motor systems

Question 1

what, where, how

Answer 1

what - decide and initiate; basal ganglia and frontal cortex, where - reach; cerebellum and primary motor area, how - grasp; primary motor cortex and spinal circuits

Question 2

motor systems organisation

Answer 2

• hierarchical and parallel organisation, cerebellum and basal ganglia control movement by regulating descending cortical and brainstem pathways, next is the cortical pathways, then brainstem pathways, interneurones that project to motor neurones and finally motor neurones.
• basal ganglia cerebellum and cortical pathways also have parallel organisation with separate circuits controlling distinct, overlapping, functions.
• both premotor areas and primary motor cortex project directly to motor circuits of spinal cord and brain stem (another example of combined hierarchical and parallel control)
• premotor areas regulate both primary motor cortex (hierarchical; premotor>primary motor>brain stem and spinal motor centres) and the brain stem and spinal motor centres directly (parallel)

Question 3

cortical paths

Answer 3

lateral corticospinal tract - involved in many different aspects of skilled limb movements, ventral corticospinal tract - participates in axial muscle control, corticobulbar tract - controls primarily facial muscles, jaw muscles and muscles for swallowing and speaking

Question 4

brainstem motor pathways

Answer 4

rubrospinal tract - originates from red nucleus and involved in limb control, tectospinal tract - originates from superior colliculus and participates in head-eye coordination during gaze, reticulospinal tract - participates in diverse automatic movements and posture, vestibulospinal tract - for posture and balance

Question 5

lower motor neurone

Answer 5

spinal motor output: alpha motor neurones>muscles, each motor neurone is associated with muscle fibres forming a functional entity called the motor unit; motor units vary in size, about of tension produced, speed of contraction and degree of fatigability

Question 6

motorpool

Answer 6

motor neurones innervating axial (proximal) musculature are located medially in the ventral horn, those innervating distal musculature are located more laterally in the ventral horn

Question 7

motor unit

Answer 7

most extrafusal skeletal muscles fibres in mature mammals are innervated by by single alpha motor neurone, more muscle fibres than motor neurone’s so individual motor axons branch within muscles to synapse onto multiple extramural fibres, these fibres are typically distributed over a relatively wide area possibly to ensure contractile force is spread evenly, this arrangement also reduces chance of damage to one or more alpha motor neurone will significantly alter muscle action, action potential generated by by motor neurone typically brings to contraction threshold all muscles fibres the neurone contacts, the single alpha motor neurone and its associated muscle fibres constitute the smallest unit of force that can be activated by the muscle both motor units and the alpha motor neurone vary in size

Question 8

fast vs slow motor units

Answer 8

• slow (S) motor units - small ‘red’ muscle fibres that contract slowly and generate relatively small forces ; rich in myoglobin, mitochondria and capillary beds, resistant to fatigue, important for sustained muscular contraction (maintaining upright posture), fast fatigable (FF) motor units - large alpha motor neurones innervate larger pale muscle fibres that generate more force; sparse mitochondria so easily fatigued, important for brief exertions that require large forces (running/jumping), fast fatigue-resistant (FR) motor units - intermediate size, not as fast as FF motor units, generate 2x force of S motor units
• small slow motor units have a lower threshold for activation than the larger ones and are more tonically active during motor acts that require sustained effort, threshold for large fast motor units only reached during rapid movement requiring great force

Question 9

neuromuscular junction

Answer 9

action potential arrives>vesicles dock>release acetylcholine to synaptic clef>stimulates nicotinic receptors on muscle fibres>end result myosin contracts fibres

Question 10

alpha vs gamma motor neurones

Answer 10

alpha - innervate extramural fibres, generate muscle tension, gamma - innervate intrafusal fibres (spindles), control function

Question 11

proprioceptive feedback

Answer 11

information about position of the muscle or how stretched it is, if a muscle is contracting then there is activity in the afferent, intrafusal fibres sitting within muscle only stretch when muscle is stretched, activation of afferent only happens when intrafusal fibre is stretched, gamma motor neurone help intrafusal muscles contract and relax to an extent, define how ,such they should be relaxed by, so they don’t respond to the stretching or how contracted they should be so that they do respond to the stretching, set a threshold at which they do fire or done (intrafusals extend or don’t), if intrafusal extends you get afferent firing (afferents 1a - primary)

Question 12

tendon organ

Answer 12

sits at point which muscle joined to bone, pick top tension in muscles, tension in muscles meaning that if the muscle is already contracted then you try to pull on it further (loading the muscle beyond its the capacity to produce certain force) this could result in a tear, to avoid this tendon organ tries to shut down the muscle from firing

Question 13

spinal cord circuitry underlying muscle stretch reflexes - muscles spindles

Answer 13

sensory signal for stretch reflex originates in muscle spindles - sensory receptors embedded within most muscles, spindles comprise of 8-10 intrafusal fibres arranged in parallel with force generating extrafusal fibres, detect changes in muscle length, enables regulation of muscle contraction and precisely matches force generation to motor task, contributes to proprioception

Question 14

spinal cord circuitry underlying muscle stretch reflexes - Golgi tendon organ

Answer 14

mechanoreceptor, nerve endings interdigitated with tendons, in series; transduce loading of muscles, afferents innervated by; 1b, negative feedback system to regulate tension - Golgi tendon reflex, detects changes in muscle tension, when muscles contracts force acts directly on tendon, acts like strain gauge (monitors muscle tension and force of contraction, contributes to proprioception

Question 15

proprioceptive afferents - cutaneous

Answer 15

skin receptors - mechanoreceptors; meissner corpuscle, pacinian corpuscle, Ruffini endings (transduce stroking, pressure, vibration, skin stretch), thermoreceptors (warm and cool), nociceptors; pain (mechanical, thermal, polymodal)

