gen physio: nerve physiology

• Neurons are the basic building blocks of the nervous system.
• There are 100 billion neurons in the human nervous system.
• The function of the parts of the neuron includes the cell body which maintains the functional and anatomic integrity of the neuron, the dendrite which receives messages in form of impulses for the neuron, the axon which transmits impulses to other neurons or to end organs, the node of Ranvier which increases the speed of impulse conduction, and the terminal buttons which store synaptic granules secreted by the nerve.
• Excitation in nerve cells can be caused by mechanical, chemical, electrical, and thermal stimuli.
• Two types of physico-chemical disturbances are produced once the nerve has been stimulated: local, nonpropagated potentials called synaptic, generator or electrotonic potentials and propagated disturbances called NERVE IMPULSES which are the universal language of the nervous system.
• The impulse is normally transmitted or conducted along the axon to its termination.
• Conduction is an active, self-propagating process that requires expenditure of energy by the nerve and the impulse moves along the nerve at a constant amplitude and velocity.
• There are electrical potential changes in a nerve when it conducts impulses.
• The resting membrane potential in neurons is -70 mV.
• Stimulus artifact, current leakage from the stimulating electrodes to the recording electrodes, marks the point at which the stimulus was applied.
• The latency period corresponds to the time it takes the impulse to travel along the axon from the site of stimulation to the recording electrodes.
• The firing level marks the point at which the change in rate of depolarization occurs.
• The spike potential is a sharp rise and rapid fall.
• After-depolarization is a slower fall at the end of the process.
• The resulting net transfer of positive charge out of the cell completes repolarization.
• Another factor producing repolarization of the nerve membrane is the increase in K+ permeability that follows that of Na+, which starts more slowly and reaches a peak during the falling phase of the action potential.
• Accomodation is the process wherein the nerve adapts to the applied stimulus, meaning the stimulus fails to fire the nerve, therefore, no impulse or action potential is produced.
• Saltatory Conduction is the jumping of depolarization from one node of Ranvier to the next, increasing the speed of conduction of impulse.
• In myelinated Neurons, the speed of conduction increases directly with the diameter of the axon.
• The subthreshold intensity is the intensity of stimulus which will not produce an action potential or impulse.
• The threshold intensity is the minimum amount of intensity of stimulus that will produce an impulse, and therefore, an action potential.
• The relative refractory period follows the absolute refractory period up to the start of after-depolarization, during which stronger than normal stimuli can cause excitation.
• The absolute refractory period is the period wherein no stimulus, no matter how strong, will excite the nerve, corresponding to the period from the time the firing level is reached until repolarization is about one third complete.
• In unmyelinated Neurons, the speed of conduction increases with the square root of the axon diameter.
• The maximal stimulus is the stimulus that produces excitation of all the axons in a peripheral nerve.
• The electrical gradient for Na+ is reversed during the overshoot because the membrane potential is reversed.
• The all or none law states that a stimulus at threshold intensity will produce an impulse; increasing it will not affect the produced impulse and decreasing it will produce no impulse at all.
• A weak stimulus will not produce a response no matter how long the stimulus is applied.
• The Na+ permeability is short-lived, limiting Na+ influx and helping bring about repolarization.
• A stimulus of extremely short duration will not excite the nerve no matter how intense this particular stimulus is.
• During the action potential when the cell membrane is depolarized, there is an increase in membrane permeability to Na+, therefore, Na+ influx.
• The electrical and concentration gradients for Na+ are both directed inward, therefore, Na+ influx further lowers the membrane potential and Na+ permeability is further increased.
• This Na+ influx will overcome the repolarizing process (K+ going out of the cell) and this will produce the spike potential.