Lec / 10 The Nervous System By
Abeer hazimNervous system is the most complex system in the human body histologically and physiologically, It is formed by interconnected a network of billions nerve cells (neurons), assisted by much numerous glial cells. Each neuron has hundreds of interconnections with other neurons, forming a very complex system for processing communication and generating responses.
The nervous system is divided anatomically into:-
Slide 7.21- Central nervous system CNS consisting of:- The brain (cerebrum and cerebellum). The spinal cord. 2- Peripheral nervous system PNS consisting of:- Nerves ( cranial, spinal , peripheral). Ganglia. Nerve ending.
Both Central and Peripheral tissue consists of two cell types: 1- Nerve cells or Neurons. 2- Glial cells.
Neurons
Neurons or nerve cell is the functional unit in both CNS and PNS, which usually show numerous long processes. they are responsible for the reception, transmission, and processing of stimuli. Neurons respond to environmental changes(stimuli) by altering the ionic gradient that exists between the inner and outer surfaces of their membranes. Neurons and their processes vary in size and shape.Functionally, neurons are classified according to the direction in which they transmit impulses relative to the central nervous system into:- Sensory (afferent) neurons: carry impulses from peripheral sense receptors to the CNS. Motor (efferent) neurons: transmit impulses from the CNS to effector organs such as muscles and glands. Interneuron: forming a communicating network between sensory and motor neurons.
Their structure consists of three parts:- 1- The cell body or Perikaryon. 2- Dendrites. 3- Axon.
Slide 7.4
The cell body ( Perikaryon )
The cell body which is the part of neuron that contain the nucleus and surrounding cytoplasm. it is primarily a trophic center for the entire nerve cell. the nucleus is large, spherical, euchromatic (pale staining) with prominent nucleolus. Cell body contains rough endoplasmic reticulum (RER), Golgi apparatus, and polyribosomes and contains Nissl bodies.Dendrites
Dendrites are elongated processes specialized to receive stimuli from the environment, sensory epithelial cells, or other neurons. They are short and divided like branches of a tree; they are covered with many synapses and are the principal signal reception and processing site on the neurons. The arborization increases the receptive area of the cell, and the branches become much thinner with further division.Axon
Axons are long, single, cylindrical processes that originate from a pyramid- shaped region of the perikaryon the Axon hillock, which is devoid of nissl bodies and Golgi cisternae,and it is the site where all other organelles pass to the axon. The plasma membrane of the axon is called Axolemma and the contents are known as the axoplasm.The distal portion of the axon is branched as terminal arborization, Each branch terminates on the next cell in dilatations called End bulbs (boutons)which interact with other neurons or non nerve cells at structures called Synapses which are specialized junctions for the transmission of nerve impulses from neuron to another neuron or other effector cell (muscle and gland) in unidirectional way.
The axon is specialized in generating and onducting nerve impulses to other cell (nerve, muscle, gland cells), also may receive information from other neurons. Collateral branches: are branches of the axon that connect with other group of cells.
Neurons can be classified according to the number of processes extending from the cell body into:- 1- Multipolar neurons, which have one axon and two or many dendrites, most neurons are Multipolar.
2- Bipolar neurons, with one dendrite and one axon, found in the retina and the inner ear.
3- Unipolar or pseudounipolar neurons, which have a single process that bifurcates close to the cell body with the longer branch extending to a peripheral ending and the other toward the CNS. Pseudounipolar found in spinal ganglia.
Glial cells ( Neuroglia )
have short processes, support and protect neurons. participate in neural activity, neural nutrition, and the defence processes of the central nervous system. They are 10 times more abundant in brain than the neurons. They are smaller than neurons, and in the CNS they surround most of the neuronal cell bodies. They provide an ideal environment for the neuronal activities.In the CNS there is very little or no connective tissue. Instead, there is Neuropil, which is dense network of fibers from processes of both neurons and glial cells fills the interneuronal spaces of CNS.
There are six kinds of glial cells :
1- Oligodendrocytes. 2- Astrocytes. 3- Ependymal cells. 4- Microglial cells. 5- schwan cells. 6- satellite cells.Oligodendrocytes Small cells with rounded, condensed nuclei and unstained cytoplasm. They are predominant glial cell in CNS white matter They produce the myelin sheath that provide the electrical insulation for neurons in the CNS, these cells extend processes that form the myelin sheath wrap around many axons.
