Reflex Arc - Physiology (8 Marks)

Definition of a Reflex

Ganong's Review of Medical Physiology, 26th Edition

The Reflex Arc

Fig: The Reflex Arc showing the sequence of electrical events at each component (Ganong's, 26th Ed)

Fig: Spinal Reflex Arc showing dorsal/ventral roots, interneuron, and motor end plate (Medical Physiology)

Five Components of the Reflex Arc

1. Receptor (Sense Organ)

• The site where a stimulus is detected and transduced into a nerve signal
• Produces a graded receptor (generator) potential proportional to the strength of the stimulus
• Examples: muscle spindles (detect stretch), Golgi tendon organs (detect tension), nociceptors (detect pain), Pacinian corpuscles (detect pressure)

2. Afferent (Sensory) Neuron

• Carries impulses from the receptor to the CNS
• Cell bodies lie in the dorsal root ganglion (for spinal reflexes) or in homologous cranial nerve ganglia
• Axons enter the spinal cord via the dorsal root
• Generates all-or-none action potentials; the number of action potentials is proportional to the size of the receptor potential

3. Nerve Centre (Integration Centre / Synapse in CNS)

• Located in the spinal cord (dorsal horn and anterior horn)
• Processing occurs here; responses are graded as EPSPs and IPSPs at synaptic junctions
• The number of synapses determines reflex complexity:
  • Monosynaptic reflex - one synapse (e.g., stretch reflex)
  • Polysynaptic reflex - two or more synapses (e.g., flexor-withdrawal reflex)
• Interneurons may be interposed and can influence other motor neurons (reciprocal inhibition, crossed extension)

4. Efferent (Motor) Neuron

• Carries impulses from the CNS to the effector
• Exits via the ventral root (for spinal reflexes)
• The alpha (α) motor neurons that supply extrafusal skeletal muscle fibers are the efferent side of most somatic reflex arcs
• Called the "final common pathway" - all neural influences on muscular contraction funnel through them; up to 10,000 synaptic knobs converge on a single motor neuron

5. Effector Organ

• The organ that carries out the response
• Usually skeletal muscle (somatic reflexes) or smooth muscle/gland (visceral/autonomic reflexes)
• At the neuromuscular junction, the efferent neuron produces a graded endplate potential, which then triggers an all-or-none action potential in the muscle fiber, producing contraction

Ganong's Review of Medical Physiology, 26th Edition, Chapter 12

Electrical Events Along the Reflex Arc

Classification of Spinal Cord Reflexes

Costanzo Physiology, 7th Edition, Table 3.5

Stretch Reflex (Monosynaptic) - Detailed Example

1. Tapping the patellar tendon stretches the quadriceps muscle
2. Group Ia afferent fibers in the muscle spindle are activated; their firing rate increases
3. These Ia afferents enter the spinal cord and synapse directly on α motoneurons (monosynaptic)
4. α Motoneurons are activated and cause contraction of the quadriceps (homonymous muscle)
5. Simultaneously, inhibitory interneurons cause reciprocal inhibition of the antagonist (hamstrings)
6. γ Motoneurons are co-activated with α motoneurons, keeping the muscle spindle sensitive to length changes throughout the contraction
7. The neurotransmitter at the central synapse is glutamate

Fig: Stretch Reflex - Group Ia afferent activates α motoneurons to homonymous muscle; inhibitory interneurons suppress antagonistic muscles (Costanzo, 7th Ed)

Properties of Reflexes

1. Stereotyped - a particular stimulus always produces a particular, predictable response
2. Involuntary - do not require conscious initiation
3. Rapid - due to short neural pathways
4. Adaptable - can be modified by descending inputs from higher centers and by learning/experience
5. Ipsilateral or contralateral - most somatic reflexes act on the same side, but the crossed-extension reflex acts on the opposite limb

Clinical Significance

• Absent reflexes (hyporeflexia/areflexia) indicate damage anywhere in the reflex arc - the receptor, afferent nerve, or motor neuron. Most common cause is peripheral neuropathy (e.g., diabetes, alcoholism)
• Exaggerated reflexes (hyperreflexia) suggest interruption of descending corticospinal pathways that normally suppress reflex arc activity (upper motor neuron lesion)
• Deep tendon reflexes (DTRs) are graded 0-5+:
  • 0 = absent; 2+ = normal; 4-5+ = hyperactive with clonus
• Spinal cord levels tested: biceps (C5, C6), triceps (C7), patellar (L4), Achilles (S1)

Ganong's Review of Medical Physiology, 26th Ed, Clinical Box 12-1

• Ganong's Review of Medical Physiology, 26th Edition - Chapter 12 (Reflex Arc, Monosynaptic Reflexes)
• Costanzo Physiology, 7th Edition - Chapter 3 (Spinal Cord Reflexes, Table 3.5)
• Medical Physiology (Boron & Boulpaep) - Chapter 10 (Spinal Reflex Arc diagram)