Somatic Receptors & Spinal Cord – Comprehensive Study Notes

Somatic nervous system: gathers information about the external environment and triggers skeletal-muscle responses.
Four "general sense" receptor classes, all structurally alike whether located externally (skin, skeletal muscles) or internally (viscera); distribution density is what varies.
- Nociceptors – pain
- Thermoreceptors – temperature
- Mechanoreceptors – mechanical distortion
- Chemoreceptors – chemical composition / pH, gases
Only about $1\%$ of the sensory information that enters the CNS ever reaches conscious awareness; remainder is processed at spinal cord or brain-stem level.

Free nerve endings (usually on unipolar sensory neurons) with large receptive fields → localization of pain is often poor.
Abundant in skin & joints; sparse in visceral organs, which explains diffuse abdominal pain.
Modalities that can activate nociceptors
- Extreme temperature
- Mechanical damage
- Chemicals from injured cells (e.g., K\^+, prostaglandins)
Very intense stimuli may activate multiple receptor classes simultaneously → patients describe pain as “burning” even without true heat.
Tonic receptors – essentially no adaptation; CNS can, however, modulate perception.
- Endorphins & enkephalins suppress pain transmission (e.g., “bunny running from bobcat” ignores sprained ankle until safe).
- Certain neurotransmitters can amplify the signal.
Referred pain
- Visceral and cutaneous afferents enter the spinal cord at the same segment → cortex misinterprets visceral source as skin/muscle region.
- Classic patterns:
- Heart → left chest & medial left arm
- Liver/gallbladder → right neck/shoulder
- Kidneys → flanks, lateral hips, inner thighs

Free nerve endings in dermis and skeletal muscle; “hot” and “cold” receptors are structurally similar.
Phasic receptors – adapt rapidly (walk into a cold room → feel cold, adapt, then hallway feels warm).
Extreme heat can feel "cold" and vice-versa due to overlap in activation thresholds.
Certain chemicals directly stimulate thermoreceptors
- Capsaicin (hot peppers) triggers “warm” receptors; food feels “spicy hot” even if served cold.
- Onions can release irritants triggering thermoreceptors; hence the gas-mask joke while peeling.

Physical deformation opens ion channels → receptor potential → action potential (the “stadium wave”).

Tactile receptors – touch, pressure, vibration
- Located in internal & external epithelia.
- Examples: free nerve endings, tactile (Merkel) discs, hair root plexuses, Meissner/lamellar corpuscles.
- Dense on soles → feet are ticklish; protects against stepping on harmful objects.
Baroreceptors – monitor pressure/stretch; adapt rapidly.
- Cardiovascular system: heart & major vessels (blood pressure regulation).
- Respiratory system: lung stretch to prevent over-inflation.
- Digestive tract: stomach/small-intestine distension signals satiety; prevents gastric rupture.
- Weather analogy: “changes in barometric pressure.”
Proprioceptors – body position & movement; no adaptation (continuous reporting to cerebellum).
- Muscle spindles (muscle length), Golgi tendon organs (tension), joint capsule receptors (angle).
- Mostly subconscious; you notice them only if you suddenly lose balance (dozing off then jerking awake).

Predominantly serve the autonomic nervous system.
Detect pH, $\text{CO}<em>2$ , $\text{O}</em>2$ concentrations in blood & CSF.
- High $\text{CO}_2$ → low pH (acidic)
- Excess $\text{O}_2$ → higher pH (alkaline)
Information sent to brain-stem respiratory & cardiovascular centers → adjust breathing rate, heart output, renal function, etc.
Olfactory epithelium (nose) contains chemoreceptors that bypass the thalamus and project directly to cortex & limbic system → strong scent–memory linkage.

Extends from brain-stem (exits skull via foramen magnum) to approximately vertebral level L1/L2, then fans into cauda equina (“horse’s tail”).
$31$ pairs of spinal nerves emerge laterally (bilateral symmetry).
Meningeal layers (continuous with brain):
1. Dura mater (outer, tough)
2. Arachnoid mater (middle, web-like)
3. Pia mater (inner, delicate, adheres to cord)
Important spaces & ligaments
- Epidural space (above dura; site of epidural anesthesia)
- Subdural space (between dura & arachnoid; subdural hematoma)
- Subarachnoid space (between arachnoid & pia; filled with CSF)
- Denticulate ligaments anchor cord laterally.
- Coccygeal ligament (fusion of pia & dura) secures inferior end to coccyx.

Vertebral body (anterior) → vertebral arch → epidural fat → meninges → spinal cord.
Gray matter (central “butterfly”)
- Cell bodies + unmyelinated axons; darker.
- Dorsal (posterior) horn – sensory synapses.
- Ventral (anterior) horn – motor neuron cell bodies.
- Dorsal root ganglion (DRG) – houses sensory neuron somata before entering dorsal horn.
White matter (peripheral)
- Myelinated & unmyelinated axons conducting signals up/down cord.

Ascending tracts = sensory → brain (e.g., spinothalamic, dorsal column).
Descending tracts = motor ← brain (e.g., corticospinal, vestibulospinal).
Tracts are organized in bundles/fasciculi within the white matter; you are not required to label each specific tract in diagrams for this lecture.

Epidural anesthesia targets epidural space to block spinal nerves without piercing dura.
Subarachnoid hemorrhage vs. subdural hematoma terminology hinges on the meningeal layer involved.
Recognizing referred-pain maps is critical for differential diagnosis (e.g., left-arm pain ≠ muscle strain but potential myocardial infarction).
Understanding capsaicin’s dual action (chemo + thermo) informs why milk (fat) soothes “spice” better than water.
Proprioceptor deficits (e.g., cerebellar damage) manifest as ataxia—patients cannot subconsciously adjust posture.