Cns.intro-1-

The Nervous System SMS 1084 Dr. Mohanad R. Alwan

Study Questions Distinguish between the peripheral and central nervous systems. Describe a neuron, including its shape and distribution of organelles within the cell body, axon and dendrites. Describe the formation of the myelin sheath of the peripheral nerve and its appearance as seen in the light and electron microscopes. Describe two types of sensory nerve endings visible in routine histological preparations. How do neurons communicate with each other and with other cells? What is a synapse and how does it function? Define axonal transport. How do axons and dendrites regenerate? What is a ganglion and where is it found?

Functions of the nervous system Collects stimuli from the environment Transform the stimuli into nervous impulses Passes them to a largely organized reception and correlation area for interpretation and issued to receptor organs for an appropriate response.

Central nervous system (CNS) Brain Spinal cord Functions: Receive all stimuli from outside the body ( exteroceptive) Receive all stimuli from the body ( interoceptive) Acts as an integrating and communications center.

Peripheral nervous system (PNS) All other nervous tissues Function: Serves to interconnect all other tissues and organs with the CNS.

Functional division of the nervous system: With central and peripheral divisions. Somatic = concern with structure derivatives from muscles, bones, skin. Autonomic, Autonomic innervates: this function is independent from the rest of the nervous system. smooth muscles cardiac muscles glands of the body

Characteristic protoplasm of the neuron Irritability = the capacity to respond to any chemical or physical agent and initiate an impulse. Conductibility = the ability to transmit the impulse and react to the stimuli by stimulating or inhibiting other neurons they are in contact with via a “Synapse”.

Nucleus Cell body Dendrite Axon Schwann cell Synaptic knobs Node of Ranvier

Basic nerve cell structure

Nervous system terminology Nerve fibers = other name for axon and dendrites Neuron = Basic unit of nervous system Reflex arc = the integrative unit of the nervous system (e.g.,knee jerk). Afferent = sensory neuron Efferent = motor neuron

Classification of Nerve Cells (Neurons) By # of Processes: A. Unipolar – one process  B. Bipolar – one dendrite and one axon  C. Multipolar – branching to create more than one of each 

99% are Multipolar

Neuron Neuron, functional classification: motor = control effector organs – muscles and glands n sensory = receive sensory stimuli both exteroceptive or interoceptive n interneurons = connects with other neurons to establish complex functional circuits or chains of neurons. n

Types of Neurons type

connected to

sensory

receptors (eyes, ears, other sense organs) other neurons

(afferent)

carry impulses from sense organs

carry impulses to spinal cord and brain

(efferent)

effectors (muscles spinal cord and glands) and brain other neurons

muscles and glands

connector

other neurons

other neurons

motor

(interneuron )

other neurons

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3 main types of nerve cells

sensory neurone

motor neurone

Types of neurons. motor neuron cells ( anterior horn cells) = has large cell bodies with dendrites radiating in all directions and branching to forma near symmetrical dendritic area around a perikaryon. Stellate = smaller neurons with dentrites radiating in all directions, flattened in one plane. Purkinje cells = flask shaped bodies with single or branched dendrites.found in the cerebellar cortex.

Types of neurons…… Pyramidal cells = of cerebral cortex, with apical dendrites, 4 or more dendritic process, often pear shaped. Golgi type 1 = cells are well developed dendritic tree with s long axon that leaves the gray matter , enters the white matter and run in a major fiber tract of the CNS and terminate into the nerve endings in muscles or skin. Golgi type 11 = neurons are shorter axons, do not leave at the area of the perikarya. Found in the interneurons of the cerebellar and cerebral cortex

1.

Nervous Tissue Highly cellular 

How does this compare to the other tissue types?

2 cell types 1. Neurons Functional, signal conducting cells 2. Neuroglia Supporting cells

2.

