Session 5 - Group work Flashcards
What happens to muscle power in UMNL
Patients complain of muscle weakness
because they have lost voluntary control but their
muscles are still innervated and contract reflexly.
If the lesion e.g. a pyramidal lesion occurs
above the decussation) weakness is seen on the
contralateral side. For long tract lesions due to spinal
damage the weakness may be ipsilateral (Brown
Sequard syndrome) but more often is bilateral.
What happens to muscle power in LMNL
- Patients complain of muscle weakness
because motor units are denervated. The pattern of
weakness (or paralysis in total nerve transection)
necessarily has the distribution of the ‘final common
pathway’ i.e. of the alpha motor neurones.
What happens to muscle tone in UMNL and LMNL?
UMNL
Increase in muscle tone because the
inhibitory effect of descending tracts is removed. As
the upper limb flexors are stronger than the
extensors, the arms tend to flex. In the lower limb
extensors tend to predominate.
The increase in tone is characterised by
changes in the resistance to passive movement.
giving rise to clinical features such as ‘clasp knife’
and ‘cog wheel’ rigidity.
LMNL
Muscle tone will be reduced. The
reduction may range from a barely discernible
change to complete flaccidity, in proportion to the
number of motor neurones denervated.
What happens with muscle wasting in UMNL and LMNL?
UMNL
Although the unused muscles may lose
some of their bulk no muscle wasting occurs because
they are still innervated and contract reflexly.
LMNL
3. Muscle wasting depends upon the
length of time the affected motor neurones have had
to degenerate and the number denervated. In actual
nerve transection, wasting appears early and is
obvious.
What happens to reflexes in UMNL and LMNL?
UMNL
4. Hyperreflexia: The “damping” effect of the
descending tracts upon reflex activity in the cord is
removed so reflexes become exaggerated. Flexor
reflexes predominating in the upper limb and
extensor responses in the legs (see above). In
consequence the normal flexor response of the foot
becomes extensor. This means that when the sole of
the foot is stimulated the normal plantar flexion is
replaced by a dorsiflextion and a fanning of the toes
(a positive Babinski sign)
LMNL
4. Hyporeflexia: weak or absent reflexes.
Hyporeflexia is the only possible outcome as the
efferent side of the muscle stretch reflex is lost.
What happens to the plantar response in UMNL and LMNL?
UMNL
5. A positive Babinski sign where the normal
curling of the toes and plantar response to a stimulus
applied to the sole is replaced by an extension of the
toes and dorsiflexion of the foot. The sign of Bing is
also present, i.e. when a sharp stimulus is applied to
the dorsum of the foot the normal reaction, to extend
the foot away from the stimulus, is replaced by a
flexion which drives the foot toward the stimulus.
LMNL
5. There is NO Babinski sign. The plantar
response remains normal i.e. flexor response.
A key is used to elicit the plantar response
because it simulates walking on rough ground. A
stronger stimulus would elicit the reflex withdrawal of
the whole leg just as would happen if we stood on a
sharp stone.
Give three associated features of UMNL
Drift of limbs when eyes closed
Loss of abdominal and cremasteric reflex
Sensory deficits involve quadrants or halves of the body
Give three associated features of LMNL
Fasciulations as a result of increased sensitisation of muscle fibres to Ach
Muscle contractures occur at the end stage of a lmn deficit as muscle cells are replaced by fibrous tissue
Associated sensory deficits are likely to have a peripheral nerve pattern
What would be the consequence of a pure lesion of the cell body of the α-motoneurone
a) acutely and b) chronically on the ability of the muscle to produce force? What
disease(s) can give rise to this scenario?
a)and b) Death of the cell body of an α-motoneurone will result in the muscle losing its
neuronal innervation, hence all its ability to produce force both acutely and chronically. There
will be no chance of recovery from this situation. In this regard, there is no difference between
the acute and chronic states. Polio myelitis is a fitting example of a disease that selectively
destroys α-motoneurone cell bodies. Limb muscles or those for respiration are equally
vulnerable to this disease
What would be the consequence of a pure lesion of the axon of the α-motoneurone a)
acutely and b) chronically on the ability of the muscle to produce force? What pathology
is likely to result in this kind of scenario
a)and b) Lesions of an axon of an α-motoneurone will result in the nerve dying and the muscle
losing its neural innervations, hence all its ability to produce force both acutely and chronically.
