Spinal reflexes & the descending pathway Flashcards
What is a motor control system and what is its function?
Made up of cerebral cortex, cerebellum, basal ganglia, thalamus, motor cortex & brain stem, + neurones* going to the muscles.
3 functions:
1. Posture & Balance e.g. sitting & standing.
2. Goal-directed movements e.g. picking up glass.
3. Communication e.g. pointing, waving.
Describe the classifications of movement.
- Voluntary movements:
- Complex actions e.g. reading or writing.
- Purposeful goal-directed movements e.g. picking up an object.
- Learned movements - Reflexes:
- Involuntary, rapid movements e.g. kneejerk or eye blink. - Rhythmic motor patterns:
- Movement can be voluntary AND reflexive e.g. breathing, chewing, walking, runnning.
- Initiation & termination are voluntary but once initiated, the movement is repetitive & reflexive.
State the classification of sensory and motor neurones.
Sensory neurons
- Ia fibres come from muscle spindles.
- Ib fibres come from Golgi tendon organs.
Motor neurons
1. ⍺-motor neurons supply extrafusal skeletal muscle (outside the muscle spindle).
- larger
- fastest
2. 𝛄-motor neurons supply intrafusal skeletal muscle (inside the muscle spindle).
- smaller
- slower
What is a reflex arc?
A single reflex
- Stimlus e.g. nail detected by receptor
- Sensory neurones (afferent neurons) transit signaal through peripheral nerve to CNS
- CNS- info processing
- afferent neurons synapse w/ interneuron
- interneuron synapse w/ efferent neuron (motor neuron) - Efferent neuron exits spinal cord & relays info to effector e.g. muscle
- Effector responds by contracting & moving away from stimuli
What is the muscle spindle?
RECEPTOR in muscle fibres
- respond to stretch
- innervated by sensory neurones (afferent neurons)
- detects changes in length of muscle
What is the function of muscle spindles?
- Stops damage - overstitching can damage the cross-bridges that link thin &thick filaments in muscles
- muscle spindle identifies stretch & length
- gets the opposite muscle to counteract the action of the muscle if it is stretching too far. - Proprioperception - the muscle spindle helps us plan movement because understanding the length of the muscle & its attachment can tell us where the limb is
NOTE: proprioperception is the unconscious perception of movement & spatial orientation.
What is the mytotatic reflex? Explain what occurs in a myotatic reflex. Use an example.
also called stretch reflex
- its when the stretched muscle stimulates the muscle spindle
- causing the muscle to contract
It’s an example of monosynaptic reflex
- most rapid, simple reflex w/ a single synapse between afferent & efferent neruons
- E.g. stretch reflex- knee jerking as a result of patella reflex
Reflex summarised:
- Patella tendon hit > muscle stretched > stimulates receptors in muscle (muscle spindles) > signal sent to spinal cord > quadriceps contracts > knee jerks forward
E.g. patella reflex- knee jerk
1. Tap on the patella stimulates quadricep muscles
- hitting patella mimics the quadriceps stretching
2. Dynamic nuclear bag receptors of the muscle spindle are stimulated.
3. Increases the rate of firing of group La afferent neurones to the spinal cord
4. Afferent neurons will synapse w/ efferent neuron
4. Efferent neurons supply the quadricep muscles & innervate its muscle fibres
5. Quadriceps will contracting- increase in muscle tone= knee will jerk up
6. Muscles work in pairs- Ia neurons also stimulate inhibitorary interneurones - inhibit the motor neurons of hamstrings, stopping them contracting & flexing.
What is the Golgi tendon organ?
PROPRIRECEPTOR in tendon
detects tension or force in a tendon
- activated by muscle stretch & contraction
Provides info on:
- length of muscle.
