Human Physiology/The Nervous System

6

(The downswing) is caused by the closing of sodium ion channels and the opening of potassium ion channels.

Release of positively charged potassium ions (K+) from the nerve cell when potassium gates open. Again,

these are opened in response to the positive voltage--they are voltage gated. This expulsion acts to restore the

localized negative membrane potential of the cell (about -65 or -70 mV is typical for nerves).

Refractory phase

is a short period of time after the depolarization stage. Shortly after the sodium gates open they close and go

into an inactive conformation. The sodium gates cannot be opened again until the membrane is repolarized to

its normal resting potential. The sodium-potassium pump returns sodium ions to the outside and potassium

ions to the inside. During the refractory phase this particular area of the nerve cell membrane cannot be

depolarized. This refractory area explains why action potentials can only move forward from the point of

stimulation.

Increased permeability of the sodium channel occurs when there is a deficit of calcium ions. when there is a deficit

of calcium ions (Ca+2) in the interstitial fluid the sodium channels are activated (opened) by very little increase of

the membrane potential above the normal resting level. The nerve fiber can therefore fire off action potentials

spontaneously, resulting in tetany. Could be caused by the lack of hormone from parathyroid glands. could be caused

by hyperventilation, which leads to a higher pH, which causes calcium to bind and become unavailable. Speed of

conduction. This area of depolarization/repolarization/recovery moves along a nerve fiber like a very fast wave. In

myelinated fibers, conduction is hundreds of times faster because the action potential only occurs at the nodes of

Ranvier (pictured below in 'types of neurons') by jumping from node to node. This is called "saltatory" conduction.

Damage to the myelin sheath by the disease can cause severe impairment of nerve cell function. Some poisons and

drugs interfere with nerve impulses by blocking sodium channels in nerves. See discussion on drug at the end of this

outline.

Brain

A color-coded image of the brain, showing the

main sections.

The brain is found in the cranial cavity. Within it are found the higher

nerve centers responsible for coordinating the sensory and motor

systems of the body (forebrain). The brain stem houses the lower nerve

centers (consisting of midbrain, pons, and medulla),

Medulla

The medulla is the control center for respiratory, cardiovascular and

digestive functions.

Pons

The pons houses the control centers for respiration and inhibitory functions. Here it will interact with the cerebellum.

Cerebrum

The cerebrum, or top portion of the brain, is divided by a deep crevice, called the longitudinal sulcus. The

longitudinal sulcus separates the cerebrum in to the right and left hemispheres. In the hemispheres you will find the

cerebral cortex, basal ganglia and the limbic system. The two hemispheres are connected by a bundle of nerve fibers

called the corpus callosum. The right hemisphere is responsible for the left side of the body while the opposite is true

of the left hemisphere. Each of the two hemispheres are divided into four separated lobes: the frontal in control of

specialized motor control, learning, planning and speech; parietal in control of somatic sensory functions; occipital in