Spinal Reflexes Flashcards
What is the simplest reflex?
stretch reflex- found in all muscles
for e.g. patellar tendon or knee-jerk reflex
Where does stretch reflex get info from?
info from muscle spindles which monitor muscle length
Describe knee jerk reflex?
knee jerk follows a sharp tap to an inelastic tendon
force is transmitted to the muscle fibres – they are more elastic than tendons and so are more able to stretch.
stretch activates 1a afferent sensory nerves in the muscle spindle :
this increases the number of APs in 1a afferents projecting through the dorsal horn into the spinal cord.
What are the three types of connections that spindle sensory afferents are divided?
1.many directly contact alpha-motoneurones in the stretched muscle : causing rapid contraction of the AGONIST muscle.
this is a MONOSYNAPTIC REFLEX – one synapse, no interneurones involved – it is the only such connection known.
- Because muscles use agonists and antagonists to move joints - sensory fibres from the stretched spindle also connect indirectly with the antagonist muscles.
So, when the AGONIST muscle contracts,
the ANTAGONIST muscle relaxes (stretches)
This happens because spindle afferents connect with and activate inhibitory interneurones which decreased activation of alpha -motoneurones to the antagonist muscle - which then relaxes
this process called RECIPROCAL INHIBITION.
- Spindle afferent firing also travels up the dorsal columns to thalamus and somatosensory cortex - to tell the brain about length of muscles
What is involved in the inverse stretch reflex (golgi-tendon organ or clasp-knife reflex)?
caused by 1b afferent nerves from the Golgi Tendon Organs (GTO) – which monitor muscle tension.
muscle contracts and shortens – this pulls on the tendon and the sensory 1b afferent nerves from the GTOs increasing firing of APs.
this causes -
Activation of inhibitory interneurones to the agonist muscle and a decrease in contraction strength.
Activation of excitatory interneurones to antagonist muscles.
Again, information about muscle tension ascends in the dorsal columns to the somatosensory cortex.
Describe Golgi-tendon organ reflex : inverse stretch reflex : clasp-knife reflex
in weightlifting?
weightlifters lifting heavy weights, using legs for force.
Contract quads and pulls hard on tendon
And therefore golgi tendon afferents increasing firing rate to spinal cord- synapse onto inhibitory interneuron which dampens activity of motor neurons - tendon relaxes
Another branch of afferent axon activates an excitatory interneuron- which increases activity of motor neuron that go to antagonist muscle and cause contraction.
reflex is polysynaptic and protective
avoids muscle contracting so hard tendon is ripped from the bone
What is the flexor/ withdrawal reflex- with crossed extension?
use information from pain receptors (nociceptors) in skin, muscles and joints.
they are polysynaptic and protective
they withdraw part of the body away from the painful stimulus and in towards the body - so flex the affected part.
How does the withdrawal reflex happen?
Increased APs in nociceptor nerves cause 5 things to happen:
1) increased activity in the flexor muscles of the affected part via a number of excitatory interneurones.
2) At the same time, via a number of excitatory and inhibitory interneurones, the antagonistic extensors are inhibited
Describe circuitry of flexor reflex?
class a delta fibres detect pain and info sent to spinal cord
sensory afferent branch a lot and activate interneurons in several spinal segments above entry point
which activate α motoneurones controlling all the flexor muscles of the affected limb
How to prevent falling over- as a result of simply withdrawing limb from pain and nothing else?
the contralateral limb extends to support whole body weight now one leg is lifted off floor
How does contralateral limb extend?
excitatory interneurones cross the spinal cord and excite the contralateral extensors
Other interneurones cross the spinal cord, synapse with inhibitory neurones and they inhibit the contralateral flexors
this helps to maintain an upright posture by extending the limb opposite the flexed one - to bear the shift in body weight.
Sensory information ascends to the brain in the contralateral spinothalamic tract.
This basic circuitry is similar to that used for walking
and is hardwired at spinal cord level.
What happens when you ask someone to lift an ever increasing load?
The load stretches the muscle and its spindles, causing reflex contraction to restore the arm position (the stretch reflex)
How do we override the golgi tendon reflex?
each α motoneurone receives >10,000 synapses
many are from the thalamus and cortex and cause excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs).
some make direct contact, but most act through interneurones
each α motoneurone has to integrate these signals. Their net effect at the cell body is summed (total excitation minus total inhibition).
“I don’t want to drop this” - and descending voluntary excitation of motoneurones overrides the inhibition from the GTOs and maintains muscle contraction – so preventing the GTO reflex
How is stretch reflex overriden?
strong descending inhibition in an anxious patient hyperpolarizes α-motoneurones and the stretch reflex can be hard to evoke