Astrocytes
Star-shaped cells, because of their multiple radiating processes, and they are unique to the CNS. Astrocytes compared to other glial cells are by far the most numerous and exhibit considerable morphological and functional diversity. In addition to their supporting function, Astrocytes have major roles in controlling the ionic environment of neurons. They have processes that extend between blood vesselsand neurons, the processes areexpanded, forming Perivascular feet that cover capillary endothelial Cells and contributeto the Blood – Brain barrier.Two kinds of astrocytes have been identified:-
- Protoplasmic astrocytes, which are more prevalent in the gray matter, have short and branched processes. - Fibrous astrocytes, which are more common in the white matter, have few and long processes.Ependymal cells
Ependymal cells are low columnar or cuboidal cells lining the ventricles of the brain and central canal of the spinal cord. ependymal cells have cilia which facilitate the movement of the cerebrospinal fluid (CSF), or long microvilli which are involved in absorption.Microglia
small cells with short irregular processes and less numerous than oligodendrocytes or astrocytes cells. but more Distributed throughout gray and white matter. Have dense elongated nuclei which contrast with the spherical nuclei of other glial cells. Microglia are phagocytic cells, originate from blood monocytes and belongs to the same family as macrophages and antigen presenting cells. Secretes a number of immunoregulatory cytokines and constitute the major mechanism of immune defense in CNS tissues.Schwan cells (neurolemmocytes)
They are found only in PNS. have trophic interactions with axons and allow for their myelination. one schwan cell forms myelin around a segment of one axon, in contrast to the ability of oligodendrocytes to branch and sheath parts of more than one axon.Satellite cells of ganglia
small satellite cells form a covering layer over the large neuronal cell bodies in PNS ganglia. Closely associated with the neurons. The satellite cells exert a trophic or supportive role.Lec / 11 The Nervous System
Central Nervous SystemThe principal structures of the CNS are: The brain ( cerebrum and cerebellum ) which is enclosed in the skull. 2- Spinal cord which is contained within the vertebral canal.
It is soft and gel like organ because there is no connective tissue, in freshly sectioned tissues (cerebrum cerebellum and spinal cord ) show regions of white (white matter) and gray (gray matter). This difference in color is due to the distribution of the nerve cell bodies, glial cells and the myelinated nerve axons.
The white matter is composed mainly of the myelinated axons and the myelin- producing oligodendrocytes. It does not contain neuronal cell body, but microglia are present. Gray matter is composed mainly of the nerve cell bodies, dendrites, the initial unmyelinated portions of axon, astrocytes and the glial cells. This is the region where synapses occur. Aggregates of neuronal cell bodies forming island of gray matter embedded in the white matter are called nuclei. Gray matter prevalent mainly at cortex of cerebrum and cerebellum, and the white matter present in the central region.
In cross section of the spinal cord, white matter is peripheral and gray matter is internal and has general shape of an H (butterfly shape). In the center is an opening of the central canal. The gray matter forms: - The anterior horns, which contain the motor neurons whose axon make up the ventral roots of spinal nerves. - The posterior horns which receive sensory fibers from neurons in the spinal ganglia (dorsal root). Spinal cord Neurons are large And multipolar, especially the motor neurons in the anterior horns.
Peripheral Nervous System
The main components of PNS are:- 1- Nerves. 2- Ganglia. 3- Nerve endings. Nerves are bundles of nerve fibers (axons) surrounded by glial cells and C.T.Nerve fibers
In PNS nerve fibers consist of axons sheathed by schwann cells and it is divided into: 1- Myelinated nerve fibers The multiple layers of Schwann cell membrane unite as a myeline layer. Myelin sheath is a lipid layer, It is composed of concentrically wrapped layers of plasma membrane of Schwann cells (neurolemmocytes), pushing the nucleus and cytoplasm to the periphery.Between adjacent Schwann cells the myelin sheath shows small nodal gaps along the axon called the node of Ranvier devoid of myelin. The distance between two nodes is called an "Internode" and consists of one Schwann. The Myelin sheath is segmented due to numerous Schwann cells arranged along the length of the axon. Myelin sheath serves to protect axons and maintain a constant ionic environment required for action potentials.
2- Unmyelinated nerve fibers:-
The Unmyelinated nerve fibers are more abundant in the CNS, where axons run free, while in the PNS the axons are enveloped within simple folds of schwann cells.Peripheral nerves In PNS nerve fibers are grouped into bundles to form nerves ( Myelinated and Unmyelinated ). They have a whitish, glistening appearance because of their myelin and collagen content. The nerves establish communication between centers in the brain and spinal cord and the sense organs and effectors (muscles, glands, etc) by containing both afferent and efferent fibers.
Axons and Schwann cells of nerves are enclosed within connective tissue layers:Epineurium: which is an external dense irregular fibrous coat of C.T., which surrounded the bundles of nerve fibers.Perineurium: a sleeve of specialized C.T. formed by layers of flattened epithelium- like cells surrounded each bundle.Endoneurium: is thin layer of C.T. within the perineurial sheath run the Schwann cell – sheathed axons.
Ganglia
- Ganglia are small group of nerve cells outside the CNS. - They are nodular masses of neuronal cell bodies, glial cells and Satellite cells supporting by C.T, ganglia transmit nerve impulses.- According the direction of the nerve impulse, there are two kinds of ganglia in the PNS:
Sensory ganglia: It receive afferent impulses that go to the CNS. The neurons of these ganglia are pseudounipolar neurons and relay information from the ganglion's nerve endings to the gray matter of the spinal cord via synapses with local neurons. Satellite cells surround ganglion cells with large number. The sensory ganglia include: Cranial ganglia, which are associated with some of the cranial nerves. - Spinal ganglia, which are associated with dorsal root of the spinal nerve.
2- Autonomic ganglia: - which are small bulbous dilatations in autonomic nerves. - The neurons of these ganglia are multipolar. Satellite cells are less in number than the sensory ganglion.
- The autonomic nerves are effect the activity of smooth muscle, secretion of some glands, and the modulation of cardiac rhythm. - It’s function to maintain constant internal environment ( homeostasis ).