Histology:2nd stage 2016-2017Department of Anatomy &Histology Dr. Raja Ali --------------------------------------------------------------------------------------------

Nerve Tissue & the Nervous system I:

Chapter Outline:

Neurons (Cell Body (Perikaryon) , Dendrites , Axons ) .Synaptic Communication. Glial cells & Neural Activity :( Oligodendrocytes , Astrocytes , Ependymal cells ,Microglia ,Schwann cells(Neurolemmocytes), Satellite cells of Ganglia).Myelin.Central Nervous System (Brain &Spinal Cord , Meninges ). Periperal NervousSystem : ( Nerves , Ganglia ) .

Learning Objective:

Name and describe the divisions of nervous system. State the function of nerve cells. Name , locate &describe the structures of synapses. State the function of glial cells. Name , locate &describe the structures of the brain and spinal cord. Describe the peripheral nervous system.

Terminology:• Input: sensory = sensory input.

– Receptors monitor changes.– Changes called “stimuli” (sing., stimulus)– Information sent by “afferent” nerves.

• Integration:– Info processed.– Decision made about what should be done.

• Output: motor = motor output.

– Effector organs (muscles or glands) activated.– Effected by “efferent” nerves.

Remember the difference between the English words “affect” and “effect”.• Neuron = nerve cell.• Neuroglia = supporting cell.• Nerve fiber = long axon.• Nerve = collection of nerve fibers (axons) in PNS.• Tract = collections of nerve fibers (axons) in CNS.• Nucleus = cluster of cell bodies in CNS.• Ganglia = cluster of cell bodies in PNS.• Unilateral: on one side.• Ipsilateral: on the same side.• Contralateral: on the opposite side.

Remember also:• CNS vs PNS.• Input: sensory: afferent: to brain.• Output: motor : efferent: from brain.• Dense network of fibers from processes of both neurons and glial cells fills

the interneuronal space of the CNS and is called the neuropil .

Introduction: The nervous tissue is composed of interconnecting network of specialized

cells called neurons (nerve cells) supported by neuroglial cells. There are about

10 million neurons in human beings. The function of neurons is to receive stimuli

and conduct them to a central site, the central nervous system (CNS), where they

are analysed and integrated to produce a desired response in the effector organs.

Divisions of Nervous System:

Flowchart .. Divisions Nervous System:

Structure of a neuron:

Cell body/Soma/Perikaryon (5–150 _m): The cell bodies of all neurons are situated in the grey matter of the CNS

and in the ganglia of PNS. The cell body of a neuron contains the nucleus and the following

cytoplasmic organelles and inclusions (fig.1): Nucleus—is large, euchromatic, spherical and centrally located. Nissl bodies or Nissl substance—are composed of large

aggregations of rough endoplasmic reticulum:

are observed as basophilic clumps by light microscopy . extend into dendrites but not into axon and axon hillock. disintegrate as a result of injury to axon (chromatolysis).

Golgi complex—are found near the nucleus. Mitochondria—are numerous and rod shaped. Neurofi laments (10 nm dia) and microtubules (25 nm dia)—form neuronal

cytoskeleton provid ing structural support and intracellular transport. Melanin pigments—dark brown granules. Lipofuscin pigments—residual bodies not digested by lysosomes (increase

with age).

fig.(1):Ultrastructure of neuron

Dendrites: Fig.(2) Are highly branched, tapering processes of a neuron. So their diameter is

not uniform. Are covered by thorny spines (gemmules) which are sites of synaptic

contact. Receive stimuli from sensory cells and other neurons and transmit them

towards the soma. So they can be regarded as major sites of information input into neuron.

Axon: Fig.(2) Single, long, cylindrical process of a neuron. So its diameter is uniform. Does not branch profusely; but may give rise to collaterals. Arises from a cone-shaped portion of the cell body called axon hillock,

which is devoid of Nissl bodies, but contains bundles of microtubules. The cytoplasm of the axon is called axoplasm and the plasma membrane is

called axolemma. Terminates by dividing into many small branches, axon terminals, ending

in small swellings—terminal buttons.

