Organization of the Motor System Flashcards
Which structures are involved in the following?
a) Reflexes
b) Posture
c) Voluntary movement
a) LMN and spinal cord circuits
b) Brainstem and spinal cord
c) cortex, brainstem and spinal cord
What is the difference between a closed and open loop?
Closed loops are reflex control done by our axial muscles (muscles of trunk and head) for balance, posture
and locomotion
Open loops are for sensory cue or desire to move done by distal muscles for fine motor skills
Match the following structures or control centres with its corresponding structure or control centre
a) postcentral gyrus
b) precentral gyrus
c) body sensations
d) temporal
e) Vision
f) Frontal lobe
a) Primary somatosensory cortex
b) Primary motor cortex
c) Parietal lobe
d) Hearing, advanced visual processing
e) Occipital
f) Planning of movements, recent memory and some aspects of emotion
What are the following structures responsible for?
- Insert photo after
a) Premotor cortex
b) Primary motor cortex
c) Primary somatosensory cortex
a) motor planning and sequencing for contralateral body
b) controls contralateral body motor functions
c) receives contralateral sensory input from the body
What are the following structures responsible for?
a) Posterior parietal cortex (superior lobule)
b) primary visual cortex
c) Primary auditory cortex
a) Integration of sensory input - stereognosis
b) receives contralateral visual field info from both eyes
c) receives bilateral auditory sensory input
a) What is the special term for the terminal branches of the axon?
b) What is the special term for the sheath of schwann?
a) telodendria
b) Neurilemma
Which structures have direct control over skeletal muscle fibres? What effect will it have on skeletal muscle fibres when stimulated?
The axons of lower motor neurons from the brainstem or spinal cord has direct control over skeletal muscle fibres, and stimulation of the lower-motor neuron always has an excitatory effect on skeletal muscle fibres
Describe the basic overview of normal motor function/the motor system
Transmission of signal from cortex - muscle goes via an UMN; which are neurons of the cerebral motor cortex and brainstem nuclei to a LMN; defined as cells of the ventral horn of the spinal cord OR cranial nerve motor nuclei which give rise to axons supplying skeletal muscle
Signals may travel to LMNs via the conscious pathway (pyramidal) or unconscious pathways (extrapyramidal)
Once reached the spinal cord -> anterior horn cell -> peripheral nerve -> NMJ -> Muscle
*insert photo from workbook
Where does the corticospinal tract begin?
The motor cortex on the pre-central gyrus
Which tracts are conscious and which are unconscious? Where do they originate and end?
Conscious: pyramidal or corticospinal tract:
The pyramidal system consists of UMNs originating in the cerebral cortex, descending through the internal capsule and medullary pyramids to synapse with LMNs in the spinal cord
Unconscious: Extrapyramidal tracts originating in the brainstem, basal ganglia and cerebellum which target the lower motor neurons in the spinal cord
What are the four subconscious extrapyramidal tracts?
- Rubrospinal
- Anteromedial tracts: these subconsciously control the axial and proximal muscles
a) . Tectospinal
b) . Vestibulospinal
c) . Reticulospinal
*insert photo after
What are Betz cells and where are they? What do they synapse with?
Upper motor/pyramidal neurons with long axons in the primary motor cortex which travel along the corticospinal/pyramidal tract to synapse with interneurons or alpha motor neurons
What are alpha motor neurons?
A LMN is the cell body and axon of an alpha motor neuron.
Name the three tracts of the corticospinal tract, what do they each control?
- Corticobulbar - conscious control over eye and face muscles
- Lateral corticospinal - conscious control over limb skeletal muscles
- Anterior corticospinal - conscious control over axial skeletal muscle
Describe the pathway of the corticospinal tract(s), including its origin and decussations
Originates in the primary motor cortex on the precentral gyrus.
Axons descend through the internal capsule to the medulla via the pyramids of the medulla
- 75-90% decussate in the pyramids of the medulla oblongata and descend in the lateral corticospinal tract to distal musculature/limbs
- 10-25% remain ipsilateral and don’t decussate until they are nearer to where they terminate in the spinal cord. These descend in the ventral corticospinal tract and go mainly to axial skeletal muscle
They all synapse onto the ventral horn of the spinal cord onto their LMN targets
Name five general functions that the extrapyramidal motor pathways are responsible for
Involuntary actions, reflexes, locomotion, complex movements and posture
Where does the rubrospinal tract originate from and descend through/to?
