Leona Melodia T. Matheus, MD, FPCS,FPSGS

1. To know the different sensory and motor nerve endings.

2. Describe the different sensory nerve endings for ordinary sensations found in the skin, skeletal muscles, tendons and joints.

3. Differentiate the motor nerve endings for skeletal, smooth, and cardiac muscles.

Sensory Nerve Endings (Receptors)

Sensory receptors are nerve endings which convert stimuli from external or internal environments into afferent nerve impulsesAfferent impulses pass into the CNS and

initiate voluntary or involuntary responses

Sensory ReceptorsFunctional classification:

A. Exteroceptors – respond to stimuli from outside the body = receptors for: touch light pressure and deep pressure

cutaneous pain temperature smell, taste, sight, hearing

Sensory ReceptorsFunctional classification:

B. Proprioceptors – located within the skeletal system = provide conscious and unconscious information about orientation, skeletal position, tension and movement

= vestibular apparatus of the ear tendon organs neuromuscular spindles

Sensory ReceptorsFunctional classification:

C. Interoceptors – respond to stimuli from the viscera = chemoreceptors of blood vascular (pressure) baroreceptors receptors for the state of distention of hollow viscera (GIT, urinary bladder) receptors for nebulous senses as visceral pain, hunger, thirst, well-being, and malaise.

Sensory ReceptorsStructural classification: (poorly

understood)

A. Simple receptors (Non-encapsulated) – free, branched or unbranched nerve endings = responsible for cutaneous pain and temperature = rarely visible w/ light microscopy

- need special staining methods

Sensory ReceptorsStructural classification: (poorly

understood)

B. Compound receptors (Encapsulated) – involve non-neural tissues complementing neural receptors = Degree of organization: encapsulated to highly sophisticated arrangements as in the eye and ear

* Eye and ear receptors and receptors for smell and taste are described as Organs of Special Sense

Sensory ReceptorsNon-encapsulated Nerve endings:

1. Free Nerve endings = Receptor for pain

= Location: found in the skin, cornea, alimentary tract and most connective tissues

= Description: afferent nerve fibers from free nerve endings are either myelinated or unmyelinated

- Before traversing the basement membrane of an epithelium, it becomes unmyelinated

- Once it passes through the lamina, it breaks into branches that end in knob-like expansions (terminal bouton)

Sensory ReceptorsNon-encapsulated Nerve endings:

2. Hair Follicle Nerve endings = Receptor for touch

= Location: wind around the follicle in its outer connective tissue sheath below the sebaceous gland- Some branches run parallel along its long axis- Many naked axon filaments terminate among cells of the outer root sheath

= Description: similar to free nerve endings

Sensory ReceptorsNon-encapsulated Nerve endings:

3. Merkel”s Tactile Discs = Receptor for touch

= Location: found in the epidermis of skin and in other stratified epithelia and borders of the tongue

= Description: Nerve fiber pass into the epidermis and terminates as a disc-shaped expansion

- Disc-shaped expansion is applied closely to a dark-staining epithelial cell (Merkel cell) in deeper part of the epidermis

-In hairy skin, clusters of Merkel’s disc (tactile domes) are found in the epidermis between hair follicles

Sensory ReceptorsEncapsulated Nerve endings:

1. Vater-Pacinian corpuscle = Receptor for deep pressure or coarse touch, vibration and tension = Location: deeper layers of the skin, ligaments and joint capsules, mesenteries, and in some serous membranes, viscera and erogenous areas

= Description: 1-4 mm long; In section, appear like an onion

- Consist of a delicate capsule enclosing concentric lamellae of flattened cells (modified Schwann cells), separated by interstitial fluid spaces and collagen fibers- At the center, lamellae are closely packed

- As nerve fiber enters the capsule, it becomes a single large unbranched non-myelinated fiber w/ several club-like terminals (neuropodia)

Sensory ReceptorsEncapsulated Nerve endings:

2. Golgi-Mazzoni corpuscles = Receptor for touch

= Location: similar to Pacinian corpuscle

= Description: resemble Pacinian corpuscles except they are smaller and their capsule are thinner

- Has an abundant granular core of protoplasm

- As the nerve fiber enters the capsule, it becomes unmyelinated, giving off several branches that end in neuropodia

Sensory ReceptorsEncapsulated Nerve endings:

3. Meissner’s corpuscles = Receptor for light discriminatory touch

= Location: dermal papillae of the skin of the palms and soles and tips of fingers and toes, nipples, eyelids, lips and genitalia

= Description: small and oval

- Consists of a delicate capsule surrounding a mass of oval, plump cells arranged transversely (specialised Schwann cells)

