Chapter 2
Structure and function of the nervous system
The structure of neurons
Neurons: Basic signalling units that transmit information
throughout the nervous system.
● Cell body: Contains nucleus, endoplasmic
reticulum, a cytoskeleton, mitochondria, Golgi
apparatus and other intracellular organelles.
● Dendrites: Extensions of the neuron that receive
inputs from other neurons.
● Axon: Output of the neuron. Electrical signals
travel along the axon to the axon terminals.
● Synapse: Transmission of two neurons, come into
close contact so that chemical or electrical signals
can be passed from one cell to the next.
● Axon collaterals: Can transmit signals to more than one cell.
Neuronal signalling
Within a neuron, transferring information involves changes in the electrical state of the
neuron as electrical currents flow through the volume of the neuron.
Between neurons, information transfer occurs at synapses, typically mediated by chemical
signalling molecules (neurotransmitters) but, in some cases, also by electrical signals.
● Presynaptic: When axons make a connection onto other neurons.
● Postsynaptic: When other neurons make a connection onto their dendrites.
The membrane potential:
● Resting membrane potential: The neuronal cytoplasm has a potential that is -70mV
compared to the extracellular space → neuron not actively signalling.
○ Lipid membrane: Maintains the separation of intracellular and extracellular
ions and electrical charge that ultimately permits neuronal communication.
● Ion channels: These hydrophilic channels selectively permit one type of ion to pass
through the membrane (Na+, K+, Ca 2+, and Cl−).
○ Permeability: Extent to which a particular ion can cross the membrane
through a given ion channel.
○ Gated ion channels: Ion channels that are capable of changing their
permeability for a particular ion.
○ Nongated ion channels: Ion channels that are unregulated, and hence
always allow the associated ion to pass through.
● Ion pumps: Active transport proteins → neurons use a
Na+/K+ pump that pumps Na+ ions out of the cell and K+
ions into the cell → transporting ions up their
concentration gradients → requires energy → pump is an
enzyme that hydrolyses ATP) → each molecule of ATP
that is hydrolysed, energy is used to pump the ions.
○ The force of the Na+ concentration gradient
pushes Na+ from an area of high concentration (outside) to one of low
, concentration (inside) → K+ concentration gradients push K+ from an area of
high concentration (inside) to one of low concentration (outside).
○ Electrical gradient: The force of the K+ concentration gradient pushes some
K+ out of the cell → inside neuron slightly more negative than the outside.
Action potential: Rapid depolarization and repolarization of a small region of the membrane
caused by the opening and closing of ion channels → can travel for meters.
● Excitatory postsynaptic potentials (EPSP): Postsynaptic potential that makes the
postsynaptic neuron more likely to fire an action potential.
● Inhibitory postsynaptic potentials (IPSP): Postsynaptic potential that makes the
postsynaptic neuron less likely to fire an action potential.
● Electronic conduction: Electrical current diminishes with distance from its origin →
synapse → action potential needed.
● Voltage-gated ion channels: These open all by themselves when the membrane
potential is -55 mV (threshold) or higher.
(Numbers correspond with the numbers in the figure)
1. Threshold reached → -55 mV.
2. Voltage-gated Na+ channels open and Na+ flows
rapidly into the neuron → further depolarizes →
more voltage-gated Na+ channels opened →
more depolarized → more channels open → etc.
3. Na+ channels close → voltage-gated K+
channels open → K+ flows out of neuron down
concentration gradient → membrane potential
shifts back toward resting potential →
repolarization.
4. Membrane temporarily hyperpolarizes → K+ channels close.
5. Membrane potential gradually returns to its resting state → absolute refractory
period → Na+ channels unable to open → another action potential cannot be
generated.
● Myelin sheets: Higher speeds are obtained in axons → work as insulators around
the axons → make axons super resistant to voltage loss.
● Saltatory conduction: Combines speed of electrotonic conduction with the distance
that can be travelled by action potentials: high speed, long distance communication
→ from one node of Ranvier to the next.
Synaptic transmission
A neuron communicates with other neurons, muscles, or glands at a synapse, and a transfer
a signal from the axon terminal to the next cell.
Chemical transmission:
, Neurotransmitters:
○ Synthesized by and localized within the presynaptic neuron and stored in the
presynaptic terminal before release.
○ It is released by the presynaptic neuron when action potentials depolarize the
terminal (mediated primarily by Ca2+).
○ The postsynaptic neuron contains receptors specific for the neurotransmitter.
