Somatic Motor System Flashcards
What comprises the neural element of the somatic motot system?
Upper motor neurones within the brain and lower motor neurones with soma within the brain stem and ventral horn of spinal cord
What do UMNs do when they provide the input to LMNs?
Modulate their activity
What do LMNs receive input from?
UMNs
Proprioceptors
Interneurons
What comprises LMNs?
Alpha motor neurones-innervate bulk of fibres within a muscle that generate force
Gamma motor neurones- innervate muscle spindles within muscle
Where do axons of LMNs exit the spinal cord?
In the ventral roots (or via cranial nerves)
Are motoneurones distributed equally?
No- greater number in cervical enlargement (C3-T1) supplying arm and lumbar enlargement (L1-S3) supplying leg
What do motoneurones belong to?
A spinal segment- C1-8, T1-12, L1-5, S1-5
What makes up a motor unit?
An α-MN and all of the skeletal muscle fibres that it innervates are collectively
What is the collection of α-MNs that innervate a single muscle called?
Motor neurone pool
What are the two principle mechanisms of force of muscle contraction graded by α-MNs?
Frequency of action potential discharge of the α-MN (each ap causes a twitch)
Recruitment of additional synergistic motor units
Do LMNs show a somatotopic distribution in the ventral horn?
Yes
What are the 3 sources of input to an α-MN that regulate it’s activity?
Central terminals of dorsal root ganglion cells whose axons innervate muscle spindles
UMNs in the motor cortex and brain stem
Spinal interneurones
What does muscle strength depend on?
Neuromuscular activation
Force production by innervated muscle fibres
What does neuromuscular activation depend on?
Firing rates of the LMNs involved
Number of LMNs that innervate a muscle
The coordination of the movement
What does the force production by innervated muscle fibres depend on?
Fibre size (hypertrophy) Fibre phenotype (fast/slow)
What are some features of motor units?
Variable size
Fast and slow types
What is the difference between fast and slow muscle fibres?
Differ in how fast myosin ATPase splits ATP to provide energy for cross bridge cycling
Describe slow oxidative (type I) fibres
ATP derived from oxidative phosphorylation.
Slow contraction and relaxation.
Fatigue resistant.
Red fibres because of high myoglobin content
Describe fast type IIa fibres
ATP derived from oxidative phosphorylation
Fast contraction and relaxation
Fatigue resistant
Red and reasonably well vascularised
Describe fast type IIb fibres
ATP derived from glycolysis
Fast contraction but not fatigue resistant
Pale in colour and poorly vascularised
Which has a higher threshold, fast or slow fibres?
Fast
What is the increase in force in whole muscle contraction due to?
1st. Temporal summation in individual fibres/motor units
2nd. Recruitment of more motor units (via different threshold α-MNs)
What is the velocity of contraction due to?
Muscle fibre types
Load on muscle (faster shortening at lower loads, slower at higher)
What is the Henneman size principle?
The susceptibility of an α-MN to discharge aps is a function of its size
What is the myotatic reflex?
When a skeletal muscle is pulled, it pulls back
What do spindles consist of?
Fibrous capsule
Intrafusal muscle fibres (extrafusal generates force)
Sensory afferents (Ia class, myelinated/fast conducting) that innervate intrafusal fibres
γ-MN efferents that innervate intrafusal fibres
What type of reflex arc is the myotatic reflex?
Monosynaptic
What happens during the myotatic reflex?
Stretch of muscle spindle Activation of Ia afferent Excitatory synaptic transmission in spinal cord (monosynaptic, mediated by glutamate release) Activation of α-MN Contraction of homonymous muscle
What spinal level is assessed in the supinator reflex?
C5-6
What spinal level is assessed in the biceps reflex?
C5-6
What spinal level is assessed in the triceps reflex?
C7
What spinal level is assessed in the quadriceps reflex?
L3-4
What spinal level is assessed in the gastrocnemius reflex?
S1
What do intrafusal fibres consist of?
