Spinal Reflexes (8.1)

Question 1

List the components of a simple reflex arc and describe the role of each component

Answer 1

Question 2

Describe how input from the brain is able to modulate some reflexes

Answer 2

Descending pathways arising from the brainstem (corticobulbar) and cortex (corticospinal) are able to modulate spinal reflexes - the same pathways which control voluntary motor movements.

Rubrospinal, vestibulospinal and reticulospinal tracts also modulate reflexes.

• Rubrospinal tract: Inhibits extensor LMNs, excites flexor LMNs
• Vestibulospinal tract: Excites extensor LMNs, inhibits flexor LMNs
• Reticulospinal tract: Pontine has an excitatory output, medullary has an inhibitory output

Example: When touching an iron to see if it is hot the flexor reflex may be hypersensitive.

NOTE: In spinal cord lesions spinal shock causes an initial absence of reflexes below the level of the lesion. Following this period hyper-reflexia would be observed.

Question 3

Describe the potential effects of spinal cord injury on reflexes

Answer 3

UMN lesion ⇒ Hyper-reflexia

Resultant of the loss of descending upon spinal reflexes

LMN lesion ⇒ Hyporeflexia

Question 4

Outline the knee jerk reflex

A monosynaptic (stretch) reflex

Answer 4

The primary receptor in the reflex is the muscle spindle/stretch receptor, signals via fast conducting Ia sensory fibres. Consists of intrafusal fibres which lie parallel to extrafusal fibres.

Question 5

State the moto-neurones associated with intrafusal and extrafusal muscle fibres

Gamma moto-neurone: Special function

Answer 5

Intrafusal muscle fibres are innervated by gamma moto-neurones, at polar ends.

• Gamma moto-neurones are able to adjust the sensitivity of muscle spindles through stretching the muscle spindles and ‘re-setting’ the spindle mechansism

Extrafusal muscle fibres are innervated by alpha moto-neurones.

Question 6

Outline the Golgi Tendon Organ reflex

Answer 6

• Proprioceptive input from muscles - detect tension within muscles
• Signals via 1b afferent fibres
• A high threshold receptor
• Seen at the junction between the skeletal muscle and its associated tendon

Question 7

Outline the flexor/withdrawal reflex

Answer 7

• A polysynaptic pathway activated by nociceptors
• Activation seen at several spinal levels
• Allows for withdrawal of the limb

Question 8

Outline the crossed extensor reflex

Answer 8

• ‘Compliments’ the flexor reflex
• Flexor reflex: Contraction and withdrawal of the effector muscles in right leg (following nociceptive stimulus)
• Extensor reflex: Contraction and extension of leg leg to allow balance to be maintained