- Nonmyelinated branches of large myelinated sensory fibers ramify throughout the cell mass in a helical manner

Sensory ReceptorsEncapsulated Nerve endings:

4. Ruffini’s corpuscles = Receptor for hot sensation

= Location: deep in the skin or even subcutaneous tissue of the fingertips and toes

= Description: robust, spindle-shaped structures

- Has a thin capsule w/ a crumpled appearance

-There is a core of

granular cytoplasm, connective tissue fibers and fluid space

- As nerve fiber enters the capsule, it becomes unmyelinated and branches off ending into neuropodia

Sensory ReceptorsEncapsulated Nerve endings:

5. Krause end bulb = Receptor for cold sensation

= Location: lining of oropharynx and conjunctiva of the eye, external genitalia, tendons and ligaments

= Description: spherical or oval- Capsule is lamellated containing

flattened connective tissue cells and fibers surr0unding a central cavity = inner bulb

- Within the inner bulb, unmyelinated fibers give off a number of branches which twist and interlace forming a spherical mass = glomerulus

- Simplest end bulbs are found in the connective tissue of external genitalia = genital corpuscles

Sensory ReceptorsEncapsulated Nerve endings:

6. Neuromuscular spindles (Muscle spindles) = Receptor for stretch reflex

(proprioception)

= Location: lie parallel to muscle fibers, embedded in endomysium or perimysium

= Description: lymph-filled, fusiform structures up to 6 mm long but less than 1 mm in diameter

- Each spindle contains 2-10 modified skeletal muscle fibers = Intrafusal fibers

- Intrafusal fibers are smaller than skeletal muscle fibers proper (Extrafusal fibers)

- Intrafusal fibers have a central non-striated area where the nuclei are concentrated

- 2 types of Intrafusal fibers:

1. Nuclear Bag fibers – central nuclear area is dilated

2. Nuclear Chain fibers –

no dilatation and nuclei are

arranged in a single row

- 2 types of sensory receptors:

1. Annulospiral endings – branched nonmyelinated endings of large, myelinated sensory fibers wrap around the central non-striated area of the intrafusal fiber

2. Flower-spray endings – smaller, myelinated nerve fibers located on the striated portions of the intrafusal fibers

Sensory ReceptorsEncapsulated Nerve endings:

7. Neurotendinous organ of Golgi (Tendon spindles)

= Receptor for stretch (proprioception)

= Location: found in the junctions of muscles and tendons

= Description: resembles muscle spindles except that collagenous tendon fibers form intrafusal tendon fibers

- 2 Forms:1. Simple – naked nerve

fibers and their branches spread on the surface of collagen bundles; may give rise to pain sensations

2. Composite - tendon spindles consist of several tendon fascicles surrounded by a capsule supplied by myelinated nerve fibers

Motor Nerve Endings in Skeletal Muscle

3 Types of Motor fibers: 1. Large alpha myelinated fibers - supply

extrafusal fibers that form the main mass of the muscle

2. Small gamma myelinated fibers - supply the intrafusal fibers of the neuromuscular spindles

3. Fine, unmyelinated C fibers are

postganglionic autonomic efferents that supply the smooth muscle in the walls of blood vessels

Motor UnitDefined as the single alpha motor neuron

and the muscle fibers that it innervates

As each alpha myelinated nerve fiber enters a skeletal muscle, it branches many times

A single branch terminates on a muscle fiber at a site called Neuromuscular junction or Motor end-plate

On reaching the muscle fiber, the nerve fiber loses its myelin sheath and breaks up into a number of branches

Neuromuscular Junction or Motor End-Plate

Each branch ends as a naked axon and forms the neural element of the motor end-plateAxon slightly expanded: contains vesicles and

mitochondria

At the site of the motor end-plate, the surface of the muscle fiber is elevated slightly to form the muscular element of the plate (also known as sole plate)Sole plate contains numerous nuclei and

mitochondria

Nonmyelinated postganglionic autonomic nerves terminate as a series of branches

At the site of transmission, the Schwann cell is retracted; part of the axon thus is naked, permitting free diffusion of the transmitter from axon to muscle cell

Motor Nerve Endings in Cardiac Muscle

Nonmyelinated postganglionic autonomic nerves extend into the connective tissue between muscle fibers and terminate in close proximity to individual cardiac muscle fibers.

At the site of transmission, axon becomes naked because of retraction of the Schwann cell, permitting free diffusion of neurotransmitter from axon to muscle fiber.

(+) of desmosomes and gap junctions causes excitation and contraction of one muscle fiber to rapidly spread from fiber to fiber.