This web page contains notes to accompany lectures in Vertebrate Physiology, Biology 410, taught by Dr. Peter King in the Department of Biology, Francis Marion University, Florence, South Carolina, 29502, USA.

The nervous system is the "other half" of the control network for an animal along with the endocrine system.
The nervous system is very quick to respond, (milliseconds).
The nervous system tends to be short term control as opposed to long term control by the endocrine system (generalization).

Function
Receiving information about the internal and external environment.
Integration of information.
Coordinating reaction to the information received.
Stimulating effectors.

Organization
Structurally the nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system, CNS, contains the brain and the spinal cord.
It integrates input from the peripheral nervous and is the source of consciousness.
For example: the cerebral cortex has an area that controls voluntary movement. Another area in the cerebellum coordinates appropriate patterns of movement. An animal with a damaged cerebrum may be able to walk but may not be able to initiate walking!

The peripheral nervous sytem is divided into two divisions: sensory and motor .
The sensory nerves receive stimuli and transfer the information to the central nervous system and the motor nerves transfer information from the CNS to effectors (muscles).

The motor division is again divided into the somatic or voluntary nervous system and the autonomic or involuntary nervous system.

The simplest circuit is known as a reflex arc and involves only 3 neurons.

The somatic motor system conducts nerve impulses to skeletal (striated or voluntary) muscle.

The autonomic nervous system controls smooth and cardiac muscle and in doing so control the function of many internal organs.
This system is divided into the sympathetic and parasympathetic systems.
These 2 systems are generally antagonistic to each other.

The nervous system is made up of neurons (nerve cells) that transmit nervous impulses and glial cells (supporting cells).
Glial of the CNS
Astrocytes - provide a cellular connection between the neurons and capillaries, transport nutrients and waste.
Control the chemical environment around the neuron.
Absorb and recycle neurotransmitters.
Not the blood brain barrier.
Oligodendrocytes - processes wrap around neurons and insulate them from stimulation from other neurons.
They form a myelin sheath around axons.
Microglia - small phagocytic white blood cells that migrate through CNS.
Ependymal cells - line the cavities of the CNS that are filled with cerebrospinal fluid (CFS). Have role in monitoring CSF and producing CSF.

Glial of the PNS
Satellite cells - surround the cell bodies of neurons. Thought to assist the neuron in maintaining homeostasis, e.g. chemical environment.
Schwann cells - wrap around the axon and forms a myelin sheath.
Myelin sheath increases the spread of transmission of a nerve impulse along the axon.

Neurons (and muscles) are different from other cells and are excitable cells. Neurons transmit nerve impulses.
Functionally neurons are divided into sensory neurons, interneurons and motor neurons.
Each have an input zone (dendrites), a transmission zone (axon) an output zone (axon terminal) and a cell body (contains the nucleus).

So how does a neuron work?
As mentioned earlier neurons are excitable cells.
Neurons maintain an imbalance in charged particles inside and outside of their cell membrane.
The cell traps large negatively charged organic molecules inside the cell, and actively controls the movement of positive ions to produce an electrical potential across the membrane.

Protein pumps in the membrane maintain the potential by pumping out 3 Na⁺ ions in exchange for 2 K⁺ ions.
The outside of the cell is more positive than the inside.
This imbalance is called the resting potential of the cell.
For a somatic motor neuron it is commonly about -70 millivolts, but varies in the range of neurons from about -30 mV to about -100 mV.

At their resting potential, neurons are referred to as polarized, because the inside is negative compared to the outside.

Allowing Na⁺ and K⁺ diffusion leads to a depolarization of the membrane potential.
Such a depolarization event is localized to the area around the point of excitation. Na⁺/K⁺ pumps soon return the potential back to the original resting potential.
The effect of this local depolarization diminishes with distance away from the site of excitation like a ripple from a pebble dropped in a pool.

What causes a local excitation?
Depending on the nature of the dendrites it can be many different criteria.
Such as:touchstretchpressureheata chemical

A nerve impulse is triggered when the membrane potential reaches a critical threshold level at the axon hillock.
This is usually attained by the summation of many local depolarization events.
A nerve impulse or action potential travels down the axon away from the axon hillock but does not diminish with distance.

Action potentials are either on or off.

The threshold is the level of depolarization that will cause special ion channels to open that allow diffusion of Na⁺ and K⁺.
These channels are called voltage gated channels.
An action potential results in the axon.

Voltage gated ion channels are located all along the axon.
Opening one voltage gated channel causes a local change in potential that opens adjacent voltage gated channels - these then cause a local change in potential that open adjacent voltage gated channels - these then cause a local change in potential that open adjacent voltage gated channels
- these then cause a local change in potential that open adjacent voltage gated channels
- these then cause a local change in potential that open adjacent voltage gated channels
- these then cause a local change in potential that open adjacent voltage gated channels
- these then cause a local change in potential that open adjacent voltage gated channels
- these then cause a local change in potential that open adjacent voltage gated channels ...

