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LECTURE NERVOUS TISSUE
NERVE TERMINATIONS
The nerve fiber traveling in peripheral nerves
terminate in peripheral structures to which or
from which they convey nerve impulses.
Accordingly the nerve terminations are classified
into two major groups:
1. Motor nerve endings
2. Sensory nerve endings
1.MOTOR NERVE ENDINGS
• The motor nerve fiber terminate in tissue in which they excite
activity by releasing neurotransmitter substances.
• At the termination of somatic efferent fiber(supplying skeletal
muscles) the transmitter released is acetylcholine.
• On the other hand at the termination of the visceral efferent
fibers (supplying smooth muscle and glandular epithelium) two
different transmitter are released
1. Norepinephrine (noradrenaline) which is released at the most of
the sympathetic nerve endings
2. Acetylcholine which is released at parasympathetic nerve
endings
A-TERMINATIONS OF SOMATIC EFFERENT NERVE FIBERS
MOTOR END PLATES(MYONEURAL JUNCTIONS):
• Every skeletal muscle fiber has a motor end plate at the site
where an axonal ending comes in close association with the
fiber.
• As the myelinated nerves fiber reach a muscle they divide into
several branches each passes to different muscle fibers,
terminating there at the motor end plate.
• Close to the termination the myelin sheath disappears and
the axon ends in a series of bulbous expansions that lie in
troughs on the surface of the muscle fiber. These troughs are
called synaptic troughs or synaptic gutters.
• Within the bulbous axonal termination are present numerous
small, round, clear synaptic vesicles containing acetylcholine.
• In the base of the synaptic gutter the sarcolemma has folds
which are called sub neural clefts or junctional folds.
Motor end plate
B-TERMINATION OF THE VISCERAL EFFERENT NERVE FIBERS
They are postganglionic unmyelinated nerve fibers .
From autonomic ganglia they terminate into following effectors:
I.
Heart muscles(cardiomotor)
II. Smooth muscle of blood vessels(vasomotor)
III. Smooth muscle of viscera(viscermotor)
IV. Smooth muscle of hair(pilomotor)
V. Glandular epithelium(secretomotor)
• Near termination the visceral efferent fibers branch repeatedly forming
complicated networks.
• Very fine fibers arise from these networks and terminate in bulbous
enlargement.
• These enlargements contains vesicles of acetylcholine in case of
postganglionic parasympathetic fibers and noradrenaline in most of the
postganglionic sympathetic fibers.
• No specialized neuromuscular junctions are formed at the termination of
visceral efferent nerve fibers.
• The neurotransmitter released at these terminations directly influence the
target cells.
2.SENSORY NERVE ENDINGS(RECEPTORS)
The sensory nerve endings respond to stimuli in the
periphery and send nerve impulses to the central
nervous system (hence they may be called beginnings).
Morphologically the sensory nerve endings can be
divided into two main types:
1. Encapsulated nerve endings
2. Non-encapsulated (free) nerve endings
1-ENCAPSULATED NERVE ENDINGS
In this group of sensory nerve endings the nerve fiber terminals
are enveloped by specialized connective tissue capsules.
Classified into following six types:
1: TACTILE CORPUSCLES OF MEISSNER:
• Meissner’s corpuscles are oval bodies covered by a connective
tissue capsule.
• Within the capsule are present multilayered stacks of
transversely placed flattened cells.
• Two or more myelinated nerve fibers supply each corpuscle
from which both myelinated and unmyelinated branches pass
upward through the stacked cells, pursuing a zigzag or spiral
course.
Location:
• Located in dermal papillae
• Found in nonhairy skin,espicially that of finger
tips, palms and soles.
Function:
It is clear from their name that the tactile
corpuscles of meissner are concerned with
perception of touch sensation.
ENCAPSULATED NERVE ENDINGS….
2.PACINIAN CORPUSCLES (CORPUSCLES OF VATER PACINI):
• Pacinian corpuscles respond to pressure and vibration.
• They are large structures 1-2mm in length and 0.5-1mm in
diameter.
• Characterized by highly developed connective tissue capsule
which is composed of concentric lamellae of epithelioid
fibroblast.
• In transverse section a pacinian corpuscles gives the
appearance of a sliced onion.
• Each pacinian corpuscle is supplied by a single myelinated
nerve fiber that looses its myelin sheath at the edge of the
corpuscle and the naked axon passes through the core of the
corpuscle to the end in bulbous swelling.
LOCATION:
• Found in subcutaneous tissue especially that
of palms, soles and digits.
• Also occur in external genitelia, periostium,
mesentery, tendons, ligament.
ENCAPSULATED NERVE ENDINGS..
