THE NERVOUS

SYSTEM: NEURAL

TISSUE

• Nervous system

– Swift, brief responses to stimuli

• Endocrine system

– Adjusts metabolic operations

– Directs long-term changes

Two organ systems coordinate and

direct activities of body

Nervous system includes all neural

tissue in body

• Central Nervous System

– Brain and spinal cord

• Peripheral Nervous System

– All neural tissue outside CNS

Histology of Neural Tissue

• Neurons

Cells in Nervous Tissue

• Neuroglia

• about half the volume of cells in the CNS

• smaller than neurons

• 5 to 50 times more numerous

• do NOT generate electrical impulses

• divide by mitosis

• Four types in the CNS

– Astrocytes

– Oligodendrocytes

– Microglia

– Ependymal cells

Neuroglia (Glia)

Neuroglia (Neuroglial Cells)

Central Neuroglia

Astrocyte protoplasmic astrocyte

fibrous astrocyte

Oligodendrocyte perineuronal satellite cell

interfascicular cell

Microglia

Ependymal Cell

Peripheral Neuroglia

Schwann Cell

in peripheral nerve

and ganglion

Capsular (Satellite) Cell

in ganglion

• Largest of glial cells

• Most numerous

• Star shaped with many processes

projecting from the cell body

• Help form and maintain blood-brain barrier

• Provide structural support for neurons

• Maintain the appropriate chemical

environment for generation of nerve impulses/action potentials

• Regulate nutrient concentrations for neuron survival

• Regulate ion concentrations - generation of action potentials by neurons

• Take up excess neurotransmitters

• Assist in neuronal migration during brain development

• Perform repairs to stabilize tissue

Astrocytes

Oligodendrocytes

• Most common glial cell

type

• Each forms myelin sheath

around the axons of

neurons in CNS

• Analogous to Schwann

cells of PNS

• Form a supportive

network around CNS

neurons

• fewer processes than astrocytes

• round or oval cell body

Microglia

• Small cells found near blood vessels

• Phagocytic role - clear away dead cells

• protect CNS from disease through phagocytosis of microbes

• migrate to areas of injury where they clear away debris of

injured cells - may also kill healthy cells

• few processes

• derived from mesodermal cells

that also give rise to monocytes

and macrophages

Ependymal Cells

• Form epithelial membrane lining cerebral cavities (ventricles) & central canal

- that contain CSF

• Produce & circulate the cerebrospinal fluid (CSF) found in these chambers

• CSF = colourless liquid that protects the brain and SC against

chemical & physical injuries, carries oxygen, glucose and other necessary

chemicals from the blood to neurons and neuroglia

• epithelial cells arranged in a

single layer

• range in shape from cuboidal

to columnar

• Flat cells surrounding PNS axons

• Support neurons in the PNS

PNS: Satellite Cells

PNS: Schwann Cells

• each cell surrounds multiple unmyelinated PNS axons with a single

layer of its plasma membrane

• Each cell produces part of the myelin sheath surrounding an axon in

the PNS

• contributes regeneration of PNS axons

Neurons

•have the property of electrical excitability - ability to produce

action potentials or impulses in response to stimuli

•what is the main defining characteristic of neurons?

Representative Neuron

1. cell body or soma -single nucleus with prominent nucleolus

-Nissl bodies

-rough ER & free ribosomes for protein

synthesis

-proteins then replace neuronal cellular

components for growth

and repair of damaged axons in the PNS

-neurofilaments or neurofibrils

give cell shape and support -

bundles of

intermediate filaments

-microtubules move material

inside cell

-lipofuscin pigment clumps

(harmless aging) - yellowish

brown

2. Cell processes =

dendrites (little trees)

- the receiving or input

portion of the neuron

-short, tapering and

highly branched

-surfaces specialized

for contact with other

neurons

-cytoplasm contains

Nissl bodies &

mitochondria

Neurons

3. Cell processes = axons

• Conduct impulses away from cell body-

propagates nerve impulses to another neuron

• Long, thin cylindrical process of cell

• contains mitochondria, microtubules &

neurofibrils - NO ER/NO protein synth.