From the red nuclei of the midbrain (which receives inputs from the cerebral cortex and cerebellum);
Since it doesn’t pass through the pyramids of the medulla its an extrapyramidal pathway: It descends in the lateral part of the brainstem and then in the lateral funiculus adjacent to the lateral corticospinal tract
What happens if there is a lesion to the rubrospinal tract?
Minimal effects (demonstrating the presence of considerable overlap!)
What is the rubrospinal tract especially in control of?
Voluntary control of upper limb movements; specifically large muscle flexor movement and inhibiting extensor tone of the upper limbs
*workbook also said for fine motor activity of upper limbs
What is the reticulospinal tract responsible for?
Automatic movement: Body posture, controlling reflexes and muscle tone
What is the reticulospinal tract comprised of?
Pontine and medullary reticulospinal tracts
Where does the pontine reticulospinal tract originate from and what is it responsible for?
It’s the medial pathway originating in the pontine reticular nucleus responsible for exciting extensor muscles
Where does the medial reticulospinal tract originate from and what is it responsible for?
It’s the lateral pathway originating in the medullary reticular nuclei responsible for inhibiting excitatory axial extensor muscles and automatic breathing
What is the vestibulospinal tract responsible for?
Involuntary control over head balance and body posture
Where does the vestibulospinal tract receive information from and what does it control?
Recieves info from the vestibulocochlear nerve about changes in head orientation. It then descends via the vestibulospinal tract and alters muscle tone, extension and maintains posture
What is the tectospinal tract responsible for?
Orientation and positioning of the head for balance in response to stimuli - “eye-head coordination”
Where does the tectospinal tract originate from?
The tectum of the midbrain
Where is a LMN’s cell body? How does it interact with muscle?
Either in the ventral horn of the spinal cord or cranial nerve nuclei of the brainstem and directly innervates a muscle with its axon
*another question like this in other quiz
What is a motor unit and how do they influence control over musculature?
A motor unit is the number of muscle fibres innervated by a single axon, a smaller number means finer control
What are the two types of LMNs? What do they do and what is their primary difference?
They feedback to the brain and the reflex arc
alpha-MN: innervates the muscle itself
gamma-MN: innervates the muscle spindle and keeps it taught
What is a muscle spindle and what does it detect and allow for/provide?
An encapsulated sense organ found within the muscle
Since it detects muscle contraction it’s co-activated with alpha-motor neurons and tightens as the muscle contracts - and is therefore responsible for tendon reflex
Allows for finer motor control and can detect the extent of muscle tension (contraction/relaxation) and inform the cerebellum (unconscious), therefore it can provide limb positional information to the CNS (proprioception)
What does a muscle spindle activate (and what stimulates this activation)?
Activates LMNs in response to muscle stretch
Describe what takes place in the neuromuscular junction and how it leads to a muscular contraction
- A motor nerve impulse causes the release of Ach into the synaptic cleft, which activates nicotinic Ach receptors on muscle fibre membranes
- This opens ligand-gated sodium channels depolarising the sarcolemma
- AP is propagated deep into muscle fibre through T tubules (invaginations in the sarcolemma) as voltage gated sodium channels open
- T tubule depolarisation activates ryanodine receptors in the membrane of the SR - releasing stored Ca2+ into the cytoplasm
- Ca2+ stimulates muscle contraction by binding a troponin subunit (TnC) allowing myosin binding sites to be revealed
* Ca2+ is returned to the SR via ATPase pumps
How does the activation of a single motor neuron compare to the activation of multiple motor neurons?
Increased FOC the more motor neurons are activated
Activation of a single motor neuron will lead to a weak but distributed contraction
Where are golgi tendon organs found?
At the myotendinous junction between muscle and tendon
What do golgi tendon organs do?
They monitor the degree of muscle contraction by sensing changes in the collagen within the tendon (as the collagen stretches and straightens with muscle contraction)
They prevent OVER contraction of muscles as when they’re activated they inhibit the muscle contraction and activate its antagonist
What is muscle tone and what does it rely on?
The minimal background muscle power that allows us to maintain posture and minimal stiffness in skeletal muscles, it relies entirely on the muscle stretch reflex and rises/falls depending on the number and size of motor units involved.
