motor function Flashcards
spinal reflexes: recognise a range of spinal reflexes, including stretch reflex, flexion / withdrawal reflex, crossed extension reflex); distinguish hypo- and hyperreflexia; explain the concept of supraspinal control of reflexes
define reflex and feature of reflex
automatic and often inborn response to a stimulus, involving a nerve impulse passing inward from a receptor to a nerve centre, and then outward to an effector (as a muscle or gland) without reaching the level of consciousness; involuntary coordinated pattern of muscle contraction and relaxation elicited by peripheral stimuli; once started, can’t be stopped
what is magnitude and timing of reflex determined by
intensity and onset of stimulus
components of reflex arc, and significance of afferent signal
sensory stimulus -> pain receptor -> dorsal root ganglion -> dorsal horn (if cut dorsal roots, no generation of reflex) -> interneurone (integrating centre in spinal cord) -> motor neurone from ventral horn -> ventral root -> effector muscle
monosynaptic stretch reflex: patellar reflex pathway
patellar reflex: hit patellar tendon -> receptor -> sensory neurone excited -> dorsal root ganglion -> dorsal horn -> (up to brain) and integrating centre (spinal cord) -> ventral horn -> spinal nerve -> effector motor neurone -> contraction of effector agonist muscle, while inhibitory interneurone causes inhibition of outflow to antagonist muscle
what is the Hoffman (H-) reflex
a monosynaptic reflex which is elicited by electrical stimulation
polysnaptic reflex: flexion withdrawal and crossed extensor, and pathway
flexion withdrawal and crossed extensor: pain stimuli from foot -> receptor -> sensory neurone excited -> dorsal root ganglion -> dorsal horn -> stimulation of flexor, and up and down spinal cord, to promote flexor contraction and allow withdrawal of leg (interneurone excited to allow different motor neurones to be activated at different levels) -> also interneurones excited which cross spinal cord at same level and cause stimulation of motor neurones, which cause flexion in other leg to maintain gait
what is supraspinal control of reflexes
where reflex response (automatic e.g. knee jerk, or stereotyped e.g. sneeze) can be modified
how to show supraspinal control in clinic
potentiate patellar response when clenching teeth or making fist and tapping patellar tendon; this causes hyperexcitability causing bigger patellar swing (Jendrassik manoeuvre)
how is supraspinal control of reflexes shown using decerebration
higher CNS centres exert inhibitory and excitatory reflex upon stretch reflex (inhibitory dominated in normal conditions); removing cerebellar input (decerebration) shows excitatory control from supraspinal areas, as remains rigid
supraspinal control of reflexes: how do higher centres influence reflexes (5 wyas)
activate alpha motor neurones, activate inhibitory interneurones, activate propriospinal neurones (up/down spine), activate gamma motor neurones (intrafusal so respond to movements of muscle; control muscle fibre length based on afferent input), activate terminals of afferent fibres
supraspinal control of reflexes: 4 higher centres, pathways involved and function
cortex - corticospinal (fine control of limb movements, body adjustments), red nucleus - rubrospinal (automatic movements of arms in response to posture/balance changes), vestibular nuclei - vestibulospinal (altering posture to maintain balance), tectum - tectospinal from roof of midbrain at superior colliculi down to spinal cord (head movements in response to visual information)
gamma reflex loop: extrafusal vs intrafusal
extrafusal: under control of alpha motor neurones and is voluntary control of muscle; intrafusal: receive information via sensory axons and respond to changes in lengths of muscle spindles using gamma motor neurones (change length based on input)
upper motor neurone lesion: hyper-reflexia cause, pathophysiology and clinical signs
e.g. due to stroke, causing loss of descending inhibition, so same reflex requires very little stimulus; clonus and Babinski sign
describe Babinski sign: normal vs upper motor neurone lesion
stimulus to lateral aspect of foot and across balls of toes: big toe flexes, and small toes fan out; if normal should flex dorsally
lower motor neurone lesion: hypo-reflexia meaning
below normal or absent reflexes
