle contraction.

How does Ca²⁺ (calcium ion) entry trigger neurotransmitter release at the synapse?

When an action potential reaches the presynaptic terminal, voltage-gated Ca²⁺ channels open and calcium ions enter the neuron. The rise in Ca²⁺ causes synaptic vesicles to fuse with the presynaptic membrane, releasing neurotransmitter into the synaptic cleft by exocytosis.

What do excitatory neurotransmitters do?

Increase the chance that the postsynaptic cell will fire an action potential, usually by making the inside of the cell more positive (depolarization).

What do inhibitory neurotransmitters do?

Decrease the chance that the postsynaptic cell will fire an action potential, usually by making the inside of the cell more negative (hyperpolarization).

Neurotransmitter action is stopped by: (3)

• - Reuptake: The neurotransmitter is taken back into the presynaptic neuron for reuse.

• - Enzymatic breakdown: Enzymes in the synaptic cleft break down the neurotransmitter.

• - Diffusion: The neurotransmitter diffuses away from the synaptic cleft.

What are the five main components of a reflex arc?

• 1. Receptor: Detects the stimulus

• 2. Sensory neuron: Carries the signal to the central nervous system (CNS)

• 3. Integration center: Processes the information (usually in the spinal cord)

• 4. Motor neuron: Carries the response signal away from the CNS

• 5. Effector: Muscle or gland that carries out the response

What is a reflex, and why are reflexes fast and automatic?

A rapid, automatic response to a stimulus that does not require conscious thought. Reflexes are fast and automatic because the nerve signal travels through a simple pathway called a reflex arc, often involving only the spinal cord (not the brain), which allows for quick, involuntary responses.

What is an example of a reflex?

The withdrawal reflex (also called the flexor reflex) is a common example. If you touch something hot or sharp, your hand quickly pulls away before you even feel pain. This rapid, automatic response helps protect your body from injury.

What is a monosynaptic reflex?

A simple reflex pathway that involves only one synapse between a sensory neuron and a motor neuron. The most common example is the knee-jerk (patellar) reflex.

What is a polysynaptic reflex?

A reflex pathway that involves two or more synapses and at least one interneuron between the sensory and motor neurons. This allows for more complex responses, such as the withdrawal reflex.

What is the survival value of reflexes?

Provide survival value by allowing the body to respond quickly and automatically to potentially harmful stimuli, often before the brain is even aware of the danger. This rapid response helps protect the body from injury and increases the chances of survival in dangerous situations.

What is the homeostatic value of reflexes?

Reflexes help maintain homeostasis by automatically regulating important body functions without conscious control. For example, reflexes control heart rate, blood pressure, breathing, and pupil size, keeping the body’s internal environment st

What is the role of interneurons in the integration center of a reflex arc?

Interneurons in the integration center (usually in the spinal cord) process and interpret the incoming sensory information and help determine the appropriate response. They connect sensory neurons to motor neurons and allow for more complex reflexes and modulation of the response.

What does the somatic nervous system control?

Controls voluntary movements by carrying signals from the central nervous system to skeletal (voluntary) muscles.

What does the skeletal muscle system control?

Controls voluntary movements of the body, such as walking, running, and lifting objects. It allows you to move your bones and maintain posture through the contraction of skeletal (voluntary) muscles.

What does the voluntary (somatic) nervous system control?

Controls conscious, intentional movements of skeletal muscles—such as walking, talking, and writing. It allows you to decide and carry out body movements under your control.

What does the autonomic nervous system control?

Controls involuntary functions of the body, such as heart rate, digestion, breathing, blood pressure, and gland activity. It automatically regulates internal organs and smooth and cardiac muscles without conscious effort.

What does the smooth and cardiac muscle and glands system do?

Smooth muscle, cardiac muscle, and glands control involuntary activities in the body.

What does the somatic nervous system control?

Controls voluntary movements of skeletal muscles.

What does the autonomic nervous system control?

Controls involuntary functions by regulating smooth muscle, cardiac muscle, and glands.

What is voluntary control?

Refers to movements and actions that are consciously directed by the brain through the somatic nervous system, mainly involving skeletal muscles. Examples include deciding to walk, write, or speak.

What is involuntary control?

Refers to body functions and movements that happen automatically, without conscious effort. These are regulated by the autonomic nervous system and include activities such as heartbeat, digestion, breathing, and gland secretion, mainly involving smooth muscle, cardiac muscle, and glands.

What effectors are targeted by the somatic and autonomic nervous systems?

• - Somatic nervous system: Targets skeletal muscles (voluntary effectors).

• - Autonomic nervous system: Targets smooth muscle, cardiac muscle, and glands (involuntary effectors).

How do the motor pathways of the somatic and autonomic nervous systems differ?

• - Somatic nervous system: Uses a single motor neuron that extends from the central nervous system (CNS) directly to the skeletal muscle (no synapse in a ganglion).

• - Autonomic nervous system: Uses a two-neuron chain. The first (preganglionic) neuron leaves the CNS and synapses in a ganglion with the second (postganglionic) neuron, which then goes to the target organ (smooth/cardiac muscle or gland).

