4 nervous tissue

Compiled by Dr. D. Simon

NERVOUS TISSUE• Nervous tissue consists of two main cell types:

neurons (nerve cells) and neuroglia (non-neuronal, glial cells).

• Neurons are the structural and functional units of the nervous system specialized for rapid communication.

• A neuron is composed of a cell body with processes

(extensions) called dendrites and an axon.



• The cytoplasm contains a large central nucleus with a prominent nucleolus, numerous mitochondria, lysosomes and a Golgi complex.

• The cytoplasm also shows the presence of a granular material that stains intensely with basic dyes called Nissl substance or Nissl bodies or Nissl granules.

• The dendrites terminate near the cell body.• They are irregular in thickness, and Nissl granules

extend into them.• The axon may extend for a considerable distance

away from the cell body.


• The longest axons may be as much as a meter long.• Each axon has a uniform diameter, and is devoid of

Nissl substance.• The Nissl-free zone of the axon extends for a short

distance into the cell body: this part of the cell body is called the axon hillock.

• The dendrites carry impulses to and the axons away from the cell body.


• The most common type of neuron gives off several processes and the cell body is therefore multipolar.

• Some neurons have only one axon and one dendrite called bipolar.

• Another type of neuron has a single process. After a very short course this process divides into two.

• One of the divisions represents the axon; the other is functionally a dendrite. This type is called as unipolar.


Neurons


• During development, each axon comes to be associated with certain cells that provide a sheath for it.

• The cells providing this sheath for axons lying outside the central nervous system are called Schwann cells.

• Axons lying within the central nervous system are provided a similar covering by a kind of neuroglial cell called an oligodendrocyte.

• An axon lying near a Schwann cell invaginates into the cytoplasm of the Schwann cell.


• In this process the axon comes to be suspended by a fold of the cell membrane of the Schwann cell. This fold is called the mesaxon.

• The mesaxon becomes elongated and comes to be spirally wound around the axon.

• Thus the axon is surrounded by several layers of cell membrane.

• Lipids and proteins are deposited between adjacent layers of the membrane.

• These layers of the mesaxon, along with the lipids and proteins, form the myelin sheath.

• Outside the myelin sheath a thin layer of Schwann cell cytoplasm persists to form an additional sheath which is called the neurolemma.



• There are some axons that are devoid of myelin sheaths.

• These unmyelinated axons invaginates into the

cytoplasm of Schwann cells, but the mesaxon does not spiral around them.

• Another differnce is that several such axons may invaginate into the cytoplasm of a single Schwann cell.





• Neurons communicate with each other at synapses, points of contact between neurons.

• The communication occurs by means of neurotransmitters, chemical agents released or secreted by one neuron, which may excite or inhibit another neuron, continuing or terminating the relay of impulses or the response to them.



Nodes of Ranvier:• The myelin sheath is in the form segments. Each

segment is formed by one Schwann cell.• At the junction of any two such segments there is a gap

in the myelin sheath called a node of Ranvier.• The part of the nerve fiber between two such nodes is

called the internode.

• When an impulse travels through a nerve fiber, it does not proceed uniformly along the length axis, but jumps from one node to the next. This is called saltatory conduction.


• In unmyelinated neurons the impulse travels along axolemma. Such conduction is much slower than the saltatory conduction and consumes more energy.

• Immediately next to a node the myelin sheath shows an expansion called the paranodal bulb.

• There are longitudinal furrows on the surface of the paranodal bulb.

• These furrows are filled in by Schwann cell cytoplasm.• Finger-like processes of this cytoplasm extend towards

the axon and come in contact with it.• They interdigitate with processes from the

neighbouring Schwann cell.


Node of Ranvier


• In the intervals between these processes the axon is covered by a gap substance that plays a role in regulating the flow of the nerve impulse by influencing the passage of ions into, and out of, the axon.

• At a node of Ranvier the axon itself is much thinner than the internode.

• The part of the axon passing through paranodal bulb shows infoldings of its axolemma (cell membrane) that correspond to the grooves on the surface of the paranodal bulb.


• Blood-Nerve Barrier:

• Peripheral nerve fibers are separated from circulating blood by a blood-nerve barrier.

• Capillaries in nerves are non-fenestrated and endothelial cells are united by tight junctions.

• There is a continuous basal lamina around the capillary.

• This barrier is reinforced by cell layers present in the perineurium.


• Neuroglia (glial cells or glia) are approximately five times as abundant as neurons, are non-neuronal, non-excitable cells that form a major component of nervous tissue.

• Neuroglia support, insulate, and nourish the neurons.


• Synapse:• Synapses are sites of junction between neurons.• In the most common, an axon terminal establishes

contact with the dendrite of a receiving neuron to form an axo-dendritic synapse.

• Synapses on dendrites may be located on spines or on the smooth areas between spines.

• The axon terminal may synapse with the cell body axo-somatic synapse.


• Axo-axonal synapse• Dendr-oaxonic synapse• Dendro-dendritic synapse• Somato-somatic synapse• Somato-dendritic


• A synapse transmits an impulse only in one direction.

