Spinal Reflexes Flashcards
What is the stretch reflex
Simplest reflex and found in all muscles. Uses info from muscle spindles which monitor muscle length
Give an example of a stretch reflex
knee jerk reflex
How does the stretch reflex work summary
- Muscle stretch - stimulates muscle spindles
- Causes reflex muscle contraction - muscle shortens to previous length (e.g. heavy object stopped fromdropping on outstretched arms)
stretch activates 1a afferent sensory nerves in muscle spindle –> inc number of APs in 1a affernets projecting through dorsal horn into the spinal cord
How do spindle sensory afferents divide?
3 types of connections (2 in spinal cord, 1 in brain)
Give 3 connections made by spindle sensory afferents
- many directly contact a-monorneurones in the stretched muscle: causing rapid contraction of the agonist muscle - is monosynaptic reflex with 1 synapse and no interneurons involved
- Since muusles use agonist and antagonists to move joints, sensory fibbres from stretched spindle also connect indirectly with antagonist muscles 9when agonist conntracts, antagonist relaxes). Happens because spindle afferents connect with and activate inhibitory interneurones which decrease activation of a-motorneurones to the antagonist muscle - which then relaxes - process called RECRIPROCAL INHIBITION
- Spindle afferent firing also travels up the dorsal columns to thalamus and somatosensory cortex - to tell brain aout length of muscles
describe nerual pathway of stretch reflex
see diagram/sheet
afferent neuron
sensory neuron
Other names for the inverse stretch reflex
Golgi-tendon organ or clasp-knide reflex
Explain the inverse stretch/Golgi-tendon organ reflex
what causes it, what happens and what this causes
- Caused by 1b afferent nerves from the Golgi Tendon Organs (GTO) which monitor muscle tension
- Muscle contracts and shortens, pulls the tendon and sensory 1b afferent nerves from the GTOs, inc firing of AP
- This causes:
1. Activation of inhibitory interneurones to the agonist muscle and a dec in contraction strength
2. Activation of exitory interneurones to the antagoist muscles
3. Info about muscle tension ascends in the dorsal columns to the somatosensory cortex
Why is the GTO called the “clasp-knife reflex” and what is it’s function
greatly inc tension leads to collapse of resistance - protective mechanism to prevent muscle/tendon damage
briefly describe process of GTO reflex
Pull hard on tendon, GTO afferents inc firing but synaps with inhibitory interneurons so reduce motoneurone firing - muscle inhibited and relaxes rapidly.
What is the GTO reflex and what does it prevent
Reflex is polysnaptic and protective and avoids muscle contacting so hard tendons are ripped from bone
Flexor-withdrawal reflex
uses info from pain receptors (nociceptors) in skin/muscles/joints and withdraw part of the body away from stimulus and towards the body (flex affected part)
What is the flexor/withdrawal reflex
Polysynaptic and protective
What do inc Ap in nociceptor nerves cause in the flexor/withdrawal reflex
5 steps
- Inc activity in flexor muscle via number of excitatory interneurons
- At same time, via number of extiroy/inhibitory interneurones, the antagonistic extensors are inhibited
- Excitatory interneurons cross spinal cord and excite the contralateral extensors
- other interneurones cross the spinal cord, synapse with inhibitory neurons and they inhibit the contralateral flexors - helps maintain an upright posture by extending the limb opposite the flexed one (shifts body weight)
- Sensory info ascends to the brain in the contralateral spinothalamic tract
What is the flexor reflex also associated with
crossed extensor reflex
What does flexor reflex cause
Ipsilateral flexion in response to pain (on same side of body)
??? as also have crossed extensor reflex
briefly explain the circuitry of the flexor reflex
- Small diameter A# nociceptive fibres triccering pain enter cord
- branch and activate interneurons in several spinal segments above entry point
- activate alpha motorneurons controlling all the flexor muscles of the affected limb
Why is the flexor and crossed extensor reflex slower than the stretch reflex
Several interneurons in pathway of flexor/crossed extensor, each with a small synaptic delay. Nociiceptive sensory receptors have a small diameter compared to muscle spindle afferents (stretch reflex), meaning they conduct slower
Summarise what the flexor/crossed extensor reflex actually looks like
Leg flexes from pain, other one extends to maintain balance
What else does our brain do to these inbuilt spinal reflexes
Uses them as building blocks and adds voluntary control from the thalamus and cortex to reshape them into finely controlled movements
What can we do to reflexes
Override them consciously
How does the GTO reflex work
- Hold heavy load
- Neuron from GTO fires
- Motor neuron inhibites
- Muscle relaxes
- load is released
if was child then could override
4 types of reflex
- Stretch reflex
- Golgi tendon reflex
- Withdrawal reflex
- Crossed extensor reflex
how can we override the GTO reflex
descending voluntary exitation of alpha motorneurons overriides the inhibiton from the GTO s and maintains muscle contraction - preventing GTO reflex
Explain the synapses that each alpha motorneuron recieves and where they come from
Thalamus and cortex, causing exitory/inhibitory postsynaptic potentials (EPSP/IPSP). Some make direct contact but most act through interneurons.
Each alpha motorneurone must intergrate these signals, net effect at cell body is summed (exitation minus inhibition). Membrane potential changes on distant dendrites will havbe less of an effec than those from nearer the cell body
How can we override the stretch reflex?
Strong descending inhibition in patient hyperpolarizes alpha motoneurones and the stretch reflex is hard to evoke
High activity in upper motor neurones spreads to and depolarises lwer level motorneurones and this overcomes the descending inhibition
What does the absence of the stretch reflex not necessarily show
evidence of nerve damage
Can we override the withdrawal reflex
Yes, if for ex, dropping a hot object would cause more harm (we can override it)
What is the clinical relevane of reflexes
From reflexes can work out spinal level of problem
In a stretch reflex, what is spindle input
highly localised and affects only alpha motoneurones at 1 or 2 spinal segments
What happens to pain fibre input in the withdrawle reflex
Diffues, and spreads over several spinal segments. Powerful pain causes greater segmental spread, more muscles recreuited and response is larger
For ex, if finger touch potato, small responese, but if hand touch then withdraw whole arm = big response
this is facilitation
What does facilitation do?
Enhance the effectiveness of sensory inputs
Explain the process of facilitation
If have A.B.C as neuronal inputs, each casuing AP in 4 motorneurons.
Input A will cause slight depolarisation of B, but not to threshold: said to be in subliminal zone of input A.
if have input in A and C then overlap big enough to fire AP in B also. This activates 12 motorneurones (not just 4 or 8). This is FACILITATION
Jendrassik manoeuvre
a medical maneuver wherein the patient clenches the teeth, flexes both sets of fingers into a hook-like form, and interlocks those sets of fingers together. The tendon below the patient’s knee is then hit with a reflex hammer to elicit the patellar reflex. - should stop knee reflex
role of interneurons in reflexes
coordinate the motor response necessary to address a stimulus. Additionally, these neurons are responsible for communicating with the brain for cognition.
Ipsilateral
same side (e.g. side of body stood on pin like right leg)
Contralateral
Opposite side of body (e.g. right leg flexes after standing on pin so contralateral response in left leg by extending)
Role of higher centres of reflex
Not sure about answer???
brain overriding reflex (e.g. inhibiting or facilitating alpha motorneurons to intiiate desired response
Function of stretch reflex
To relieve stretch on your tendons/muscles
