Nervous system. 1. Neurons :

Transcription

1 Nervous system nervous system is composed of billions of cells, the most essential being the nerve cells or neurons. There are estimated to be as many as 100 billion neurons in our nervous system. Two major components of nervous system 1- central nervous system (CNS) - brain, spinal cord. These neurons cannot regenerate if damaged. 2- peripheral nervous system (PNS) - nerves extending toward or away from CNS and ganglia. These neurons can regenerate if damaged. Nervous tissue consists of two major types of cells: 1. neurons - responsible for conduction, propagation, and reception of nervous impulses. Processes called axons or dendrites extend from these cells. 2. glial cells - (neuroglia) cells associated with neurons. No axons or dendrites. These cells are involved in nutrition, support, insulation, protection of neurons. 1. Neurons : Neurons are nerve cells (neurocytes), which, together with neuroglial cells, comprise the nervous tissue making up the nervous system. Neurons receive nerve signals (action potentials), integrate action potentials, and transmit these signals to other neurons or effector organs, such as muscles and glands.

2 1- Neuron A. Cell body or soma (perikayon) : which is the synthetic center for entire nerve cell and is receptive to stimuli, contains nucleus and typical cell organelles Nucleus - large, central in most, large amount of euchromatin, Barr body. rough endoplasmic reticulum (RER) - lots for synthesis of structural and transport proteins, Nissl bodies seen with light microscope and free ribosomes. Golgi apparatus - only found near nucleus in perikaryon. Expected, since intense synthetic activity of neurotransmitters and/or neurohormones. Mitochondria - abundant for high energy requirements Neurofilaments, microtubules - neurofilaments are intermediate filaments (10 nm) Microtubules - important in transport of materials (e.g. neurotransmitters). Inclusions - pigment vesicles - function unknown. Lipofuscin deposits - residual bodies from autophagosome activity. Increase with age. B. dendrite : many elongated processes specialized to receive stimuli from environment, sensory epithelial cells,or other neurons, may be branched, forms receptive area for synaptic contacts from other neurons, has tiny rough projections or spines called gemmules that may be points of synaptic contact, dendrites from larger neurons may be lightly myelinated by oligodendroglia. Neurons may have more than one dendrite. Cytoplasm in these processes similar to that of perikaryon, but no golgi bodies. C. axon : which is a single process specialized in generation and conducting nerve impulses to other cells (nerve, muscle and gland cells). axons carry electrical impulses (action potentials) to synapses at end of axon. Many axons are enclosed in sheaths formed by other cells; axons of PNS commonly

3 have sheaths composed of Schwann Cells also called Neurolemmocytes, which can tightly surround the nerve fibers (axon), while axons lying within the CNS are provided with a similar covering by Oligodendrocytes. The nerve fibers with enclosed sheaths are known as myelinated nerve fibers, while those lacking the sheaths are called unmyelinated nerve fibers. In junction between any two segments,there is a gap in myelin sheath (Lack of myelin); these gaps are called Nodes Of Raniver The ends of branches form synapses with other neurons or muscle cells, may be myelinated by either oligodendroglia in CNS or Schwann cells in PNS. Each neuron has only one axon. axon hillock (pyramid shaped region where axon originates from the cell body or perikaryon). initial segment (unmyelinated intitial portion of axon). remainder of axon (may be myelinated or unmyelinated, may be branched). D. Synapse : it s a special site of contact between neurons or between neurons and other effector cells which are responsible for nerve impulse transmission from neuron to another cell and insures that transmission is unidirectional. Various type of synapse are seen between neuron : 1- axosomatic : if an axon form synapses with cell body ( soma ). 2- axodendritic : axon synapses with dendrite. 3- axoaxonic: axon synapses with another axon terminal branch for presynaptic inhibition) or beside the initial segment of an axon. 4- dendrodendritic : dendrite synapsing on another dendrite (very localized effect). According to the way of impulse transmission: Electrical synapses (uncommon in mammals) Chemical synapses (by the release of neurotransmitters)

4 Synapses are comprised of three elements: a) Presynaptic nerve terminal contains synaptic vesicles which house a chemical neurotransmitter that is released after vesicle fusion with the presynaptic terminal plasma membrane. b) Postsynaptic element : adendrite, a cell body, or a target cell receiving the synaptic input. Receptor protein molecules, to which neurotransmitter molecules bind, are embedded in the postsynaptic plasma membane. c) Synaptic Cleft: a gap between pre- and post-synaptic elements into which neurotransmitter molecules are released. TYPES OF NEURONS: There are three basic shapes to the neurons: 1- bipolar (single axon and single dendrite). 2- Unipolar or pseudounipolar (single axon and dendrite arise from a common stem) - the primary general sensory neurons are usually pseudounipolar. 3- multipolar (the commonest) - most motor neurons are multipolar (single axon and numerous dendrites). Neuron classification based on function: 1- Motor neurons - efferent, action potential moves from CNS to effector organ (e.g. muscle). 2- Sensory neurons - afferent, action potential moves from sensory organ to CNS, associated with pacinian corpuscles, touch, pressure). 3- Interneurons - form connections between neurons

