5. the motor system Flashcards
what is the internal capsule?
A bidirectional white matter pathway connecting the cerebral hemisphere with the rest of the CNS.
what structures is the internal capsule continuous with?
Superiorly continuous with the corona radiata
Inferiorly continuous with the cerebral peduncle of the midbrain
what neurones lie in the internal capsule?
descending axons of upper motor neurones
ascending axons of third order sensory neurones
what are the 3 anatomical divisions of the internal capsule?
anterior limb
genu
posterior limb
what is contained in the anterior limb of the internal capsule?
Contains axons connecting the motor cortex with the
cerebellum
what is contained within the genu of the internal capsule
the axons of the upper motor neurones supplying the face
what is contained within the posterior limb of the internal capsule?
axons of upper motor neurones supplying the upper limb, trunk and lower limb in that order from anterior to posterior
also contains third order sensory axons connecting thalamus to postcentral gyrus
what is the arterial supply of the internal capsule?
the lenticulostriate arteries
what is the function of neurones in the lateral corticospinal tract?
fine motor control of the distal muscles. Control the limbs, especially the distal extremities
how many neurones are in a chain of the somatic motor system
2 - upper and lower motor neurones
what is the structure of the upper motor neurone?
multipolar neurone - many dendrites, single axon
what is the continuation of the grey matter of the spinal cord into the brain stem?
cell columns
what happens when the lmn are activated?
muscle contraction
where are the cell bodies of lower motor neurones found?
ventral horn of the spinal cord
cranial nerve motor nuclei (oculomotor nucleus, trochlear nucleus, trigeminal motor nucleus etc)
which motor neurones participate in spinal reflexes?
lower motor neurones, particularly the deep tendon reflexes
what is meant by primitive spinal reflexes?
the spinal reflexes exhibited in babies before the maturation of descending upper motor neurones pathways e.g. up going plantar reflex
what signs can be seen in the muscles supplied by damaged lower motor neurones?
o Weakness (due to denervation - no Ach released from motor end plates) o Areflexia (due to denervation) o Wasting/ atrophy (due to loss of GF and electrical stimulation to the muscle from the LMN across the neuromuscular junction) o Hypotonia (due to loss of muscle activation) Flaccid o Fasciculation / fibrillation (due to up-regulation of muscle nAChRs to try to compensate for denervation)
where are the cell bodies of upper motor neurones found?
in the primary motor cortex
what is the net effect of UMN on LMN?
inhibition
what structures must the axons of umn pass through to synapse on the lmn?
- Corona radiata
- Internal capsule
- Cerebral peduncle in the midbrain
- Pons
- Medullary pyramids
- Decussation of the pyramids (in the caudal medulla)
- Lateral corticospinal tract (in the lateral funiculus of the cord)
- Ventral horn
- Synapse (directly but usual indirectly via inhibitory interneurones) on LMNs
describe the pathway of axons of umn that supply facial structures
corona radiata internal capsule genu cerebral peduncle in the midbrain pons corticobulbar tract
describe the structure of the facial motor nucleus
split into two halves – one supplies the superior face (mostly occipitofrontalis) and
one the inferior face (most of the remaining muscles of facial expression)
why are UMN lesions involving the face described as forehead sparing?
as the half of the facial motor nucleus that supplies the superior face receives umns from both hemispheres, whereas the part that supplies the lower face only receives a contralateral umn input. Therefore in a umn lesion, only the inferior half of the contralateral facial motor nucleus will lose innervation
how do umn lesions involving the face differ from true facial palsies?
UMN lesions involving the face will spare the forehead
true facial nerve palsies which affect all of the muscles of facial expression resulting in a complete facial hemiparesis
what are the signs of upper motor neurone damage?
- Weakness (due to loss of direct excitatory inputs onto LMNs from UMNs)
- Hypertonia (due to loss of descending inhibition – remember that the net effect of UMNs on LMNs is inhibition) results in rigidity
- Hyperreflexia (same as hypertonia – an overactive reflex arc) occurs after spinal shock
- Extensor plantar reflexes (this is a reversion to the situation in a baby, due to loss of descending modulation of spinal reflexes)
- No fasciculations of fibrillations
what is spinal shock?
occurs in the days immediately following a UMN lesion. Initially there is flaccid paralysis with areflexia (like in LMN lesions) but then tone increases (becoming hypertonia) and reflexes become exaggerated (hyperreflexia). The mechanism of this is unclear, but is related to neuroplasticity in the spinal cord
describe how the patellar reflex occurs
- Tap patella ligament with tendon hammer and stretch the fibres of the quadriceps.
- Stretch detected by muscle spindles present within the quadriceps muscles. Muscle spindles detect muscle stretch by detecting muscle length.
- Information concerning the length of the muscle fibres is carried to the CNS through the L3 spinal nerve, into the dorsal root.
- In the DRG there are the cell bodies of the 1st order sensory neurones. The sensory neurone continues to the dorsal horn and projects down to the ventral horn. Synapses upon a LMN cell body.
- LMN receives impulse concerning muscle stretch. LMN sends its axons out through the ventral horn of the spinal cord, through the ventral root into the spinal nerve and down to the quadriceps muscle. This forms the reflex arc.
Hamstrings antagonise the action of the quadriceps, therefore for quadriceps to contract, the hamstrings must relax. - Cord level dominant for innervation of the hamstrings is at the L5 level of the spinal cord.
