Session 4 - The Motor System

Question 1

What is an upper motor neurone?

Answer 1

An UMN is a motor efferent fibres, with a cell body in the motor region of the cerebral cortex or brainstem, which remain within the CNS and synapse with lower motor neurones.

Question 2

What is a lower motor neurone?

Answer 2

A LMN is a somatic motor efferent fibre, with a cell body in either Lamina IX of the spinal cord (spinal motorneurone) or Cranial Nerve Motor Nucleus (cranial motorneurone). LMNs leave the CNS and directly supply the skeletal muscles of the body.

Question 3

What are the two types of lower motor neurone?

Answer 3

a and gamma

Question 4

What is a motor unit?

Answer 4

A motor unit is made up of an α-Motorneurone (a type of LMN) plus all of the muscle fibres it supplies. The number of muscle fibres can vary greatly (e.g. extra-ocular muscles 10, quadriceps 1,000). It is the minimal functional unit of the Motor System.

Question 5

What is a spinal reflex?

Answer 5

A reflex is an involuntary, unlearned, repeatable autonomic reaction to a specific stimulus that does not require the brain.

Question 6

Outline the anatomical pathways of a spinal reflex

Answer 6

o	A receptor (or transducer)
o	An afferent fibre
o	An integration centre
o	An efferent fibre
o	An effector

Question 7

What is an a-motor neurones

Answer 7

o Innervate the extrafusal muscle fibres of skeletal muscle

o Directly responsible for initiating skeletal muscle contraction

Question 8

What is a g-motor neurones

Answer 8

o Innervate intrafusal muscle fibres of muscle spindles

o Keep muscle spindles taut

Question 9

What are muscle spindles?

Answer 9

Muscle spindles are connective tissue capsules that contain muscle fibres (Intrafusal muscle fibres).

Question 10

What are the middle and end portions of muscle spindles innervated by?

Answer 10

The middle portion of the muscle spindle is innervated by afferent sensory neurones.
The end portions of the muscle spindle are innervated by efferent γ-LMNs.

Question 11

What does the stretching of muscle spindles do?

Answer 11

The stretching of the muscle spindle increases the firing of the afferent sensory neurones. When the muscle shortens (contracts) firing rate decreases.

Question 12

Why do gamma neruones prevent spindle fibres from going slack?

Answer 12

as this would remove feedback from the sensory neurones and thus provide no information about the muscle length.

Question 13

What are golgi tendon organs?

Answer 13

Golgi tendon organs are found at the junction between muscle and tendon and are innervated by sensory neurones. They are composed of a network of collagen fibres inside a connective tissue capsule with the sensory axon winding around the collagen.

Question 14

What do golgi tendon organs do?

Answer 14

Firing rate increases when tendon is stretched. Sensory neurones branch extensively in the spinal cord and synapse with inhibitory synapses with a-LMNs that innervate the muscle the sensory afferent came from.

As the muscle contracts, tension through the golgi tendon organ increases, causing increased inhibition on the α-LMNs, reducing their firing and the muscle’s contraction.

Question 15

What is the stretch reflex?

Answer 15

The stretch reflex is the hardwired connection between a α-LMN and the sensory afferents of muscle-length stretch organs. The α-LMN supplies the muscle fibres the sensory afferents arise from.

Question 16

Outline the muscle stretch reflex

Answer 16

o When a muscle is not contracted, it relaxes, increases in length and is stretched
o Muscle length receptors detect a stretch, and fire action potentials via afferent axons to keep the CNS appraised of muscle length at all times (proprioception).
 Action Potentials to the Brain via the Dorsal Columns
 Action Potentials directly to spinal LMNs that supply the muscle – reflex contraction of the muscle
o Monosynaptic Reflex Arc – No interneurons involved
o Polysynaptic Reflex Arc – Interneurons involved in the spinal cord

Question 17

Outline the knee jerk reflex

Answer 17

o Strike the patellar tendon with a reflex hammer
o This causes stretching of muscle spindles within the quadriceps
o Firing of sensory afferent nerve
o Stimulation in the spinal cord of α-LMN supplying the quadriceps
o Contraction of the quadriceps
o There is also an inhibitory neurone in the spinal cord, which relaxes opposing muscle (hamstring)

Question 18

What is the Golgi Tendon Organ Reflex?

Answer 18

The golgi tendon organ reflex acts as a protective feedback mechanism to control the tension of an active muscle by causing relaxation before the tendon tension becomes high enough to cause damage.
o Muscle contracts, stretching golgi tendon organ
o Afferent sensory neurone fires, synapsing with inhibitory interneurons in the spinal cord
o Inhibitory interneurons reduce the firing of α-LMNs, thus reducing contraction of the muscle and preventing damage from over-contraction

Question 19

Define muscle tone

Answer 19

Muscle tone is the continuous, passive, partial contraction of all skeletal muscle. It allows us to maintain body posture and hold our heads upright. Muscle tone is observed as a muscle’s resistance to passive stretch during resting state.

Question 20

How does muscle tone change during development?

Answer 20

o Present but low in utero, absent in new-born and returns a few months after
o Present in all skeletal muscles of the body

Question 21

What happens to muscle tone during sleep?

Answer 21

o	Inhibiting during deep (REM) sleep in all muscles except
	Muscles of breathing
	Extra-ocular muscles
	Urinary sphincter
	Anal sphincter

Question 22

What is hypotonia?

Answer 22

o Body becomes limp and is unable to support its own weight
o Body posture is lost
o LMN lesion sign

Question 23

What is hypertonia?

Answer 23

o Muscles and joints become stiff
o Reciprocal inhibitory relationships between agonists and antagonists is disrupted, both become equally stiff, simultaneously
o UMN lesion sign