NERVOUS TISSUE Types of Cells: 1. NEURON

- structural & functional unit Parts: Cell Body ; Dendrites ; Axon MYELIN SHEATH lipid protein complex covering axons SYNAPSE neuronal connections

NERVOUS TISSUE 2. NEUROGLIA - 5x more abundant than neurons - non-neuronal & non-excitable Functions: Support Insulation Nutrition Types: CNS - Oligodendroglia, Astrocytes, Ependymal Cells, Microglia PNS - Satellite Cells, Neurolemma (Schwann Cells)

Neuroglia Neuro = nerve, glia = glue  binds together the nervous tissue of the CNS  generally small, nuclei is 3 to 10 um dm  stain with special silver or gold impregnation technique.  Includes astrocytes, oligodendrites ( macroglia), micgroglia and ependymal cells  Form the myelin sheath, phagocytic and provide supporting framework for the neurons.  Some are mobile, retains capacity to subdivide.

Neuroglia 6 types of supporting cells 

2.

4 are found in the CNS:

Astrocytes Star-shaped, abundant, and versatile cytoplasm has golgi complex Guide the migration of developing neurons Act as K+ and NT buffers Involved in the formation of the blood brain barrier Function in nutrient transfer

2 types of astrocytes: protoplasmic astrocytes = found in gray matter of brain, spinal cordwith short thick processes.. Fibrous astrocyte = found in white matter, with long slender processes with few branches.

Macroglia or Oligodendrocytes Oligodendrocytes  

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smaller than astrocytes. Develop from spongioblasts ( primary ectodermal cells) Fewer than astrocytes with fewer and shorter processes. Nuclei are small, ovoid, irregular, scanty cytoplasm On electron microscopy, more dense with numerous free and attached ribosomes, extensive golgi complex Occur mainly in gray matter and among bundles of axons in white matter (interfascicular oligodendrocytes)

Macroglia… 

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acts as supporting structural element of the CNS. After damage, it removes debris and seal the damage area, might lead to scarring. Produce the myelin sheath which provides the electrical insulation for certain neurons in the CNS

Neuroglia 3. Microglia Specialized immune cells that act as the macrophages of the CNS Why is it important for the CNS to have its own army of immune cells?

4. Ependymal Cells Low columnar epithelial-esque cells that line the ventricles of the brain and the central canal of the spinal cord Some are ciliated which facilitates the movement of cerebrospinal fluid

Neurons

The functional and structural unit of the nervous system Specialized to conduct information from one part of the body to another There are many, many different types of neurons but most have certain structural and functional characteristics in common: - Cell body (soma) - One or more specialized, slender processes (axons/dendrites) - An input region (dendrites/soma) - A conducting component (axon) - A secretory (output) region (axon terminal)

Soma Contains nucleus plus most normal organelles. Biosynthetic center of the neuron. Contains a very active and developed rough endoplasmic reticulum which is responsible for the synthesis of ________.  The neuronal rough ER is referred to as the Nissl body. Contains many bundles of protein filaments (neurofibrils) which help maintain the shape, structure, and integrity of the cell.

In the soma above, notice the small black circle. It is the nucleolus, the site of ribosome synthesis. The light circular area around it is the nucleus. The mottled dark areas found throughout the cytoplasm are the Nissl substance.

Somata Contain multiple mitochondria,Why?

Acts as a receptive service for interaction with other neurons. Most somata are found in the bony environs of the CNS. Clusters of somata in the CNS are known as nuclei. Clusters of somata in the PNS are known as ganglia.

Soma…… . Nucleus       

large up to 20 um dm. euchromatic and spherical, centrally located in soma chromatin is fine and dispersed only one large nucleoli indicating active synthesis Barr bodies present Nuclear envelop is distinct with numerous pores. With large pale vesicular nucleus and prominent nucleulos “owls eye”.

Soma…. . Cytoplasmic organelles  

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plasmalemma is 7 – 8 nm thick mitochondria usually small ovoid, or spherical with cristae of both tubular and lamellar type. Large golgi apparatus & para-nuclear in position. Primary lysosomes are common, located near a golgi apparatus, associated with hydrolysis of end products of cellular metabolism Secondary lysosomes increase in number with age, some becomes lipofuscin granules.