Why does the body need both the somatic and autonomic nervous systems for coordinated function?

The body needs both systems to work together for overall coordination and survival. The somatic nervous system allows for conscious, voluntary control of movements (like walking or grabbing objects), while the autonomic nervous system automatically regulates vital involuntary functions (like heartbeat, digestion, and breathing). Together, they ensure you can interact with your environment while your internal organs and processes remain balanced and responsive to your needs.

What is the general role of the sympathetic division of the autonomic nervous system?

Prepares the body for “fight or flight” in stressful situations by increasing heart rate, dilating pupils, and directing blood flow to muscles.

What is the general role of parasympathetic division of the autonomic nervous system?

Promotes “rest and digest” activities by slowing the heart rate, stimulating digestion, and supporting relaxation and recovery.

Give an example of how the sympathetic and parasympathetic divisions have opposite effects on the same organ.

• - Sympathetic division: Increases heart rate (“fight or flight”).

• - Parasympathetic division: Decreases heart rate (“rest and digest”).

How do the sympathetic and parasympathetic divisions work together to maintain homeostasis?

Back:

The sympathetic and parasympathetic divisions have opposite effects on many organs, allowing them to balance each other’s actions. The sympathetic division prepares the body for action in stressful situations ("fight or flight"), while the parasympathetic division calms the body and promotes rest and recovery ("rest and digest"). By working in opposition, they keep the body’s internal environment stable and responsive to changing needs.

What is autonomic tone, and how does it differ from an on/off switch?

Autonomic tone is the continuous, dynamic balance between sympathetic and parasympathetic activity. Both divisions are always active at low levels, adjusting their influence as needed. Rather than one system being completely on or off, the body shifts the balance (tone) between them to respond to different situations and maintain homeostasis.

What is the main neurotransmitter used by the sympathetic division of the nervous system?

Norepinephrine (also called noradrenaline) is the main neurotransmitter at most target organs.

What is the main neurotransmitter used by the parasympathetic division of the autonomic nervous system?

Acetylcholine is the main neurotransmitter at target organs.

What are the body-wide effects of activating the sympathetic division?

Increases heart rate and blood pressure, dilates pupils, opens airways, inhibits digestion, and redirects blood flow to muscles—preparing the body for action or stress.

What are the body-wide effects of activating the sympathetic parasympathetic division?

Decreases heart rate and blood pressure, constricts pupils, stimulates digestion and gland activity, and promotes energy storage—helping the body relax, recover, and maintain normal functions.

What are the major regions of the brain?

• Cerebrum

• Cerebellum

• Brainstem

• Diencephalon

What is the general function of the cerebrum?

The largest part of the brain; responsible for processing sensory information, voluntary movement, learning, memory, and conscious thought.

What is the general function of the cerebellum?

Coordinates and fine-tunes voluntary movements and maintains balance and posture.

What is the general role of the brainstem?

Connects the brain with the spinal cord and controls vital automatic functions such as breathing, heart rate, and reflexes.

What is the general role of the diencephalon?

Acts as a relay center and regulates homeostasis; includes the thalamus (sensory relay) and hypothalamus (controls autonomic functions, hormones, and basic drives).

What is the role of the hypothalamus in the nervous system?

Acts as a key homeostatic control center. It regulates vital functions such as body temperature, hunger, thirst, sleep, hormone release, and the autonomic nervous system, helping keep the body’s internal environment stable.

What are the general functions of the spinal cord?

Serves as a conduction pathway, carrying nerve impulses between the brain and the rest of the body. It also acts as a reflex center, processing some reflexes independently of the brain for quick, automatic responses.

What is the role of the brainstem in controlling vital autonomic functions?

The brainstem controls vital autonomic functions such as heart rate, breathing, and blood pressure. It contains centers that automatically regulate these essential body processes, ensuring survival without conscious effort.

How does the hypothalamus link the nervous and endocrine systems?

The hypothalamus links the nervous and endocrine systems by controlling the pituitary gland. It receives information from the nervous system and uses hormones to regulate the pituitary, which in turn controls other endocrine glands. This connection allows the brain to influence body functions through both nerve signals and hormones.

What is the role of the cerebellum in the nervous system?

Coordinates voluntary movements and helps maintain balance and posture. It ensures movements are smooth, precise, and well-timed by processing sensory input and fine-tuning motor activity.

In what direction does a signal normally travel through a neuron?

Dendrites → cell body (soma) → axon → axon terminals

How does myelin speed up conduction along an axon?

The signal jumps between gaps (nodes) in the myelin, a process called saltatory conduction

What is the sequence of events at a chemical synapse?

An action potential arrives, neurotransmitter is released, it binds receptors on the next cell, and an effect follows

What directly triggers neurotransmitter release from the presynaptic terminal?

Calcium (Ca2+) entering the terminal, which causes vesicles to release neurotransmitter by exocytosis

Which brain region is a key homeostatic control center, regulating temperature, hunger, and thirst?

The hypothalamus