Conducts impulses away from the cell body to the axon terminals from which impulses are transmitted to another neuron or another target cell.

Axons are commonly referred to as nerve fibers. Are often surrounded by myelin sheath, which is derived either from

Schwann cells (PNS) or oligodendrocytes (CNS). When an axon is cut, peripheral part degenerates. Regeneration of the axon is possible only when the cell body of the neuron

is intact. Neurons do not regenerate in the event of cell body death, i.e. they do not

multiply.

Fig.(2):Structure of a neuron

Classification of Neurons:

A. Morphological (based on the number of processes)1. Unipolar neuron—has a single process (rare), e.g. mesencephalic nucleus of V cranial nerve.

2. Bipolar neuron—has two processes (an axon and a dendrite; fig.(3)), e.g. spiral ganglion, bipolar cells in retina .3. Multipolar neuron—has many processes (an axon and many dendrites; Fig. (4)), e.g. autonomic ganglia motor neurons.4. Pseudo-unipolar neuron—has a single process that divides into an axon (central process) and a dendrite (peripheral process; Fig. (5)), e.g. cranial and spinal ganglia (sensory neurons).

Fig.(3):Bipolar neuron Fig.(4):Multipolar neuron

Fig.(5):Paseudopolar neuron

B. Functional (based on the function performed):

1. Sensory neuron—receives stimuli from receptors and conducts impulses to CNS, e.g. sensory ganglia.2. Motor neuron—conducts impulses from CNS to effecter organs (muscles), e.g. ventral horn cells.

3. Interneuron—connects sensory and motor neurons and completes the functional circuit.

Synaptic Communication: The synapse( Gr. Synapsis , union) is responsible for the transmission of

nerve impulses from neuron to another cell and insure that transmission is unidirectional.

The function of the synapse is to convert an electrical signal (impulse) from the pre synaptic cell into a chemical signal that acts on the postsynaptic cell. Most synapses transmit information by releasing neurotransmitters.

A synapse has the following structure: Fig.(8) Presynaptic axon terminal (terminal button) from which neurotransmitter

is released, Postsynaptic cell membrane with receptors for the transmitter and ion

channels or other mechanisms to initiate a new impulse. . 20-30 nm wide intercellular space called the synaptic cleft separating

the pre synaptic and postsynaptic membranes. The synapses are called excitatory, because their activity promotes

impulses in the postsynaptic cell membrane. In some synapses the neurotransmitter-receptor interaction has an opposite effect, promoting membrane hyperpolarization with no transmission of the nerve impulse. These are called inhibitory synapses. Thus, synapses can excite or inhibit impulse transmission and thereby regulate nerve activity.

Neurons can synapse with:Neurons , Muscles , or Glands (fig.(6)).

Synapses between neurons may be classified morphologically as:

• axodendritic, occurring between axons and dendrites; (fig.(7a)).• axosomatic, occurring between axons and the cell body(fig.(7b)).; or• axoaxonic, occurring between axons and axons (fig.(7c)).

1-Neurons

2-Muscle

3-Glands

Fig.( 6)

Fig.(7 ):Types of Neuroneural Synapses

Fig.( 8 ): Diagram of a chemical axodendritic synapse.

a-Diagram showing the current view of neurotransmitter release from a presynaptic knob by a fusion of the synaptic vesicles with presynaptic membrane.

b. Diagram showing a new proposed model of the neurotransmitter release via porocytosis.

The neurotransmitter carries the impulse across the space (the synapse) and onto the next neuron, or onto the organ the impulse is meant to stimulate.

Some examples of neurotransmitters are : Acetylcholine . Dopamine , endorphins Serotonin ,and Norepinephrine .

These neurotransmitters have a variety of functions, including muscle movement, mood, and stress release.