Histological Organisation of Receptors of General Sensation Flashcards
General receptors can be classified based on 3 characteristics. Name them.
- origin of stimulus (exteroceptor, interoceptor, proprioceptor)
- nature of stimuli/functional classification
- anatomical classification [dependent on the structural organization of the terminal portion of the sensory nerve (non-encapsulated nerve endings and capsulated nerve endings e.g. Meissner’s and Pacinian corpuscles)
Somatic senses can be categorised into 3 physiologic types, which are?
a) mechanoreceptive sensations: stimulated by mechanical displacement of body tissues; include tactile sensations (touch, pressure, vibration, and tickle senses) and position sensations (static position and rate of movement senses)
b) thermoreceptive sensations: stimulated by degree of thermal energy; for heat and cold sensations
c) pain sensations: activated by factors that cause damage to body tissues
State the properties of sensory receptors.
- Excitability.
- Require an adequate stimulus, just enough to cause excitation of the sensory receptor leading to generation of impulses in the afferent nerve.
- Specificity (differential sensitivity).
- Intensity discrimination: intensity of stimuli are discriminated by (i) number of receptors stimulated, and (ii) frequency of action potentials generated.
- Receptor adaptation i.e. when a stimulus is applied for a prolonged duration, some of the sensory receptors may stop responding to it in the course of time.
Sensory organs are organs that house a sensory receptor. A single sensory organ may house one type or multiple types of sensory receptors. TRUE or FALSE?
TRUE.
What are free nerve endings, and what do they detect?
- they consist of dendritic nerve endings that lack connective tissue capsule or myelin sheath investment around them.
- they are found everywhere in the skin as well as in many other body tissues (mostly terminate in stratum granulosum in skin).
- can detect touch and pressure sensations
- also detect temperature and pain sensations
Briefly discuss Meissner’s corpuscles (tactile corpuscles). [hint: location, structure, function]
⚚ found in the dermal papillae of glabrous skin (hairless skin), being particularly dense at the tips of the fingers and toes.
⚚ [smaller and more cylindrical than the Pacinian corpuscles]
⚚ consist of many branching terminal nerve filaments within the capsule, hence you’ll observe multiple stacked lamellae within a thin fibrous connective tissue forming the outer coat.
⚚ they are phasic receptors for light touch, and texture.
⚚ have great sensitivity to movement of objects over the surface of skin and to low-frequency vibration.
⚚ [Histological slide 1] [Histological slide 2]
NB: Phasic simply means, fast adapting.
Brielfy discuss Merkel’s discs.
- consists of a Merkel cell in close apposition with an enlarged sensory nerve terminal.
- Merkel’s discs are responsible for giving steady-state signals that allow one to determine continuous touch of objects against the skin.
- detects crude touch.
- provide information on pressure to the body surface, and texture of objects the body is in contact with.
- [Histological slide: some receptors found in skin]
[Histological slide: Merkel’s discs]
Pro-tip on observing it on a light microscope: A Merkel’s cell appears to have a halo around it.
Peritrichial nerve endings/hair root plexuses/hair follicle receptors consist of sensory nerve terminals that wrap around the root of hair follicles. What are their roles?
- fast adapting receptors that are sensitive to any slight movement of the hair follicles.
- detects mainly movement of objects on the surface of the body or initial contact with the body.
Ruffini terminals are spindle-shaped, multi branched and encapsulated nerve endings located in the _____(a)____ dermis of skin as well as deeper internal tissues and joint capsules.
(b) State the role of the Ruffini terminals.
(a) reticular
(b) Role of Ruffini terminals
- very slow adapting hence signal continuous states of deformation of the tissues e.g. heavy prolonged touch and pressure signals.
- also signal the degree of joint rotation and finger position (from stretch of skin and joint capsule).
- it may be associated with detection of heat.
Pacinian corpuscles are encapsulated receptors that consist of concentric connective tissue lamellae around a nerve terminal. They are mainly found at the junction of the dermis and subcutaneous tissue and they are widely distributed throughout the body. State their roles.
- These receptors are extremely fast adapting and are only stimulated by rapid local compression of the tissues.
- They are sensitive to light pressure.
- Those in the subcutaneous tissues respond to pressure in the skin.
- Pacinian corpuscles in and near the joint capsules are proprioceptors; through excitation of these endings and non-encapsulated endings in the same tissue, there is accurate monitoring of the position and movement of the body parts.
- They also serve as receptors for vibration.
- The corpuscles in viscera and mesenteries may contribute to sensations of fullness of hollow organs.
- [Histologial slide 1] [Histological slide 2] [Histological slide 3]
Neuromuscular spindle- detection of muscle stretch (myostatic stretch reflex)
The neuromuscular spindle comprises of encapsulated receptors parallel to skeletal muscle cells. It comprises of two types of fibres: intrafusal fibres and extrafusal fibres. The motor innervation of intrafusal fibres is from ____(a)____ motor neurons of anterior horn cells whereas sensory innervation comes from both ____(b)____ and ____(c)____ nerve endings. Extrafusal fibres are located oustide the spindle capsule. Their motor innervation is by ____(d)____ motor neurons.
(a) gamma
(b) annulospiral
(c) flower-spray
(d) alpha
Name two types of intrafusal muscle fibres and outline 2 differences between them.
- nuclear bag and nuclear chain fibres
- Nuclear chain fibres have a single row of centrally arranged nuclei; nuclear bag fibres show an accumulation of nuclei.
- Nuclear chain fibres are static and are responsible for detecting the static length of the muscle; nuclear bag fibres are dynamic and are sensitive to both the length and velocity of muscle stretch.
- [Diagram]
- Where are Golgi tendon organs located?
- What is their function?
- They are located in the origins and insertions of skeletal muscles/at musculotendinous junctions.
- They sense changes in muscle tension.
What is the Golgi tendon reflex?
The Golgi tendon reflex, also known as the inverse stretch reflex is a protective feedback mechanism that controls the tension of an active muscle by causing relaxation before the tendon tension becomes high enough to cause damage.
