1.17 - Spinal Reflex & Muscle Control

Question 1

What is the function of alpha motor units?

Answer 1

Generate force by signalling muscle contraction

Question 2

What is a motor unit?

Answer 2

Describes a single alpha motor neurone and all the fibres it innervates

Question 3

What is the motor neurone pool?

Answer 3

The collection of motor units innervating a single muscle

Question 4

What two ways can the CNS control the amount of contraction?

Answer 4

1. Increasing the frequency of ACh release, which will llow the summation of twitches to maintain a force
2. Recruitment of more synergistic motor units, thus increasing the total number of contracting fibres

Question 5

What is a lower motor neurone?

Answer 5

Neurons that originate in the ventral horn of the spinal cord and control the somatic muscle

Question 6

What is an upper motor neurone?

Answer 6

Originate in the cerebral cortex and command lower motor neurons

Question 7

What are the three sources of input to the alpha motor neurons?

Answer 7

Command (descending) inputs: coming from
the upper motor neurons (primary motor
cortex)
Sensory (afferent) inputs: from muscle
spindles coding for muscle length, and play a
role in sensory feedback
Interneurons: in the spinal cord which are
responsible for processing information, such as
reflexes, as well as coordinating motor programs in response to many inputs

Question 8

Sensory inputs relay the status from the muscles and joints to the nervous system. Proprioceptors are receptors that provide information of location and position in space, and include, what?

Answer 8

Muscle spindles (stretch receptors) that prodive feedback of muscle stretch/length
Golgi tendon organs that provide feedback of joint position and load/forces in tendons
Mechanoreceptors located in connective tissues of joint capsules and ligaments relaying information on angle, velocity and movement of joints

Question 9

What is central processing?

Answer 9

It is the coordination of sensory inputs and effector outputs with the use of spinal interneurons (can be both excitatory and inhibitory).

Question 10

Which motor units are the first to be recruited in a muscle contraction?

Answer 10

The first motor units to be recruited are those with lower threshold, these have smaller cell bodies and low innervation rations (slow twitch fibres).
We wouldn’t want fast twitch fibres to contract first as this would cause excessive and uncontrolled movement.

With progressively more stimulation, the larger motor units are recruited, this is called the size principle. It refers to the order of recruitment of motor units based on size.

Question 11

Describe Extrafusal and Intrafusal muscle fibres

Answer 11

Extrafusal
 - The majority of muscle
 - Produce the force of contraction
 - Innervated by α motor neurons
Intrafusal
 - Fibres that contain sensory muscle spindles (stretch receptors) for proprioceptive feedback
 - The contractile portion is contracted by gamma
motor neurons

Question 12

Describe Muscle Spindles

Answer 12

Mucles spindles (a type of stretch receptor), provide
proprioceptive feedback
Contained in the fibrous capsule of the specialised
intrafusal muscle fibres

Question 13

Describe the Innervation of Muscle spindles

Answer 13

Sensory innervation by Ia and II sensory fibres (fast conducting sensory fibres). Sensory endings are in the middle. Each muscle spindle synapses on virtually all α motor neurons in the motor neuron pool, innervating its muscles.

Motor innervation by gamma motor neurons (fusimotor fibres). Fusimotor fibres bificate to contract polar segments of the muscle spindle

Question 14

Describe the gamma loop.

Answer 14

Indirect muscle contraction via gamma motor neurons to set muscle length and the sensitivity of the stretch reflex

Gamma motor neurons innervate the intrafusal muscle fibres, and keep the muscle spindles with same tone as extrafusal muscles:
o If the spindles are slack, group Ia fibres will be silentàtheα
motor neurons stop firing
o Gamma motor neurons contract the intrafusal fibres to absorb the slack of the muscle spindles during extrafusal muscle contraction
o Gamma motor neuron activity set the length that the stretch reflex will work
o Gamma motor neuron activity changes the set point of stretch reflex during muscle contraction, maintaining the set point relative to the muscle length, irrespective of its contractile status
o Provides additional control of α motor neurons and muscle contraction

In short:

