Nerve Endings and Receptor Types

Nerve Endings

• Nerve endings are either receptors or effectors
• Receptors receive external or internal stimuli and convert them.
• Effectors bring efferent nerve impulses to effectors (muscle or gland).

Receptor Classifications

• Exteroceptors receive external stimuli
• Proprioceptors receive stimuli from muscles
• Interceptors receive internal stimuli

Nerve Endings in Epithelium

• The receptors, categorized as exteroceptors, include:
• Free nerve endings
• Merkel endings
• Peritrichial nerve endings
• Neuroepithelium endings

Free Nerve Endings

• Mechanoreceptors for pain, temperature, touch, pressure, and itching
• Located in the epidermis of the skin and cornea of the eye.
• Myelinated nerves lose myelin below the basement membrane, passing between epithelial cells

Merkel Endings

• Mechanoreceptors for prolonged touch or pressure on the skin
• Present in the epidermis of the skin of the palm and sole
• Nerves lose their myelin sheath and form a disc-like expansion under Merkel cells near the base of the epidermis

Peritrichial Nerve Endings

• Free nerve endings incorporated in hair follicles
• They respond to touch and movement of hair, acting as mechanoreceptors.

Neuroepithelim Endings

• Taste buds in the tongue are receptors for taste
• The Organ of Corti in the ear is required for hearing
• Macula utriculi, macula sacculi, and cristae ampullaris contribute to equilibrium
• Important to the retina and the olfactory epithelium of the nose

Effectors in Glandular Epithelium

• Autonomic nerve endings supply glandular epithelium
• Found in lacrimal and salivary glands
• Unmyelinated nerve fibers form networks just outside the basal lamina
• Branches penetrate the lamina, ending between the bases of glandular cells

Nerve Endings in Connective Tissue

• Nerve endings in connective tissue are receptors
• Receptors include:
• Free nerve endings
• Meissner's corpuscle
• Krause's end bulb
• Ruffini's end organ
• Pacinian corpuscle
• Golgi tendon organ (Tendon spindle)

Free Nerve Endings in Connective Tissue

• Similar to those in the epithelium
• Present in the dermis of skin and stroma of the cornea

Meissner's Corpuscles

• Encapsulated and pear-shaped
• Found in the dermal papillae of the skin, especially sensitive as tips of fingers
• Axons enter corpuscles after losing their myelin and spiral between cell layers, ending at the upper pole
• Mechanoreceptor for touch

Krause's End Bulbs

• Encapsulated and have spherical bodies
• Located deep in the dermis of the skin
• Axons enter the corpuscle after losing myelin and branch repeatedly inside
• Mechanoreceptor for touch

Ruffini's End Organs

• Encapsulated and have fusiform bodies
• Located deep in the dermis of the skin, especially in the sole
• Axons enter the capsule after losing their myelin sheath, branching between parallel collagen fibers inside
• Mechanoreceptor responding to tension forces

Golgi Tendon Organs

• Proprioceptor nerve endings in tendons near the insertion sites of muscle fibres
• Connective tissue sheath surrounds large bundles of collagen fibres
• Fibres are continuous with collagen fibres of the tendon
• Sensory nerve penetrates and ends around collagen bundles within the capsule
• Collect information about differences in tension among tendons
• relays data to the CNS to help coordinate muscular contraction

Pacinian Corpuscles

• Encapsulated, large ovoid structures up to 1 mm in length
• Located in the dermis and hypodermis of skin, periosteum of bone, joint capsules, and connective tissue of organs like the pancreas
• Have a thin connective tissue capsule enclosing 20-60 concentric lamellae
• Lamellae comprise very thin flat cells separated by narrow spaces filled with gel-like material
• The lamellae are closely packed toward the centre
• Myelinated nerve fibres enter the corpuscle at one pole
• Schwann cell sheaths become continuous with the capsule
• Myelin sheaths end inside the corpuscle
• Naked nerve fibres run parallel to the longitudinal axis, ending in a small expansion
• Mechanoreceptor for pressure, vibration, and tension and is a type of proprioceptor.

Nerve Endings in Muscular Tissue: Receptors

• Muscle spindles are proprioceptors within skeletal muscle
• Responsible for regulating muscle tone via the stretch reflex
• Participate in the control of body posture and coordination of opposing muscles
• Sites are more numerous in fine-movement muscles as intrinsic muscles of the hand, and in anti-gravity muscles
• Lie parallel to muscle fibres
• Fusiform in shape
• Up to 6 mm long but less than 1 mm in diameter

Muscle Spindle Structure

• A capsule surrounds a lymph-filled space
• Contains intrafusal and nerve fibres
• Consists of intrafusal fibres, which are smaller than skeletal muscle fibres, and have a central non-striated area containing nuclei
• Two types:
• Nuclear bag type: central nuclear area is dilated
• Nuclear chain type: no dilation, nuclei are in the form of a chain.

Afferent Nerves

• Annulospiral endings: myelinated large nerve fibres enveloping the non-striated central portion of intrafusal muscle fibers
• Flower-spray endings: myelinated small nerve fibres located on the striated peripheral portion of intrafusal muscle fibres

Efferent Nerves

• Gamma motor nerve fibres innervate the striated portion of intrafusal muscle fibres
• Alpha motor nerve fibres innervate the extrafusal muscle fibres

Nerve Endings in Muscular Tissue: Effectors

• Motor end plate (neuromuscular junction)
  • A myelinated motor nerve branches into several terminal branches
  • A nerve loses myelin sheath and forms a dilated terminal, sitting in a surface depression
  • This depression is the sole plate
  • The basement membrane of Schwann cells fuses with the muscle fibres
  • The terminal swelling is rich in mitochondria and synaptic vesicles

Motor End Plate Details

• In the Neuromuscular junction, the space between the terminal swelling and muscle fibre is the primary synaptic cleft
• The post-synaptic membrane folds to form secondary synaptic clefts
• The sole plate of the muscle fibre contains many mitochondria and nuclei