• joins the soma at a cone-shaped elevation =

axon hillock

• first part of the axon = initial segment

• most impulses arise at the junction of the

axon hillock and initial segment = trigger

zone

• cytoplasm = axoplasm

• plasma membrane = axolemma

• Side branches = collaterals arise from the

axon

• axon and collaterals end in fine processes

called axon terminals

• Swollen tips called synaptic end bulbs

contain vesicles filled with neurotransmitters

Functional Classification of Neurons

• Sensory (afferent) neurons

– transport sensory information from skin, muscles,

joints, sense organs & viscera to CNS

• Motor (efferent) neurons

– send motor nerve impulses to muscles & glands

• Interneurons (association) neurons

– connect sensory to motor neurons

– 90% of neurons in the body

• Afferent division of PNS

• Deliver sensory information from sensory receptors to CNS

– free nerve endings: bare dendrites associated with pain, itching, tickling,

heat and some touch sensations

– Exteroceptors: located near or at body surface, provide information about

external environment

– Proprioceptors: located in inner ear, joints, tendons and muscles, provide

information about body position, muscle length and tension,

position of joints

– Interoceptors: located in blood vessels, visceral organs and NS

-provide information about internal environment

-most impulses are not perceived – those that are,

are interpreted as pain or pressure

Sensory Neurons

Sensory Neurons

• Sensory receptors cont…

– mechanoreceptors: detect pressure, provide sensations of touch, pressure,

vibration, proprioception, blood vessel stretch, hearing and equilibrium

– thermoreceptors: detect changes in temperature

– nociceptors: respond to stimuli resulting from damage (pain)

– photoreceptors: light

– osmoreceptors: detect changes in OP in body fluids

– chemoreceptors: detect chemicals in mouth (taste), nose (smell)

and body fluids

-analgesia: relief from pain

-drugs: aspirin, ibuprofen – block formation of prostaglandins that

stimulate the nociceptors

-novocaine – block nerve impulses along pain nerves

-morphine, opium & derivatives (codeine) – pain is felt but not perceived in

brain (blocks morphine and opiate receptors in pain centers)

• Efferent pathways

• Stimulate peripheral structures

– Somatic motor neurons

• Innervate skeletal muscle

– Visceral motor neurons

• Innervate all other peripheral effectors

• Preganglionic and postganglionic neurons

Motor Neurons

-both divisions use two neurons and one ganglion

-first neuron has its cell body within the CNS

(Pre-ganglionic)

-second neuron has its cell body within the

ganglion (Post-ganglionic)

-sympathetic division:

-preganglionic fibers arise from middle

of the cord and are very short

-long post-ganglionic neurons

-flight or fight response

-NT = norepinephrine

-parasympathetic division:

-cranial nerves: vagus

-preganglionic fibers arise from bottom

of the cord and are very long

-short post-ganglionic neurons

-housekeeper division - relaxed state

e.g. pupil dilation, food digestion, slows

heartbeat

- NT = acetylcholine

Motor Units • Each skeletal fiber has only ONE

NMJ

• MU = Somatic neuron + all the

skeletal muscle fibers it innervates

• Number and size indicate precision of

muscle control

• Muscle twitch

– Single momentary contraction

– Response to a single stimulus

• All-or-none theory

– Either contracts completely or not at

all

• Muscle fibers of different motor units are intermingled so that net distribution of force

applied to the tendon remains constant even when individual muscle groups cycle

between contraction and relaxation.

• Motor units in a whole muscle fire asynchronously

some fibers are active others are relaxed

delays muscle fatigue so contraction can be sustained

Structural Classification of Neurons

• Based on number of processes found on cell body

– multipolar = several dendrites & one axon

• most common cell type in the brain and SC

– bipolar neurons = one main dendrite & one axon

• found in retina, inner ear & olfactory

– unipolar neurons = one process only, sensory only (touch, stretch)

• develops from a bipolar neuron in the embryo - axon and dendrite fuse and

then branch into 2 branches near the soma - both have the structure of axons

(propagate APs) - the axon that projects toward the periphery = dendrites

• Named for histologist that first described them or

their appearance

Structural Classification of Neurons

•Purkinje = cerebellum

•Renshaw = spinal cord

• others are named for shapes

e.g. pyramidal cells

Classification of neurons by cell size

• 1. golgi type I :

– Neurons have a long axon and large soma

• 2. Golgy type II :

– Neurons have short axon undergoes extensive

terminal aeborization and small soma

The Nerve Impulse

Continuous versus Saltatory Conduction

• Continuous conduction

(unmyelinated fibers)

