THE NERVOUS SYSTEM 1

Structure of the Nervous System The nervous system is formed of two parts :- Central Nervous System (CNS) Includes Brain and Spinal cord Peripheral Nervous System (PNS) Includes Cranial nerves ; Spinal nerves and ganglia . PNS further subdivided into: Sensory division (input) and Motor division (output) 2

Functions of Nervous System Sensory Function Sensory receptors gather information Information is carried to the CNS Motor Function Decisions are acted upon Impulses are carried to effectors Integrative Function Sensory information used to create: Sensations Memory Thoughts Decisions 3

Central nervous system Nerve cells (neurons) Conduct electrical impulses Gilal cells (neuroglia) Supports and protect neurons

In peripheral nervous system Nerve fibers Nerve cells, only in ganglia. Supported by connective tissue

Classification of neurons according to function Sensory neurons Motor neurons Association neurons (interneurons)

Neuroglia Nerve cells

Neuron It is the structural and functional unit of the nervous tissue. Consists of the nerve cell body and its processes. Processes are formed of dendrites axon

Neuron structure Neuron structure Cell body Cell processes Dendrites Axon

Neuron Cell Body Nucleus: large, spherical vesicular with a prominent nucleolus. Central in most neurons,

Neuron Cell Body The cell body contains the usual organelles except the centrioles. why? (not divide) Nissl bodies: rER + free ribosomes & polysomes (protein and membrane making).

Neuron Cell Body Golgi apparatus: around the nucleus, And function to Synthesize neuro-transmitters. Mitochondria: cellular respiration. Lysosomes: contain hydrolytic enzymes. Lipofuscin: pigment that accumulate with age.

Neuron Cell Body contains: Neurofilaments: intermediate filaments support. Neurotubule: maintains shape intracellular transport mechanism.

Differences betwen the axon & denderites: Single Long, thin of uniform diameter. Branches at terminal end may give collateral arise at right angle. Axoplasm lacks Nissl granules and Golgi complex. Carries impulses away from cell body; centrifugal May be myelinated. Multiple Short, irregular, wide base and tapering ends Gives extensive branches arise at acute angle. Cytoplasm contains all structures as the cell body except: no Golgi. Carries impulses towards cell body; centripetal Non-myelinated.

Neuroglial cells Oligodentrocytes Schwan cells Astrocytes Ependymal cells Microglia cells

Types of Neuroglial Cells in the PNS 1) Schwann Cells Produce myelin found on peripheral myelinated axons . Speed up neurotransmission 2) Satellite Cells Support clusters of neuron cell bodies in ganglia . 19

Types of Neuroglial Cells in the CNS 1) Microglia Phagocytic cell 3) Oligodendrocytes Responsible for Myelinaion In the CNS 4) Ependyma or ependymal Ciliated Line central canal of spinal cord Line ventricles of brain help regulate composition of CSF 2) Astrocytes aid in metabolism of certain substances form blood-brain barrier. Supportive in function.

Oligodendrocytes

Myelination of an Axon Formed in peripheral nervous system ( PNS ) by Schwann cells

CNS Axons Oligodendrocytes form the myelin sheaths around the fibers in the central nervous system (CNS ) .

Astrocytes

Ependymal cells Lining the ventricle found in Brain and . spinal cord , in some places are ciliated to move the cerebrospinal fluid.

Microglial cells Short elongated cells with short irregular process (elongated nucleus ). The other have spherical nucleus .

Central Nervous tissue CNS Spinal cord Brain Cerebrum Cerebellum

CNS White matter Gray matter Myelinated axons , Myelin- producing oligodendrocytes Not contain neuronal cell bodies , Present in central region Nuclei (aggregates of neuronal cell bodies forming islands of gray matter embedded in white matter ) Neuronal cell bodies , dendrites and the initial unmyelinated portion of the axons and glial cells Found at the surface of the cerebrum and cerebellum forming cerebral and cerebellar cortex. White matter Gray matter

Cerebral Cortex Has 6 layers of cells with different forms and sizes. Some neuron are afferent (sensory) , other are efferent (motor).

