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Chapter 12: The Somatic Sensory System (Nociception (Most nociceptors are…
Chapter 12: The Somatic Sensory System
Somatic Sensory system has broadly distributed receptors & responds to many different types of stimuli
Sensory Neurons: one end extends to periphery, the other through dorsal root to CNS (pseudounipolar). Cell body in dorsal root ganglion
Somatosensory Receptors
Nociceptors
Thermoreceptors
Mechanoreceptors
Two types of skin- hairy & glabrous
Two layers- epidermis & dermis
Types:
Pacinian Corpuscles
: Vibration, fast adaptation
Have football-shaped capsule w/ layers of connective tissue- causes fast adaptation
Ruffini's endings: Rapid indentation, stretch felt, slow adaptation
Merkel's Disks: Steady indentation, pressure felt, slow adaptation
Meissner's Corpuscles: Low frequency vibration, Gentle fluttering felt, fast adaptation
Meissner & Merkel: Small RFs; Pacinian & Ruffini Large RFs
Specific channels are still unidentified
Two-Point Discrimination: measure fo spatial resolution; function of density of mechanoreceptors, small receptor fields, more brain area devoted to space, special neural mechanisms. Lateral inhibition also increases resolution
Either free nerve endings or encapsulated
Spinal Cord Organization:
Axon Types
Aβ- Mechanoreceptors of skin
Aδ- Pain, temperature
Aα- Proprioceptors of skeletal muscle
C- Temperature, pain, itch
30 spinal segments- correspond to vertebrae through which they pass
Dermatomes- areas of skin innervated by one dorsal root
Cutting one dorsal root does not completely eliminate ipsilateral sensation- overlapping dermatomes
Shingles infects one DRG
Dorsal Horn (where 2nd order neurons synapse), Intermediate zone, ventral horn
Dorsal Column- Medial Lemniscal Pathway
Aβ axons ascend via ipsilateral dorsal column
Terminate in dorsal column nuclei in medulla
Decussate & ascend via medial lemniscus
Medial Lemniscus axons synapse in Ventral Posterior Nucleus (VPN) of thalamus- from there to primary somatosensory cortex
Trigeminal Touch Pathway
Trigeminal Nerve has three branches
Maxillary (V2)
Mandibular (V3)
Opthalmic (V1)
Enters brainstem at pons- decussates & sends to medial VPN thalamus, then to V1
Trigeminal Pain Pathway: Axons synapse at trigeminal nucleus- axons decussate & ascend via trigeminal lemniscus
Trigeminal Neuralgia: "Suicide DIsease"
Somatosensory Cortex- in the parietal cortex
3B is primary SMC- receives input from VPN
Layer IV receives input
Somatotopic map is plastic
Posterior Parietal Cortex, BA 5 and 7, involved in somatic sensation, visual stimuli, movement planning, attentiveness
Agnosia: inability to recognize object even though sensation is normal
Astereognosia: normal sense of touch, but cannot identify objects
Neglect Syndrome: a part ofvisual field or body is ignored or denied- more common in right hemisphere
Nociception
Different path to the brain than mechanoreceptors
Ion channels can be opened by: Mechanical stimulation, temp. extremes, oxygen deprivation, chemicals, Proteases (bradykinin), ATP, K+ ion channels, histamine
Most nociceptors are polymodal, but show selectivity for one;
Mechanical
Thermal
Chemical
Capsaicin activates TRPV1 receptors- perception of heat
Two 'types' of pain
"First pain"- Aδ, fast & sharp
"Second Pain"- C, duller, longer lasting
Both synapse in substantia gelatinosa of dorsal horn- decussation & ascension
Glutamate is main NT
Gate Theory of Pain: Projection neuron inhibited by interneuron "gate"
Mechanoreceptor stimulates inhibitory interneuron
Periaqueductal Gray Matter (PAG) in midbrain regulates pain
Sends axons to dorsal raphe
Referred Pain: Stimuli from viscera get "mixed up" with cutaneous pain
Itch
C-fibers- selectively responsive to histamine released to mast cells in response to inflammation (TRPV1 channels). Not all itch is histamine-mediated
Spinothalamic Pathway- conveys pain & temperature
Axons decussate in spinal cord, ascend via spinothalamic tract- ascends to thalamus without synapsing
Thermoreceptors
6 distinct TRP channels, confer different sensitivities to temperature
Cold Receptors connected to Aδ & C fibers, hot receptors to C fibers
Receptors adapt to extended stimulus
Temperature pathway is identical to Pain Pathway
Touch ascends ipsilaterally to medulla, pain ascends contralaterally