Spinal cord physiology; Sensory physiology (Somatosensory system) & clinical physiology of pain and referred pain

Question 1

What are the three protective meningeal layers of the spinal cord?

Answer 1

Dura mater, arachnoid mater, and pia mater.

Question 2

: How many pairs of spinal nerves are there, and how are they categorized?

Answer 2

31 pairs: 8 cervical (C1-C8), 12 thoracic (T1-T12), 5 lumbar (L1-L5), 5 sacral (S1-S5), 1 coccygeal (C0).

Question 2

What is the function of the dorsal root and ventral root of a spinal nerve?

Answer 2

Dorsal root: Sensory neurons (afferent). Ventral root: Motor neurons (efferent).

Question 2

What are the four classes of sensory receptors based on stimulus type?

Answer 2

Mechanical, thermal, electromagnetic, and chemical.

Question 3

Name the four functional zones within the spinal cord’s gray matter.

Answer 3

Somatic sensory (SS), visceral sensory (VS), visceral motor (VM), somatic motor (SM).

Question 4

What is the somatosensory cortex, and how is it organized?

Answer 4

The brain region receiving sensory input from the body; organized somatotopically (e.g., medial lemniscus: dorsal = neck, ventral = foot).

Question 5

Define proprioception, enteroreception, and exteroreception.

Answer 5

Proprioception: Sense of body/limb position.

Enteroreception: Perception of internal body states.

Exteroreception: Detection of touch, temperature, and pain.

Question 6

What four elements are involved in sensory coding?

Answer 6

Modality (stimulus type), location (receptive field), intensity (firing frequency), duration (adaptation).

Question 7

What is a dermatome?

Answer 7

An area of skin innervated by the cutaneous branches of a single spinal nerve (all except C1).

Question 8

What distinguishes allodynia from hyperalgesia?

Answer 8

Allodynia: Pain from non-painful stimuli (e.g., light touch).

Hyperalgesia: Exaggerated pain response to painful stimuli.

Question 9

What are the effects of spinal cord injuries at different levels (e.g., paraplegia vs. quadriplegia)?

Answer 9

Paraplegia: Lower limb paralysis (thoracic/lumbar injury).

Quadriplegia: Paralysis of all four limbs (cervical injury).

Question 10

Define hypesthesia, paresthesia, and anesthesia.

Answer 10

Hypesthesia: Reduced sensitivity.

Paresthesia: Abnormal sensations (tingling, numbness).

Anesthesia: Complete loss of sensation.

Question 11

What is the role of the medial lemniscus pathway?

Answer 11

It transmits proprioceptive and fine touch signals to the thalamus, organized dorsally (upper body) to ventrally (lower body).

Question 12

How do sensory receptors respond to stimuli outside their specialization?

Answer 12

At very high thresholds, they may respond to non-preferred stimuli (e.g., mechanoreceptors activated by intense heat).

Question 12

What is the significance of the spinal cord ending at the second lumbar vertebra?

Answer 12

The spinal cord is shorter than the vertebral column, so lumbar/sacral nerves form the cauda equina below this level.

Question 13

What distinguishes somatic sensory (SS) from visceral sensory (VS) pathways?

Answer 13

Detects body surface/muscle signals (e.g., touch). VS: Detects internal organ signals (e.g., stomach stretch).

Question 14

Why is the somatosensory cortex described as “plastic”?

Answer 14

Its organization can adapt based on sensory input (e.g., enlarged representation for frequently used body parts).

Question 15

What causes sensory adaptation/desensitization?

Answer 15

Continuous stimulation reduces receptor response over time (e.g., not noticing clothing after wearing it).

Question 16

Which spinal nerve does not contribute to dermatomes?

Answer 16

C1 (first cervical nerve).

Question 17

What are the three types of paralysis based on affected regions?

Answer 17

Monoplegia (one limb), hemiplegia (one side), paraplegia (lower limbs), quadriplegia (all limbs).

Question 18

A patient reports feeling intense pain from light brushing of their skin. What is this condition, and what mechanism causes it?

Answer 18

Allodynia — non-painful stimuli (e.g., touch) perceived as pain due to aberrant sensory pathway sensitization.

Question 18

What is the role of dorsal root ganglia?

Answer 18

Contain cell bodies of sensory neurons that transmit signals to the spinal cord via dorsal roots.

Question 18

How is stimulus intensity encoded in sensory systems?

Answer 18

Increased stimulus strength raises action potential frequency and recruits more receptors in the receptive field.

Question 19

A trauma at T10 causes paralysis of the lower limbs. What term describes this, and why are upper limbs unaffected?

Answer 19

Paraplegia — injury below cervical levels spares upper limb innervation (T1-L1 controls lower limbs).

Question 20

A patient with a herniated L4-L5 disc exhibits numbness in the anterior thigh. Which spinal nerve is likely compressed?

Answer 20

L3 or L4 — these contribute to the femoral nerve, which innervates the anterior thigh.

Question 21

Why is a lumbar puncture performed below L2?

Answer 21

The spinal cord ends at L2; sampling CSF here avoids cord damage (needle enters cauda equina).

Question 22

Which spinal nerve lacks a dermatome, and why?

Answer 22

C1 — it primarily innervates neck muscles, not skin.

Question 23

What happens if the dorsal root ganglia are damaged?

Answer 23

Loss of sensory input (e.g., touch, pain) in the associated dermatome, but motor function remains intact.

Question 24

A lesion in the medial lemniscus disrupts which sensory modalities

Answer 24

Proprioception and fine touch — this pathway transmits these signals to the thalamus.

Question 25

How does the nervous system distinguish between a gentle touch and a pinch?

Answer 25

Intensity coding: Pinch triggers higher action potential frequency and recruits more receptors.

Question 26

Why might a burn victim feel "tingling" after healing?

Answer 26

Paresthesia — nerve regeneration causes aberrant signaling (common in sensory nerve damage).

Question 27

What is Brown-Séquard syndrome, and what deficits occur ipsilateral vs. contralateral to the injury?

Answer 27

Hemisection of the cord causes: Ipsilateral: Loss of fine touch, proprioception (damaged dorsal columns), and motor paralysis (damaged corticospinal tract). Contralateral: Loss of pain/temperature (damaged spinothalamic tract decussates).

Question 28

A diabetic patient has reduced sensation in their feet. What term describes this?

Answer 28

Hypesthesia — diminished sensitivity, often from peripheral neuropathy.

Question 28

What explains the "phantom limb" phenomenon after amputation?

Answer 28

Cortical plasticity — the somatosensory cortex reorganizes, creating aberrant sensory maps.

Question 28

Which spinal cord region has the largest gray matter, and why?

Answer 28

Lumbar enlargement — innervates lower limbs (high neuron density for motor/sensory control).

Question 29

Why can intense heat sometimes activate mechanoreceptors?

Answer 29

At very high thresholds, receptors may respond to non-preferred stimuli (e.g., TRPV1 receptors activated by heat and mechanical stress).

Question 30

Which sensory receptor type is absent in visceral organs?

Answer 30

Proprioceptors — organs lack position sense, relying on stretch receptors (e.g., baroreceptors).

Question 31

A patient with a C5 injury can breathe unaided. Why?

Answer 31

The phrenic nerve (C3-C5) remains partially intact, allowing diaphragm function.