Note
Studied by 61 people
3. Somatosensory Receptors Study Notes
Learning Objectives
Understand the general principles of sensation
Identify and differentiate between receptor types:
Mechanoreceptors
Chemoreceptors
Thermoreceptors
Nociceptors
Describe nerve pathways involved in somatosensory perception
Explain the role of the somatosensory cortex in sensory processing
Review common somatosensory defects
Overview of Sensory Receptors
Definition:
Sensory receptors are specialized cells that detect specific forms of energy (stimuli) from the environment. They transduce (convert) this energy into electrical signals (action potentials), which travel via sensory nerves to the spinal cord and CNS, where they are interpreted.
🧠 Somatosenses:
Pain
Temperature
Touch
Pressure
Vibration
Proprioception (body position awareness)
✅ The brain interprets these signals based on where they terminate:
Somatosensory cortex → interpreted as cutaneous sensations.
Visual cortex → interpreted as visual signals.
Classification of Sensory Receptors
By Transduction Mechanism
By Transduction Mechanism:
Mechanoreceptors: Respond to physical compression/stretching.
Thermoreceptors: Detect temperature changes (hot/cold), located in the skin, hypothalamus, spinal cord, liver, and gut.
Electromagnetic receptors: Detect light (retina); some species detect other wavelengths or magnetic fields.
Chemoreceptors: Detect chemical changes:
O₂/CO₂/H⁺ → brainstem (respiratory/cardiovascular control).
Osmolarity → hypothalamus (water balance).
Taste and smell receptors.
🩻 By Function:
Nociceptors: Detect pain (can be mechanoreceptors, thermoreceptors, or chemoreceptors).
Proprioceptors: Provide information on joint position and body movement.
Include mechanoreceptors and other senses (vision, vestibular system, and skin hair detecting air displacement).
💡 Key Concept: Modality
Receptors respond best to their specific stimuli but can respond to other stimuli if strong enough.
However, the afferent nerve interprets the signal as the receptor's primary sensation.
Example: Pressing on the eye → activates photo-receptors → perceived as a white flash (light) visual sensation, even though the stimulus is pressure.
LAW OF SPECIFIC NERVE ENERGY
Receptive Field:
The area monitored by a single receptor.
Smaller fields (e.g., fingertips, tongue, lips) → greater discrimination (can detect two closely spaced stimuli). → high sensitive areas
larger recpetive feild - skin on torso, leg, arm
Tested using a 2-point discrimination brush.
⚡ Receptor Potential and Signal Coding:
Transduction Process:
e.g Pacinian corpuscle - receptor consist a bare neurone tip surrounded by concentric tissue layers
local pressure cause deformation of tissue
transferred to unmyeliated fibre tip
deformation of firber tip → stimulus
Stimulus → Na⁺ or Ca²⁺ influx → local membrane depolarization → Receptor Potential.
If depolarization reaches threshold, it generates an action potential → sent to the CNS.
Graded Response:
Stronger stimulus → larger receptor potential → more frequent action potentials.
Stimulus strength is coded by the frequency of action potentials.
🔥 Receptor Response Types:
consider constant stimulus to receptor
Phasic Response:
Rapid, transient firing at the stimulus onset, then adapts quickly.
gives transient information
Example: Pacinian corpuscle (detects vibrations).
Tonic Response:
Little or no adaptation → constant firing and sustained sensation.
gives constant sensation
Example: Ruffini corpuscle (detects skin stretch) and some proprioceptors.
Somatosensory Receptors – 3 grps
🛠 A) Mechanoreceptors → Detect physical stimuli (touch, pressure, stretch)
have stretch sensitive membrane channel, respond to distortion of membrane
Tactile Receptors (Skin & Deep Tissue)
Free nerve endings: Detect pain, temp, and light touch. Found in the skin and cornea.
Root hair plexus: Around hair follicles, detect hair movement, rapidly adapting.
Tactile (Merkel’s) discs: Detect fine touch, slow-adapting, small receptive fields.
Tactile (Meissner’s) corpuscles: Detect light touch and low-frequency vibration, fast-adapting.
Laminated (Pacinian) corpuscles: Detect deep pressure and high-frequency vibration, rapidly adapting.
Ruffini corpuscles: Detect skin stretch and pressure, slow-adapting.
Proprioceptors (Body Position)
Muscle spindles: In muscle bodies, detect muscle length and prevent overstretching via stretch reflex.
Golgi tendon organs: In tendons, detect muscle tension, prevent muscle damage.
Joint receptors: In joint capsules, detect joint pressure, movement, and tension.
Baroreceptors (Pressure Sensors)
Free nerve endings in vessel walls → monitor wall stretch → indirectly detect blood pressure.
🌡 B) Temperature Receptors
free nerve endings located in dermis
sensitive to hot or cold temperature range
alspLocated in the hypothalamus (major role in temperature regulation)
phaic receptor
Use TRP channels:
TRPM8 → Activated by cold (<27°C) and menthol.
