Week 11 Flashcards
What is a spinal reflex?
- automatic, stereotypes motor response in reaction to a sensory stimulus
- stimuli from peripheral receptors
- circuitry entirely within spinal cord
What are the basic components of the spinal reflex loop?
- sensory neurom
- interneuron
- motor neuron(s)
Which spinal reflex is often referred to as the stretch reflex?
the spindle mediated reflex
The spindle-mediated reflex is elicited by…
stretch of muscle fibres (e.g. clinical tendon tap, vibration of tendon)
The spindle-mediated reflex is mediated by…
muscle spindles via 1a afferent, ascend via Dorsal Column Medial Lemniscal tract
What is the goal of the spindle-mediated reflex?
maintain muscle length
The spindle-mediated reflex is comprised of two simultaneous reflexes. What are they?
- autogenic excitation
- reciprocal inhibition
What is autogenic excitation?
- spindle-mediated
- 1a afferents diverge and project to:
→ all alpha motor neurons of the same (homonymous/agonist) muscle
→ many motor neurons of synergists - amplification effect of single 1a afferents
- monosynaptic reflex (occurs ~20-30ms)
What is reciprocal inhibition?
- spindle-mediated
- to facilitate the activation of the agonist/synergist muscles → inhibit antagonist muscle
- 1a afferents diverge onto 1a inhibitory interneuron
- 1a inhibitory interneuron projects onto alpha motor neuron of the antagonist muscle → inhibits antagonist
What is the 1b inhibition reflex?
GTO-mediated
The GTO-mediated reflex is elicited by…
active (i.e. self-generated) tension in muscle fibres
The GTO-mediated reflex is mediated by…
GTO via 1b afferent, ascend via dorsal Column Medial lemniscal tract
What is the goal of the 1b inhibition reflex
relax/inhibit muscle
The GTO-mediated reflex is comprised of two simultaneous reflexes. What are they?
- autogenic inhibition
- reciprocal excitation
What is autogenic inhibition?
- GTO mediated
- 1b afferent projects to 1b inhibitory interneuron
- 1b inhibitory interneuron projects to alpha motor neuron of agonist muscle → inhibits agonist
- disynaptic reflex (occurs ~40-50 ms)
What is reciprocal excitation?
- GTO mediated
- to facilitate relaxation agonist muscle → activates antagonist
- 1b inhibitory interneuron diverges onto other inhibitory interneuron
-second inhibitory interneuron projects on alpha motor neuron of the antagonist muscle → facilitates (i.e. activates) antagonist muscle - facilitation via inhibition of an inhibitor
Cutaneous-mediated reflexes are also known as…
polysynaptic reflex pathways
The cutaneous-mediated reflex is elicited by…
painful stimulus (e.g. touching hot surface, stepping on nail)
The cutaneous-mediated reflex is mediated by…
nociceptors (pain receptors) via A- or C- fibres, ascend via spinothalamic tract
What is the goal of the cutaneous-mediated reflex?
withdraw from painful stimulus
The cutaneous mediated reflex is comprised of two simultaneous reflexes. What are they?
- flexor (withdrawl) reflex
- crossed-extensor reflex
What is the flexor (withdrawl) reflex?
- cutaneous-mediated
- coordinated response in limb flexor muscles - withdrawl from pain
→ A-fibres project to excitatory interneuron → excite flexor alpha motor neurons (i.e. hamstring muscles) - extensor motor neurons inhibited - allow flexors to pull away from pain
→ A-fibres project to inhibitory interneuron → inhibit extensor alpha motor neurons (i.e. quadriceps muscles)
What is the crossed-extensor reflex?
- cutaneous mediated
- opposite response in opposite limb of the flexor reflex
- functions to stabilize body so other limb can move away from pain
- A-fibers projects to excitatory interneuron → excite extensor alpha motor neurons (i.e., quadriceps muscles)
- A-fibers projects to inhibitory interneuron → inhibit flexor alpha motor neurons (i.e. hamstring muscles)
How is the flexor & crossed-extensor reflex a protective mechanism?
- flexor (withdrawal) reflex → withdraw from stimulus
- crossed-extension reflex → stabilize body
Recurrent inhibition is mediated by what type of cells?
renshaw cells
Recurrent inhibition is a _______ feedback mechanism
negative
How do the renshaw cells mediate recurrent inhibition?
- inhibitory neurons within spinal cord
- release glycine (inhibitory neurotransmitter) onto ⍺ motor neuron
- activated by collateral from ⍺ motor neuron (allows motor neuron to “stay informed” about its own activity
- inhibition onto the motor neuron pool (acts as a “limiter” or “governor” to prevent overactivity of the muscle
- rate of discharge is proportional to the rate of discharge of the associated motor neuron (ie.e. as MN increases activity, renshaw cell activity increases)
What is C. Tetani?
- bacteria causing tetanus (lockjaw)
- release toxin which prevents renshaw cells from releasing glycine (prevents recurrent inhibition)
- leads to hyperactivity of motor neurons
→ severe, persistent muscle activation
→ tetanic spasm
What is pre-synaptic inhibition?
- one pre-synaptic neuron inhibits another by releasing GABA
- leads to downstream decrease in post-synaptic neuron activity
example: 1a afferents can be inhibited before they can excite the ⍺ MN
How does pre-synaptic inhibition work?
communication via an axo-axonal synapse
- inhibition on the pre-synaptic neuron
- prevents Calcium channels from opening
- decreases calcium influx in presynaptic neuron
- reduction in neurotransmitter release onto post-synaptic neruon
- selective inhibition of only the presynaptic neuron
How does studying the H-reflex lead to showing how reflexes can be trained?
- The H reflex is a stretch reflex mediated by the spinal cord. It is elicited by electrical stimulation of the Ia afferent neurons, bypassing the muscle spindle receptors.
- This method allows researchers to study reflex plasticity and neural adaptation.
- The triceps surae EMG (electrical muscle activity of the calf) is recorded in response to tibial nerve stimulation.
What is the take home of the experiment involving pre-synaptic inhibition and the H reflex
able to train pre-synaptic inhibition to “condition” the
H reflex (i.e., stretch reflex) depending on what a task requires
What do vestibular end organs do?
- sense head motion (acceleration)
- project information regarding head motion to vestibular nucleus
Vestibular nucleus project to…
muscles in the body to elicit reflexive movements to compensate for head motion
What is the vestibuloocular reflex?
function: stabilize gaze
projections from vestibular end organs to ocular muscles
What is the vestibulocolic reflex?
function: stabilize head
projects from vestibular end organs to muscle of the neck to bring head back to neutral position
What is the vestibulospinal reflex?
function: maintain upright balance
projections from vestibular end organs to muscles of the limbs
What is electrical vestibular stimulation?
- small electrical currents activate vestibular afferents
→ mimics vestibular afferent activity we would see if vestibular end organs were actually sensing head motion - CNS believe vestibular afferent activity is coming from actual head motion → think we are falling
- produce vestibulospinal reflexes in our lower limbs to ‘counteract’ this sensation of falling and maintain upright posture