Question 16

spinal cord circuitry underlying the regulation of muscle force - Golgi tendon organ

Answer 16

Golgi tension organs are encapsulated afferent nerve endings located at the junction of a muscle and tendon, each tendon organ innervated by single group 1b sensory axon (1b slightly smaller than 1a that innervate spindles), when muscle actively contracts the force acts differently on tendon leading to increase in tension of collagen fibrils in the tendon organ and consequent compression of intertwined sensory nerve endings, activation of non-selective cationic mechxnosensitive ion channels in the nerve endings of Golgi tension organ result in generator potential, if supra-threshold triggers action potentials that are propagated along the group 1b axon tot he spinal cord, 1b axons from Golgi tendon organ s contact GABAergic inhibitory local circuit neurones in spinal cord (1b inhibitory interneurones) that synapse with he alpha motor neurones that innervate same muscle, thus negative feedback system (Golgi tension organ), regulates muscle tension; decreases activation of muscle when exceptionally large forces generated, protects muscle.

Question 17

sensory input (afferents) from muscles

Answer 17

stretching of spindle is measure of degree of stretch of the muscle itself, tendon organ stretch is proportional to the tension exerted by the muscle, from muscle spindle - primary afferent; group 1a senses stretch and rate of change in stretch (terminates in motorpool directly, monosynaptic), secondary afferent; group 2 stretch only, group 3 and above are finer than the two previous; noiceceptive from muscle, from tendon organs - group 1b signal force change in the muscle

Question 18

flexion reflex pathways

Answer 18

stimulation of noiceceptive fibres leads to withdrawal of the limb form the source of pain by excitation of ipsilateral flexor muscles and reciprocal inhibition of ipsilateral extensor muscles, flexion of the stimulated limb is also accompanied by an opposite reaction in the contralateral limb (contralateral extensors excited, flexors inhibited), crossed extension reflex provides postural support during withdrawal of the affected limb from thew painful stimuli

Question 19

reflexes

Answer 19

• mono synaptic reflex; stretch reflex - sensory afferent>motor neurone>output, primary spindle 1a afferents excite alpha motor neurones of the same muscle (homonymous)
• poly synaptic reflex; crossed extensor reflex - sensory afferent>interneurone>motor neurone>output, Golgi tendon 1b afferents inhibit (via interneurones) alpha motor neurones of the same muscle

Question 20

spinal cord circuitry and locomotion

Answer 20

studies of rhythmic movements (locomotion and swimming) demonstrate local spinal circuits called central pattern generators are capable of controlling timing and coordination of complex patterns of movement and adjusting them in response to altered circumstances, movement of a single limb during locomotion can be thought of as a cycle with a stance phase; limb is extended and placed in contact with ground to propel animal forward, swing phase; limb is flexed to leave ground then brought forward to begin next stance stage, given the precise timing of the movements of individual limbs and the necessity of coordinating these movements its natural to assume locomotion is accomplished by higher centres that organise the spatial and temporal activity patterns of the individual limbs, activation of centres in brainstem, such as mesencephalic locomotor region can trigger locomotion and change the speed and pattern of the movement by changing level of activity delivered to spinal cord

Question 21

descending motor control (upper motor neurones)

Answer 21

most upper motor neurones that project to the medial part of the ventral horn also project to the medial part of the intermedia zone, axons of upper motor neurones course through anterior-medial white matter of the spinal cord and give rise to collateral branches that terminate over many spinal cord segments among medial cell groups on both sides of the spinal cord, sources of these projections are mainly in the brainstem, as their terminal zones in the medial spinal cord grey matter suggests they are concerned primarily with proximal muscles that control posture, balance, orientating mechanisms and initiation and regulation of stereotyped rhythmic behaviour, contrasting this a large majority of axons that project from motor cortex to the spinal cord course through the lateral white matter of the spinal cord and terminate in the lateral parts of the ventral horn, terminal fields restricted to only a few spinal cord segments, major component of this corticospinal pathway concerned with voluntary expression of precise, skilled movements involving more distal parts of the limbs

Question 22

corticospinal and corticobulbar tracts

Answer 22

upper motor neurones in the cerebral cortex reside in serval adjacent and highly interconnected areas in the posterior frontal lobe, which together mediate the planning and initiation of complex temporal sequences of voluntary movements, these cortical areas all receive regulatory inputs from the basal ganglia and cerebellum via relays in ventrolateral thalamus and inputs from sensory regions of parietal lobe

Question 23

reticular formation

Answer 23

complicated network of circuits in core of brainstem, extends from rostral midbrain to caudal medulla, similar in structure and function to local circuitry in the intermediate grey matter of the spinal cord, temporal and spatial coordination of limb and trunk movements - particularly those controlling rhythmic stereotypical behaviours such as locomotion, descending motor control pathways from reticular formation to spinal cord terminate primarily in medial part of grey matter where they influence the local circuit neurones that coordinate axial and proximal limb muscles, motor centres in reticular formation controlled largely by motor systems in cerebral cortex, hypothalamus or brainstem, relevant neurones in reticular formation initiate feedforward adjustments that stabilise posture during ongoing movement

Question 24

babinski reflex

Answer 24

typical response in adult to sharply stroking sole of foot is flexion of big toe and often other toes, damage to descending upper motor neurone pathway (corticospinal) stimulus may elicit extension of big toe and fanning of other toes, similar response occurs in human infants before maturation of corticospinal pathway and presumably indicates incomplete upper motor neurone control of local motor neuronal circuitry