Nervous system 4 - reflexes & spinal pathways

What is a reflex?

A rapid, automatic, predictable response to a stimulus, performed without conscious control.

What are the five components of a reflex arc?

• Receptor – detects stimulus.

• Afferent neuron – transmits sensory signal to CNS.

• Integration centre – spinal interneurons process input.

• Efferent neuron – sends command to effector.

• Effector – performs the response (muscle/gland).

What are monosynaptic and polysynaptic reflexes?

• Monosynaptic: Direct sensory-to-motor neuron connection (e.g. stretch reflex).

• Polysynaptic: Involves one or more interneurons (e.g. withdrawal reflex).

What is the stretch (myotatic) reflex?

A muscle contraction in response to stretching to prevent overextension; mediated by muscle spindles (e.g., patellar reflex).

Describe the patellar (knee-jerk) reflex.

• Tap stretches quadriceps → activates muscle spindle → sensory neuron synapses with motor neuron → quadriceps contract.

• Reciprocal inhibition prevents hamstring contraction.

What is the withdrawal reflex?

A protective reflex that pulls a limb away from painful stimuli; polysynaptic and ipsilateral.

What is the crossed-extension reflex?

Complements the withdrawal reflex — while one limb flexes, the opposite limb extends for balance (contralateral response).

What are descending motor tracts?

• Corticospinal tracts: Voluntary motor control; most fibres decussate at pyramids.

• Reticulospinal tracts: Muscle tone, posture, autonomic functions.

• Vestibulospinal tracts: Balance and head position adjustments.

What are ascending sensory tracts?

• Dorsal column: Fine touch, vibration, proprioception.

• Spinothalamic tract: Pain, temperature, crude touch.

• Spinocerebellar tract: Muscle coordination and proprioception.

What are first, second, and third-order sensory neurons?

1. First-order: Detects stimulus and sends signal to spinal cord.

2. Second-order: Ascends to thalamus.

3. Third-order: Projects to postcentral gyrus for perception.

What are upper and lower motor neurons?

• Upper motor neuron: From precentral gyrus → spinal cord.

• Lower motor neuron: From spinal cord → effector muscle.

What happens after spinal cord injury?

• Initial stage: Spinal shock — flaccid paralysis, loss of sensation and reflexes.

• Later stage: Spastic paralysis — reflexes return but uncontrolled by brain.

• Reflex bladder/bowel emptying below lesion.

What are somatic vs autonomic reflexes?

• Somatic: Skeletal muscle response (e.g. patellar reflex).

• Autonomic: Visceral response (e.g. heart rate, pupil size)

What are Babinski and plantar reflexes?

• Plantar (normal): Toes curl downwards when sole stroked.

• Babinski (abnormal in adults): Toes fan upward — indicates corticospinal tract damage.

What is proprioception?

Awareness of body position and movement, provided by muscle spindles and Golgi tendon organs.

What are spinal cord disorders related to tracts?

• Anterior horn damage: Polio (motor neuron destruction).

• Demyelination: Multiple sclerosis (affects tracts).

• Lesion on one side: Loss of contralateral pain/temp and ipsilateral motor function (Brown-Séquard syndrome).