• The two elements taking part in a synapse can, therefore, be of presynaptic and postsynaptic.

• In an axo-dendritic synapse, the terminal enlargement of the axon called as presynaptic bouton or synaptic bag.

• The region of the dendrite receiving the axon terminal is the postsynaptic process.

• The two are separated by a space called the synaptic cleft.

• Delicate fibers or granular material may be seen within the cleft.


Synapse


• On either side of the cleft there is a region of dense cytoplasm.

• On the presynaptic side this dense cytoplasm is broken into several bits.

• On the postsynaptic side the dense cytoplasm is continuous.

• The thickened areas of membrane on the presynaptic and postsynaptic sides constitute the active zone of a synapse. Neurotransmission takes place through this region.

• Within the presynaptic bouton numerous synaptic vesicles can be seen.


• The transmission of impulses through synapses involves the release of chemical substances called neurotransmitters, that are present within synaptic vesicles.

• When a nerve impulse reaches a terminal bouton neurotransmitter is released into the synaptic cleft.

• Under the influence of the neurotransmitter the

postsynaptic surface becomes depolarized resulting in a nerve impulse in the postsynaptic neuron.


• Central Nervous System:• The central nervous system consists of the brain and

spinal cord. • Grey matter and white matter:• Sections through the spinal cord or through any part of

the brain show certain regions that appear darker greyish colour, and others that have a whitish.

• These constitute the grey matter and white matter respectively.

• Cell bodies of neurons are located only in grey matter that also contains dendrites and axons starting from or ending on the cell bodies.


• Most of the fibers within the grey matter are unmyelinated.

• On the other hand the white matter consists predominantly of myelinated fibers.

• Isolated masses of grey matter anywhere in the CNS are referred to as nuclei.

• Aggregations of cell bodies outside the CNS are referred to as ganglia.

• Aggregations of axons in the CNS are called as tracts.

• Neuroglia and blood vessels are present in both grey and white matter.


• In transverse sections of the spinal cord, the grey matter appears roughly as an H-shaped area embedded in a matrix of white matter.

• The struts (supports) of the H are horns; therefore,

there are right and left posterior (dorsal) and anterior (ventral) grey horns.


Spinal cord and meninges


• There are paired (right and left) intermediolateral cell columns (IMLs) are a part of the grey matter, extending between the first thoracic (T1) and the second or third lumbar (L2 or L3) segments of the spinal cord.

• In horizontal sections of this part of the spinal cord,

the IMLs appear as small lateral horns of the H -shaped grey matter, looking somewhat like an extension of the cross-bar of the ‘H’ between the posterior and the anterior horns of grey matter.


• Structure and Components of a Typical Spinal Nerve:

• A typical spinal nerve arises from the spinal cord by nerve rootlets, which converge to form two nerve roots:

• the anterior (ventral) root consists of motor

(efferent) fibers passing from nerve cell bodies in the anterior horn of the spinal cord grey matter to effector organs located peripherally.


• The posterior (dorsal) root consists of sensory (afferent) fibers that convey neural impulses to the CNS from sense organs (e.g., the eyes) and from sensory receptors in various parts of the body (e.g., in the skin).

• Both types of sensory fibers - visceral sensory and general sensory have their cell bodies in spinal ganglia (sensory ganglia) (dorsal nerve root ganglia) of spinal nerves.


• The anterior and posterior roots unite at the

intervertebral foramen to form a spinal nerve, which immediately divides into two rami (branches):

• an anterior ramus and a posterior ramus.

• As branches of a mixed spinal nerve, the anterior and posterior rami also carry both motor and sensory nerves, as do all their branches.


• The anterior rami supply nerve fibers to a much larger area, consisting of anterior and lateral regions of the trunk and the upper and lower limbs arising from them.

• The posterior rami supply nerve fibers to synovial joints of the vertebral column, deep muscles of the back, and the overlying skin.


• Sensory Ganglia:• Neurons are large and arranged in groups.• The neurons are unipolar.• The groups of cells are separated by groups of

myelinated nerve fibers.• The cell body is surrounded by a layer of flattened

capsular cells or satellite cells.• The satellite cells are continuous with the Schwann cells

covering the processes arising from the neuron.

• Outside the satellite cells there is a layer of delicate connective tissue.

• The connective tissue covering each neuron is continuous with the endoneurium.


Sensory Ganglia (Neurons in groups)


Sensory Ganglia (Neurons Unipolar)


• Autonomic Ganglia:• The neurons are smaller than those of sensory

ganglia.• Neurons are multipolar.• The neurons are not arranged in groups. But

scattered throughout the ganglion.• The nerve fibers are non-myelinated and thinner.• Satellite cells are present but not so well defined.• The ganglion is permeated by connective tissue that

also provide a capsule for it (just as in sensory ganglia).

• The Nissl substance of the neurons is much better defined.


Autonomic Ganglia (Neurons scattered)


Autonomic Ganglia (Neurons Multipolar)


• Neuroglia:• Supporting cells in the nervous system.• (a) Neuroglial cells found in the parenchyma of the

brain and spinal cord.• (b) Ependymal cells lining the ventricular system.• (c) Schwann cells forming sheaths for axons of

peripheral nerves.• (d) Capsular cells (satellite cells) that surround

neurons in peripheral ganglia.• (e) Various types of supporting cells found in relation

to motor sensory terminals of nerve fibers.