– An action potential spreads

(propagates) over the surface of

the axolemma

– as Na+ flows into the cell

during depolarization, the

voltage of adjacent areas is

effected and their voltage-gated

Na+ channels open

– step-by-step depolarization of

each portion of the length of

the axolemma

Saltatory Conduction

• Saltatory conduction

-depolarization only at nodes of

Ranvier - areas along the axon

that are unmyelinated and

where there is a high density of

voltage-gated ion channels

-current carried by ions flows

through extracellular fluid from

node to node

• Properties of axon

• Presence or absence of myelin sheath

• Diameter of axon

Rate of Impulse Conduction

Synaptic Communication

• Synapse

– Site of intercellular communication between 2 neurons or between a neuron and an effector (e.g. muscle)

• Originates in the soma

• Travels along axons

• Permit communication between neurons and other cells

– Initiating neuron = presynaptic neuron

– Receiving neuron = postsynaptic neuron

• Most are axodendritic axon -> dendrite

• Some are axoaxonic – axon > axon

Synapse

Tipes of synapses

• Axodendritic:

– Between an axon and a dendrite

• Axosomatic:

– Between an axon and a soma

• Axoaxonic:

– Between two axon

• Dendrodendritic:

– between two dendrites

Synaptic morphology

• Presynaptic membrane:

– Contains metochondria, a few elements of SER,

and an abundance of synaptic vesicles.

• Synaptic cleft

• Postsynaptic membrane:

– Contains neorotransmitter receptors

SYNAPSE

Impuls transmission at synapse can

occur:

• Electrically

• Chemically

• Chemical

– Membranes of pre and postsynaptic neurons do not touch

– Synaptic cleft exists between the 2 neurons – 20 to 50 nm

– the electrical impulse cannot travel across the cleft – indirect method is required – chemical messengers (neurotransmitters)

– Most common type of synapse

– The neurotransmitter induces a postsynaptic potential in the PS neuron – type of AP

– Communication in one direction only

– Is the conversion of an electrical signal (presynaptic) into a chemical signal back into an electrical signal (postsynaptic)

• 1. nerve impulse arrives at presynaptic end bulbs

• 2. fusion of synaptic vesicles to PM and release of NTs

• 3. binding of NT to receptors on postsynaptic neuron

• 4. opening of channels in PM of postsynaptic neuron (e.g. sodium)

• 5. postsynaptic potential develops – depolarization

• 6. triggering of AP in postsynaptic neuron

Synapses

• Electrical

– Direct physical contact between cells required

– Conducted through gap junctions that permit

free movement of ions from one cell to another

– Two advantages over chemical synapses

• 1. faster communication

• 2. synchronization between neurons or

muscle fibers

– E.g. heart beat, brain stem, retina, and

cerebral cortex

Synapses

• axon terminal swell to form synaptic end bulbs or form swollen bumps called varicosities

• release of neurotransmitters from synaptic vesicles

– multiple types of NTs can be found in one neuron type

Synapse

Neurotransmitters

• Are signaling molecules that are released at

the presynaptic membranes and activate

receptors on postsynaptic membranes.

• More than 100 identified

• Some bind receptors and cause channels to open

• Others bind receptors and result in a second

messenger system

• Results in either excitation or inhibition of the

target

• Represented by three groups:

– Small molecules transmitters

– Neoropeptides

– Gases

Neurotransmitters

Neorotransmitters

Small molecules

1. Acetylcholine (ACh)

• -All neuromuscular junctions use ACh

• -ACh also released at chemical synapses in the PNS and by

some CNS neurons

• -Can be excitatory at some synapses and inhibitory at others

• -Inactivated by an enzyme acetylcholinesterase

• -Blockage of the ACh receptors by antibodies = myasthenia

gravis - autoimmune disease that destroys these receptors and

progressively destroys the NMJ

– -Anticholinesterase drugs (inhibitors of acetylcholinesterase)

prevent the breakdown of ACh and raise the level that can

activate the still present receptors

2. Amino acids: glutamate & aspartate & GABA

– Powerful excitatory effects

– Stimulate most excitatory neurons in the CNS (about ½ the

neurons in the brain)

– Binding of glutamate to receptors opens calcium channels = EPSP

– GABA (gamma amino-butyric acid) is an inhibitory

neurotransmitter for 1/3 of all brain synapses

Neurotransmitters

Valium is a GABA agonist - enhancing its inhibitory effect

Neurotransmitters 3. Biogenic amines: modified amino acids

– catecholamines: norepinephrine (NE), epinephrine, dopamine (tyrosine)

