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P & A Notes Nervous System Part 1

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P & A Notes--Nervous System Part 1

Nervous System
Part #1
*Two major subdivisions:

1.) Central Nervous System (C.N.S.)

2.) Peripheral Nervous System (P.N.S.)

These systems provide three general functions-

-Sensory function:

-Integrated function:

-Motor function:

*The functional unit of the nervous system is the neuron.
Three types: (pg. 230)

-Sensory Neuron- carry impulses from body to the brain or spinal cord.

-Motor Neuron- Carry impulses from C.N.S. to the “effecters.”

-Interneurons- Direct impulses to the parts of the brain for processing, also transfers impulse to motor neurons. Found only in the brain or spinal cord.

Neurons vary in size and shape, but all have a cell body and nerve fibers. (pg.218, Fig.9.6)
-Cell body- An enlarges area with a nucleus and a prominent nucleolus. In a mature neuron, this nucleus seems incapable of mitosis... Reproduction.

-Nerve Fibers- Dendrites... usually short and highly branched... providing a good receptive surface. It carries the impulse toward the cell body. Axon... Carries the impulse away from the cell body. At its terminal end the axon has many fine extensions that contact the dendrites of another cell.

*Myelin- is a lipoprotein that forms an insulation barrier around a nerve. Myelinated fibers form the white matter of the nervous system. Unmyelinated fiber form the gray matter.

*Neuroglial Cells

-Schwann cells are found in the P.N.S They are responsible for forming a myelin shealth around the axons of peripheral nerves. (pg. 216-217) The narrow gaps between Schwann cells is called nodes of Ranvier

Neuroglial cells of the C.N.S. greatly outnumber the neurons. Neuroglial cells have the ability to divide by mitosis. (pg.215, Fig.9.3)

-Astrocytes- provide structural support. Form scar tissue following an injury.
-Oligodendrocytes- form myelin in the C.N.S.
-Microglia- Pagocytize bacteria and cellular debris.
-Ependyma- These cells form the choroid plexuses... which in turn produce cerebrospinal fluid. (fig. 9.13 pg.240)

Impulse Conduction:

An unmyelinated nerve conducts an impulse over its entire surface.  In a myelinated nerve, the myelin serves as an insulator.  The nodes of Ranvier are “very” permeable to ions and the impulse appears to jump from node to node.  This impulse is many times faster than in an unmyelinated neuron. (Termed, saltatory)

Nerve Impulse:

The resting state or “resting potential” is when there is a (+) charge on the outside of the fiber and a (-) charge on the inside.  
When a nerve is at rest its membrane is imperable to sodium ions (Na+) and potassium (K+).
The nerve fiber has a high number of Na+ on the outside of the membrane and a huge number of K+ on the inside.  Along with the K+ is a very high number of negatively charged ions.  Active transport keeps Na+ and K+ in their respective locations.
This gives the nerve a “+” charge on the outside and a negative charge on the inside, giving it a polarized state. ( Nerves may respond to changes in temperature, light, pressure, chemicals, neurotransmitters....when strong enough...threshold potential)
When this nerve is stimulated some of the Na+will move across the membrane and depolarize it in that region.
Immediately some K+ will move out and repolarize the membrane.  This is an action potential.
A chain reaction of action potentials moves down the nerve giving as a nerve impulse.
The sequence of events takes about one-thousandth of a second.

-Synapse- The junction between the dendrites of one neuron and the axon of another. The gap between these two cells is called the synaptic cleft.

Synaptic Transmission: (pg.220, Fig. 9.9 & 9.10)

When an action potential reaches the synaptic knob it increases the permeability of the knob’s membrane to calcium....Calcium moves in causing the synaptic vesicles to release their contents into the cleft.
The neurotransmitter  will then diffuse across the synaptic cleft into the dendrite of a connecting nerve.
-Neurotransmitter: Acetylcoline (stimulates skeletal muscles), Monoamines (epinephrine, norepinepherine, dopamine and serotonin) The vesicles are quickly resupplied with neurotransmitters through the process of protein synthesis.
The neurotransmitters are usually decomposed in the synaptic cleft by another enzyme.
-Acetylcoline is decomposed by cholinesterase.
-Monomine is decomposed by monomine oxidase.
-others are removed from the cleft back into the vesicles by active transport.
*Removal of the neurotransmitter prevents a continuous stimulation.

Reflexes are automatic responses to changes inside or outside the body designed to help maintain homeostasis.
*Heart rate, breathing rate, blood pressure, and digestion are all involuntary processes controlled by reflexes.  Sneezing, coughing & vomiting are also reflex actions.

Reflex arc... the simplest of nerve pathways.

Withdrawal reflex: ....three neurons
You tough something hot/sharp... your sensory neurons take the impulse from your skin receptors to the spinal cord.  It is then relayed to an association neuron which will send it out to a motor neuron... to a flexor muscle of the arm and you jerk your arm away.
The association neuron also carries the sensory impulse to the brain and we feel pain.
Reflex arc... the simplest of nerve pathways.

Knee jerk:... two neurons.

Automatic bladder: .. two neurons.  (pg. 468)

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