When the patellar tendon reflex is activated, the information sent into the L3 segment of the spinal cord. The first order sensory fibres from the quadriceps also send descending fibres down through the spina cord. At L5 cord segment they synapse on inhibitory interneurones. The inhibitory interneurones are activated and go on to inhibit the lower motor neurones at the L5 level supplying the hamstrings. This selectively inhibits the hamstrings whilst the quadriceps are contracting. Spinal reflexes does occur autonomously, in the absence of supra spinal input.
what is the babinski reflex? How might this reflex change in a patient that has had a stroke?
Babinski reflex occurs when there is stimulation of the sole of the foot leading to flexion of the toes. This reflex is modulated from fibres descending from the brain. If a patient has a stroke the modulatory fibres can be damaged. Reflex manifests in a different way. In babinski reflex the toes will extend instead of flex
what is the somatotopic organisation of the dorsal column pathway?
lower body maps to the medial portion of the tract - leg most medially, arm most laterally
what is the somatotopic organisation of the spinothalamic tract?
the lower body maps to the lateral/superficial parts of the tract - leg is most lateral, arm is most medial
what is the somatotopic organisation of the lateral corticospinal tract?
the upper body maps to the medial tract, the legs are most lateral
how are the lateral and ventral corticospinal tracts formed?
the axons of the UMNs travel through the pons and then the medullary pyramids. At the decussation of the pyramids, around 85% of axons decussate to
form the lateral CST. The remaining 15% remain ipsilateral and descend in the ventral funiculus as the ventral CST
what is the function of axons in the ventral corticospinal tract?
The ventral CST supplies proximal and trunk muscles
At the level of the target LMN, the ventral CST axons decussate.
why is the spinal cord wide at the cervical enlargement?
due to the presence of the brachial plexus. There are many LMN cell bodies and second order sensory neurone cell bodies at these levels to supply the upper limbs
why is the spinal cord narrow at the thoracic levels?
Narrow since there are relatively few LMN cell
bodies and second order sensory cell bodies due
to relatively small dermatomes and myotomes at thoracic levels
what significant feature is present in the spinal cord at the thoracic levels?
These levels of the cord
(as well as down to about L2) have the lateral horn
which contains sympathetic preganglionic cell bodies)
why is the spinal cord wide at the lower lumbar/upper sacral levels?
due to the presence of the the lumbosacral enlargement. This correlates with the presence of the lumbosacral plexus
There are many LMN cell bodies and second order
sensory neurone cell bodies at these levels to supply the lower limbs
how does the amount of white matter vary on descent of the spinal cord?
At the top of the cord (C1) there is the maximum
amount of white matter since it contains UMN axons yet to be distributed as well as the
convergence of all sensory axons from levels below
As we descend the cord the UMN axons are distributed to LMNs in the cord grey matter
(hence as we descend number of UMN axons decreases)
As we ascend the cord from the bottom sensory axons are gradually added
The net effect of the above is a gradual increase in total white matter as we ascend the cord
what is the somatotopic organisation of the grey matter ventral horn of the spinal cord?
Sitting most laterally in the ventral horn are LMNs
supplying distal muscles
- This makes sense as they sit closest to the lateral CST which supplies distal muscles
Sitting most medially in the ventral horn are LMNs
supplying proximal muscles
- This makes sense as they sit closest to the anterior/ventral CST which supplies proximal muscles
what are peduncles?
white matter structures that connects a hemisphere to the brain stem. There are cerebral and cerebellar peduncles.
what structures sit on either side of the posterior limb of the internal capsule
thalamus medially
lentiform nucleus on the lateral side
what is the nucleus ambiguus?
The nucleus ambiguus is a group of large motor neurones, situated deep in the medullary reticular formation. The nucleus ambiguus contains the cell bodies of motor neurones distributed through CNX, that innervate the muscles of the soft palate, pharynx, and larynx which are strongly associated with speech and swallowing.
what are the thalamic radiations?
Thalamic radiations are parts of the internal capsule that contain ascending third order sensory fibres. Sensory fibres in the thalamic radiations of the posterior limb are part of the somatosensory system. Information runs from the posterior limb in the thalamic radiations to the primary somatosensory cortex.
the structure of the midbrain vaguely resembles mickey mouse. Describe the structure of mickey mouse?
Ears = cerebral peduncles
Sad eyebrows = substantia nigra
eyes = red nucleus
streaming tears = parts of the sensory system including the medial lemniscus and spinothalamic fibres. Ascending secondary sensory fibres on the way to the thalamus
nose = 3rd nerve nucleus, oculomotor nucleus. contains the edinger westphal nucleus
lips = peri aqueductal grey matter. Important roles in micturition and pain management
mouth = cerebral aqueduct
chin = superior calliculus
what is the red nucleus?
Involved in motor control. If damaged can get a course tremor
what is the function of the superior calliculus?
Involved in reflex actions involved in the visual systems. E.g. looking away from bright lights.
what is the function of the inferior calliculus?
involved in reflex activity to do with auditory stimuli.
how are muscle fibrillations discovered?
not visible - detectable by electromyography
why do stroke patients adopt a similar flexed position of their arm?
In a patient that has had a stroke that affects the upper limb, the extensor and flexor muscles are affected equally.
The reason we get the typical posture (flexed elbow flexed wrist and flexed fingers) in the limb affected by spasticity is because in the upper limb the flexors are more powerful than the extensors. Therefore even though the flexors and the extensors are affected equally the flexors overpower the extensors. Hypertonicity.
in what common clinical conditions is rigidity seen?
stroke
parkinsons
what is the reticular formation?
A diffuse network of white longitudinal nerve fibers interspersed with gray matter, located in the brainstem, that regulates various autonomic functions, such as sleep and waking.