– serotonin - concentrated in neurons found in the brain region = raphe nucleus

• derived from tryptophan

• sensory perception, temperature regulation, mood control, appetite, sleep induction

• feeling of well being

– NE - role in arousal, awakening, deep sleep, regulating mood

– epinephrine (adrenaline) - flight or fight response

– dopamine - emotional responses and pleasure, decreases skeletal muscle tone

Other types:

a. ATP - released with NE from some neurons

b. Nitric oxide - formed on demand in the neuron then release (brief lifespan)

-role in memory and learning

-produces vasodilation - Viagra enhances the effect of NO

Neuropeptides • widespread in both CNS and PNS

• excitatory and inhibitory

• act as hormones elsewhere in the body

-Substance P -- enhances our perception of pain

-opoid peptides: endorphins - release during stress, exercise

enkephalins - analgesics

(200x stronger than morphine)

-pain-relieving effect by blocking the release of

substance P

dynorphins - regulates pain and emotions -Hypothalamic-releasing hormones (thyrotropin-releasing hormone and

somatostatin)

-Hormones stored in and teleased from the neurohypophysis (ADH and

oxytocin)

**acupuncture may produce loss of pain sensation because of

release of opioid-like substances such as endorphins or

dynorphins

Neurotransmitters

• Gases

– May act as neuromodulators. The ones that do

are nitric oxide (NO) and carbon monoxide

(CO).

Removal of Neurotransmitter

• Diffusion

– move down concentration gradient

• Enzymatic degradation

– acetylcholinesterase

• Uptake by neurons or glia cells

– neurotransmitter transporters

• NE, epinephrine, dopamine, serotonin

Peripheral Nerve

Nerve Fiber

Myelinated Nerve Fiber

Axon, Myelin sheath, Schwann cell

Unmyelinated Nerve Fiber

Axon, Schwann cell

Connective Tissue Sheath

Endoneurium

Perineurium – blood vessels

Epineurium

Composition of Peripheral Nerve

Connective tissue investment

• Connective tissue investments of peripheral

nerves include the:

– Epineurium

– Perineurium

– Endoneurium

Epineurium

• Is the outermost layer

• Is composed of dense irregular, collagenous

connective tissue containing thick elastic

fibers that completely ensheathe the nerve.

Collagen fibers within the sheath are

aligned and oriented to prevent damage by

overstretching of the nerve bundle.

Perineurium

• The middle layer of connective tissue

investments, covers each bundle of nerve

fibers (fascicle) within the nerve.

• Composition:

– Dense connective tissue but is thinner

than epineurium.

Endoneurium

• The innermost layer connective tissue

investment of a nerve, surrounds individual

nerve fibers (axons).

• Is a loose connective tissue composed of a

thin layer of reticular fibers (produced by

Schwann cells), scattered fibroblasts,

macrophages, and mast cells.

• The endoneurium is in contact with the

basal lamina of the Schwann cells.

Somatic motor and autonomic

nervous systems

• Functionally, the motor component is divided into

the somatic and autonomic nervous systems

• The somatic nerves systems provides motor

impulses to the skeletal muscles

• The autonomic nerves systems provides motor

impulses to the smooth muscles of the viscera,

cardiac muscle and secretory cells of the exocrine

and endocrine glands.

Motor component of the somatic

nervous system

• Motor innervation to skeletal muscle is

provided by somatic nerves from spinal and

selected cranial nerves.

• The cell bodies of these nerve fibers

originate in the CNS

Autonomic nervous system = ANS

(involuntary , visceral)

• Is generally defined as a motor system.

• Controls the viscera of the body by supplying the general visceral efferent (visceral motor) component to smooth muscle, cardiac muscle, and glands.

• The autonomic nervous system possesses two neurons between the CNS and the effector organ.

• Cell bodies of the first neuron lie in the CNS and their axons are usually myelinated.

• These preganglionic fibers (axons) seek an autonomic ganglion located outside the CNS, where they synapse on multipolar cell bodies of postganglionic neurons.

• Postganglionic fibers usually unmyelinated although they always are enveloped by Schwnn cells, exit the ganglion to terminate on the effector organ.

