BI111 Lecture Notes - Lecture 19: Occipital Lobe, Peripheral Nervous System, Grey Matter
Biology- nervous system
• Nerve impulses move like electricity through a wire, except that they do not lose their signal strength
over distances
• The nerve impulse is due to the movement of ions through the nerve tissue
• The ion movement creates a potential difference (v) between the inside and outside of the nerve
• K+ ions tend to move out of the nerve tissue, while Na+ ions tend to move into the nerve tissue.
• The permeability for K+ is greater than that of Na+ (so more K+ ions move out of the tissue than Na+
move in)
• This creates a slightly negative charge on the inside of the nerve, compared to the exterior (-70 mV);
this is the resting potential
Sending a signal
• When stimulated, the nerve becomes more permeable to Na+ (ion gates open) and K+ ion gates close
• This quickly reverse the charge of the nerve to a slightly positive charge (+40mV), this is called
depolarization
• Once enough of a charge difference has been reached, a signal is triggered
• This is referred to as the action potential
• After the signal has passed by, the sodium gates close and potassium gates open to remove + ions
from the nerve
• A sodium-potassium pump restores the resting conditions of the nerve, removing Na+ and
replacing with K+ (repolarization)
• There is a time lapse between depolarization and repolarization when the nerve cannot conduct a
signal
• This is called the refractory period, which can last for 1-10 ms.
• The depolarization/action potential of the nerve travels along the entire length of the axon in one
direction
o Resting state
o Depolarization
▪ Threshold (only if enough stimulation)
▪ Action potential
o Repolarization
▪ Refractory period (takes 1-10 ml) - in one direction
• Once enough of a charge difference has been reached, a signal is triggered, this is referred to as the
action potential
• Sodium goes in, potassium comes out
Impact of myelin on signals
• Myelin helps to protect, and insulate, the signal from a nerve
• Dendrite --> cell body --> axon
• The signals of a myelinated nerve travel much faster than a nerve without myelin
• Ion transport jumps from node to node, instead of one continuous steam
• Happens in nodes
• When you have myelin on the nerve, it happens from node, to node, to node
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Threshold levels
• There is a minimum amount of stimulation required in order to trigger a nervous system (threshold
level)
• Once the stimulus has reached the required level, the nerve will fire (the all - or- none response)
• The frequency of nerve impues will give the sensation of strength of the stimulus
• The brain recognizes more impulses in a time as a strong signal
• E.g. a hot substance will trigger more impulses than a warm substance
• You either send the signal or you don’t- no half way
Define:
✓ Dendrites: projections of cytoplasm that carry impulses toward the cell body
✓ Axons: extension of cytoplasm that carries nerve impulses away from the cell body
✓ Myelin sheath: insulated covering over the axon of a nerve cell
✓ Schwann cells: special type of glial cell that produces the myelin sheath
✓ Nodes of Ranvier: regularly occurring gaps between sections of the myelin sheath along the axon
✓ Neurilemma: delicate membrane that surrounds the axon of some nerve cells
• Synaptic transmission
• Nerves must be able to send/ receive signals with other nerve to span your entire body
• Synapses are the spaces between neurons
• Signals are sent out from the presynaptic neuron, across the synaptic cleft, and into the post synaptic
neuron
• Neurotransmitters are chemicals released by the nervous and across the synaptic cleft
• Acetylcholine is one of the many Neurotransmitters
• It is able to make the post synaptic nerve more permeable to Na+, this triggers a new nerve signal
• Cholinesterase is released to destroy the acetylcholine after the signal passes
• In some cases several nerves may meet up and join with one new nerve this is called summation
• Some nerves will have an inhibitory effect
• Coordinating excitatory and inhibitory effects can allow for proper control of muscles and tissues
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Document Summary
Ion transport jumps from node to node, instead of one continuous steam. Dendrites: projections of cytoplasm that carry impulses toward the cell body. Axons: extension of cytoplasm that carries nerve impulses away from the cell body. Myelin sheath: insulated covering over the axon of a nerve cell. Schwann cells: special type of glial cell that produces the myelin sheath. Nodes of ranvier: regularly occurring gaps between sections of the myelin sheath along the axon. Neurotransmitters are chemicals released by the nervous and across the synaptic cleft. It is able to make the post synaptic nerve more permeable to na+, this triggers a new nerve signal: cholinesterase is released to destroy the acetylcholine after the signal passes. In some cases several nerves may meet up and join with one new nerve this is called summation. Coordinating excitatory and inhibitory effects can allow for proper control of muscles and tissues.
