Chapter 39: Neurons and Synapses

Functions of the nervous system

Detects changes in the external and internal environment, center of mental activity, coordinates with the endocrine system to maintain homeostasis, responds to sensory information by issuing motor responses, body’s communication system.

Central nervous system (CNS)

The Brain and the spinal cord, and is the integration center.

Peripheral nervous system (PNS)

All neuronal tissue outside the CNS, including cranial and spinal nerves.

Pathway of information processing in the nervous system

Stimulus-sensory receptors-afferent neurons (sensory)-integration center (CNS)-efferent neurons (motor)-effector (muscles and glands).

Neuron

Electrically excitable cell that transmits impulses.

Role of dendrites

Receive neuronal impulses and transmit them to the cell body.

Role of axons

Conduct neuronal impulses away from the cell body.

Neuroglia (glial cells)

Non-neuronal cells having supportive functions.

Types of neuroglia cells

Astrocytes, Oligodendrocytes, Schwann cells, Microglia, Ependymal cells, and Satellite cells.

Function of afferent (sensory) neurons

Carry sensory information towards the CNS.

Function of efferent (motor) neurons

Carry the response signals from the brain to responsive networks.

Function of interneurons

Integrate the neural messages.

Synapse

Small gap or junction where two neurons (or a neuron and another cell like a muscle or gland) communicate, transmitting electrical or chemical signal

Neuromuscular junction (NMJ)

Synapse between a neuron and a muscle.

Action potential at the synapse

Action potential (electric current) is converted into a chemical signal (in the form of neurotransmitters) at the synapse.

Types of synapses

Electrical and Chemical.

Electrical synapses

Ions flow from one cell to another via gap junctions and plasma membranes are in direct contact, allowing rapid signal conduction.

Chemical synapses

Presynaptic and postsynaptic neurons are separated by a synaptic cleft; neurotransmitter release upon arrival of an action potential.

Membrane potential

Electrical potential in the cell’s interior.

Contributors to membrane potential

Difference in the ionic concentration between the cytosol and the ECF.

Resting membrane potential

Membrane potential of an unstimulated resting cell.

Resting membrane potential of a neuron

typically -70mV (negative inside).

Cells respond to stimuli

By changing the resting membrane potential.

Passive ion channels

Leaky channels that are always open.

Active ion channels

Gated channels that open or close in response to specific stimuli.

Chemically-gated or ligand-gated channels

Respond to binding to a ligand.

Voltage-gated channels

Respond to changes to voltage or potential difference.

Mechanically-gated channels

Respond to changes in mechanical pressure or vibration.

Graded potential

A temporary and localized change in the resting membrane potential.

Action potential

A travelling electrical potential along the surface of the axon.

Threshold potential

The membrane potential at which an action potential begins.

Neurotransmitters

Endogenous chemicals that allow neurons to communicate with each other throughout the body.

Principal excitatory neurotransmitter in the brain

Glutamate.

Major inhibitory neurotransmitters

Gamma-aminobutyric acid (GABA) and glycine.

Acetylcholine release

At the neuromuscular junction.

Excitatory postsynaptic potential (EPSP)

Results from neurotransmitter binding causing gated Na+ channels to open and cause depolarization.

Inhibitory postsynaptic potential (IPSP)

Results from neurotransmitter binding to voltage-gated potassium / chloride channels and causing hyperpolarization.

Temporal summation

Occurs on a membrane that receives multiple depolarizing stimuli from the same source in rapid succession.

Spatial summation

More than 2 stimuli arrive at the same time but at different locations.

Reception

Detection of stimuli by specialized sensory neurons those in eye and skin (Sensory receptors).

Transmission

Is the sending of a message along a neuron, and then to another neuron or to a muscle or gland.

Integration

Neuronal processing of signals from sensory neurons.

Response

Output or action produced by the brain; activating muscles and hormone-secreting glands (Effector organs).

Functional unit of the nervous system

Neuron.

Dendrites

Densely branched structures that receive stimuli from other nerve cells.

Axon hillock

The point where the membranes of an axon contacts the membrane of the cell body.

Afferent neurons

Send signals from the stimuli to the brain.

Efferent neurons

Carry the response signals from the brain to the responsive networks.

Interneurons

Integrate the neural messages.

Largest and most numerous neuroglia in the CNS

Astrocytes; form Blood Brain Barrier (BBB).

