Parts of the Nervous SystemThe two halves of the nervous system work together in order for your body to properly communicate its sensations and needs. The forebrain, midbrain, hindbrain, and spinal cord form the central

nervous system (CNS), which is one of two great divisions of the nervous

system as a whole. The brain is protected by the skull, while the spinal cord,

which is about 17 inches (43 cm) long, is protected by the vertebral column.

The other great division of the human brain is the peripheral nervous

system (PNS), which consists of nerves and small concentrations of gray

matter called ganglia, a term specifically used to describe structures in the

PNS. Overall the nervous system is a vast biological computing device

formed by a network of gray matter regions interconnected by white matter

tracts.

The brain sends messages via the spinal cord to peripheral nerves

throughout the body that serve to control the muscles and internal organs.

The somatic nervous system is made up of neurons connecting the CNS

with the parts of the body that interact with the outside world. Somatic

nerves in the cervical region are related to the neck and arms; those in the

thoracic region serve the chest; and those in the lumbar and sacral regions

interact with the legs.

The autonomic nervous system is made of neurons connecting the CNS with

internal organs. It is divided into two parts. The sympathetic nervous

system mobilizes energy and resources during times of stress and arousal,

while the parasympathetic nervous system conserves energy and resources

during relaxed states, including sleep.

Messages are carried throughout the nervous system by the individual units

of its circuitry: neurons.

How impulses move through nerves:

Impulses move from one nerve cell to another because of a difference in electrical "action potential" caused by ions inside and outside the cell. The cell membrane is selectively permeable to potassium ions, K+, and highly impermeable to sodium ions, Na+.

These are the basic steps:

1. Resting state:

A neuron is not conducting an impulse. The K+ concentration is higher inside the cell than out. The Na+ concentration is higher outside the cell than in.

2. Depolarization:

A nerve cell is stimulated. At the point of stimulation, the membrane becomes permeable

to Na+ for an instant and they quickly move into the cell. The inner surface of the cell membrane is now more positively

charged than the outside.

3. Repolarization:

When the cell membrane becomes depolarized, K+ automatically leave the cell until the cell is back to its resting state.

4. The impulse travels:

This quick movement of ions causes a similar change or wave all across the cell and down the axon.

Vertebrate nerves are covered by a myelin sheath with openings called nodes. The myelin sheath is an insulator and causes the ion exchange to occur only at the nodes. This speeds up the process by several orders of magnitude.

5. Transmission across a synapse:

Neurons to not actually touch. The axon terminals of one neuron stop before reaching the dendrites of the next neuron. This gap between the two cells is called a synapse.

Impulses are carried across a synapse by chemical messengers called neurotransmitters.

Approximately 30 different neurotransmitters have been identified, but they all do one of two things:

 

1. Stimulate the action potential in the next neuron cell.

Speed up the impulse.

2. Inhibit the action potential in the next neuron.

Slow down or completely stop the impulse.

6. Refractory period:

The period of time it takes a neuron to return to its resting potential after being stimulated.

A neuron cannot be stimulated during its refractory period. This period of time is about 0.004 of a second.

 

How Fast Are Nerve Impulses?

Action potentials can travel along axons at speeds of 0.1 to 100 m/s. This means that nerve impulses can get from one part of a body to another in a few milliseconds, which allows for fast responses to stimuli.

Impulses are MUCH slower than electrical currents in wires, which travel at close to the speed of light, 3x108 m/s.

The speed is affected by 3 factors:

Temperature - The higher the temperature, the faster the speed. So warm-blooded animals have faster responses than cold-blooded ones.

Axon diameter - The larger the diameter, the faster the speed. So marine invertebrates, who live at temperatures close to 0oC, have developed thick axons to speed up their responses. This explains why squid have their giant axons.

Myelin sheath - Only vertebrates have a myelin sheath surrounding their neurones. The voltage-gated ion channels are found only at the nodes of Ranvier, and between the nodes the myelin sheath acts as a good electrical insulator. The action potential can therefore jump large distances from node to node (1 mm), a process that is called saltatory propagation. This increases the speed of propagation dramatically, so while nerve impulses in unmyelinated neurones have a maximum speed of around 1 m/s, in myelinated neurones they travel at 100 m/s.