A non-contractile equatorial region innervated by the Ia sensory neurones
Contractile polar ends that receive efferent inputs from γ-MNs with cell bodies in the ventral horn of spinal cord
During voluntary movement when α- and γ-MNs are co-activated what happens to intrafusal and extrafusal fibres?
They contract in parallel
What are the different intrafusal fibres?
Nuclear bag fibres
Chain fibres
What are the 2 types of nuclear bag fibres?
Bag 1 or dynamic- very sensitive to rate of change of muscle length. Innervated by dynamic γ-MNs
Bag 2 or static- more sensitive to absolute length of muscle. Innervated by static γ-MNs
Describe chain fibres
Sensitive to absolute length of muscle. Innervated by static γ-MNs
What two types of afferent fibre innervate the intrafusal fibres?
Ia afferents
II fibres
What do Ia afferents form winding around the centre of all intrafusal fibres?
A primary annulospiral nerve ending
What do II fibres form on all intrafusal fibres except bag 1 dynamic?
Flowerspray ending
How is the activity in dynamic and static γ-MNs set?
At levels appropriate to activity. Slow predictable changes= static. Rapid unpredictable changes= dynamic
Where are Golgi tendon organs?
Located at the junction of muscle and tendon
What do Golgi tendon organs do?
Monitor changes in muscle tension, in series with extrafusal fibres
What are Golgi tendon organs innervated by?
Group Ib sensory afferents (myelinated, slighty slower than Ia)
Why do Golgi tendon organs act to regulate muscle tension?
To protect muscle from overload (weight lifting)
To more generally regulate muscle tension to an optimal range
What is the pathway of Group Ib afferents?
Enter spinal cord and synapse upon inhibitory interneurons
They in turn synapse upon the α-MNs of homonymous muscle forming basis of reverse myotatic reflex
What kind of pathway is present in coding of force by Golgi tendon organ?
Polysynaptic pathway in which an inhibitory interneurone is interposed between the Ib afferent and α-MN
Where are proprioceptive axons present?
Connective tissue of joints
What do proprioceptive axons respond to?
Changes in angle, direction and velocity of movement of a joint. Also prevent excessive flexion or extension
What are proprioceptive axons made up of?
Mixture of rapidly adapting (RA) and slowly adapting (SA) units that have either high threshold (HT), or low thresholds (LT) for activation (thus information concerning movement and the static, or resting, positon of a joint is transmitted to the CNS)
What are some particular structures that make up proprioceptive axons?
Free nerve endings, found in capsule and connective tissue – most numerous – HT – SA – nociceptive function
Golgi-type endings, found only in ligaments – HT – SA – protective role?
Paciniform endings, found in periosteum near the articular attachments and the fibrous part of the joint capsule – LT – SA – acceleration detectors
Ruffini endings, found mainly in joint capsule – LT – SA - static position and speed of movements
What do spinal interneurons receive input from?
Primary sensory axons (Ia and Ib)
Descending axons from brain
Collaterals of LMNs
Other interneurons
What do inhibitory interneurons mediate?
Inverse myotatic response
Reciprocal inhibition between extensor and flexor muscles
What is reciprocal inhibition?
At a joint, voluntary contraction of an extensor will stretch an antagonist flexor, initiating the myotatic reflex. Descending pathways that activate the α-MN controlling the extensor muscles also, via inhibitory interneurons, inhibit the α-MNs supplying the antagonist muscles allowing unopposed extension
What do excitatory interneurons mediate?
Flexor reflex
Crossed extensor reflex
What is the flexor reflex?
Noxious stimulus causes limb to flex by:
Contraction of flexor muscles via excitatory interneurons
Relaxation of extensor muscles via excitatory and inhibitory interneurones
What is the crossed extensory reflex?
Noxious stimulus causes limb to extend by:
Contraction of extensor muscles via excitatory interneurons
Relaxation of flexor muscles via excitatory and inhibitory interneurones
What structures are involved in the strategy of motor control?
Neocortical association areas
Basal ganglia
What structures are involved in the tactics of motor control?
Motor cortex
Cerebellum
What structures are involved in the execution of motor control?