*The smaller the size of the motor units, the less energy is expended (i.e maintaining normal posture)
What generates resting motor tone?
When we’re awake LMNs continually activate muscle to generate the resting motor tone
Describe the golgi tendon reflex, which other reflex does it act opposingly to?
It’s polysynpatic:
1. 1b afferent sensory fibres detect overstretch of a tendon (straightening/stretching of its collagen)
- Connects to inhibitory and excitatory interneurons to cause relaxation of the muscle and contraction of the antagonist muscle - opposite to stretch/myotatic reflex
Where are motor programs generated?
Basal ganglia
Describe the hierarchy of the motor system
Highest: cerebral cortex
Intermediate: brainstem nuclei and cerebellum
Lowest: motor neurons of cranial and spinal nerves
What are LMNs defined as?
Cells of the ventral horn of the spinal cord OR cranial nerve motor nuclei which give rise to axons supplying skeletal muscle
How should alpha motor neurons be influenced in a healthy nervous system?
They SHOULD be under constant inhibition from UMNs, particularly the extrapyramidal system, this inhibition is only temporarily lifted to allow us to carry out voluntary movements.
The intensity of this constant inhibition varies continually; i.e during sleep; descending inhibition paralyses virtually all skeletal muscles apart from those responsible for breathing or those supplying extraocular muscles
What is the role of the cerebellum in the motor cortex?
Involved in the timing and coordination of the motor cortex
What happens to stretch reflexes in an UMN lesion?
Since UMNs (especially extrapyramidal ones) are inhibiting most stretch reflexes, a lesion will lead to less descending inhibition and a rise in motor tone
*muscle tone to the antagonistic muscle will be equally affected
What may happen to the upper and lower limbs when there is a severe UMN lesion?
Since there is now LESS descending inhibition; spastic paralysis may occur in both upper and motor limbs in severe cases
Since flexors are usually stronger than extensors in the upper body, there will be overall flexion of the limb. However, a lower limb with spastic paralysis will have overall extension, since extensors are dominant.
In addition to alterations of muscle tone, name four other signs which may occur as a result of an extrapyramidal disorder?
Excessive movements such as
- Pill rolling tremor
- Choreiform movements (jerking) and hemiballismus (unilateral, usually violent chorea)
- Tics
- Bradykinesia which can progress to akinesia (absence of movement) and tardive dyskinesia - an iatrogenic cause of parkinsonism like effects (oral grimaces, sucking, chewing)
Describe what is meant by the term “orderly recruitment” of motor units
The recruitment of motor units is always orderly and is based on the cell body size, i.e; recruitment begins with motor units capable of minimal forces and ends with those capable of heaviest forces
Where would you expect to find an alpha motoneuron 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 all alpha motor neurons are within the CNS in either the cranial nerve motor nuclei of the brainstem or the ventral horn of the spinal cord
Activation of an alpha motoneuron will result in
a) shortening of muscle it innervates and development of force
b) increase in the muscle’s motor tone
Describe the pathway of the axons of alpha motoneurons, where do they terminate?
Axons of alpha motor neurons leave the CNS to course the PNS as either motor divisions of cranial nerves or spinal segmental nerves. They terminate on skeletal muscles at the NMJ
Where would you expect to find the cell body of a gamma-motor neuron? What would be the effect of direclty activating it on a) muscle length and b) muscle tone?
Cell bodies of gamma motor neurons are found in the ventral horn of the grey matter
Activation will result in
a) shortening of the intrafusal muscle fibres of the muscle spindle sense organ it innervates -> this activates the muscle spindle afferent -> recruiting the alpha motoneuron via the stretch reflex which shortens the muscle length
b) Contraction results in increased development of force and thus increase in motor tone
Identify whether the following are pyramidal (UMN), extrapyramidal and LMN
a) Festinating gait (PD)
b) Clasp knife rigidity
c) dysdiadochokinesia
e) marked muscle wasting
f) intention and pill rolling tremor
g) Babinski sign
h) Paucity of facial expression
i) complete absence of unilateral facial expression
j) Cogwheel rigidity
k) intention tremor
a) extrapyramidal
b) UMN
c) extrapyramidal
e) LMN
f) extrapyramidal
g) UMN
h) extrapyramidal
i) LMN
j) extrapyramidal
k) extrapyramidal