There will be no chance of recovery from this situation. In this regard, there is no difference
between the acute and chronic states. Presentational signs are exactly the same as those of
lesions affecting cell bodies. Any disease processes that selectively damage axons of α-
motoneurones will produce such an impairment of the motor system. The usual suspect in such
a scenario has to be a complete transection of a peripheral nerve as a result of trauma to it.
Lesion of either cell bodies or axons of all α-motoneurones to a muscle result in
paralysis of that muscle. By what name is this kind of paralysis commonly known?
Flaccid paralysis
Imagine strong descending tonic synaptic inhibition on all α-motoneurones supplying a
muscle or limb. a) How will this affect the ability of that muscle to produce force? b)
Under what physiological conditions does this scenario occur? c) What muscles are
exempt from this form of inhibition?
a) and b) α-motoneurones of all muscles of the body are said to be subject to constant and tonic
inhibition from extrapyramidal descending (motor) system. Careful consideration of the physics
attending this subject predicts that motoneurones with large diameter cell bodies will be
relatively more severely inhibited than those with smaller diameter cell bodies. As such ,when at 5 - 4
rest, most α-motoneurones are under constant tonic inhibition, thus making them unable to
recruit their respective muscles, hence there is not much muscle force to speak of when
muscles are at rest. C) Muscles of respiration and those of extra-ocular muscles are not subject
to the tonic descending inhibition from descending motor systems. The biggest benefit to be
derived from this arrangement is that when we are asleep, descending inhibition on the motor
system is extremely heavy and leads to general paralysis of our bodies, however, our ability to
breathe whilst in deep sleep is not impaired. The rapid eye movements seen in REM sleep
explain this lack of descending inhibition to extra-ocular muscles. There are many debates as to
the necessity of such movements.
Lesion of either cell bodies or axons of some but not all α-motoneurones to a muscle
result in partial paralysis of that muscle. How will this affect the ability of that muscle to
produce maximum force?
This will result in reduction of maximum force production by the limb in question. The severity of
the resulting disability is quantified using the MRC scale for force production in which a score of
0 suggests complete paralysis (i.e.no force production at all) whilst that of 5 suggests that the
muscle is able to produce maximum force that should be expected.
How would you describe the state of tonus of the muscle described in a) Q6-4 and b) Q6-
6?
a) Atonia
b) Hypotonia
What would be the consequence of a pure lesion of either cell bodies or axons of α-
motoneurones a) acutely and b) chronically on the appearance of the muscle they
supply?
a)There would be no obvious perceptible change in the appearance of the muscle soon after
lesioning either cell bodies or axons of α-motoneurones
b) This is a little more involved and can be answered as follows (thorough treatment of the
question):
Intermediate :The muscle will first exhibit spontaneous and continuous ripples referred to as
fasciculations. It will also start to lose its bulk, though this could be subjective.
Long-term: The muscle will lose most of its bulk and the fasciculations will eventually
disappear as the muscle tissue dies through loss on neurotrophins that are normally produced
by the nerve so as to keep the muscle tissue alive .
What is the explanation for the progressive changes that take place between acute and
chronic denervation of a muscle as outlined in Q6-8 above?
The denervated muscle up-regulates its production of nicotinic Ach receptors. These receptors
are said to be super-sensitive and will detect minute amounts of Ach circulating in the blood
stream, (note that these Ach molecules would have escaped destruction by synaptic as well as
blood-borne circulating Ach-esterase). On being detected by these super-sensitive nicotinic
receptors, these trace amounts of Ach produce unco-ordinated and random minute contractions
of individual fascicles of the muscle, hence the fasciculations. The muscle will lose its bulk as a
result of loss of trophic support which it receives from α-motoneurones supplying it. Eventually,
the muscle will die and only fat or connectivetissue would occupy the compartment of the
muscle.
Where would you expect to a find cell body of an α-motoneurone in the spinal cord and
what would be the effect of directly activating it on a) muscle length and b) motor tone
Cell bodies of α-motoneurones are found in the ventral horn of the grey matter of the spinal cord
(lamina IX of Rexed)
a)Activating an α-motoneurone will result in shortening of the muscle it innervates, development
of force and b)an increase in the motor tone of the muscle in question
Where would you expect to find a cell body of a γ-motoneurones in the spinal cord and
what would be the effect of directly activating it on a) muscle length and b) motor tone?
