Spinal Reflexes Flashcards
motor unit
single motor neuron and the muscle fiber that it innervates
size principle
states that as motor units are recruited progressively a larger number of motor neurons are involved and greater tension will be generated
Type I (slow twitch fatigue resistant) fibers
- many mitochondria and myoglobin present
- slow contraction, little tension and high endurance
- posture maintenance
Type IIb (fast twitch fatigable)
- glycolytic (abundant glycogen)
- large fiber diameter
- fast contraction, high tension and low endurance
- bursts of tension and precise movements
Type IIa (fast twitch fatigue resistant)
- mix between I and IIb
- both oxidative and glycolytic
- IIb fibers can develop into IIa fibers through training
Henneman’s Size Principle
- slow twitch fatigue resistant fibers are recruited first (type I)
- fast fatigue resistance are recruited second (type IIa)
- fast fatigable are recruited last (type IIb)
during sustained activity, motor units will
cycle in rotation
reflexes are
involuntary subconscious responses mediated by “hard-wired” pre-existing nerve circuits
modulation of reflex activity
higher control signals are sent by interneurons to the motor nerves that control the reflex to modulate or change it (enhance or diminish)
renshaw cells
type of interneuron located in the anterior horn of the spinal cord and are inhibitory for surrounding motor neurons
anterior motor neurons
- alpha (⍺-): for extrafusal muscle fibers
- gamma (γ-): for intrafusal muscle fibers
involuntary and voluntary motor control is modulated by the
- spinal cord (involuntary)
- higher brain centers (voluntary)
intrafusal vs extrafusal muscle fibers
- intra: too small to generate force (muscle spindles)
- extra: generate force
gamma-motor neurons (Type Aγ)
innervate muscle spindle to sense muscle length (static) and rate of change (dynamic) and adjust to the sensitivity of the muscle spindle
alpha-motor neurons (Type A⍺)
Innervate extrafusal muscle fibers that cause the muscle to contract and shorten, producing movement
what does it mean that alpha- and gamma-motor neurons are co-activated
when the brain sends a movement signal, it activates both ⍺-motor neurons (extrafusal contraction) and γ-motor neurons (intrafusal adjustment) simultaneously
why is co-activation important?
- Prevents the muscle spindle from becoming slack during contraction, ensuring it remains sensitive to further changes in muscle length
- Prevents the spindle from resisting movement, allowing smooth execution
- Dampens unnecessary muscle oscillations, ensuring coordinated and precise movements
Muscle spindles contain two types of specialized intrafusal length receptor muscle fibers
- Nuclear Bag Fibers → phasic response
- Nuclear Chain Fibers → static response
Nuclear Bag Fibers (Dynamic, Phasic Response)
- detect the velocity of changes in muscle length
- highly compliant and quickly adapt to prolonged stretch
- Innervated by Group Ia afferent neurons
- Innervated by dynamic gamma-motor neurons, which have flower-spray endings and help regulate spindle sensitivity
mechanism of nuclear bag fibers
when the muscle stretches, mechanoreceptors in the Ia afferents open sodium channels, leading to depolarization and signaling stretch to the CNS
Nuclear Chain Fibers (Static Response)
- detect sustained muscle length
- innervated by Group II afferents, which provide tonic (sustained) sensory input about limb position, even when the limb is not moving
- Innervated by static gamma-motor neurons, which have flower-spray endings to regulate sensitivity
- innervated by group Ia afferent to providing continuous feedback about muscle length over time
signal averaging function of the muscle spindle
the damping or smoothing out the stretch reflex that helps to prevent oscillation or jerkiness of body movements
If the muscle spindle is not functioning correctly, then the muscle contraction is
jerky and not smooth during the course of a stimulus
monosynaptic reflex
- stretch, myotatic or jerk reflex
sequence of stimulation of monosynaptic reflex
- When muscle is stretched, group Ia afferents are activated and firing increases.
- Group Ia synapse on alpha-motor neurons and these alpha-motor neurons innervate the extrafusal fibers of homonymous muscle.
- The alpha-motor neurons will cause contraction which will shorten both the intra- and extra-fusal fibers Group Ia afferents return to baseline.
- While there is contraction of synergistic muscles, the antagonistic muscles will relax.
what are the 3 neural circuits that can modulate monosynaptic reflex
- reciprocal innervation
- renshaw arc
- gamma efferents
reciprocal innervation
when interneurons activate synergistic muscles and at the same time the antagonist muscles are inhibited by the interneuron releasing glycine to increase chloride conductance
renshaw arc
alpha-motor neurons can activate Renshaw
cells causing feedback inhibition that will inhibit other parallel motor nerves as well as the primary motor neuron by releasing inhibitory neurotransmitters, GABA and glycine
Function of Renshaw arc is to
confine reflex to one group of muscles without interference from other muscle groups that will prevent an over-exuberant response (i.e., muscle spasticity).
Baclofen
an agonist for the GABA type B receptor used in the treatment of muscle spasticity
multi-synaptic reflex
Golgi Tendon reflex (feedforward inhibition) and withdrawal reflex
primary and secondary function of the golgi tendon reflex
- primary: equalize muscle tension between agonist and antagonist muscles across a joint
- secondary: to prevent the muscle and/or tendon from tearing from within or from bone due to rapid induced high levels of muscle tension
flexor-withdrawal reflex is the motor response to
protopathic stimuli that causes the perception of pain
crossed extensor reflex
On the contralateral side of spinal cord (opposite side of pain stimulus), the flexor muscles are inhibited, and the extensor muscles are stimulated to contract.
what CNS circuits are present in the withdraw reflex
- divergence
- reciprocal inhibition
- after discharge
Afterdischarge is the continued rate of
flexor contraction due to recurrent pathways
Central pattern generators (CPGs) have
patterns of repetitive and alternating activity
Initiation and control of CPGs originates in
- Motor cortex for initiation
- Basal ganglia for maintaining the repetitive movement
