PHYS: Posture, balance, and reflexes Flashcards
What are the three classes of sensory input contributing to postural control?
- Somatosensory (including proprioceptive)
- Vestibular
- Visual
What is a vestibule?
A small space or cavity at the beginning of a canal.
What does the vestibular system control?
Sense of balance and spatial orientation.
What is a monosynaptic reflex?
Reflex involving transmission of information from a sensory neuron to a motor neuron across a single synapse in the spinal cord.
How do polysynaptic reflexes differ from monosynaptic reflexes?
Polysynaptic reflexes involve two to several synapses in the spinal cord, involving one or more interneurons.
- Where do interneurons exist?
- What is their function?
- In the spinal cord.
- Relays signals between afferent and efferent neurons.
Name the two sensory receptor organs of note from this lecture.
- Muscle spindles.
- Golgi tendon organs.
Describe a feed-forward strategy
Postural responses are triggered centrally before voluntary movements
What is a spinal reflex?
Entire neural circuit bypasses higher centres
To what are Golgi Tendon organs sensitive?
Tension/change of tension
~16 micrometre diameter
What do muscle spindles monitor?
- Change in muscle length
- Rate of change in muscle length
Where are Golgi tendon organs located?
Within myotendinous junctions
Where are Muscle Spindles located?
Encapsulated within the muscle belly
What is the mechanism of action of Golgi tendon organs?
They monitor tendon tension (both static and dynamic) and transmit via large, fast, lb type fibres to synapse with inhibitory interneurones.
These inhibit alpha motor neurone firing, preventing damage
What is another phrase for muscle spindles?
Intrafusal fibres.
Extrafusal fibres = regular muscle fibres
What are the two types of sensory endings of muscle spindles?
Where are they located?
- Primary ending (Ia) - encircles central portion of each intramural fibre, input from nuclear bag & chain, senses velocity and length.
- Secondary ending (II) - innervate the receptor region on sides, input only from nuclear chain, senses length.
What are the two types of intrafusal fibres?
Hint: one relates to static, one relates to dynamic information
- Nuclear bag fibres (1-3 per spindle); dynamic - provide a rapidly adapting response
- Nuclear chain fibres (3-9 per spindle); static - provide a slowly adapting response
What are the three levels of hierarchy in reflex circuitry?
- Control of individual muscles
- Control of muscles around a joint
- Coordination of muscles at several joints
What is the monosynaptic stretch reflex?
Hint: most simple reflex
A 2 neuron circuit: single synaptic connection between a Ia afferent fibre from a muscle spindle and an alpha motor neuron
e.g. patellar tendon tap
What is the inverse myotactic reflex?
Hint: requires an inhibitory interneuron
Stimulation of Ib afferent neutron from a Golgi tendon organ causes reflex inhibition of the motoneurons via Ib inhibitory interneuron
Explain coordination about a joint
Hint: involves synergists and antagonists
Requires divergence of sensory input:
- Inhibitory interneuron synapse at antagonist muscle
- Excitatory synapses also with synergist muscles
Explain an example of complex multi-joint reflexes
Hint: e.g. stepping on a nail
- Flexion withdrawal reflex
- Reflex withdrawal from noxious stimuli
- Flexor muscles activated, extensor muscles inhibited - Crossed extensor reflex
- Enhance postural supporting during flexion withdrawal on contralateral limb
- Flexor muscles inhibited, extensor muscle activated
Explain modulation of reflex strength
Descending input from higher motor centres can modify sensitivity of of reflex response
Occurs via gamma motor neurons to muscle spindles - tightening or relaxing fibres within spindle
What is a motor unit?
A single motor neurone and the multiple muscle fibres that it innervates
What is the innervation ratio of motor units?
The ratio between the alpha motor neurone and the number of muscle fibres it innervates.
Associated with the degree of dexterity needed in the movement
Explain the difference in motor unit ratios between big strong muscles (e.g. gastrocnemius) and fine control muscles (e.g. rectus lateralise controlling the eye)
Gastroc = 579 MUs, and 1,934 Myofibres/MU –> Generates a lot of force, but not good dexterity
Rectus lat. = 4,150 MUs, and 5 Myofibres/MU –> Not a lot of force, but perfect dexterity
What are the 3 skeletal muscle fibre types?
Type I - slow contracting, fatigue resistant (e.g. postural muscles)
Type IIa - fast contracting, fatigue resistance
Type IIb - fast contracting, fast fatigue
What is the metabolic type and mitochondrial density of the 3 skeletal muscle fibre types?
Slow = oxidative, high mitochondrial density
Fast, fatigue resistant = intermediate, medium mitochondrial density
Fast, fatiguable = glycolytic, low mitochondria density