TRPV3/4 → Activated by warmth (25–31°C).
Some TRP channels detect noxious temperatures → involved in pain sensation.
🧪 C) Chemoreceptors
Detect chemical stimuli:
Taste and smell receptors.
O2, CO2, and H⁺ receptors in the brainstem and carotid bodies → control respiration and blood gases.
GI chemoreceptors → detect nutrients (fats, glucose, proteins) → regulate digestive hormone release.
⚡ D) Electromagnetic Receptors
Detect light in the retina (covered in eye lectures).
🔥 Nociceptors (Pain Receptors)
Detect tissue damage or potential damage.
Respond to:
Chemicals: K⁺, prostaglandins, ATP, adenosine, serotonin (5-HT).
Mechanical stress: From tissue damage.
Extreme temperatures: >43°C or <18°C (via TRP channels).
Do not adapt (continuous pain signaling).
Two types of pain fibers:
Type C fibers: Slow conduction → slow, dull, aching pain.
Type A fibers: Fast conduction → sharp, localized pain.
Nerve Classification – Summary
🛤 1) Primary Afferent Fibres → Classified by Conduction Velocity
with receptor endings, cell bodies in dorsal root ganglia, enter spinal cord via dorsal roots
Conduction speed depends on axon diameter and myelination.
Larger, myelinated fibres → faster conduction.
Smaller, unmyelinated fibres → slower conduction.
Fibre Types:
Aα fibres:
Fastest conduction velocity.
Carry signals from Golgi tendon organs (GTOs) and muscle spindles → crucial for proprioception and motor coordination.
Aβ fibres:
Fast conduction.
Carry touch, pressure, and vibration signals.
Aδ fibres:
Medium conduction speed.
Carry sharp, localized pain and temperature signals.
C fibres:
Slowest conduction velocity, unmyelinated.
Carry crude touch, aching (slow) pain, and temperature signals.
🛣 2) Path to the Spine
Afferent signals travel from receptors to the spinal cord via ascending pathways.
Pathway:
Primary afferent fibres → enter the spine via the dorsal root.
Cell bodies are located in the dorsal root ganglion (outside the spinal cord but within the spinal column).
Slower axons (C fibres) → travel more dorsally in spinal lamina
Faster axons (Aα, Aβ) → travel more ventrally in spinal lamina
Synapse with secondary nerves → signals ascend to the brain in specific tracts.
Different receptors → travel via different ascending tracts.
connect to cerbral cortex - 3rd order neurons
(1st - sensory recpetor to spinal cord. 2nd spinal cord to thalamus, 3rd thalamus to sensory cortex
Dermatomes & Ascending Pathways – Summary
🩻 1) Dermatomes
Definition:
A dermatome is a skin region whose sensory input is carried by a specific spinal nerve entering the spinal cord at a particular level.
Pattern:
Trunk & head: Dermatomes form transverse bands.
Arms & legs: Dermatomes run longitudinally.
When visualized in a quadruped position (all fours), the banding pattern is more apparent.
Clinical relevance:
Shingles (Herpes Zoster):
The virus resides in primary afferent cell bodies in the dorsal root ganglion.
Upon reactivation, it travels antidromically (backward) down the afferent nerve, causing pain and blistering in the affected dermatome.
⚠ 2) Referred Pain
Definition:
Pain from internal organs is perceived as arising from the dermatomes linked to the spinal level of entry.
Mechanism:
Internal organ afferents share spinal entry points with somatosensory afferents → the brain misinterprets the origin.
Example:
Cardiac pain:
Myocardial ischemia (narrowed arteries → hypoxic myocardium) stimulates afferents entering the spine at T1-T4.
Interpreted as pain from the T1-T4 dermatomes → causing left pectoral and left arm pain (classic referred pain of a heart attack).
Diagnostic clues:
Referred pain patterns help identify underlying organ issues.
🛤 3) Ascending Sensory Pathways
Spinothalamic Tract:
Function:
Carries signals for temperature, pain, tickle, and itch.
Path:
Primary afferent → spine → synapse with secondary neurone (DHN).
Crosses the midline in the spine → ascends contralaterally in the spinothalamic tract.
Synapse in thalamus → 3 order neurone → sensory cortex → sensation beccomes conscious
Features:
Poor spatial discrimination, crude sensation (diffincult in locating the position of signal
Slow conduction velocities (0.5–40 m/s) via smaller fibres.
Dorsal Column Pathway:
Function:
Carries proprioception, fine touch, and vibration signals.
Path:
Primary afferents ascend ipsilaterally (same side) in the dorsal column. (does not synapse in spine)
Synapse in the dorsal column nucleus (brainstem).
Secondary neurone crosses midline in the medulla.
Ascends contralaterally → synapse in the thalamus → tertiary neurone → sensory cortex.
Features:
Good spatial discrimination.