• The ANS is subdivided into two

functionally deferent divisions:

– The sympathetic nervous system

– The parasympathetic nervous system

Ganglia

• Are aggregations of cell bodies of neurons

located outside the CNS, there are two types

of ganglia:

– Sensory

– autonomic

Sensory ganglia

• Sensory ganglia house cells bodies of

sensory neurons.

• Cell of the sensory ganglia are

pseudounipolar which enveloped by

cuboidal capsule cells. These capsule cells

are surrounded by connective tissue capsule

composed of satellite cells and collagen.

Autonomic ganglia

• Autonomic ganglia house cells bodies of

postganglionic autonomic nerves.

• Nerve cells bodies of autonomic ganglia are

motor in function.

Central nervous system

• The CNS, composed of the brain and the

spinal cord, consist of white matter and gray

matter without intervening connective tissue

elements ; therefore, the CNS has the

consistency of a semifirm gel.

Continued

• White matter is composed mostly of myelineted fibers a long with some unmyelineted fibers and neoroglial cells.

• Gray matter is consist of aggregation of neuronal cells bodies, dendrites, and unmyelineted portion of axons as well as neuroglial cells.

• Gray matter in the brain is located at the periphery (cortex) of the cerebrum and cerebellum. Whereas the white matter lies deep to the cortex and surrounds the basal ganglia.

continued

• Spinal cord:

– White matter is located in the periphery,

whereas grey matter lies deep in the spinal

cord, where it forms an H shape in cross

section.

– Central canal lined by ependymal cells.

Meninges

• Are three connective tissue covering the

brain and spinal cord.

• Meninges consist of:

– Dura mater : the outermost layer

– Arachnoid : the intermediate layer

– Pia mater : the innermost layer

Dura mater • The dura mater is the dense outermost layer

of the meninges.

• Cerebral dura:

– Is a dense, collagenous CT composed of two

layers that are closely apposed in the adult.

– 1. Periosteal dura mater, the outer layer, is

composed of osteoprogenitor cells, fibroblast

and collagen fibers. Periousteal dura mater

serves as the periosteum of the inner surface of

the skull, and as such it is well vascularized.

continued

2. Meningeal dura :

– Inner layer of the dura is composed of fibroplast and

collagen fibers.

– This layer contains small blood vessels

– Internally meningeal dura covered by a layer of cells

called border cell layer, is composed of fibroblast.

Spinal dura mater

Does not adhere to the walls of the vertebral canal.

The epidural space : the space between the dura and the

bony walls of the vertebral canal, is filled with epidural

fat and a venous plexus.

Dura mater

Strongest

2 layers :

- Periosteal

- Meningeal

Layers fuse

except at dural

sinuses

Dura mater

Layers fused except at sinuses

Forms :

- Falx cerebri

- Falx cerebelli

- Tentorium cerebelli

Arachnoid mater

• Spaces

– Subdural

• Between dura and arachnoid

• Little CSF

– Subarachnoid

• between arachnoid and pia

• CSF and blood vessels

Arachnoid

• Is the intermediate layer of the meninges.

• Is avascular although blood vessels course

through it.

• It consist of fibroblast, collagen, and some

elastic fibers.

• Subdural space located between dura and

arachnoid, is a potential space because it

appears only after injury resulting subdural

hemorrhage

Arachnoid mater

* Arachnoid Villi

Projections through dura

Pass into superior sagittal

sinus

Passage of CSF

* Web-like attachments to pia

continued

• In certain regions the arachnoid extend

through the dura to form arachnoid villi,

which protrude into the dural venous

sinuses. The function of the arachnoid villi

is transporting CSF from the subarachnoid

spaces into the venous system.

Pia mater

• Is the innermost highly vascular layer of the

meninges, is in close contact with the brain,

following closely all of its contours.

• The pia mater does not contact with the

neural tissue because a thin layer of

neuroglial processes is always interposed

between them.

continued

• Composition : a thin layer of flatened, modified fibroblast.

• Blood vessels, abundant in this layer, are surrounded by pia cells interspersed with macrophage, mast cells, and lymphocytes.

• The pia mater is completely separated from the underlying neural tissue by neuroglial cells.

• Blood vessels penetrate the neural tissue and are covered by pia mater until they from the continuous capillaries characteristic of the CNS.

• Pedicels of the astrocytes, cover capillaries within the neural tissue.

