Spinal reflexes Flashcards
what is a stretch reflex
muscle stretch stimulates muscle spindles and this causes reflex muscle contraction which shortens the muscle to previous length eg knee jerk
3 types of connections that spindle sensory afferents divide into
monosynaptic reflex, reciprocal inhibition, connections to thalamus and somatosensory cortex
monosynaptic reflex
one synapse, no interneurones involved. many directly contact alpha motoneurons in the stretched muscle which causes rapid contraction of the agonist muscle
reciprocal inhibition
sensory fibres from the stretched spindle also connect indirectly with the antagonist muscles- when the agonist muscle contracts, the antagonist muscle relaxes because spindle afferents connect with and activate inhibitory interneurones which decrease activation of alpha motoneurons to the antagonist muscle which then relaxes
connections to thalamus and somatosensory cortex
spindle afferent firing also travels up the dorsal columns to thalamus and somatosensory cortex to tell the brain about length of muscles
inverse stretch reflex causes
activation of inhibitory interneurones to the agonist muscle, activation of excitatory interneurones to antagonist muscles and information about muscle tension ascends in the dorsal column to the somatosensory cortex
inverse stretch reflex is caused by
1b afferent nerves from the golgi tendon organs. muscle contracts and shortens and this pulls on the tendon and the sensory 1b afferent nerves from the GTOs increase firing of action potentials
how is inverse stretch reflex protective
reflex is polysnyaptic and protective as avoids muscle contracting so hard tendon is ripped from bone. uses antagonist muscles
circuitry of flexor reflex works by
small diameter A delta nociceptive fibres triggering pain enter the spinal cord, they branch a lot and activate interneurons in several spinal segments above entry point this activates alpha motoneurons controlling all the flexor muscles of the affected limb and then contralateral limb extends to prevent u falling over (stand on tack)
how does the contralateral limb extend in circuitry of flexor reflex
excitatory interneurones cross the spinal cord and excite the contralateral extensors, other interneurones then cross the spinal cord and inhibit the contralateral flexors which helps to maintain an upright posture by extending the limb opposite the flexed one
what is included in the sum of alphamotoneurons net effect at the cell body
total excitation minus total inhibition
why do we not drop heavy but important things
you think you don’t want to drop it so descending voluntary excitation of alpha motoneurons overrides the inhibition from Golgi Tendon Organs and maintains muscle contraction so prevents the Golgi tendon organ (inverse stretch) reflex
stretch reflex does not work does this mean nerve damage
no as strong descending inhibition hyperpolarizes alpha motoneurons and can mean stretch reflex is hard to evoke. interlock fingers and pull and it should work. high activity in upper motor neurones spread and depolarises lower level motor neurones and this overcomes the descending inhibition. jendrassik manoeuvre
why are reflexes clinically relevant
they are important in assessing the integrity of the whole spinal cord circuit, helps to find spinal level localisation of a problem. reflexes evoked above but not below a given level may localise a problem
what is facilitation
the enhancement of the effectiveness of sensory inputs. occurs between the same stimuli, pain fibres and between different stimuli. hot plate one finger vs whole hand. firing of input A= 4 motoneurons activated, firing of input B= 4 motoneurons activated, firing of inputs A and B= 12 motoneurons activated as they have sufficient excitation together to bring C to threshold too
