Nervous System III
I. Nerves travel in bundles
A. Gray matter
B. White matter=bundles of nerve fiber axons
C. Nerve fibers= fine fibers which extend throughout the dendrites, cell body and axon.
D. Tracts= Groups of axons travelling together to and from. They are considered white matter and are in (CNS)
1. ascending tracts are sensory
2. descending tracts are motor found in white matter of cord.
E. Nerves=group of axons carring sensory information
II. Resting state of a neuron:
A. The cytoplasm is + compared to the Extracellular fluid
B. Na+/K+ pump maintains this resting membrane potential
1. 30% of all energy used per day is used by this pump
2. It sends 3 Na+ ions out for every 2K+ ions it sends in
C. The membrane is impermeable to Na+ ions, but is permeable to K+ ions
D. The voltage difference across the cell membrane is 60millivolts
We say the resting membrane potential is = -60mV inside
III. Action Potential (AP)
A. This is what must occur in order for an impulse to be sent
B. Stimulus for an AP can be a conscious or unconscious thought or action
C. Sequence of events: (ALL OR NONE RESPONSE)
depolarization
1. Stimulus meets threshold value to cause...
2. Na+ gates to open in cell membrane=depolarization
3. Excess Na+ ions rush into neuron at the site of stimulus
4. Neighboring gates will open (become depolarized)
repolarization
5. When cytoplasm inside neuron reaches +40mV the Na+ gates automatically close.
6. Na+/K+ pump continues to work pumping Na+ back out of neuron bringing inside charge back to -60mV
refractory period
7. During the time just following repolarization, the membrane is very difficult to stimulate and usually no impulse can be conducted during this time.
IV. Factors effecting nerve impulse
A. Ca++ must be present in the cell in order for the Na+ gates to close. If Ca++ is low, Na+ gates don't close completely.
1. Ca++ tetany: Causes muscle spasms (can occur due to pregnancy or fatigue)
B. Local Anesthetic like novacaine: decrease membranes permeability to Na+. This blocks sensory impulses.
C. General Anesthetics like vicoden or codein: block the neurotransmitter release so the impulse doesn't get to the brain. Prevents perception of pain, causes sleepiness.
D. Epideral- below Lumbar section 3