3.RUFFINI’S ENDINGS:
• They are fusiform structures of 1-2mm in length.
• Each ruffini’s ending is consist of a thin connective
tissue capsule enclosing a fluid filled space.
• A single myelinated nerve fiber enters the capsular
space ,looses its myelin sheath and breaks up into a
large number of unmyelinated branches .
LOCATION:
Dermis of skin, subcutaneous tissue and in joint
capsule.
FUNCTION:
• They function as mechanoreceptors which are
activated by displacement of surrounding
connective tissue.
• According to some investigators Ruffini’s
endings are also responsible for the
perception of warmth sensation.
ENCAPSULATED NERVE ENDINGS….
4.END BULBS OF KRAUSE:
• They are spherical and consist of a thin connective tissue
capsule that surrounds a central cavity.
• A myelinated fiber enter the cavity, looses its myelin sheath
and divides into a number of branches which terminate into
club-like endings.
Location:
Mainly found in dermis of the skin.
Function:
The end bulbs are primarily responsible for the cold sensation.
ENCAPSULATED NERVE ENDINGS….
5.MUSCLE SPINDLES(NEUROMUSCULAR
SPINDLES):
• Fusiform structures ,ranging in length from 0.5-5mm.
• Each spindle consist of several striated muscle fibers
enclosed in a connective tissue capsule these fibers are
called intrafusal fibers .
• Intrafusal fibers are much smaller in diameter and
length than extrfusal fibers.
Two varieties of intrafusal fibers may be distinguished in a
neuromuscular spindle:
1. nuclear bag fibers
• They have a central bag like dilation which has a cluster
of myonuclei.
• The central dilated portion is devoid of myofibrils and
hence does not exhibit cross striations.
2. Nuclear chain fibers:
The nuclear chain fibers are thinner than the nuclear bag
fibers and exhibit only single row of myonuclei in their
central portion which is devoid of myofibrils and cross
striations.
ENCAPSULATED NERVE ENDINGS….
• A muscle spindle is supplied by both afferent (sensory) and
efferent (motor) nerve fibers that enter the spindle by
piercing the capsule.
• Each spindle receives a single large myelinated afferent fiber.
• This myelinated nerve fiber then looses its myelin sheath
inside the capsule and divides into several branches that end
in series of spirals which encircle the central portion of each
nuclear bag and nuclear chain fiber.These endings are called
annulospiral nerve endings.
ENCAPSULATED NERVE ENDINGS….
• ENCAPSULATED NERVE ENDINGS….
• Many spindles also receive one or more smaller
afferent fibers.
• These fibers lose their myelin as they branch within
the spindle capsule and terminate on nuclear chain
fibers in small, multiple endings which are known as
flower spray nerve endings.
• The motor nerve terminate at small motor end plates
on striated portion of the intrafusal fibers .
ENCAPSULATED NERVE ENDINGS….
5.MUSCLE SPINDLES(NEUROMUSCULAR SPINDLES)………..
Location:
Found in all striated muscles, often near a
tendon.
Function:
They are concerned with regulation of reflex
muscle tone.
• ENCAPSULATED NERVE ENDINGS….
ENCAPSULATED NERVE ENDINGS….
6.Tendon organs of Golgi
(neurotendinous organs):
• Consist of small bundles of collagenous fibers
enclosed in a connective tissue capsule.
• A large myelinated nerve fiber enter the organ
usually at its middle and divides into numerous
smaller unmyelinated branches which form an
extensive network around the collagenous
bundles of tendon organ.
Location:
Located in tendons usually close to the muscletendon junctions.
Function:
• They respond to an increase in the muscle tension.
• A heightened activity of these receptors exert an
inhibitory effect, through interneurons of the central
nervous system, upon the alpha motor neuron of the
same muscle.
• Thus this reflex provides a negative feedback
mechanism that prevents the development of too
much tension on muscle.
2.NON-ENCAPSULATED (FREE) NERVE ENDINGS
• The most common type of sensory nerve endings in
the body.
• Here the sensory nerve fibers are either
unmyelinated or finely myelinated in type.
• The nerve fibers loose their covering before
terminating and the naked axons divide into several
branches which end blindly between the epithelial
cells.
• Different fibers are functionally specialized to
respond to the sensation of pain, temperature or
light touch.
2.NON-ENCAPSULATED (FREE) NERVE ENDINGS
• The stratum basal of epidermis has some modified
epidermal cells called Merkel cells.
• A Merkel cell and its associated nerve ending is
collectively called as Merkel capsule.
• The Merkel capsule function as mechano-receptor,
detecting the mechanical displacement of the skin.
• 2.NON-ENCAPSULATED (FREE) NERVE ENDINGS