Pia mater

* Delicate

* Vascular

* Clings to surface of brain

Blood-brain barrier

• Endothelial cells of CNS capillaries prevent

the free passage of selective blood-borne

substances into the neural tissue.

• This barrier is established by the endothelial

cells lining the continuous capillaries that

course through the CNS.

• These endothelial cells form zonula

occludentes with one another, retarding the

flow of material between cells.

continued

• These endothelial cells have relatively few

pinocytotic vesicles and vesicular traffic is

almost completely restricted to receptor

mediated transport.

Choroid plexus

• Is composed of folds of pia mater within the

ventricles of the brain, produces CSF.

• Are formed by folds of pia mater countain

abundant of fenestrated capillaries and

invested by the simple cuboidal

(ependymal) lining extend into the third,

fourth, and lateral ventricles of the brain.

• Are produced CSF.

Cerebrospinal fluid

• Cerebrospinal fluid bathes, nourishes, and

protects the brain and spinal cord.

• Is produces by the choroid plexus.

Cerebral cortex

• Is responsible for learning, memory,

sensory integration, information analysis,

and initiation of motor responses.

• Is divided into six layers as follows:

1. Molecular layer : contains horizontal cells and

neuroglia

2. External granular layer : contains mostly

granule(stellate) cells and neuroglial cells

continued

3. External pyramidal layer : contains

pyramidal cells and neuroglial cells.

4. Internal granual layer contains small

granule cells (stelate cells), pyramidal

cells, and neuroglia.

5. Internal pyramidal layer contains larges

pyramidal cells and neuroglia

6. Multiform layer consist of various shapes

(Martinotti cells), and neuroglia.

Cerebellar cortex • Is responsible for balance, equilibrium,

muscle tone, and muscle coordination.

• Is divided into three layers:

1. Molecular layer, lies directly below the pia mater.

2. Purkinje cell layer, contains the large, flask-shaped Purkinje cells, which are present only in the cerebellum.

3. Granular layer, consist of small cells and glomeruli (cerebellar islands).

Neural Regeneration

Nerve regeneration

• Nerve cells, unlike neuroglial cells, cannot

proliferate but can regenerate their axons,

located in the PNS.

• When a traumatic event destroy neurons,

they are not replaced because neurons

cannot proliferate ; therefore the damage to

the CNS is permanent.

continued

• However, if a peripheral nerve fiber is

injured or transected, the neurons attempts

to repair the damage, regenerate the

process, and restore function by initiating a

series of structural and metabolic events,

collectively called the axon reaction.

Axon reaction

• The reactions to the trauma are

characteristically localized in three regions

of the neurons:

1. Local changes: at the site of damage.

2. Anterograde changes: distal to the site of

damage

3. Retrograde changes: proximal to the site of

damage.

Local reaction • Local reaction to injury involves repair and

removal of debris by neuroglial cells.

• The severed ends of the axon retract away from

each other, and the cut membrane of each stump

fuses to cover the open end, preventing loss of

axoplasm.

• Macrophages and fibroblast infiltrate the damaged

area, secrete cytokines and growth factors, and up-

regulate the expression of receptors.

• Macrophages invade the basal lamina and assisted

by Schwann cells, phagocytose the debris.

Anterograde reaction

• In the anterograde reaction process, that portion of the axon distal to an injury undergoes degeneration and is phagocytosed

• The axon undergoes anterograde changes as follows:

1. The axon terminal becomes hypertrophied and degeneretes within a week. Schwwan cells prolivered and phagocitose the remnants of the axon terminal, and the newly formed Schwann cells occupy the synaptic space.

Continued

– 2. The distal portion of the axon undergoes

wallerian degeneration, distal to the lesion, the

axon and the myelin disintegrate, Schwann

cells dedifferentiate and myelin synthesis is

discontinued. Macrophages and Schwann cells

phagocytose the disintegrated remnants

– 3. Schwann cells proliferate, forming a column

of Schwann cells ( Schwann tubes ) enclosed

by the original basal lamina of the

endoneurium.

Retrograde reaction and regeneration

• In these process, the proximal portion of the

injured axon undergoes degeneration followed by

sprouting of a new axon whose growth is directed

by Schwann cells.