Brain stem
Spinal cord
Where do descending spinal tracts originate from?
Cerebral cortex and brain stem
What are descending spinal tracts concerned with regarding movement?
Control of movement Muscle tone Spinal reflex Spinal autonomic functions Modulation of sensory transmission to higher centres
What are the two important descending pathways?
Lateral and ventromedial
What controls the lateral pathways?
Cerebral cortex
What are the lateral pathways important for?
Voluntary control of distal musculature, particularly discrete, skilled movements
What controls the ventromedial pathways?
Brainstem
What are the ventromedial pathways important for?
Control of posture and locomotion
What is the major lateral pathway?
Corticospinal (pyramidal_ tract
Where are the corticospinal tract cell bodies located?
Motor cortex (BA4 and 6, about 2/3) Somatosensory areas of parietal cortex (about 1/3)
The axons of the corticospinal tract course to the base of the medulla forming what?
Medullary pyramid
Where do the fibres of the lateral pathway decussate?
Most (75-90%) at pyramidal decussation to form lateral corticospinal tract. Remainder stay ipsilateral to form ventral corticospinal tract and decussate more caudally
Where do axons of the lateral pathways terminate?
In the dorsolateral region of the ventral horn and intermediate grey, the location of LMNs and interneurons controlling distal muscles, particularly flexors
What is the rubrospinal tract?
A minor lateral pathway
Where are the cell bodies of the rubrospinal tract located?
Red nucleus, which receives input from motor cortex and cerebellum
Where do axons of the rubrospinal tract decussate?
At the ventral tegmental decussation, then descending the spinal cord ventrolateral to the lateral corticospinal tract, terminating in the ventral horn
What does the rubrospinal tract control?
Limb flexor muscles, exciting LMNs of those muscles
What are lesions of the lateral columns associated with?
Loss of fractionated movements (shoulders, elbow, wrist and fingers cannot be moved independently)
Slowing and impairment of accuracy of voluntary movements
Little effect on normal posture
What do lesions of the corticospinal tract alone cause?
Defecits like lesion of lateral columns, but major recovery can occur (although weakness can persist)
Where do the cell bodies of the vestibulospinal tract reside?
In the vestibular nuclei (lateral and medial) that receive input, via CNVIII, from vestibular labyrinths. Cerebellar input also important
Where do axons from the lateral vestibular nucleus descend?
Ipsilaterally as the lateral vestibulospinal tract as far as the lumbar spinal cord
What do lateral vestibular nucleus axons do?
Help to hold upright and balanced posture by facilitating extensor MNs of antigravity muscles
Where do axons from the medial vestibular nucleus descend?
As the medial vestibulospinal tract as far as the cervical spinal cord
What do medial vestibular nucleus axons do?
Activate cervical spinal circuits that control neck and back muscles guiding head movements
Where are the cell bodies of the tectospinal tract?
In the superior colliculus (optic tectum) which receives direct input from the retina and also from the visual cortex and afferents conveying somatosensory and auditory information
Where do axons of the tectospinal tract decussate?
In the dorsal tegmental decussation and sescending close to the midline as the tectospinal tract to cervical spinal cord influencing muscles of neck, upper trunk and shoulders
What does superior colliculus act as?
A map of the external world guiding orientation of the head and eyes to an important new visual stimulus
Where do the pontine (medial) and medullary (lateral) reticulospinal tracts arise from?
The reticular formation
What is the reticular formation?
A diffuse mesh of neurones that are located along the length and at the core of the brainstem
Where does the pontine reticulospinal tract descend?
Ipsilaterally
What does the pontine reticulospinal tract do?
Enhances antigravity reflexes of the spinal cord
Helps to maintain a standing posture by facilitating contraction of the extensors of the lower limbs
Where does the medullary reticulospinal tract descend?
Bilaterally
What does the medullary reticulospinal tract do?
Opposes the action of the pontine tract
Releases antigravity muscles from reflex control
What controls activity in both reticulospinal tracts?
Descending signals from the cortex
