Spinal reflexes

Question 1

What is a spinal cord reflex?

Answer 1

• A very rapid, autonomic and highly sterotyped response

Question 2

What are the components of spinal reflexes?

Answer 2

1. Sensory receptor - responds to stimulus by producing a generator or receptor potential
2. Sensory neuron - axon conducts impulses from receptor to integrating centre
3. Integrating center(one or more regions within the CNS that relay impulses from sensory to motor neurons)
4. Motor neuron (Axon conducts impulses from integrating centre to the effector)
5. • Muscle or gland that responds to motor nerve impulses

Question 3

What are monosynaptic stretch reflexes ?

Answer 3

• Basis of the knee jerk response routinely tested in neurological examinations
• Plays a major role in maintaining posture
• Primary receptor involved in this reflex = muscle spindle (stretch receptor)
• Present in nearly all skeletal muscle
• Consists of intrafusal fibres
• Lie parallel to the extrafusal fibres
• Fast conducting myelinated axons (Group Ia) encircle intrafusal muscle - these are the primary sensory endings
• When muscle is stretched, muscle spindles fire action potentials

Question 4

Explain the monosynaptic stretch reflex

Answer 4

1. Sensory receptor -muscle spindle stretch
2. Sensory afferent neurone (1a afferent)
3. Integrating centre (Spinal cord)
4. Motoneurone efferent
5. Effector - same muscle contracts and relieves stretch

Descending motor pathways modulate the reflex circuits in the spinal cord
- Weak or absent reflex - possible lower motoneurone lesion
- Exaggerated reflex - possible upper motoneurone lesion

Question 5

What does the inhibitory input of the hamstrings involve?

Answer 5

• Inhibitory interneurones and is polysynaptic
• Reciprocal innervation reduces resistance to stretch reflex

Question 6

What are gamma motorneurones?

Answer 6

• They adjust the sensitivity of muscle spindles
  1.) Indirectly lead to muscle contraction
  2.They stretch muscle spindles and reset spindle mechanism
  3.Increases likelihood of afferent discharge of 1a afferents
  4.Adjust sensitivity of muscle spindles via descending control

Descending fibres modulate muscle tone by exciting or inhibiting gamma motor neurones
Excitation - facilitates stretch reflex
Inhibition - inhibits stretch reflex

Question 7

What is the reversed myotatic / clasp-knife of the golgi tendon reflex?

Answer 7

• Protects the muscle from producing tension (overstretching) and tearing of breaking tendons
• Motorneurone inhibited by inhibitory interneurone of the same muscle
• Spinal cord : activate excitatory interneurone that excites motorneurone of antagonistic muscle
• Antagonist muscle is excited
• Interneurones can be excitatory or inhibitory

Question 8

What is the flexor reflex (withdrawl reflex)?

Answer 8

• Primarily modulated by pain receptors (nociceptors)
• 1.) Sensory pain receptors activated
• 2.) Nociceptor afferents excited
• 3.) Spinal cord - interneurones activated that excite flexor motorneurones
• OR
• Spinal cord collaterals activated that activate interneurones in several spinal segments that excite flexor motoneurones
• 4.) Effector flexor muscle contracts
• 5.) Limb withdrawal
• Reflex rapidly withdraws body part from painful stimulus
• ## Synergists = muscles that work together

Question 9

What is the crossed extensor reflex?

Answer 9

• crossed extension to maintain balance
• When withdrawing one foot, quadriceps in opposite leg extend knee to bear additional weight (prevents falling)

Question 10

What is the descending control of the spinal reflex?

Answer 10

Descending info - info that travels back down from the brain to the spinal cord
- Info acts to modulate motor output
- Most descending messages are inhibitory in nature = they work to dampen down the signal in the spinal cord

Question 11

How does spinal cord injury effect the reflexes?

Answer 11

• Injuries above thsi level = nothing
• Below the level of motor descending tracts - initially absent (spinal shock) then exaggerated reflexes