○ When artificially applied to a postsynaptic cell, the neurotransmitter elicits the
same response that stimulating the presynaptic neuron would.
● Biochemical classification of neurotransmitters:
○ Amino acids: Aspartate, GABA, glutamate and glycine.
○ Biogenic amines: Dopamine, norepinephrine, epinephrine and histamine.
○ Neuropeptides: Larger molecules made up of strings of amino acids.
● Functional classification of neurotransmitters:
○ Excitatory: Catecholamines, glutamate, histamine and some neuropeptides.
○ Inhibitory: GABA, glycine and some peptides.
○ Conditional: Action is conditioned on the presence of another transmitter in
the synaptic cleft or activity in the neuronal circuit.
Inactivation of neurotransmitters after release: Monitor level of neurotransmitter in
synaptic cleft → presynaptic neurons have autoreceptors → regulate release of transmitter.
● Active reuptake of the substance back into the presynaptic terminal.
● Enzymatic breakdown of the transmitter in the synaptic cleft.
● Diffusion of neurotransmitter away from the region of the synapse or site of action.
Electrical transmission:
● Gap junction: Neuronal membranes are touching → create pores connecting the
cytoplasm’s of the two neurons.
● Isopotential: Electrical changes in one neuron are reflected instantaneously in the
other.
● Pro: Useful when information must be conducted rapidly and
when groups of neurons should operate synchronously.
● Con: They cannot amplify a signal.
The role of glial cells
Astrocytes: Help form the blood-brain barrier and have an active role
in modulating neural activity.
Microglial cells: Devour and remove damaged cells.
Oligodendrocytes: Forms myelin in the central nervous system.
Schwann cells: Forms myelin in the peripheral nervous system.
Overview of the nervous system structure
The central nervous system: Consists of brain and spinal cord.
● Three protective membranes (outer - inner): Dura mater, arachnoid mater, pia mater.
● Cerebral cortex: Continuous sheet of layered neurons in different parts of the brain.
Peripheral nervous system: Consist of nerves and ganglia (groups of neurons) outside of
the CNS.
● The autonomic nervous system: Involved in controlling the involuntary action of
smooth muscles, the heart and various glands.
○ Sympathetic: Uses norepinephrine → “fight or flight” activation → increased
heart rate, divert blood from digestive tract to somatic muscular and prepares
Structure and function of the nervous system
The structure of neurons
Neurons: Basic signalling units that transmit information
throughout the nervous system.
● Cell body: Contains nucleus, endoplasmic
reticulum, a cytoskeleton, mitochondria, Golgi
apparatus and other intracellular organelles.
● Dendrites: Extensions of the neuron that receive
inputs from other neurons.
● Axon: Output of the neuron. Electrical signals
travel along the axon to the axon terminals.
● Synapse: Transmission of two neurons, come into
close contact so that chemical or electrical signals
can be passed from one cell to the next.
● Axon collaterals: Can transmit signals to more than one cell.
Neuronal signalling
Within a neuron, transferring information involves changes in the electrical state of the
neuron as electrical currents flow through the volume of the neuron.
Between neurons, information transfer occurs at synapses, typically mediated by chemical
signalling molecules (neurotransmitters) but, in some cases, also by electrical signals.
● Presynaptic: When axons make a connection onto other neurons.
● Postsynaptic: When other neurons make a connection onto their dendrites.
The membrane potential:
● Resting membrane potential: The neuronal cytoplasm has a potential that is -70mV
compared to the extracellular space → neuron not actively signalling.
○ Lipid membrane: Maintains the separation of intracellular and extracellular
ions and electrical charge that ultimately permits neuronal communication.
● Ion channels: These hydrophilic channels selectively permit one type of ion to pass
through the membrane (Na+, K+, Ca 2+, and Cl−).
○ Permeability: Extent to which a particular ion can cross the membrane
through a given ion channel.
○ Gated ion channels: Ion channels that are capable of changing their
permeability for a particular ion.
○ Nongated ion channels: Ion channels that are unregulated, and hence
always allow the associated ion to pass through.
● Ion pumps: Active transport proteins → neurons use a
Na+/K+ pump that pumps Na+ ions out of the cell and K+
ions into the cell → transporting ions up their
concentration gradients → requires energy → pump is an
enzyme that hydrolyses ATP) → each molecule of ATP
that is hydrolysed, energy is used to pump the ions.
○ The force of the Na+ concentration gradient
pushes Na+ from an area of high concentration (outside) to one of low
, concentration (inside) → K+ concentration gradients push K+ from an area of
high concentration (inside) to one of low concentration (outside).