5 2. glial cells There are many more glial cells in the nervous system than there are neurons. These cells are situated among the neurons and are generally smaller. Glial cells have been shown to have nutritive and support functions, but in general, the functions of these cells within the CNS are not well understood. Supporting cells ( neuroglia) in the central nervous system : In the central nervous system, there are four types of supporting cells. And in PNS there are two type. Supporting cell in CNS: 1. Astrocytes - two types a. protoplasmic astrocytes : granular cytoplasm, many branches on short processes, some of processes are closely applied to neurons, while others form intimate contacts with blood vessels. Thought to form a duct for nutrients from blood vessels to neurons. found in gray matter. b. fibrous astrocytes: long slender processes,function not well understood,found chiefly in white matter. 2. oligodendroglia (also called oligodendrocytes): Smaller than astrocytes, fewer processes, found in both gray and white matter. In white matter, these cells form the myelin sheaths that are around many axons, in gray mater they may lightly myelinate some dendrites. 3. Microglia :small cell body that is elongated. Elongate nucleus with mostly heterochromatin.can be differentiated from other glia by elongate nucleus. Other have a spherical nucleus. Many of what were thought to be microglia under the light microscope, have turned out to be oligodendroglia when cells were examined with EM. 4. ependymal cells : ciliated cells forming single layer of cuboidal epithelium that lines the entire neurocoel, ciliary action acts to circulate cerebral spinal fluid.

6 Supporting cell in PNS: 1-Schwann cells :A type of support cell that surrounds all axons in the PNS. Support myelinated and unmyelinated nerve fibers in the PNS Produce the myelin sheath in the PNS cleaning up the PNS debris regrowth of PNS axons. Single Schwann cell myelinate only one axon Schwann cell can envelope several unmyelinated axons Schwann cell is covered by a basal lamina 2-Satellite cells : Small cuboidal, flattened cells found in the ganglia of the PNS that help regulate and stabilize the environment around ganglion cell bodies. Peripheral Nervous System It consists of Peripheral Nerves and Ganglia. 1-Peripheral Nerves: - are composed of bundles of nerve fibers (mostly the axons) held together by C.T and include spinal nerves connected to the spinal cord and cranial nerves connected with the brain. Surrounding the entire nerve is a sheath of relatively strong C.T termed the epineurium it is composed of fibroblast and collagenous fibers and few elastic fiber, the epineurium contains the major blood vessels to the nerve. Within the epineurium, nerve fibers are grouped into bundles or fascicles, each fascicle is surrounded by a C.T perineurium which has the important function of protecting the nerve fibers and helping maintain the internal microenvironment. 2-GangIia:- are typically ovoid structures containing nerve cell bodies and glial cells supported by C.T, They serve as stations to transmit nerve impulses, one nerve enters and another exists from each ganglion. There are 2 types of ganglia: A- Sensory Ganglia: are associated with cranial and spinal nerves and composed of nerve cells' bodies which are enveloped by thin sheet of satellite cells which nourish and regulate the microenvironment of the nerve cells bodies. Those ganglia are supported by CT capsule which is continuous with the CT layers of nerves. The capsule sends trabeculae into the ganglion. Sensory ganglia receive afferent impulses that go to the CNS.

7 B- Autonomic Ganglia: are small bulbous dilatations in autonomic nerves. Autonomic nerves affect the activity of smooth muscle, the secretion of some glands, modulate cardiac rhythm and other involuntary activities. These ganglia usually have multipolar neurons, unlike sensory ganglia. Central Nervous System (Brain and Spinal Cord). The brain lies within the cranial cavity of the skull and the spinal cord occupies the vertebral canal within the vertebral column. Beneath the bony coverings the brain and spinal cord are protected and nourished by 3 membranes called meninges that are located between the bone and soft tissue of the nervous system. The only C.T within CNS is very small amount of delicate areolar tissue associated with numerous capillaries and hence the substance of CNS is soft. The CNS has a characteristic tissue arrangement called grey matter and white matter. Grey matter contains the cell bodies (perikarya) of neurons and the supporting cells (neuroglia) as well as unmyelinated dendrites. White matter does not contain any cell bodies, but mostly contains myelinated nerve fibres. The central region of the spinal cord is grey matter, and the surrounding region is white matter. A-The Meninges: protective connective tissue sheaths surrounding the brain and spinal cord. Contain 3 layers, dura mater, arachnoid and pia mater. 1-Dura mater: - is the thick external layer consisting of dense fibro-elastic C.T continuous with the periosteum of the skull around spinal cord the dura mater is separated from periosteum of the vertebra by the epidural space. 2-Arachnoid:- has 2 components : a sheet of C. T in contact with dura mater. And a system of Loosely arranged trabeculae containing fibroblasts and collagen fibers. 3-Pia mater:- is the inner most layer, it is lined by flattened mesenchymal derived cells.

8 B-Cerebrum Consist of 2 large masses called cerebral hemispheres connected by a deep bridge of nerve fibers; covered by thin layer of gray matter called the cerebral cortex form. The outermost portion of the cerebrum containing nerve cell bodies, fibers, neuroglia and blood vessels. Just beneath the cortex, masses of white matter are present making up the thick bulk of cerebrum. C-Cerebellum The cerebellar cortex which coordinates muscular activity throughout the body has 3 layers: a- Outer layer : composed of small nerve cells bodies and many unmyelinated nerve fibers b- The central layer: composed of Purkinji cells which are large flask-shaped neurons with extensively branched dendrites c- Inner most layer composed of very small neurons packed together densely. D-The spinal cord In cross sections of the spinal cord, white matter is peripheral and gray matter is Internal and has the general shape (H). The arms of the H are made up of the bilateral anterior and posterior horns of the cord. A horizontal bar of gray matter in the middle of spinal cord, this bar surrounds the central canal which contains C S F and lined by ependymal cells.