Session 5

Question 1

What are the 5 components of the reflex arc?

Answer 1

1) Receptor
2) Afferent fibre
3) Integration centre
4) Efferent fibre
5) Effector e.g. muscle

Question 2

What are the two categories of lower motor neurones of the spinal cord?

Answer 2

• Alpha motor neurones: trigger the generation of force by extrafusal muscle fibres.
• Gamma motor neurones: provide motor innervation to intrafusal fibres within the fibrous muscle capsule. They work to keep the muscle spindles taut.

Question 3

What are the two types of tetanic contractions?

Answer 3

1) Unfused tetanus - muscle fibres don’t completely relax before the next stimulus. Only partial relaxation between twitches.
2) Fused tetanus - high rate of stimulation where the muscle fibres do not relax between stimuli.

Question 4

How does motor unit recruitment vary between muscles?

Answer 4

The number of fibres per unit is directly related to the precision control that we have over that particular muscle. E.g. many more motor axons in the exterior rectus of the eye compared to the gastrocnemius.

Question 5

What are the three types of motor units?

Answer 5

S: slow contracting, very resistant to fatigue, very small force. Typically in the antigravity muscles of the legs and torso.

FR: fatigue resistant, faster contracting, low force.

FF: fast fatiguable, fast contracting, anaerobic respiration, high force. Typical of muscles involved in escape reflexes.

Question 6

What are muscle spindles?

Answer 6

Consists of connective tissue capsules in which there are intrafusal fibres. Innervated by gamma motor neurones and they detect changes in the length of the muscle.

Question 7

What are golgi tendon organs?

Answer 7

Receptors at the junction of muscles and tendons. A large myelinated fibre subdivides, which then intermingles and encircles the collagenous fibres.

Question 8

What activates golgi tendon organs?

Answer 8

Active contraction of the muscle or stretching of the muscle.

Question 9

What are Ib axons?

Answer 9

They intertwine among the coils of collagen fibres. When the muscle contracts, the tension of the collagen fibres increases, squeezing the Ib axons. They enter the spinal cord and synapse on interneurones in the ventral horn. They regulate muscle tension. As muscle tension increases, the Ib interneurones inhibit alpha motor neurones to slow muscle contraction.

Question 10

Give three examples of proprioceptors.

Answer 10

Muscle spindles
Tendon organs
Joint receptors
Skin

Question 11

What is the myotatic reflex?

Answer 11

Stretch reflex. As the muscle is stretched, action potentials are produced by the muscle spindle, these synapse with alpha-motor neurones in the spinal cord to innervate extrafusal muscle fibres. This results in contraction of the muscle.

Question 12

What is the flexion reflex?

Answer 12

Withdrawal reflex. Mediated by pain receptors. The free nerve endings are stimulated, and the resulting impulses are conducted through myelinated afferent fibres and unmyelinated afferent fibres. These synapse with alpha-motor neurones in the spinal cord, resulting in contraction of the ipsilateral flexor muscles. The limb in withdrawn in response to noxious stimuli.

Question 13

What is the difference between a negative neurological sign and a positive neurological sign?

Answer 13

Positive = emergence of a feature
Negative = Loss of function or capacity.

Question 14

Give three lower motor neurone signs.

Answer 14

Muscle wasting
Muscle weakness
Atonia
Areflexia
Fasciculations

Question 15

What are the two types of descending tracts?

Answer 15

Pyramidal and Extrapyramidal tracts.

Question 16

What are upper motor neurones?

Answer 16

Have their cell bodies in the brain and synapse within the CNS. Pyramidal motor neurones have their cell bodies in the cerebral cortex, Extrapyramidal motor neurones have their cell bodies in the sub-cortical brain areas .

Question 17

What is the role of upper motor neurones?

Answer 17

Provide descending inhibition to the lower motor neurones and run from the higher regions of the brain to the spinal cord.

Question 18

Give four upper motor neurone signs.

Answer 18

Hypereflexia
Hypertonia
Spastic paralysis
Babinski sign
Myoclonus
Choreas
Bradykinesia

Question 19

What is an autonomous bladder?

Answer 19

Caused by LMN lesions, with damage above S2-4. Loss of parasympathetic and afferent neurones. Overflow incontinence and no ability to micturate.

Question 20

What is an automatic reflex bladder?

Answer 20

Damage above the sacral region can cause damage to the UMN and result in automatic reflex bladder. Loss of descending inhibitory control, hence leakage of urine and urge urinary incontinence.

Question 21

Name the pyramidal tracts.

Answer 21

Corticospinal (lateral and ventral) and Corticobulbar.

Question 22

Name the extrapyramidal tracts.

Answer 22

Vestibulospinal
Tectospinal
Reticulospinal
Rubrospinal
Olivospinal

Question 23

What are the roles of the corticospinal tracts?

Answer 23

Control of voluntary, discrete, skilled movements, especially those of the distal part of the limbs.

Question 24

Where do corticospinal tract neurones arise from?

Answer 24

Motor cortex, premotor cortex and supplementary motor area and somatosensory cortex.

Question 25

Why is the internal capsule vulnerable?

Answer 25

It's associated with branches of the middle cerebral artery, hence a bleed into this region can knock out the corticspinal tract, affecting one side of the body.

Question 26

Describe the path of the lateral corticospinal tract.

Answer 26

Decussates then descend into the spinal cord, terminating in the ventral horn.

Question 27

Describe the path of the anterior corticospinal tract.

Answer 27

Remains ipsilateral and descends into the spinal cord. Then decussates and terminates in the ventral horn of the cervical and upper thoracic segmental levels.

Question 28

Describe the path of the corticobulbar tract.

Answer 28

Originates in the cerebral cortex, descends through the internal capsule into the brainstem. Decussates in the brainstem and terminates on contralateral cranial nerve motor nuclei in the midbrain, pons and medulla.

Question 29

What is clinically important about corticobulbar tracts?

Answer 29

Many innervate motor neurones bilaterally. Except upper motor neurones from the facial and hypoglossal nerve only provide contralateral innervation.

Question 30

Describe the vestibulospinal tract.

Answer 30

Responsible for balance and posture. Fibres do not decussate. Innervate anti-gravity muscles via lower motor neurones.

Question 31

Describe the reticulospinal tract.

Answer 31

Fibres arise from the medulla and pons, descend bilaterally and facilitate the extensor spinal reflexes.

Question 32

What are the two reticulospinal tracts?

Answer 32

Medullary (lateral) - from medulla and inhibits voluntary movements. Facilitates the flexor reflex. Pontine (medial) - from pons and facilitates voluntary movements and increases muscle tone. Also facilitates extensor reflex.

Question 33

Describe the rubrospinal tract.

Answer 33

Arises from the red nucleus of the midbrain, decussates in the midbrain and fibres descend contralaterally in the spinal cord. Controls flexor tone in distal muscles and facial muscles.

Question 34

Describe the tectospinal tract.

Answer 34

Aids directing of head movements in response to visual and auditory stimuli. Neurones arise from the tectum of the midbrain and decussate within the brainstem. Terminate in upper cervical segments of the spinal cord.

Question 35

What do lesions of extrapyramidal tracts often lead to?

Answer 35

Types of dyskinesias or disorders of involuntary movement. Generation of unwanted or uncontrollable movements or aberrant reflexes.