ANATOMICAL TYPE OF RECEPTORS

Dr. Abdul QadeerMBBS;FCPS;FICS

Assistant Prof. of SurgeryLCMD, Karachi.

Types of sensory receptors

Non-capsulated receptors

1. Free nerve endings

2. Merkel discs

3. Hair follicle receptors

Capsulated receptors

1. Meissner’s corpuscles

2. Pacinian corpuscles

3. Ruffini corpuscles

4. Neuromuscular spindles

5. Neurotendinous spindles

Free nerve endings

• Widely distributed throughout the body

• They are present in:

1. Epithelial tissue e.g. dermis, cornea, GIT

2. Connective tissues e.g. dermis, fascia, ligaments, joint capsules, tendons, periosteum, perichondrium, haversiansystem of bone, tympanic membrane and dental pulp and muscle

Free nerve endings

• Afferent nerve fibers may be myelinated or un-myelinated

• But terminal nerve endings are without myelin sheath i.e. no Schwann cells covering their tips

Free nerve endings

• Free nerve endings detect:

1. Pain (most common)

2. Crude touch

3. Pressure

4. Tickle sensation

5. Cold & heat may be

Merkel discs

• Found in hairless skin e.g. fingertips

• Also found in hair follicles

• Merkel cell: when nerve fiber expands as a disc in epidermis

• Tactile domes: clusters of Merkel discs in hairy skin

Merkel discs

• Slowly adapting touch receptors

• Transmit information about the degree of pressure exerted on the skin e.g. when one is holding a pen

Hair follicle receptors

• Nerve fibers wind around the follicle below sebaceous gland

• Bending and release of hair will stimulate the follicle receptor

Meissner’s corpuscles

• Ovoid in shape

• Located in the dermal papillae of the skin

• Especially skin of palm & sole, nipple, external genitalia

• The corpuscles reduce in number with age

• Very sensitive to touch and able for two-point discrimination

• They are rapidly adapting mechanoreceptors

Meissner’s corpuscle

Pacinian corpuscles

• Widely distributed throughout the body

• Abundant in dermis, subcutaneous tissue, ligaments, joint capsules, pleura, peritoneum, nipples and external genitalia

• Ovoid in shape, 2 mm long

• Consists numerous concentric lamellae

• It is sensitive to vibration up to 600 stimuli/sec

• They are rapidly adapting mechanoreceptors

Ruffini corpuscles

• Located in the dermis of hairy skin

• They are stretch receptors of skin

• They are slowly adapting mechanoreceptors

Joint receptors

• Joints have 4 types of sensory endings supplying capsule and ligaments of synovial joints

• Three are encapsulated, resembling pacinian, Ruffini and tendon stretch receptors

• One non-encapsulated sensitive to excessive movements and pain

Joint receptors

Neuromuscular spindles

• Contains:

1. Intrafusal fibers

2. Extrafusal fibers

3. Nuclear bag fibers

4. Nuclear chain fibers

• Stretching & relaxing the spindle causes impulses to pass along the axons

Neuromuscular spindles

Neuromuscular spindle

Neurotendinous spindle(Golgi tendon organ)

• Present in tendons

• Located near the junction of tendons with muscles

• Provide CNS with sensory information regarding the tension of muscles

• Has intrafusal fibers

• Myelinated nerve fibers pierce the capsule, loose their myelin sheath, branch & terminate in club shaped endings

Neurotendinous spindle(Golgi tendon organ)

• Unlike the neuromuscular spindle, which is sensitive to muscle length, the neurotendinous organ detects changes in muscle tension

• Unlike the muscle spindle reflex, this reflex is inhibitory & inhibits muscle contraction (hence protective)

• It can influence voluntary muscle activity

Neurotendinous spindle(Golgi tendon organ)

Neurotendinous spindle(Golgi tendon organ)

The end