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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