- Rate of change in length of muscle
- Force generated
What is the inverse myotatic reflex? Explain what happens in the inverse myotatic (Golgi Tendon) reflex.
inverse myotatic= stimulation of the Golgi tendon organ due to prolonged stretch causes stretched muscle to relax
- Also called reciprocal activation- stretch of 1 muscle stimulates activity of opposing muscle (contract)
- this reflex is a polysnaptic reflex
Process:
1. Muscle contracts e.g. quadriceps- i.e. it gets shorter
2. results in increased tension/ force in muscle
3. This is detected by Golgi tendon organ
4. Stimulation of Golgi tendon organ is picked up by 1b afferent fibres & sent to dorsal root ganglion
5. in the dorsal & ventral horn, the 1b afferent fibres split into 2 & synapse w/ 2 different interneurons
- 1= inhibitory interneuron- synapses w/ ⍺-motor neurone of the quadriceps (extensor muscle) & inhibits it, Stopping it from shortening any further (will cause it to snap off) & RELAXING)
- 2= excitatory interneuron- synapse w/ ⍺-motor neurone of hamstring (antagonist muscle- flexor muscle), causing it to contract
6. This stops damage & tells CNS where the limb is (proprioception).
How does the inverse myotatic reflex help maintain posture?
- During maintained posture i.e. standing, quadricep muscles will start to fatigue
- Force in patella tendon will start to decline
- So activity in the Ib afferent fibres in Golgi Tendon Organ will decline
- Normal inhibition of ⍺-motor neurons supplying the quadriceps is removed
- quadriceps will contract more strongly = increases the force in the patellar tendon.
- This keeps knee extended, allowing us to stand.
NOTE: view diagram on notes
Explain the crossed extensor reflex (withdrawal reflex).
Example of a polysynaptic reflex
- means it involves interneurons, is more complex & involves reciprocal inhibition?
- e.g. withdrawal reflex
Response from both legs- leg where stimulus occurs & leg opposite
Involves shifting body weight to opposite leg away from pain, to support body & prevent falling over
- Pain is detected through Aδ sensory fibres
- Signal sent to dorsal horn of spinal cord where sensory fibre splits into 2
- synapse w/ both inhibitory & excitatory interneurons - In the side that feels the pain, inhibitory interneurones inhibit ⍺-motor neurones of extensor muscles (quadriceps)
- causes them to relax - Exitatory interneurons excite the ⍺-motor neurons of antagnoistic flexor muscles(semitendinosus)
- causes them to contract - The net effect is co-ordinated flexion = removes leg from stimulus
- On the side not in pain, inhibitory interneurons inhibit the ⍺-motor neurons of flexor muscles (semitendinosus)
- causing them to relax - Excitatory interneurons excite ⍺-motor neurons of extensor muscles (quadriceps), casuing them to contract
- The net effect is co-ordinated knee extension = allows weight to be shifted to other leg
Name the descending pathways involved in motor control.
Descending tract splits into 2:
- Pyrimidal- travels through pyramids of medulla
- responsible for voluntary movement of muscles
- originate at cerebral cortex
- Made up of:
*Corticospinal tract
* Corticobulbar tract - Extrapyrimidal- does not travel through pyramids of medulla
- responsible for involuntary movement & modulation (tone & balance) of muscles
- originate inn brainstem
- Made up of:
* Rubrospinal tract
* Vestibulospinal tract
* Reticulospinal tract
* Tectospinal tract
Describe the origin, route and innervation of the corticospinal tract.
Origin of 1st order neuron = cerebral cortex
Travel through cerebri > internal capsule> pons > medulla
Decussate to other side of the spinal cord before synapsing onto 2nd order motor neurons on ventral horn
2nd order neuron will then innervate muscle
*Contralateral innervation- left side of brain controls movement on right side of body
called lateral corticospinal tract
Describe the origin, route and innervation of the corticobulbar tract. What is it responsible?
Responsible for VOLUNTARY control of face, head & neck muscles
- swallowing + facial expressions
Origin of 1st order neuron= cerebral cortex
Pass down to internal capsule before synapsing w/ 2nd order motor neurons in brain stem of each segment
Thus, innervated billaterally- neurons from 1 side innervate muscles of both sides of face
Describe the origin, termination and innervation of the Rubrospinal tract. What is it responsible for?
Responsible for fine motor control + flexion & extension tone of large muscles
Origin= red nuclei in mid brain
Terminate at cervical & thoracic sections of spine
- so primarily innervate upper limb
Contralateral invervation