Fast conduction velocities (40–100 m/s), larger myelinated fibre
Spinocerebellar Tract:
Function:
Carries proprioceptive information to cere for movement and balance control. limb and joint position
Path:
originates in the spinal cord
Some 2nd nuerone ascend ipsilaterally, others contralaterally.
Terminate in the cerebellum.
Importance:
Crucial for coordinating motor function.
Ensures precise movement and balance adjustments.
Trigeminal Pathway
Somatosensory Innervation of the Head and Neck – Summary
🌐 1) Nerve Supply Overview
Cervical Nerves:
Carry afferent information from the back of the head and neck.
Trigeminal Nerve (Cranial Nerve V):
Provides sensory innervation to the face and most of the head.
Divides into three main branches, represented in dermatomes:
🔍 2) Trigeminal Nerve Branches & Their Sensory Areas
Ophthalmic Nerve (V1):
Sensory information from:
Scalp and forehead.
Upper eyelid, conjunctiva, and cornea.
Nose and nasal mucosa.
Frontal sinuses.
Parts of the meninges.
Maxillary Nerve (V2):
Sensory information from:
Lower eyelid and cheek.
Nares (nostrils) and upper lip.
Upper teeth and gums.
Nasal mucosa.
Palate and roof of the pharynx.
Maxillary, ethmoid, and sphenoid sinuses.
Parts of the meninges
Mandibular Nerve (V3):
Sensory information from:
Lower lip, chin, and jaw (except the angle of the jaw, supplied by C2-C3).
Lower teeth and gums.
Parts of the external ear.
Touch-position and pain-temperature sensations from the mouth.
Parts of the meninges.
primary afferents synapse in the trigeminal nucleus. 2nd order neurones cross the midline to the thalamus → 3rd order neurones travel to the sensory cortex
💉 3) Clinical Relevance
The three trigeminal dermatomes are clearly defined.
Essential for dentists as they target specific branches of the trigeminal nerve for local anaesthesia during dental procedures.
🔬 4) Neural Pathway - Trigeminal Pathway
Primary afferents → synapse in the trigeminal nucleus:
Second-order neurones:
Cross the midline of the trigeminal nucleus (pons, mudulla), Ascend to the thalamus.
Third-order neurones:
Travel to the somatosensory cortex, where the sensation becomes conscious.
Somatosensory Cortex – Summary
🌐 1) Sensory Processing in the Cortex
Location:
Somatosensory information arrives in the postcentral gyrus of the parietal lobe.
This is where conscious sensation occurs.
Somatotrophic Representation:
Different body parts are represented in specific areas of the cortex.
The amount of cortex assoicalted depends on the density of receptors and the region’s sensitivity.
Example:
Fingers (high receptor density) → large cortical area.
Back (low receptor density) → small cortical area.
Sensory Homunculus:
A map showing that in the somatosensory cortex → there are different specific ares and proportions representating differnt body parts
Larger areas represent more sensitive regions.
Contralateral Processing:
Sensations from one side of the body are processed by the opposite side of the brain.
🔍 2) Somatosensory Deficits
Peripheral Nerve Damage:
Trauma to a nerve → sensory loss in the receptive fields of the affected axons.
Neuropathy:
Gradual, widespread damage to primary afferents from:
Diabetes.
Multiple sclerosis.
Chronic alcohol consumption.
Chemotherapy.
Symptoms:
Paraesthesia (pins and needles) → progresses to anaesthesia (numbness).
All sensory modalities are typically affected.
🔬 3) Spinal Cord & Tract Damage
Spinal Root Damage:
Trauma or tumour → damages primary afferents.
Sensory deficit occurs in the dermatome associated with the affected spinal root.
Ascending Tract Damage:
Usually bilateral (both side of body) sensory loss below the lesion.
Unilateral lesions (e.g., from a spinal tumour):
Ipsilateral loss of joint position
Contralateral loss for temp, pain
🚩 4) Stroke-Related Deficits
Somatosensory damage within the brain - caused by stoke
Can affect tracts carrying nerves from the thalamus to the cortex (via the internal capsule).
Causes contralateral sensory loss.
May also cause paralysis if nearby motor areas are affected.
Trigeminal Neuralgia
🔥 1) Symptoms & Presentation
Pain Characteristics:
Trigger: Light touch, eating, cold air
Pain quality: Stabbing, shooting, excruciating, or burning.
Duration: sudden, Lasts for a few seconds but may occur in rapid bursts,
Onset:
More common in individuals over 50 years old.
Misdiagnosis Risk:
Often mistaken for toothache, making dentists the first to suspect TN.
Diagnosis considered when there are no dental issues present.
⚠ 2) Causes of TN
usually one branch is involved
Trigeminal nerve demyelination & hyper-excitability
induced by
• vascular pressure
• demyelinating disease (MS)
• tumour/cyst/vascular abnormality
• surgery/dental surgery
💊 3) Treatment Options
medication, pain killers, anticonvulsant, surgery