• The portion of the axon proximal to the damage

undergoes the following changes :

– 1. the perikaryon of the damaged neuron becomes

hypertrophied, its Nissl bodies disperse, and its nucleus

is displaced ( these events called chromatolysis). The

soma is actively producing free ribosomes and

synthesizing proteins and various macromolecule.

continued

– 2. Several “sprouts” of axon emerge from the

proximal axon stump, enter the endoneurium,

and are guided by the Schwann cells to their

target cell. For regeneration to occur, the

Schwann cells, macrophages, and fibroblasts as

well as the basal lamina must be present. These

cells manufacture growth factors and cytokines

and up-regulate the expression for the seceptors

of these signaling molecules.

continued

– 3. the sprout is guided by the Schwann cells

that redifferentiate and either begin to

manufacture myelin around the growing axon

or, in nonmyelinated axons, form a Schwann

cell sheath. The sprout that reaches the target

cell first form a synapse, whereas the other

sprout degenerate.

• Limited ability in PNS

• Severed peripheral nerve successfully regenerates a

fraction of the axons

– Function is permanently impaired

– Schwann cells participate

• Wallerian degeneration

– Loss of axon distal to damage

Regeneration

• More complicated than PNS regeneration

• Far more limited

• More axons involved

• Astrocytes produce scar tissue preventing axonal regrowth

• Astrocytes release chemicals blocking regrowth

Regeneration in CNS

Nerve ending – nerve terminal

• Two structural type :

– 1. Motor ending ( terminal of axon )

• Transmit impulses from the CNS to skeletal &

smooth muscle & to glands ( secretory ending)

– 2. Csensory ending = sensory receptor =

terminal of dendrites :

• Perceive various stimuli and transmit this input to

the CNS

continued

• These sensory receptor are classified into

three type depending on the source of the

stimulus, and are components of the general

or special somatic and visceral afferent

pathway :

– Exteroceptors

– Proprioceptors

– interoceptors

Exteroceptors

• Location : near the body surface

• Specialized to perceive stimuli from the external

environment

• These receptors sensitive to :

– Temperature

– Touch

– Pressure and

– Pain

• Are component of the general somatic afferent

continued

• Special somatic afferent :

– Specialized for light ( sense of vision) and

sound (sense of hearing)

• Special visceral afferent modality :

– Specialized for smell and taste

Proprioceptors

• Are specialized receptor located in joint

capsules, tendon and intrafusal fibers within

muscle.

• These general somatic afferent receptors

transmit sensory input to the CNS, which

translated into information that relates to an

awareness of the body in space and

movement

continued

• Vestibular (balance) mechanism, located

within the inner ear, are specialized for

receiving stimuli related to motion vectors

within the head.

Interoceptors

• Are specialized receptors that perceive

sensory information from within organs of

the body.

Specialized peripheral receptors

• Certain peripheral receptors, specialized to

receive particular stimuli, include

mechanoreceptors, thermoreceptor, and

nociceptors

• The dendritic ending located in various

regions of the body, including muscles,

tendons, skin, fascia and joint capsules

continued

• These receptors are classified into three

types :

– Mechanoreceptors, which respond to touch

– Thermoreceptors,which respond to cold and

warmth

– Nociceptors, which respond to pain due to

mechanical stress, extremes temperature

differences and chemical substance

Mechanoreceptors

• Mechanoreceptors respond to mechanical

stimuli that may deform the receptor or the

tissue surrounding the receptor.

• Stimuli that trigger the mechanoreceptors

are touch, stretch, vibration and pressure

Nonencapsulated mechanoreceptors

• Are simple unmyelinated receptors present

in the skin, connective tissues and

surrounding hair follicle

– Peritricial nerve ending, located in the

epidermis of the skin, especially in the face and

cornea of the eye

– Merckel’s disks, specialized for perceiving

discriminatory touch, located in non hairy skin

and regions of the body more sensitive to touch.

Encapsulated mechanoreceptors

• Encapsulated Mechanoreceptors exhibit characteristic

structure and are present in specific location

– 1. Meissner’ corpuscles :

• Specialist for tactile

• Location : dermal papillae of the non hair portin of

the hand, eyelids, lip, tongue, nipples, skin of the

foot and forearm.