Cerebellar Cortex Has 3 layers ; 1) outer molecular layer ; less cell dense layer 2) central layer (Purkinje cells) ;which have spicuous cell body and long dendrites reach to molecular layer . 3) inner(granular) layer ; formed by very small neurons (smallest one in the body) , compactly disposed.

Spinal cord White matter is peripheral. Gray matter is central (H shape). In horizontal part of H open a central canal lined with Ependymal cells The legs of H forming anterior horns , which contain motor neuron make up ventral root , the gray matter also form posterior horns , which receive sensory fibers make up dorsal root .

Meninges Membranous connective tissue Dura mater Arachnoid Pia mater

Dura mater External layer Composed of dense connective tissue continuous with periosteum of the skull. Epidural space contains thin walwd vein , loose CT and adipose tissue Separted from arachnoid by subdural space . Its internal surface lined with simple squam. Epith.

Arachnoid Has 2 components; Layer in contact with dura mater and system of trabeculae connecting to pia mater , the cavities between the trabeculae form subarachnoid space.

Pia mater Loose connective tissue

Choroid plexus and CSF

(BBB) Blood Brain Barrier Highly selective permeability barrier that separates the circulating blood from the brain extracellular fluid in the central nervous system. Formed by brain endothelial cells, which are connected by tight junctions with an extremely high electrical resistivity . allows the passage of water, some gases, and lipid-soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function. On the other hand, the blood–brain barrier may prevent the entry of lipophilic, potential neurotoxins by way of an active transport mechanism mediated by P-glycoprotein. Astrocytes are necessary to create the blood–brain barrier. A small number of regions in the brain, including the circumventricular organs (CVOs), do not have a blood–brain barrier.

Peripheral nerve system

Unmyelinated nerve

Myelinated nerve

CT Sheath

Nerve ganglia Is a collection of nerve cells and fibers surrounded by a connective tissue capsule outside the CNS. There are two types of ganglia. Autonomic Sensory (Spinal ) parasympathetic Sympathetic

Autonomic (Symp. & para. ) Nerve ganglia Sensory (Spinal) Autonomic (Symp. & para. ) Two types: *sympathetic. *parasympathetic. Motor. Thin capsule. Nerve cells: * multipolar * Few * Small and equal in diameter. * Scattered throughout the ganglion * Nucleus is eccentric. * Capsule of satellites is incomplete. Sensory. Thick capsule. Nerve cells: *unipolar * many * Variable in size (20-120µ) * Grouped * Nucleus large and central * Surrounded by complete capsule of satellite cells

Autonomic (Sympathetic ) Nerve ganglia Sensory (Spinal) Autonomic (Sympathetic ) Nerve fibers: * thick & myelinated * widely separated from each other. Nerve Fibers: * Thin & non myelinated * Close to each other.

Autonomic nervous system Sympatatic & parasympatatic

Main changes that take place in an injured nerve fiber. A: Normal nerve fiber, with its perikaryon and effector cell (striated skeletal muscle). Note the position of the neuron nucleus and the quantity and distribution of Nissl bodies. B: When the fiber is injured, the neuronal nucleus moves to the cell periphery, and Nissl bodies become greatly reduced in number. The nerve fiber distal to the injury degenerates along with its myelin sheath. Debris is phagocytosed by macrophages. C: The muscle fiber shows a pronounced denervation atrophy. Schwann cells proliferate, forming a compact cord penetrated by the growing axon. The axon grows at the rate of 0.5—3 mm/day. D: Here, the nerve fiber regeneration was successful. Note that the muscle fiber was also regenerated after receiving nerve stimuli. E: When the axon does not penetrate the cord of Schwann cells , its growth is not organized.