• Increased α-motor activity –> decreased Ia output –> decreased α motor neuron output
• Increased gamma activity –> Increased Ia output –> increased α mn output

Question 15

Describe Golgi Tendon Bodies

Answer 15

Strain receptors located at muscle/tendon junction wrapped around collagen fibres, and are organised in a series arrangement
Activated by muscle stretch and/or contraction
Afferent information carried by group Ib fibres (fast conducting, slightly smaller than Ia) and encodes muscle tension
Group Ib fibres synapse in interneurons in the ventral horn that have inhibitory inputs to α motor neurons

Question 16

What is a spinal reflex and what is it’s components?

Answer 16

A spinal reflex is a relatively predictable, stereotyped and involuntary responses to a stimulus
A reflex arc is the basic circuit of a reflex, and is divided into:
o Afferent component: sensory receptors and axons carrying inputs to the CNS
o Central component: synapses and interneurons to process the inputs
o Efferent component: motor unit carrying command signals to muscle fibres

Question 17

Describe the Stretch (myotatic, Ia) reflex)

Answer 17

Simple monosynaptic reflex arc
Increase in stretch –> contraction of that same muscle in response to muscle lengthening (to prevent excessive lengthening)
Other effects:
- Excite synergistic muscles
- Inhibit antagonists muscles
E.g. The Knee-jerk reflex is an example of this reflex arc

Question 18

Describe the Phasic stretch reflex

Answer 18

The phasic (Ia mediated) stretch reflex is the class knee jerk reflex, and is a protective reflex during an unexpected movement. A rapid, transient stretch results in the activation of muscle spindle group Ia afferents:
o The muscle spindle Ia afferents synapse monosynaptically on all α-motor neuron pools as well as some synergistic muscles –> contraction of the stretch muscle
o Each α-motor neurone receives muscle spindle inputs from all muscle spindles in the muscle that it innervates
o The Ia afferents do not activate the gamma-motor neurons (most other connections activate α- and gamma-motor neurons in parallel
o The CNS can control the sensitivity of the muscle spindles via a control of the gamma-motor neurons (reticulospinal tract)

Question 19

Describe the Tonic stretch reflex

Answer 19

The tonic (Ia and II mediated) stretch reflex is elicited during passive bending of a joint, and plays a role more associated with muscle tone and contributes to maintain posture
o The group II afferents also make excitatory monosynaptic connections with α-motor neurons, but they also make di- and polysynaptic connections
o Feedback from Ia and II fibres help maintain the baseline firing of α motor neurons
o The reflex provides negative-feedback to maintain muscle length at an equilibrium set point

Question 20

Describe the Reverse Myotatic (Ib) Reflex

Answer 20

Polysynaptic reflex arc
Ib afferents synapse on inhibitory neurons
causing relaxation of muscle under tension
o Increase in tension –> activation of
inhibitory interneuron –> decreased activation of α motor neurone –> reduction of muscle tone
Other effects:
o Inhibits synergistic muscles
o Excites antagonistic muscles
In extreme circumstances, protects the muscle from overload, causing inhibition of the α motor neuron

Question 21

Describe the integration of myotatic (stretch) and inverse myotatic reflexes

Answer 21

While the myotatic (stretch) reflex provides feedback to regulate muscle length, the inverse myotatic reflex provides feedback to regulate tension (force). Both reflexes work synergistically to maintain posture.

Question 22

Describe Reciprocal Inhibition

Answer 22

Used to coordinate agonist and antagonist muscle groups
o Necessary so that antagonist groups do not work against each other
o Important to overcome the stretch reflex

Question 23

Describe the Withdrawal Reflex

Answer 23

Polysynaptic reflex involving excitatory interneurons
Pain carrying afferents innervate interneurons at various spinal segments
Activate flexors that withdraw limb from aversive stimulus

Question 24

Describe the Cross Extensor Reflex

Answer 24

Polysynaptic reflex involving excitatory and inhibitory interneurons
Activates extensor muscles and inhibits flexors on opposite side
Thought to play a role in central pattern generation of locomotion