• Each corpuscle is formed by three or four nerve

terminals and their associated Schwann cells, all

which are encapsulated by connective tissue.

continued

– 2. Pacinian corpuscles

• Location : in the dermis and hypodermis in the digits of the hand, breast, connective tissue of the joint, periosteum and the mesentery

• Spezialied to perceive pressure, touch and fibration

• Morphology :

– ovoid & large receptor

– Single unmyelinated fiber as a core and its Schwann cell

– Surrounded by approximately 60 layers of modified fibroblast

– Each layer separated by a small fluid-filled space

Ruffini’s corpuscles

• Location : in the dermis of skin, nail beds,

periodontal ligament and joint capsules

• Composition :

– branched nonmyelinated terminals interspersed

with collagen fibers

– Surrounded by four to five layers of modified

fibroblast

Krause’s end bulb

• Morphology :

– Spheris

– Unmyelinated nerve ending

• Location : papilla dermis, joints, conjunctiva, peritoneum, genital regions, subendothelial c.t. of the oral and nasal cavities

• Function : unknown, they were thought to be receptors sensitive to cold

Muscle spindles and Golgi tendon organs

• Muscle spindles provide feedback concerning the

changes and the rate alteration of the muscle

length

• Golgi tendon organs monitor the tension and the

rate at which the tension is being produced during

movement

• Information from these two sensory structures is

processed at the unconscious level within the

spinal cord; the information also reaches the

cerebellum & cerebral cortex, so that individual

may sense muscle position.

Thermoreceptor

• Which respons to temperature differences of

about 2° C, are three types: warmth

receptors, cold receptors and temperature-

sensitive nociceptors.

• Specific receptors have not been identified

for warmth

• Cold receptors are derived from naked

nerve ending in the epidermis

Nociceptors

• Are receptors sensitive to pain caused by

mechanical stress, extreme of temperature, and

cytokines as bradykinin, serotonin and histamin.

• Are naked ending of myelinated nerve fibers that

branch freely in the dermis before entering the

dermis

• Divided into three groups :

– Those that respond to mechanical stress or damage

– Those that respond to extremes in heat or cold

– Those that respond to chemical compound such as

bradykinin, serotonin and histamin

Peripheral Nerve Endings: Afferent Endings

Receptor Neurons of Craniospinal Ganglion

pseudounipolar neurons of dorsal root ganglia

trigeminal (semilunar, Gasserian ganglion),

geniculate (VII), superior IX, superior X ganglia

(GSA)

geniculate (VII), inferior IX, inferior X ganglia (VA)

Morphological Classification

free nerve endings

expanded tip endings

encapsulated endings ----- CT envestment

Afferent Endings

Free Nerve Endings

- Nerve endings without special structural

organization

- pain and temperature receptor

Expanded Tip Endings

- Merkel’s Touch Corpuscle

Merkel cells in basal layer of epidermis

- Type I Hair cells of Vestibular Labyrinth

Afferent Endings

Encapsulated Endings

- Meissner’s Corpuscle

- Pacinian Corpuscle

(Corpuscle of Vater-Pacini)

- Genital Corpuscle

- Ruffini’s Ending

- End Bulb of Krause

- Golgi tendon organ: Proprioceptor

Receptor

Endings

Free nerve

ending

Expanded

tip ending Encapsulated

ending

Merkel’s

Touch Corpuscle

expanded tip ending

Merkel cell

- clear cell located in the

basal layer of epidermis

- membrane bound electron

dense granules resembles

synaptic vesicle

Meissner’s Corpuscle

Pacinian Corpuscle

Efferent Endings Somatic Efferent Endings

Neuromuscular Junction

(Myoneural Junction, Motor End

Plate)

Autonomic Efferent

Endings

Endings on smooth muscle

and blood vessels

Neuromuscular Junction

(Myoneural Junction,

Motor End Plate)

NMJ

M

N

Autonomic Efferent Endings

Neuromuscular Spindle

• Both receptor and effector

• Structure

1. Capsule

2. Intrafusal Muscle Fibers

- Nuclear Bag Fiber

- Nuclear Chain Fiber

3. Receptor and Effector Nerve

Endings

- Afferent Ending

- Efferent Ending

NB: nuclear bag fiber IF: intrafusal muscle fiber

CA: capsule EF: extrafusal muscle fiber

Afferent Endings

Encapsulated Endings

- Meissner’s Corpuscle

- Pacinian Corpuscle

(Corpuscle of Vater-Pacini)

- Genital Corpuscle

- Ruffini’s Ending

- End Bulb of Krause

- Golgi tendon organ: Proprioceptor