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 α-MN 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

This neurotransmitter diffuses across the synaptic cleft and attaches to receptors in the

post-synaptic membrane

In an excitatory synapse, this produce the opening of Na+ channels which

causes an action potential, that starts in the following neuron

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

CNSSensory neurons Relay neurons

Effectors•Skeletal Muscles

•Cardiac muscles

•Endocrine and exocrine glands

Motor 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 non-myelinated neuron

Resting Potential

Action Potential

Depolarization

From -70mV to +40mV

Na+ pores shut

K+ pores open

Repolarization

From +40mV to -70mV

K+ pores shut