5 Somatosensory system

• Dorsal root ganglion = sensory neuron cell bodies

• Dorsal root = sensory afferents entering spinal cord

• Ventral root = motor efferents leaving spinal cord

• spinal nerve + rami = mixed fibers

• mechanoreceptors

• proprioceptors

• thermoreceptors

• nociceptors

receptive field; adaptation

RA = rapidly adapting; SA = slowly adapting

• Meissner’s corpuscles (RA, small))

  • light touch, low-frequence vibration

• Pacinian corpuscles (RA, large)

  • largest receptors

  • transient pressure, high-frequency vibration

• Merkel’s disks (SA, small)

  • specialized non-neural epithelial cell

  • edges, form, texture

• Ruffini’s endings (SA, large)

  • skin stretch, Hand position

• free nerve endings (unmyelinated)

  
  

the area of skin where stimulation changes the firing of a sensory neuron

• Small

  • higher precision

• Large

  • large area, less precise

core

• unmyelinated axons

• mechanosensitive ion channels

  • convert mechanical force into neural signals

differ in diameter, myelination and conduction velocity

A/C/… system = cutaneous fibers; Group system = muscle afferents

• Aα / Group I

  → proprioceptive afferents

• Aβ / Group II (thick, myelinated, fast)

  → low-threshold cutaneous mechanoreceptors

• Aδ / Group III (myelinated, slowlier)

  → fast pain / temperature

• C / Group IV (unmyelinated, very slow)

  → slow pain, temperature, itch

Aδ fibers: fast, sharp pain

C fibers: slow, longer, dull pain

Nociception = neural detection and encoding of noxious stimuli

Pain = emotion

free, branching, unmyelinated nerve endings

brainstem regions

= afferent routes

• carry sensory information

• periphery —> brain

• Somatic Sensory Pathways

  • Dorsal Column–Medial Lemniscal Pathway = DCML

  • Spinothalamic Pathway

  • Trigeminal Pathways

• Visual Ascending Pathway

• Auditory Ascending Pathways

• Ascending Reticular Activating System (ARAS)

Unilateral damage to the spinal cord

—> loss of sensation (pain/temperature) on the opposite side of the body below the lesion

Aβ —> DRG → spinal cord

→ Ipsilateral dorsal columns (Fasciculus gracilis/cuneatus)

→ dorsal column nuclei (nuclei gracilis/cuneatus) in Medulla

→ Decussation (internal arcuate fibers) in Medial lemniscus (Medulla)

→ VPL (Ventral Posterolateral Nucleus) thalamus

→ Primary somatosensory cortex (S1)

(neurons marked fat)

CAVE: above the Medulla controlateral!

Aδ/C —> DRG —> spinal cord

—> substantia gelatinosa (dorsal horn)

—> decussation in spinal cord

—> controlateral spinothalamic tract

—> Medulla

—> VPL (Ventral Posterolateral Nucleus) u. intraluminar nuclei in Thalamus

—> primary somatosensory cortex

• primary and secondary somatosensory cortex S1/S2

  • Where? How strong?

• insula

• ACC = Anterior Cingulate Cortex

• amygdala

insula/ACC/amygdala: emotions? How does it feel?

• anterior spinothalamic pathway

  • crude touch

• lateral spinothalamic pathway

  • pain, temperature

• spinoreticular pathway —> reticular formation (brainstem)

  • arousal

• spinomesencaphalic/spinotectal pathway —> Periaqueductal grey (midbrain)

  • pain modulation

spinothalamic + spinoreticular fibers = spinal lemniscus

pathway for the sensory system of the face

The Trigeminal Nerve (CN V): Sensory Entry from the Face

—> pain and temperaute analogous to Spinothalamic Pathway

—> Fine touch, vibration, and conscious proprioception analogous to DCML

durch Pons in brainstem

CAVE: nuclei are named different!

parietal lobe

• postcentral gyrus (behind central sulcus)

  • 3a

  • 3b

  • 1

  • 2

• posterior parietal cortex

  • 5

  • 7

ventral posterior nucleus VP in thalamus —> 3a/3b

3b —> 1/2

3,1,2 —> 5

visual cortex —> 7

3a: proprioception/sense of the body position

3b: touch

1: texture information

2: emphasizes size and shape

5: somatosensory integration

7: visual somatosensory integration

—> Neglect syndrome

parts of the body or the world are ignored, sometimes unable to recognize e.g. leg as part of the body

Segmental Organization of the Spinal Cord

• 8 cervical

• 12 thoracic

• 5 lumbar

• 5 sacral

segments

= humunculus

—> body parts with greater sensory acuity occupy larger cortical areas

= brain's ability to dynamically reorganize its somatotopic maps in response to changes in sensory experiences or injuries

e.g. amputation/training

S1 = primary somatosensory cortex in Gyrus postcentralis (3a,3b,1,2

S2 = secondary somatosensory cortex, upper bank of the lateral fissure

endogenous opioids = morphine-like substances = endorphins

—> analgesia

—> natural suppression system for nociceptive (pain) signals

μ (mu), δ (delta), and κ (kappa)

= G-protein coupled receptors

Reduces nociceptive transmission by:

• decreasing transmitter release from primary afferents

• increasing postsynaptic inhibition

areas involved in pain modulation

• periaqueductal gray matter (PAG) of the midbrain

• raphe nuclei of the medulla

and

• dorsal horn of the spinal cord

Strong analgesic

Major risks include sedation and respiratory depression

• Morphine

• Codeine

• Heroine

cold: Aδ, C

warm: C

= free nerve endings, unmyelinated

specific ion channels

• cold

  • TRPA1

  • TRPM8 (Menthol)

• warm

  • TRPV4,3,1,2

  • TRPV1 (capsaicin (from chilli peppers))