Neurons and the Nervous System
T. Irving 1/21/99; revised A.Howard 2/22/00

Describe the structure and function of three major types of neurons.
Describe the nerve impulse as an electrochemical change.
Describe the structure and function of a synapse including transmission across a synapse.
Define a nerve and distinguish between spinal and cranial nerves.
List the various divisions of the nervous system and give a function for each.
Describe the path of a spinal reflex.
Describe the autonomic nervous system and show similarities as well as differences in the structure and function of the two divisions

This is generic term for cells that service the neurons in some way, nutritive or supportive functions
Schwann cells are a specialized neuroglial cell that form so-called myelinated nerves
Only peripheral neurons are myelinated
Schwann cells form an ‘anion skin’ wrapping around nerve axons or dendrites electrically insulating them from the exterior except at gaps called Nodes of Ranvier.
Surrounding the myelin layer is a neurilemma or cellular sheath.
If nerve cells are damaged, the neurilemma acts as a pathway for neuron regeneration

Impulse conduction can be very fast in myelinated nerve: ~200 m/s due to saltatory conduction
Inside of resting nerve cells is at a potential of -65 mV relative to the outside
[Na⁺] is greater outside
[K⁺] is greater inside
Caused by Na⁺/K⁺ pump in the membranes
Movements of these ions across the membrane primarily responsible for movement of electrical impulses
Polarity changes are called an Action Potential

Early in an action potential Na+ channels in the membrane open
[Na⁺] ions move inwards
Membrane potential moves from -65 mV to +40 mV
Membrane said to depolarize
Depolarization causes [K⁺] channels to open
[K⁺] ions move outwards
Membrane re-depolarizes
Membrane voltage goes slightly beyond -65 mV and then returns to its resting value
There is a refractory period during which nerve conduction is not possible
During this period the Na⁺/K⁺pump restores ionic balances to resting value.

Axons branch into small terminal branches each tipped with an axon bulb
Axon bulbs come in very close contact with either the dendrite or cell body of another neuron
Region containing the gap between them called a synapse
Gap called the synaptic cleft
Transmission across synapse is by way of neurotransmitters, small organic compounds that are stored in synaptic vesicles in the pre-synaptic cells
When a nerve impulse arrives at an axon bulb, a reaction involving Ca²⁺ ions, the cytoskeleton and integral membrane proteins causes vesicles to fuse with the pre-synaptic membrane releasing their contents

In the post synaptic cells, neurotransmitters bind to receptors
In excitation, binding of neurotransmitter causes membrane potential to move in positive direction
In sufficient neurotransmitter binds, a threshold for an action potential is exceeded and an action potential occurs
Inhibitory binding causes the membrane potential to become more negative decreasing the likelihood of an action potential
Common neurotransmitters are acetylcholine (ACh) and norepinephrine (NE)
ACh is rapidly broken down by acetylcholinesterase
Other neurotransmitters are actively reabsorbed
Neurotransmitters are only active for a short period of time.

Nerves consist of long axons or long dendrites of many neurons bundled together
Cells bodies are usually found in the CNS or in ganglia
ganglia are local clusters of cell bodies in the PNS
Somatic System:
contains nerves that control muscles, skin and joints
Autonomic System
contains nerves that control glands and smooth muscles
Cranial nerves (12 pairs):
- Nerves that originate in the brain
- Nerves for head, neck, facial regions
- Vagus nerve an exception -serves many internal organs
Spinal Nerves (31 pairs):
- Originate in spinal column
- Dorsal root contains dendrites of sensory neurons bringing impulses to the spine
- Ventral root contains axons of motor neurons carrying impulses away from the spine
- Spinal nerves are mixed - both sensory and motor
Neurons have an all or none response;
Nerves may have a graded response depending on how many individual neurons within the nerve fire.

Reflexes are automatic responses to things in the environment that do not necessarily involve the brain
e.g. eye blinking, swallowing, hand withdrawal from a hot object, knee reflex
In a typical reflex:
- A receptor generates nerve impulses
- Sensory neuron carries message along its long dendrite (spinal nerve) to its cell body located in the dorsal root ganglion
- Its axon then carries it to the spinal cord itself
- An interneuron picks up impulse in its dendrites and carry it along its axon to the dendrites of a motor neuron whose cell body lies within the spinal cord.
- Impulses travel along the axon of the motor neuron through the ventral root of the spinal cord to an effector
- The effector glands secrete or muscle contracts

Autonomic System

Motor neurons are responsible for automatically controlling internal organs
In both cases two outgoing motor neurons involved
First neuron has cell body in CNS and preganglionic fiber.
Second neuron has cell bodies are in ganglia, with axons (postganglionic fiber) extending to target organ
Sympathetic System = Fight or Flight
- preganglionic fiber short, post ganglionic long
- neurons arise from thoracic-lumbar region (middle) of spine
- Sympathetic system when required:
  accelerates heartbeat, dilates bronchi & increases breathing rate, digestion is inhibited
- neurotransmitter in sympathetic system primarily norepinephrine (NE) which similar to the drug epinephrine a.k.a. adrenaline
Parasympathetic system = Rest & Digest
- some cranial nerves (vagus) and sacral spinal nerves
- preganglionic fiber long, postganglionic short
- i.e. ganglia are out near target organs.
- Pupils contract, heartbeat slows down, digestion promoted
- neurotransmitter primarily acetylcholine