○ Electrical gradient: The force of the K+ concentration gradient pushes some
K+ out of the cell → inside neuron slightly more negative than the outside.
Action potential: Rapid depolarization and repolarization of a small region of the membrane
caused by the opening and closing of ion channels → can travel for meters.
● Excitatory postsynaptic potentials (EPSP): Postsynaptic potential that makes the
postsynaptic neuron more likely to fire an action potential.
● Inhibitory postsynaptic potentials (IPSP): Postsynaptic potential that makes the
postsynaptic neuron less likely to fire an action potential.
● Electronic conduction: Electrical current diminishes with distance from its origin →
synapse → action potential needed.
● Voltage-gated ion channels: These open all by themselves when the membrane
potential is -55 mV (threshold) or higher.
(Numbers correspond with the numbers in the figure)
1. Threshold reached → -55 mV.
2. Voltage-gated Na+ channels open and Na+ flows
rapidly into the neuron → further depolarizes →
more voltage-gated Na+ channels opened →
more depolarized → more channels open → etc.
3. Na+ channels close → voltage-gated K+
channels open → K+ flows out of neuron down
concentration gradient → membrane potential
shifts back toward resting potential →
repolarization.
4. Membrane temporarily hyperpolarizes → K+ channels close.
5. Membrane potential gradually returns to its resting state → absolute refractory
period → Na+ channels unable to open → another action potential cannot be
generated.
● Myelin sheets: Higher speeds are obtained in axons → work as insulators around
the axons → make axons super resistant to voltage loss.
● Saltatory conduction: Combines speed of electrotonic conduction with the distance
that can be travelled by action potentials: high speed, long distance communication
→ from one node of Ranvier to the next.
Synaptic transmission
A neuron communicates with other neurons, muscles, or glands at a synapse, and a transfer
a signal from the axon terminal to the next cell.
Chemical transmission:
, Neurotransmitters:
○ Synthesized by and localized within the presynaptic neuron and stored in the
presynaptic terminal before release.
○ It is released by the presynaptic neuron when action potentials depolarize the
terminal (mediated primarily by Ca2+).
○ The postsynaptic neuron contains receptors specific for the neurotransmitter.
○ When artificially applied to a postsynaptic cell, the neurotransmitter elicits the
same response that stimulating the presynaptic neuron would.
● Biochemical classification of neurotransmitters:
○ Amino acids: Aspartate, GABA, glutamate and glycine.
○ Biogenic amines: Dopamine, norepinephrine, epinephrine and histamine.
○ Neuropeptides: Larger molecules made up of strings of amino acids.
● Functional classification of neurotransmitters:
○ Excitatory: Catecholamines, glutamate, histamine and some neuropeptides.
○ Inhibitory: GABA, glycine and some peptides.
○ Conditional: Action is conditioned on the presence of another transmitter in
the synaptic cleft or activity in the neuronal circuit.
Inactivation of neurotransmitters after release: Monitor level of neurotransmitter in
synaptic cleft → presynaptic neurons have autoreceptors → regulate release of transmitter.
● Active reuptake of the substance back into the presynaptic terminal.
● Enzymatic breakdown of the transmitter in the synaptic cleft.
● Diffusion of neurotransmitter away from the region of the synapse or site of action.
Electrical transmission:
● Gap junction: Neuronal membranes are touching → create pores connecting the
cytoplasm’s of the two neurons.
● Isopotential: Electrical changes in one neuron are reflected instantaneously in the
other.
● Pro: Useful when information must be conducted rapidly and
when groups of neurons should operate synchronously.
● Con: They cannot amplify a signal.
The role of glial cells
Astrocytes: Help form the blood-brain barrier and have an active role
in modulating neural activity.
Microglial cells: Devour and remove damaged cells.
Oligodendrocytes: Forms myelin in the central nervous system.
Schwann cells: Forms myelin in the peripheral nervous system.
Overview of the nervous system structure
The central nervous system: Consists of brain and spinal cord.
● Three protective membranes (outer - inner): Dura mater, arachnoid mater, pia mater.
● Cerebral cortex: Continuous sheet of layered neurons in different parts of the brain.
Peripheral nervous system: Consist of nerves and ganglia (groups of neurons) outside of
the CNS.
● The autonomic nervous system: Involved in controlling the involuntary action of
smooth muscles, the heart and various glands.
○ Sympathetic: Uses norepinephrine → “fight or flight” activation → increased
heart rate, divert blood from digestive tract to somatic muscular and prepares
