Receptors and sensation

Sensation is the perception of the brain in response to the processing of signals from the periphery. Peripheral receptors transduce stimulus energy into action potentials which travel up axons for neural processing.

There are two basic types of receptors (Figure 1): A) Separate receptor cell with specialised membrane activated by the stimulus. On stimulation, the receptor cell releases a chemical messenger which crosses the extracellular cleft and binds to the afferent nerve ending opening specific chemical gated channels. B) specialised nerve endings of afferent peripheral nerves. Stimulus directly opens voltage gated channels in the membrane which can be ionotropic (ligand gated ion channels) or metabotropic (acts through a second messenger)

Figure 1

Some types of receptors:

• Skin mechanoreceptors (Figure 2): Rapidly adapting (phasic) receptors has a quickly diminishing response and then stops e.g pacinian corpuscle.It does not provide information on the duration of the stimulus but can provide information on rapid changes in stimulus intensity and rate. A slowly adapting (tonic) receptor continues to produce action potentials over the duration of the stimulus so can convey information about duration e.g nociceptor, joint capsule, muscle spindle. Some are permanently active and indicate a background level.
• Pacinian corpuscle: responsible for detecting vibration and pressure in non hairy skin. They respond only to sudden disturbances and are sensitive to vibration. This may be used to detect surface texture (remember, they are sensitive to pressure so they are deeper in the subcutaneous tissue to allow pressure to reach them. Pacinian corpuscles react when the skin is rapidly indented but not constant pressure due to the layers of connective tissue that cover the nerve ending)
• Meissner's corpuscle: responsible for sensitivity to light touch especially in thick, hairless skin e.g finger pads. They are located just below the epidermins within the dermal papillae. (Remember: their more superficial position compared to Pacinian corpuscles means they respond to lighter touch compared to the deep pressure of Pacinian corpuscles)
• Hair follicle receptor: Is a very sensitive mechanoreceptor for touch sensation. A nerve plexus forms around a hair follicle in the dermis.
• Ruffini's corpuscle: A slowly adapting mechanoreceptor between the dermal papilla and the hypodermis sensitive to skin stretch.
• Merkel's receptorL Found in the basal epidermis and provide information on mechanical pressure, position, and deep static touch features such as shapes and edges.
• Tactile corpuscles: Located in the dermal papillae, respond to moderate vibration and light touch

Figure 2

• Periodontal ligament mechanoreceptors (Figure 3): 
• Ruffini-like (unencapsulated) endings in periodontal ligament is closely associated with collagen fibres. These have A Beta (type 2) sensory axons. Cell bodies are in the trigeminal ganglion and some in mesencephalic ganglion.  They are directionally sensitive to mechanical loads on teeth, slowly adapting type 2 receptors. Respond to low forces, 80% saturate at moderate forces (3-4N) with the remaining 20% responding linearly over a larger range of forces. (This all makes sense clinically as teeth have good proprioception and tactile sensitivity to light, medium and heavy forces. Sensation can be felt on a tonic clench.

Figure 3

• Muscle spindle (Figure 4): Stretch receptors within the body of a muscle that primarily detect changes in the length of a muscle. The several fibres that make up the muscle spindle are called intrafusal (fusal meaning spindle) fibers and are located between extrafusal muscle fibres which are the fibres of the muscle itself.  Spindles have a capsule of connective tissue and run parallel to the extrafusal muscle fibres. Muscle spindles have both sensory and motor components. Annulospiral (meaning spiral around) type 1a afferent nerve endings wrap around the non contractile centre of the intrafusal fibres. Secondary flower spray (type 2) afferent nereve endings branch out onto the surface of the intrafusal fibres.  Fusimotor neurons composed mainly of gamma motor neurons and few beta motor neurons activate the muscles within the spindle. Gamma motor neurons are specific to muscle spindle fibres but beta motor neurons also innervate extrafusal muscle fibres.

Figure 4

• Golgi tendon organ: Another type of receptor found in muscle. They are receptors located in a tendon close to the junction of the main tendon and corresponding muscle. A capsule surrounds a bundle of tendon fascicles (collagen bundles) which connect to muscle. A tendon organ will be connected in series between the tendon and the muscle so muscles stretch will excite the receptor. Each tendon organ is innervated by a fast conducting type Ib (AAlpha) afferent fibre which respond to mechanical deformation and which enters the capsule and forms spraylike endings in contact with the tendon fascicles). Golgi tendon organs signal passive stretch of muscle and active muscle tension but appear to have very low thresholds to active muscle forces therefore are more sensitive to active contractions. They appear to be absent in jaw muscles where their function may be served by periodontal ligament mechanoreceptors.

Figure 5

• Joint receptors (Figure 6): 4 types found in the joint capsule and ligaments. Golgi types are located in ligaments (connect bone to bone). Ruffini terminals and paciniform corpuscles are found in the joint capsule. Free nerve endings are found in ligaments and joint capsules. The action of these receptors to signal joint position is not fully understood especially as most receptors respond only at maximum flexion or extension (Figure 6 indicating receptor discharge is highest at maximum position and minimal at intermediate angles). Currently it is thought that they signal limits of motion rather than detail the angle of the joint
• Type 1 Ruffini terminals: innervated by medium diameter ABeta fibres. Respond to movement and position
• Type 2 Paciniform corpuscles: reseble pacini corpuscles but are smaller. Innervated by medium diameter ABeta fibres. Respond only to movement
• Type 3 Golgi type endings: innervated by large diameter AAlpha fibres. Slowly adapting receptors that respond to joint position.
• Type 4 Free nerve endings: Supplied by small A delta fibres and unmyelinated C fibres and are thought to respond to extreme, painful movement of the joint.

Figure 5

• Orofacial nociceptors (Pain and analgesia lecture)
• Pulp
• Dentine
• Periodontal ligament
• Oral mucosa
• Peri-oral skin
• TMJ and masticatory muscles