Functional hierarchy of the motor system Flashcards
What controls voluntary muscle movement?
-Basal ganglia: involved in initiation of movement
-Cerebellum: involved in propagation of movement (fine tuning)
-Descending motor pathways:
From cortex: control fine movement
From the brainstem: control posture and balance
-Local brainstem and spinal cord circuits
Stretch reflex : how does it happen?
- Tendon of Knee is hit by a soft hammer
- The force from the hammer is transmitted to the muscle fibres - more elastic than the tendons so can stretch more
- The stretch is detected by SENSORY receptors in the MUSCLE SPINDLE
- Muscle spindles are primarily made up of primary and secondary sensory (Ia and IIa)
- APs are generated and sent into the dorsal horn of the spinal cord
The sensory afferents make 3 types of connections:
-To alpha motor neurones of the AGONIST muscle causing it to contract : this is a mono-synaptic reflex: no other interneurones involved- this is the only type of reaction which is like this
- To the inhibitory interneurones of the the ANTAGONIST muscle- causing it to STRETCH- by preventing APs being sent to the alpha motoneurons = RECIPROCAL INHIBITION
- To the dorsal columns: up to the somatosensory cortex to inform the brain on the length of muscles
It is an example of a negative feedback loop
Inverse stretch (clasp-knife reflex): how does it work
- Muscle contracts and shortens
- Pulls on tendon and causes Primary sensor Ib afferents from the GTO to increase the firing of APs
- These synapse with the inhibitory interneurones to agonist muscle and DECREASE CONTRACTION
- Stimulate the excitatory interneurones of the antagonist muscle and INCREASE CONTRACTION
- synapse into the dorsal root to inform the somatosensory cortex of the length of muscles
Leads to:
- Agonist muscle stretches
- Antagonist muscle contracts
- Prevents the muscle from contracting so hard that it pulls the tendon away from the bone
what us the inverse stretch reflex controlled by?
The Golgi tendon organ
Flexor withdrawal reflex : how does it work?
- Increased APs from noci receptors
- Synapse in spinal cord and increase activity of excitatory interneurones (alpha motoneurones) to flexor AGONIST muscle- cause it to flex
- Synapse with inhibitory interneurones of the ANTAGONIST extendors- cause them to be inhibited
- Excite contralateral extensor muscles
- Inhibit contralateral flexor muscles
- Sensory information in cortex up contralateral spinothalmic tract
From which receptors is information supplied in the flexor- stretch reflex
Noci receptors in bones, joint and tendons
These receptors respond to pain
Reflexes and voluntary motor control from the brain
Reflexes can be over- ridden by voluntary control of the Brain
Alpha motoneurons are involved in both pathways: they receive information from over 10,000 synapses.
Hence why it is so easy for the reflexes to be over-ridden
Define Spinal Shock?
A result of spinal injury: - Hyporeflexia - Clonus - Autonomic Dysfunction
The reflexes gradually return over a few weeks and autonomic dysfunction takes longer to resolve, possibly it never properly does
How do gamma-motorneurons affect muscle tone?
Excess stimulation of muscle spindles –> Excess stimulation of 1a afferent fibres –> Excess muscle activity –> Spasticity And vice versa
What do gamma-motorneurons do?
They come on descending pathways from the brain and innervate muscle spindle fibres
How does pain facilitation work?
When multiple impulses come along one after another they increase in strength. This occurs between the same inputs e.g. nociceptors And between different inputs e.g. pain coupled with a stretch reflex can cause it to be very exaggerated
What stretch reflexes can we test and what vertebral level do they originate?
Biceps Jerk - C6
Triceps Jerk - C7
Patellar Tendon - L4
Achilles Tendon - S1
