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IMPOERTANT POINTS
The central nervous system is the control center for the body. It regulates organ function, higher thought, and
movement of the body. The central nervous system consists of the brain and spinal cord.
Generation & propagation of an action potential
The Nerve Impulse
When a nerve is stimulated the resting potential changes. Examples of such stimuli are pressure, electricity,
chemicals, etc. Different neurons are sensitive to different stimuli(although most can register pain). The stimulus
causes sodium ion channels to open. The rapid change in polarity that moves along the nerve fiber is called the
"ACTION POTENTIAL." This moving change in polarity has several stages:
Depolarization
The upswing is caused when positively charged sodium ions(Na+) suddenly rush through open sodium gates
into a nerve cell.The membrane potential of the stimulated cell undergoes a localized change from-65
millivolts to 0 in a limited area. As additional sodium rushes in, the membrane potential actually reverses its
polarity so that the outside of the membrane is negative relative to the inside. During this change of polarity
the membrane actually develops a positive value for a moment(+40 millivolts). The change in voltage
stimulates the opening of additional sodium channels (called a voltage-gated ion channel). This is an example
of a positive feedback loop.
Repolarization
(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
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
control of vision; and temporal lobes which consists of hearing centers and some speech. Located deep to the
temporal lobe of the cerebrum is the insula.
Cerebellum
The cerebellum is the part of the brain that is located posterior to the medulla oblongata and pons. It coordinates
skeletal muscles to produce smooth, graceful motions. The cerebellum receives information from our eyes, ears,
muscles, and joints about what position our body is currently in (proprioception). It also receives output from the
cerebral cortex about where these parts should be. After processing this information, the cerebellum sends motor
impulses from the brainstem to the skeletal muscles. The main function of the cerebellum is coordination. The
cerebellum is also responsible for balance and posture. It also assists us when we are learning a new motor skill, such
as playing a sport or musical instrument. Recent research shows that apart from motor functions cerebellum also has
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