• Neuroglial cells are in two major categories.• (i) Macroglia (or large glial cells)• Two types.• (a) Astrocytes which may be subdivided into fibrous

astrocytes and protoplasmic astrocytes.• (b) Oligodendrocytes

• (ii) Microglia (or small glial cells)

• All neuroglial cells are much smaller in size than neurons, but the number is much more than the neurons.


• Astrocytes:• Small star shaped cells give of number of processes.• The processes are often flattened into leaf-like

laminae that may partly surround neurons and separate them from other neurons.

• Fibrous astrocytes are seen mainly in the white matter.

• Their processes are thin and are asymmetrical.

• Protoplasmic astrocytes seen mainly in grey matter.• Their processes are thicker and are symmetrical.


Astrocytes and Micoglial cells


• Oligodendrocytes:• These cells have rounded or pear shaped cell bodies

with relatively few processes.• These cells provide myelin sheaths to axons that lie

within the CNS.• An oligodendrocyte may enclose several axons

(Schwann cell that ensheaths only one axon).

• Microglia:• Smallest neuroglial cells; The cell body is flattened;

The cell processes are short; Seen frequently in relation to capillaries; More numerous in grey matter.


Oligodendrocyte


• Three membranous layers – • pia mater, arachnoid mater, and dura mater

collectively constitute the meninges. • The meninges and the cerebrospinal fluid (CSF)

surround and protect the CNS.

• The brain and spinal cord are intimately covered on their outer surface by the innermost meningeal layer, a delicate, transparent covering, the pia mater (pia).

• The CSF is located between the pia and the arachnoid mater (arachnoid), in the subarachnoid space.


• External to the pia and arachnoid is the thick, tough dura mater (dura), which is intimately related to the internal aspect of the bone of the surrounding neurocranium (braincase).

• The dura of the spinal cord is separated from the

vertebral column by a fat-filled space, the epidural space.


Spinal cord and meninges


• Peripheral Nervous System:• The peripheral nervous system is made up of nerve

fibers and nerve cell bodies that connect the CNS with peripheral structures.

• Peripheral nerves consist of bundles of nerve fibers, their connective tissue coverings, and blood vessels (vasa nervorum = blood vessels supplying nerve).

• A peripheral nerve fiber consists of an axon; its

neurolemma, the neurolemma (Schwann) cells that immediately surround the axon separating it from other axons; and its endoneurium, a connective tissue sheath.


• In the PNS, the neurolemma may take two forms, creating two classes of nerve fibers:

• The neurolemma of myelinated nerve fibers have a neurolemmal (myelin) sheath that consists of a continuous series of neurolemma (Schwann) cells enwrapping an individual axon, forming myelin.


• The neurolemma of unmyelinated nerve fibers consist of multiple axons separately embedded within the cytoplasm of each neurolemma (Schwann) cell.

• These neurolemma cells do not produce myelin.

• Most fibers in cutaneous nerves (nerves that supply sensation to the skin) are unmyelinated.


• Peripheral nerves are protected by three connective tissue coverings.

• Endoneurium, a delicate connective tissue sheath

that surrounds the neurolemma cells and axons.

• Perineurium, a layer of dense connective tissue that encloses a fascicle (bundle) of peripheral nerve fibers.


• Epineurium, a thick connective tissue sheath that surrounds and encloses a bundle of fascicles, forming the outermost covering of the nerve; it includes fatty tissues, blood vessels, and lymphatics.

• A collection of nerve cell bodies outside the CNS is a ganglion. There are both motor (autonomic) and sensory ganglia.



• Peripheral nerves are both cranial and spinal nerves.

• Of the 12 pairs of cranial nerves (CN), only 11 arise from the brain; the Spinal accessory nerve(CN XI) arises mostly from the superior part of the spinal cord.

• All cranial nerves exit the cranial cavity through foramina in the cranium (G. kranion, skull).

• All 31 pairs of spinal nerves - 8 cervical (C), 12 thoracic

(T), 5 lumbar (L), 5 sacral (S), and 1 coccygeal (Co) arise from the spinal cord and exit through intervertebral foramina in the vertebral column.



• Peripheral Nerve Degeneration:

• When peripheral nerves are crushed or severed, their axons degenerate distal to the lesion because they depend on their cell bodies for survival.

• A crushing nerve injury damages or kills the axons

distal to the injury site; however, the nerve cell bodies usually survive and the connective tissue coverings of the nerve are intact.

• No surgical repair is needed for this type of nerve

injury because the intact connective tissue sheaths guide the growing axons to their destinations.


• Surgical intervention is necessary if the nerve is cut because the regeneration of axons requires apposition of the cut ends by sutures through the epineurium.

• The individual fascicles (bundles of nerve fibers) are

realigned as accurately as possible. • Compromising a nerve's blood supply for a long

period, produces ischemia by compression of the vasa nervorum, which can also cause nerve degeneration.

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4 nervous tissue