Cells that line the ventricles within the CNS

Ependymal cells; produce and circulate Cerebrospinal Fluid (CSF).

Microglia

Act as big dogs of the brain and don’t let the pathogens bother the neurons.

Oligodendrocytes

Wrap the neurons of the CNS in myelin.

Schwann cells

Wrap the neurons of the PNS in myelin.

Satellite cells

Provide nutrients and structural support for neurons in the PNS.

Myelination slows neuronal conduction

False; Myelination SPEEDS up neural conduction.

Two types of neuroglia in the PNS

Satellite cells and Schwann cells.

Afferent neurons vs Efferent neurons

Afferent: send signals to the brain; Efferent: take signals from the brain to the responsive networks.

Saltatory Propagation

Action potentials leap from one node to the other.

Nodes of Ranvier

Small segments of uncovered axons between the myelinated sections.

Oligodendrocytes vs Schwann cells

Oligodendrocytes myelinate multiple axons in the CNS; Schwann cells myelinate one axon per cell in the PNS.

Myelin

A lipid-rich material that surrounds the nerve cell axons to insulate them and increase the rate at which electrical pulses are passed along the axon.

Cells that form the blood brain barrier

Astrocytes.

Ependymal cells

Are involved in the production and circulation of cerebrospinal fluid (CSF).

Grey matter vs White matter

Grey matter contains neuronal cell bodies, while white matter contains myelinated axons.

Structure of the synapse

One side is the axon terminal of the presynaptic cell, the other side is the postsynaptic cell (dendrite or effector cell).

Types of synapses

Electrical synapse (DIRECT electrical flow) and the Chemical synapse (chemical NEUROTRANSMITTER).

Majority of synapses in mammalian CNS

Chemical synapse.

Direct contact between plasma membranes

Electrical Synapse.

Presynaptic and postsynaptic cells separated by a synaptic cleft

Chemical Synapse.

Ions flow through gap junctions

Electrical Synapse.

Release of neurotransmitter at axon terminal

Chemical Synapse.

Found in cardiac muscle, retina of eye, and pulp of tooth

Electrical Synapse.

Allows rapid signal conduction

Electrical Synapse.

Neurotransmitter binds to receptor in postsynaptic cell membrane

Chemical Synapse.

Unregulated synapse

Electrical Synapse.

Rare in adult CNS and PNS

Electrical Synapse.

Channel proteins form gap junctions

Electrical Synapse.

Synaptic connection between a neuron and muscle cell

Neuromuscular junction (NMJ).

Axon terminal releases _____ into synaptic cleft

Neurotransmitters contained in synaptic vesicles.

Arrival of action potential to a chemical synapse

Step 1 of chemical synapse process.

Activation of receptors leads to an altered flux of ions in the postsynaptic neuron

Step 7 of chemical synapse process.

Activation of receptors

Leads to altered flux of ions in the postsynaptic neuron resulting in the transmission of impulse.

Fusion of synaptic vesicle and plasma membrane

Step 4 of chemical synapse process.

Binding of Ca2+ to proteins

Step 3 of chemical synapse process.

Neurotransmitter molecules released by exocytosis

Step 5 of chemical synapse process.

Ca2+ removal via active transport
Removal of neurotransmitter via reuptake or degradation

Step 8 & 9 of chemical synapse process.

Neurotransmitters diffuse and bind to receptor proteins

Step 6 of chemical synapse process.

Ca2+ increase from opening of Ca2+ channels

Step 2 of chemical synapse process.

Resting membrane potential

The resting membrane potential of an unstimulated, resting cell.

K+ and Na+ in ECF and cytosol

K+ is the main cation in the CYTOSOL and Na+ is the main cation in the ECF.

Resting membrane potential of a neuron

Typically -70mV.

Leak channels

Passive ion channels that are always open.

Active ion channels

Opened or closed by specific stimuli.

Three classes of ion channels

Ligand-gated, Voltage-gated, Mechanically-gated. Their functions: respond to binding, to voltage changes, and to mechanical pressure, respectively.

Sodium and potassium ion channels

Voltage-gated channels.

Binding of acetylcholine at a neuromuscular junction

Ligand-gated channel response.

Sensory receptor that responds to pressure

Mechanically-gated channel.

Term for shifting membrane potential to more positive

Depolarization

Temporary period after repolarization

Hyperpolarization.