There are separate voltage gated channels for sodium and potassium.
These channels open at a threshold value and then closed by another 'gate'.
There is a refractory period of about 4 milliseconds where a channel that has closed cannot be reopened. This prevents an action potential going backwards

Why don't the Na⁺ and K⁺ cancel each other out immediately.
1. There is an electrical gradient that increases the diffusion of Na⁺ into the cell.
2. The concentration gradient id greater for Na⁺ than for K⁺.
3. The Na⁺ channels open first and then close quickly. i.e. large influx of Na⁺ to depolarize area.
4. K⁺ channels are slower to open and stay open longer, K+ flows out and membrane area is repolarized (Na⁺/K⁺ pumps also).

Myelinated axons transmit action potentials (APs) faster.
Voltage gated channels are located at spaces between Schwann cells (nodes of Ranvier).
AP skips from node to node and travels at a higher speed.
Remember: once started an action potential travels the length of the axon. It either happens or doesn't - on or off.

What happens when the AP reaches the axon endings?
The axon end are called terminal boutons or presynaptic endings.
A synapse is the communication area of a neuron and another cell.
As the AP gets to the presynaptic end another type of voltage gated channell is opened - a voltage gated Ca²⁺ channel.
Here Ca²⁺ acts like a second messenger, activates calmodulin ,which activates protein kinase

The end result of the influx of calcium is exocytosis of neurotransmitter stored in vesicles.
The result of an action potenial is the release of neurotransmitter into the synaptic cleft.

Neuron to neuron junction (chemical synapse)
There are many different neurotransmitters.
One common neurotransmitter in the CNS and the PNS is acetylcholine, ACh.
When ACh is released it enters the interstitial fluid and crosses the synaptic cleft.
The post synaptic cell has receptors for ACh.
The binding of the neurotransmitter to its receptor opens ion channels (chemically gated channels).

Opening ion channels will cause a localized change in polarity in the dendrite of the postsynaptic neuron.
Nicotinic receptors (named this because they are activated by nicotine) open sodium channels and thereby cause an influx of Na+ and a local depolarization.
This is called an Excitatory Post Synaptic Potential, or EPSP.

If the postsynaptic cell has muscarinic ACh receptors, binding of ACh opens K⁺ channels.
This causes an outflow of K⁺ and a hyperpolarization.
This is an Inhibitory Post Synaptic Potential or IPSP.

ACh dissociates quickly from its receptor but will bind again if it remains in the cleft.
An enzyme acetylcholinesterase AChE located on the post synaptic membraner or closely associated to it breaks down ACh.
Remnants of ACh are reabsorbed by the presynaptic neuron and reformed into ACh in vesicles.

Another common neurotransmitter in the CNS and PNS is norepinephrine (NE).
Its release is triggered in exactly the same way i.e. an AP opens Ca²⁺ channels and vesicles expell their contents by exocytosis.
NE binds to receptors on the post synaptic cell and a second messenger is activated inside the cell and ion channels are opened.
Depending on the receptor an EPSP or a IPSP is produced.

Enzymes, monamine oxidase (MOA) and catechol-O-methyltransferase (COMT) degrade the neurotransmitter.

Motor neurons
Acetylcholine, ACh, is the neurotransmitter used in all vertebrate neuromuscular junctions in the somatic (voluntary) nervous system.
In the autonomic system there are preganglionic and post ganglionic neurons.

Preganglionic neurons in the autonomic nervous system all use ACh as their neurotransmitter.
Parasympathetic post-ganglionic neurons use ACh.
Sympathic post-ganglionic neurons use NE.

Integration of the nervous system
Neuronal communication can be excitatory or inhibitory, depending on the neurotransmitter released and the receptors present.
An action potential in one neuron does not necessarily result in an action potential in a post synaptic neuron.
The effect at the axon hillock is a summation of all local potential changes at the dendrites.

Summation can be temporal or spatial.
e.g. the closer an excitatory synapse is to the axon hillock the greater its effect on reaching threshold.
e.g. the more frequently neurotransmitter is released the greater effect on membrane potential.

Neuronal connections (hard wiring) determine sensitivity. One neuron can have 1000s of terminal boutons that all receive an AP.
One neuron can contact 1 or 1000s of other neurons.
One neuron could receive input from 1 or 1000s of other neurons.
Endocrine/nerve intergration
Norepinephrine (sympathetic NS) is very similar structurally to the hormone epinephrine released by the adrenal medulla. They can bind to the same receptors and induce the same effects.

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This page was created by Peter King. Please contact the author at pking@fmarion.edu with comments.
http://people.fmarion.edu/pking/vertphys/nerves.html
Last edit January 10, 2011.
Copyright Peter King.