The Control of Movement - Reflexes (Prof. Yeo) Flashcards
What are α-motor neurons ?
These are neurons in the spinal chord (and in some cranial nerve nuclei) that modulate muscular contraction to produce mvnt by recruitment of motor units and changes in motoneurons firing rates.
What is the motor unit ?
The motoneuron, its axon and the muscle fibers it controls.
What is proprioception ?
Proprioception describes the sensory information that contributes to the sense of position of self and movement.
How do humans accurately control mvnt ?
By proprioceptive feedback into the brain and spinal chord. This comes largely from muscle spindles and Golgi Tendon Organs (GTOs).
Where are the muscle spindles ?
Can we thus deduct their role ?
What different types of MSs neurons exist ?
Muscle spindles (which contain intrafusal fibers) are in parallel w/ the main extrafusal fibers of the muscle, so they serve as stretch (length) receptors. MSs afferents are type Ia (v large diameters) and type II (medium diameters). Muscle spindly sensitivity is controlled by the gamma efferent system from gamma motoneurons.
Where are GTOs ?
How is this imp for their fct ?
GTOs are situated in the in the musculo-tedninous jct - in series w/ the extrafusal fires. They are are therefore tensions receptors. GTO afferents are type Ib (slightly smaller than MS !a afferents).
How are simple reflexes produced ?
Is there ever intervention of the brain ?
By afferent info from the MSs, GTOs and cutaneous receptors entering the spinal chord and activating local interneurons and motoneurons.
These simple reflexes are organized w/i the spinal chord and can be modulated by descending pathways from the higher motor centers of the brain.
How does the stretch reflex work ?
What is its purpose ?
The short latency response is monosynaptic - Ia afferents input directly to motoneurons. This can be elicited even in relaxed muscles e.g. w/ a tendon jerk.
The stretch reflex serves to control the intended muscle length during contractions against workloads.
Do Ia afferents always stimulate excitatory motoneurons ?
Absolutely not. When muscle is stretched, the Ia afferents also excite Ia inhibitory neurons in the spinal chord that innervate the antagonist across the joint, preventing unnecessary antagonism. This reflex is thus disynaptic.
What is alpha-gamma coactivation ?
If alpha motoneurons were excited alone, then extrafusal fiber contraction would “unload” the MSs (rendering them slack and floppy) and reduce or silence their output. For most mvnts, alpha and gamma motoneurons are activated together in alpha-gamma coactivation to maintain sensitivity of the spindle.
Alpha and gamma motoneurons can be separately controlled so that supra-spinal systems can control the sensitivity of the spindle system as necessary.
What is the Tendon Organ Reflex (or Inverse Myotatic Reflex) ?
Why is this reflex called as such ?
GTO afferents (Ib fibres) terminate on one group of Ib interneurons that inhibit agonist muscles (disynaptic pathway) and on other interneurons that are stimulate antagonist muscles (di- or trisnaptic pathway). This reflex opposes the spindle stretch reflex and allows fine control of mvnt.
What is the withdrawal reflex ?
The withdrawal reflex is a spinal reflex intended to protect the body from damaging stimuli. It is polysynaptic; causing stimulation of sensory-, inter-, and motor neurons.
What is the scratch reflex in furry animals ?
This is when the animals limb moves rhythmically to remove the irritation of the skin.
This pathway is less understood.
How are reflexes modulated by supra spinal control ?
Only about 5% of descending fibres from the motor cortex directly excite alpha-motoneurons. The remaining 95% of fibres act upon spinal interneurons and upon the motoneurons. Thus, supraspinal control of movement is largely done by modulating patterns of spinal interneuron activity. Many spinal reflexes are only evident when they are not controlled by supraspinal influences. In babies, the Babinski sign (toes extend upwards to tickling of the sole of the foot) and grasp reflex (fingers grasp objects placed in the palm) are seen before the descending spinal tracts have fully developed their myelin. They disappear within the first year but return if there is damage to the corticospinal tract.
What is the vestibular system ?
The vestibular system, which is the system of balance, consists of 5 distinct end organs: 3 semicircular canals that are sensitive to angular accelerations (head rotations) and 2 otolith organs that are sensitive to linear (or straight-line) accelerations.
