motor systems Flashcards
what, where, how
what - decide and initiate; basal ganglia and frontal cortex, where - reach; cerebellum and primary motor area, how - grasp; primary motor cortex and spinal circuits
motor systems organisation
- 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)
cortical paths
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
brainstem motor pathways
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
lower motor neurone
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
motorpool
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
motor unit
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
fast vs slow motor units
- 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
neuromuscular junction
action potential arrives>vesicles dock>release acetylcholine to synaptic clef>stimulates nicotinic receptors on muscle fibres>end result myosin contracts fibres
alpha vs gamma motor neurones
alpha - innervate extramural fibres, generate muscle tension, gamma - innervate intrafusal fibres (spindles), control function
proprioceptive feedback
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)
tendon organ
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
spinal cord circuitry underlying muscle stretch reflexes - muscles spindles
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
spinal cord circuitry underlying muscle stretch reflexes - Golgi tendon organ
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
proprioceptive afferents - cutaneous
skin receptors - mechanoreceptors; meissner corpuscle, pacinian corpuscle, Ruffini endings (transduce stroking, pressure, vibration, skin stretch), thermoreceptors (warm and cool), nociceptors; pain (mechanical, thermal, polymodal)