Topic 3: Sensory – Motor. Systems 1: Control of Muscle/ Topic 4: Sensory
– Motor. Systems 2: Proprioception
1. List the 3 main components of a muscle spindle?
(1) Specialized very small muscle fibers called intrafusal fibers, (2)
sensory neuron endings (Group Ia and Group II afferents) that wrap
around the central regions of these small intrafusal muscle fibers, (3)
gamma motor neuron endings that activate the polar contractile regions of
the intrafusal muscle fibers
2. Name two subtypes of intrafusal muscle fibres and differentiate between
their ability to detect static and dynamic movement.
Nuclear Bag (divided into both static and Nuclear Chain (static type)
-fiber has many spherical nuclei in its -fiber has a single row of nuclei and being
central noncontractile region (looking like less elastic stretches slowly
an elastic bag of nuclei) which stretches
quickly when lengthened because of its
3. Which afferent neurons are best able to respond quickly to stretching and
sense the rate of change? Where can they be found?
The Ia fiber sensory endings wrap around the equatorial region (most
elastic) of both the bag and chain intrafusal muscle fibers and thus respond
quickly to stretching and sensing the rate of change (dynamic muscle
length) of the muscle length. The afferent neuron endings can be found on
cell bodies in the dorsal root ganglia of the spinal cord.
4. Which intrafusal muscle fibres do type II afferents mainly innervate and why
are they less sensitive to dynamic change in muscle length when compared to
type Ia afferents?
The group II endings wrap around the region next to the equator, which is
less elastic and thus less responsive to stretching. Group II afferents go
mainly to the chain fibers and are most responsive to steady-state or static
5. What is the main difference between alpha and gamma motor neuron
function? Gamma motor neurons Alpha motor neurons
(1) gamma dynamic: activate only -innervate extrafusal fibers (regular
dynamic bag muscle fibers (enhance the skeletal muscle)
dynamic responses of the Ia afferent
(2) gamma static: innervate both static
bag and chain muscle fibers (enhance the
responses of the group II afferent
neurons, signalling steady-state length of
6. The activation of __________ enhances dynamic response of Ia afferent
The activation of gamma dynamic motor neurons enhances the dynamic
responses of the Ia afferent neurons
7. When John stretches his biceps muscle, what happens to his afferent
When a muscle is stretched, it stretches the muscle spindle, exciting the Ia
afferent neurons. The spinal stretch reflex is activated by excitatory
monosynaptic connections from the Ia afferent neurons to the alpha motor
neurons, which activate their own muscle and synergistic muscles. The Ia
afferents also excite Ia inhibitory interneurons, which then inhibit alpha
motor neurons to the antagonist muscles
8. Where are Golgi tendon organs located and what are they sensitive to?
Golgi tendon organs are located at the muscle-tendon junction and are
sensitive to tension changes that result from either stretch or contraction of
9. Which muscles have the highest concentration of spindles?
In humans the muscles with the highest spindle density (spindles per
muscle) are extraocular, hand and neck muscles (this is because we use
these muscles in head/eye coordination as we reach for obje