Somatosensory Receptors😊

Clinical Image Interpretation:

• Arrow to Dermal papilla: Meissner corpuscles.
• Dermat Miss

• Arrow to Stratum basale: Merkel cells.
• Basil ne marakkalle

• Note: Both Melanocytes and Langerhans cells are types of dendritic cells.

• Mnemonic: 
• Both M (Melanocytes and Merkel cells) → in stratum basale.
• Keettanam (Keratinocytes) Marakkathe (Merkel) → Purath (Ectoderm) ninnu
• Langeru (Langerhans) Midukkananu (Mesenchymal → blast → Vinblastine) → Spine (Stratum Spinosum) undu → badminton kalikkum (Racket shape) → Name is Birbal (Birbeck) → He is 100 - 200 yr old (100, 207) → Child in 1A (CD 1A) → Cork buttonil (button sequestra) vannu thatti → hole ayi (hole within a hole sign) → pallu poi (floating tooth) → Blasted (Vinblastine)

• Clasp Knife Spasticity
• Initial ↑ resistance → Stretch reflex
• Later ↓ resistance → Inverse stretch reflex

• Ankle Clonus
• Trigger: Sudden, sustained stretch
• Mechanism: Continuous cycle of stretch reflex and inverse stretch reflex.

Table: Temperature Sensation Details

• mnemonic Warm vanilla → TRPV

Carrageenan theory

• Inflammation and pain

Taste Perception

Five Basic Tastes in Humans

• Sweet

• Salty

• Sour

• Bitter

• Umami

Taste Receptor Types

• Ligand-gated channels
• Ionotropic ENaC receptors
• Salt and sour taste
• Salt taste:
• Na⁺ movement via ENaC
• Sour taste:
• H⁺ movement via ENaC
• Sour experience in a trip
• Mediated by the TRPV1 channel
• TRPV1 = Transient Receptor Potential Vanilloid 1
• Detects acidity of foods → contributes to sour taste
• Cation channel allowing influx of:
• Na⁺, Ca²⁺, Mg²⁺
• Activated by:
• Temperature changes
• Acidic conditions
• Capsaicin

• G-Protein Coupled Receptors (GPCRs)
• Metabotropic receptors
• Sweet, bitter, umami

Sensation	Family	Mnemonic
Sweet	T1R2 and T1R3	Sweetwo
Umami	T1R1, T1R3, Metabotropic glutamate receptor (mGluR4)	Uno (R1) and Mii (mGluR4)
Bitter	T2R family	BIt Two R → 2R

Nociception (Pain)

• Nociceptors:
• Free nerve endings of Aδ & C fibers.
• Contain ion channels for pain detection.

• A → Acrid & Allyl compounds → Awfully cold pain

Fiber Types & Characteristics:

Mechanical Nociception:

• TRP A1 
• (Transient receptor potential)

• MRGPRD 
• (sharp pain)
• (mass related g protein coupled receptor)

Pain substances vs. Nociceptor sensitizers:

Gate Control Theory of Pain

Melzack and Wall

• Pain pathway
• Aδ fibers & C fibers → Synapse with 2nd order neurons in spinal cord.
• Stimulation → Opens gate → Pain signal to thalamus.

• Inhibitory control
• G (i) interneuron in spinal cord.
• Aβ fibers (touch, pressure)
• Stimulate G neuron
• Inhibits 2nd order neuron
• Closes gate
• ⛔ projection neuron in substantia gelatinosa
• Stimulates Raphe nuclei pathways.
• ↓ Pain impulse to sensory cortex → Pain relief

Substantia gelatinosa Feature	Description
Location	Dorsal horn (Lamina II)
Input fibers	C fibers, Aδ fibers
Function	Pain & temperature modulation
Neurotransmitters	Enkephalins, GABA
Output	Spinothalamic tract neurons

Clinical applications

• Massage

• Balms

• Acupuncture

• TENS

• Applying pressure to painful area

• All stimulate Aβ fibers, reducing pain perception.

Clinical Aspect (Pain)

Local Anesthetics

• Example: Lignocaine
• ⛔voltage-gated Na⁺ channels.
• Prevents firing at free nerve endings → Analgesia.

Congenital Insensitivity to Pain Syndrome (CIPS)

• Congenital absence of voltage-gated Na⁺ channels.

Analgesia (Pain Relief)

• Endogenous Opioid Analgesic System
• Example: Acupuncture therapy
• MOA:
• Activation of periaqueductal gray matter (PAG) →
• Release endogenous morphines
• endorphins
• enkephalins
• dynorphins

• Stress-Induced Analgesic System
• Example: Fire walking (due to stress)
• MOA:
• Activation of PAG →
• Release endogenous cannabinoids (anandamide).

Raphe Nuclei: Role in Pain Relief

• Cross section at lower level of pons

• Raphe nuclei are in the brainstem.

• Involved in pain modulation.

• Contain serotonin-secreting neurons.
• Neurons descend to spinal cord.
• Release serotonin → analgesic effect

Chemical and visceral pain

Chemical Pain

Visceral Pain

• Unpleasant, poorly localized, referred pain

• A/w Nausea, vomiting, sweating

• Conveyed via: C fibers

• Pain-sensitive structures:
• Liver capsule
• parietal pleura

• Pain-insensitive structures:
• Alveoli

Pain-insensitive Brain Structures

• Note: Dura mater is pain sensitive

• Arachnoid mater

• Pia mater

• Brain parenchyma

• Ependyma

• Choroid plexus