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Neurophysiology Bibliography Campbell 3rd edition The nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that carry rapid electrical impulses The Alpha Motor Neurone The structure of a motor neuron Injury to α-MNs is the most common type of lower motor neuron lesion. Damage may be caused by trauma, ischemia, and infection, among others. In addition, certain diseases are associated with the selective loss of α-MNs. For example, poliomyelitis is caused by a virus that specifically targets and kills motor neurons in the ventral horn of the spinal cord. Amyotropic lateral sclerosis likewise is associated with the selective loss of motor neurons. Paralysis is one of the most pronounced effects of damage to α-MNs. Because α-MNs provide the only voluntary innervation to extrafusal muscle fibers, losing α-MNs effectively severs the connection between the brainstem and spinal cord and the muscles they innervate. Without this connection, voluntary and involuntary (reflex) muscle control is impossible. Voluntary muscle control is lost because α-MNs relay voluntary signals from upper motor neurons to muscle fibers. Loss of involuntary control results from interruption of reflex circuits such as the tonic stretch reflex. A consequence of reflex interruption is that muscle tone is reduced, resulting in flaccid paresis. Another consequence is the depression of deep tendon reflexes, causing hyporeflexia. Muscle weakness and atrophy are inevitable consequences of α-MN lesions as well. Because muscle size and strength are related to the extent of their use, denervated muscles are prone to atrophy. A secondary cause of muscle atrophy is that denervated muscles are no longer supplied with trophic factors from the α-MNs that innervate them. Alpha motor neuron lesions also result in abnormal EMG potentials (eg, fibrillation potentials) and fasciculations, the latter being spontaneous, involuntary muscle contractions. Diseases that impair signaling between α-MNs and extrafusal muscle fibers, namely diseases of the neuromuscular junction have similar signs to those that occur with αMN disease. For example, myasthenia gravis is an autoimmune disease that prevents signaling across the neuromuscular junction, which results in functional denervation of muscle. The synapse The principles of synaptic transmission Nerve Impulse Produce a Ca+ flow into the axon terminal Consequence: exocytosis of a neurotransmitter substance in vesicles In an excitatory synapse, this produce the opening of Na+ channels which causes an action potential, that starts in the following neuron This neurotransmitter diffuses across the synaptic cleft and attaches to receptors in the post-synaptic membrane The Reflex Arc Nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons Receptors • • • • Odor receptors Photoreceptors Mechanoreceptors Gustatory receptors Sensory neurons CNS Motor neurons Effectors •Skeletal Muscles •Cardiac muscles •Endocrine and exocrine glands Relay neurons Definition of resting potential and action potential (depolarization and repolarization) •Resting Potential The electrical potential (measured in millivolts, mV) across a cell membrane when not propagating an impulse. •Action Potential The localised reserval and then restoration of the electrical potential (measured in mV) across the membrane of a neuron as the impulse passes along it. How a nerve impulse passes along a nonmyelinated neuron Resting Potential Action Potential Depolarization From -70mV to +40mV Na+ pores shut K+ pores open Repolarization From +40mV to -70mV K+ pores shut