Cell bodies of γ-motoneurones are found in the ventral horn of the grey matter of the spinal cord
(lamina IX of Rexed) intermingling with those of α-motoneurones
a) Activating γ-motoneurones will result in shortening of the intrafusal muscle fibres of the
muscle spindle sense organ it innervates. This in turn will result in opening of spiral endings of
the sensory endings of the muscle spindle afferent on the intrafusal muscle fibre. Consequently,
this will result in activation of the muscle spindle afferent, and in turn, recruitment of the α—
motoneurone via the stretch reflex. Activation of the stretch reflex will lead to activation of the
muscle, hence shortening of its length. This long-loop pathway of activation of the muscle is
said to have occurred via the γ-loop. As in Q6-10 b) above, contraction of the muscle will result
development of force and thus, b)an increase in the motor tone of the muscle in question
Imagine a motor nucleus comprising of an equal mixture of cell bodies of α- and γ-
motoneurones, what feature would help you to distinguish between these populations of
cells?
The only distinguishing feature between them is that
α-motoneurones tend to have large cell body diameters whilst γ-motoneurones have smaller
cell body diameters.
Imagine a motor nucleus comprising of an equal mixture of cell bodies of α- and γ-
motoneurones, what would be the effect of a defined synaptic current impinging on it?
According to Ohm’s law, a defined excitatory synaptic input (current) to a motor nucleus will
produce a large voltage change on a cell body with a smaller diameter than it would on a cell
body with a larger diameter, hence lower cross-sectional resistance. Consequently, for a given
(fixed) synaptic current, smaller cell bodied motoneurones require less current and therefore,
will be recruited earlier than their large cell-bodied equivalents which will require even more 5 - 6
current before they are recruited
What do you understand by the term “orderly recruitment” of motor units?
Taking the example of the upper limb, it is interesting to note that force development by the limb
occurs on a continuously smooth scale, whereby the limb always starts by generating minimal
forces such as those required to lift a feather, followed by forces that will permit us to lift a pen
and write with it, progressing to the sort of forces needed to lift a cup of tea, to lifting a brick and
finally “pumping iron” in the gymnasium. Thus, recruitment of motor units is always “orderly”,
starting with motor units capable of only minimal forces and ending with motor units that
generate the heaviest forces that the limb in question is capable of generating. This feature of
the motor system is explained by applying the principle of motoneurone recruitment-order
based on cell body size as explained above in
How useful is orderly recruitment of motor fibres in the physiological development of a)
muscle or motor tone and b) muscle force?
a) and b) Motor tone is generated by development of minimal forces by the smallest motor units
of the body. As it so happens, this minimal amount of motor tone is adequate to maintain our
normal posture. Given the small size of the motor units that generate motor tone (hence
posture), the amount of energy required for this purpose is also minimal, thus making them less
liable to fatigue.
What is the sign of Babinski?
When the sole of the foot is stroked the normal response is to curl the toes in a plantar response. Following
an upper motor neurone lesion this is replaced by an extension and fanning of the toes to give a dorsiflexion.
Which pathway is damaged in Parkinson’s Disease?
Parkinson’s disease is associated with the degeneration of cells in the substantia nigra, a collection of
melanin containing neurones in the midbrain, which secrete dopamine. These neurones project to the
putamen and pallidum (striatum) of the basal ganglia.
Normally dopamine exerts an excitatory influence upon neurones of the medial pallidal segment (the direct
path) and an inhibitory effect upon cells in the lateral pallidal segment (the indirect pathway). Loss of
dopamine causes underactivity of the direct pathway and overactivity of the indirect pathway, which is
inhibitory upon the thalamus and cerebral cortex giving the hypokinetic symptoms of the disease.
What structures are damaged to give a chorea?
Choreiform movements e.g. hemiballismus arise because neurones in the indirect pathway of the basal
ganglia are damaged in some way, so that the direct pathway is fully expressed The usual deficit is due to a
lesion in the sub-thalamus which leads to a disinhibition of the thalamus and high activity in the cortex
leading to the abnormal movement.
In Huntington’s disease an inherited disorder (autosomic dominant) of the synaptic connections between the
striatum and the subthalamic nucleus disrupt the indirect pathway.
What would be the effect of giving a drug like levodopa to a patient with a chorea? Explain
your answer?
The symptoms of a chorea arise because the direct pathway becomes overactive so that the thalamic motor
nuclei are disinhibited giving excessive activation of the motor cortex. Giving these patients the precursors
