Transcript Motor Unit & It`s Excitation

Motor Unit & It’s Excitation
By: Baljit Brar
What Is a Motor Unit?
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A Motor Unit is described as being a
motor neuron plus the muscle fibres
that it contracts.
What Is a Motor Neuron?
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Also known as Efferent, or effector
neurons, are described as being nerve
fibres that are located in the Central
Nervous System and the nerve fibres that
elongate to the muscle fibres.
The Central Nervous System
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In the spinal cord we have two types of
matter
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White Matter
Grey Matter
In the Grey matter of the spinal cord, we
have two dorsal horns and two ventral
horns.
The Motor neurons coming out of the
spinal cord are located in the ventral
horns, therefore they can be also named
as Ventral Horn Cells.
The Central Nervous System
The Ventral Horn cells give rise to
the motor neuron that runs towards
the muscle fibres.
 Motor neurons can only come out of
the ventral aspects from the ventral
horn cells.
 Here is what this looks like:
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Motor Unit Properties: Stimulation
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The Motor Unit is stimulated generally by the
brain. The brain will send a stimulus to the
ventral horn cells.
The ventral horn cells then receive this
stimulation from the brain, and in turn become
activated to further stimulate their end muscle
fibres to respond.
The Motor Unit can also be stimulated by
something called the Reflex Arc
The Reflex Arc occurs when the sensory
receptors receive great stimuli. In turn the
sensory nerves stimulate interneuron's to further
stimulate the Motor Unit to activate.
Motor Unit Properties: Excitation
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The Motor units which lead to Skeletal
Muscle each are excitatory only.
This means any stimuli will cause the
neuromuscular junction to release
neurotransmitters into the muscle fibres
This will ultimately cause a contraction in
the muscle fibre itself.
Motor Unit Properties: Excitation
As for Cardiac muscle and Smooth
muscle, the response given from a
stimuli can be either excitatory or
inhibitory.
 This means that stimuli may be sent
to these muscles but the muscle
does not have to respond to that
stimuli.
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Motor Unit Properties: Precision
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The total area of Muscle Fibres a single
Motor Unit innervates determines how
precise those muscle fibres movements
will be.
If a single Motor Unit innervates less
muscle fibres, those muscle fibres will be
able to have more precise movements.
The more muscle fibres a single Motor
Unit innervates, the less precise the
muscle movements will be.
Eg) Motor Units in leg muscles and back muscles
must innervate more muscle fibres. Eye muscles
however have less muscle fibres to innervate.
Motor Unit Properties: Muscle
Contraction
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Some muscles have many motor units
innervating different muscle fibres of that
same muscle. We can see these certain
motor units excite their muscle fibres at
the same time.
When one motor unit for a certain muscle
is stimulated, this motor unit in turn
often will stimulate motor units running
to the same muscle.
This property is known as “Motor Unit
Recruitment”
Clinical Aspects
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Lower Motor Neuron Lesion
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This is described as being damage to the
motor neurons running from the spinal cord
to the muscle fibres that it innervates.
This damage will first cause the muscle
paralysis, along with atrophy of the muscle.
In place of the muscle shrinkage we can see
formation of fibrous tissue or fibrous
muscle.
Fibrillations, or invisible fine irregular muscle
contractions in skeletal muscle, can also be
seen in LMNL.
Also Fasciculation’s can be seen, which are
visible by the naked eye on the skin, can
occur as well.
Muscular Dystrophy
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Is described as a condition where a defect in the
Dystrophin gene leads to complications in the
Dystrophin glycoprotein complex.
The Dystrophin glycoprotein complex is described as being
a large protein which connects thin filaments of skeletal
muscle fibres to the tissue anchors in the extra cellular
fluids which help the structural integrity of the muscle
fibres to each other.
This condition comes with many symptoms generally
dealing with chronic muscle weakness and is hereditary
with different degrees is severity.
There are two main types
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Duchenne's
Becker’s
Muscular Dystrophy
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Duchenne’s – This X-linked recessively
transferred condition is the most severe
of the two types.
Patients with this type experience an
onset of muscle weakness as early as
infancy due to complete lack of the
Dystrophin glycoprotein complex.
Eventually this muscle weakness will
increase and lead to complications of the
heart muscles eventually leading to death
around the ages of 20-40.
Muscular Dystrophy
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Becker’s – This type is less severe in
contrast to Duchenne’s Muscular
Dystrophy.
Muscle weakness can be seen in this type
but the weakness is less severe because
the Dystrophin glycoprotein complex can
be seen in small amounts, or in an
altered manner.
Patients with Becker’s muscular
dystrophy however usually will live the
normal life span.
Myotonia
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This condition is described as being the
presence of abnormal genes on chromosomes
7, 17, and 19. They may be hereditary or can
also be acquired.
These abnormal genes lead to difficulty in
relaxing muscles after contraction. These
difficulties in relaxation vary in degree
depending on how mutated these genes are.
The difficulty in relaxation is caused by the
mutation of sodium, chloride, or potassium ion
channels from the mutated genes themselves.
McArdle’s Syndrome
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Also known as the glycogen storage
disease V.
Is described as the deficiency of the
enzyme myophosphorylase which breaks
down glycogen into glucose which creates
energy for the muscles.
Patients usually present generalized
symptoms such as pain in muscles during
exercise, muscle cramping, and/or
weakness in muscles when exercising.