PROMINENT FEATURES OF NEURONS NISSL bodies are basophilic component of the cytoplasm, stained by basic aniline dyes bodies are stack of granular endoplasmic reticulum with associated polysomes and ribosomes both free and attached. Present in dendrites but absent in axon and the axon hillock. Axon hillock = clear conical areas at the origin of axon from the soma. Nissl bodies react to injury by breaking up and diffusing throughout the cytoplasm called chromatolysis

Prominent features of neurons Neurofibrils    

visible by light microscopy found through out the parikaryon extend into dendrites and axons. The bundles contain microtubules ( neurotublues, 25 nm dm ) and microfilaments ( neurofilaments, 10 nm dm).

Prominent features of neuron… Cytoplasmic inclusions.      

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fat droplets commonly found in Soma glycogen not present in adult neurons. Pigments granules various type present Lipofuscin granules in large neurons. Granules are yellow brown Iron granules present in nerve cells of various regions like in globus pallidus, tend to increase in number with age. Melanin is present in: substantia nigra of midbrain, spinal and sympathetic ganglia Locus ceruleus in 4th ventricle floor.

Synapses  

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sites of transneuronal transmission of nerve impulse. Electrical synapses = electrical signals passes directly to the adjacent cells by a low resistance gap junctions. A. presynaptic element ( synaptic bout on) B. Postsynaptic element ( dendrite) with narrow extracellular or synaptic cleft 20 – 30 nm wide with fine filaments Asymmetrical synapse = 30 nm wide postsynaptic membrane Symmetrical synapse = 20 nm wide

Synapses… 

Neurotransmitter substances:

acetylcholine norepinephrine serotonin others: enkephalins, neurotensins, pituitary peptides.

Ganglia 

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A collection of nerve cell bodies located outside the central nervous system. Types of ganglia: Craniospinal ganglia ( sensory) Autonomic nerve system (visceral, motor ganglia).

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vary in size and shape with few nerve cell bodies to large one up to 50,000 or more cells. Satellite cells = capsule of single layer of cuboidal cells

Ganglia

Craniospinal ganglia spinal ganglia are fusiform or globular swellings of the posterior roots of the spinal nerves. Cranial ganglia are pseudounipolar, globular, with single process (axon) that becomes convoluted as it leaves the perikaryon to form a glomerulus. Impulse transmission in these sensory neurons appears directly from the peripheral to the central process, bypassing the soma.

Craniospinal ganglia….  appear as swelling along the sympathetic chain  has a connective tissue capsule that envelops the

ganglia.  Ganglion cells are multipolar, are adrenergic, with several dendrites and a single unmyelinated axon,  smaller than the sensory ganglia, 15 to 45 um dm  around the perikarya are capsule cells, few and small:

Contains small vesicles with dense core, has neurotransmitter noradrenaline ( norepinephrine). Has a few smaller cells with larger dense cored vesicles, are dopaminergic and release their contents directly into the blood stream on stimulation.

Nerve fibers  all nerve fibers , nerve cell processes both within and

outside the CNS are covered by one or more sheaths.  CNS cover: fibers are covered partially by Glial cells, small supporting cells of the CNS. Myelin sheath cover: * in the PNS as fine sensory fibers, postganglionic fibers of ANS * axons of the olfactory nerve. * in the PNS, it has sheath covered by Schwann sheath ( neurolemma or neurolimma ). * Myelinated fibers of the PNS have a sheath of myelin & Schwann sheath.

Schwann cells 

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envelops all nerve fibers of the PNS from attachment to the spinal cord and brain stem. With a heterochromatic nucleus, flattened centrally located in the cell, numerous mitochondria, microtubules and microfilaments ( lysosomes), some granular endoplasmic reticulum and small golgi apparatus. Unmyelinated fibers, lie singly or in groups in longitudinal gutters or invaginations of Schwann cells within the original line of invagination “ mesaxons”. Ectodermal origin, essential to the vitality and function of the peripheral nerve fibers. Necessary for axon.

Myelin 

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myelinated fibers in the PNS nerve fiber is surrounded by a tubular sheath of myelin. Myelin is responcilbe for the color of the white matter of the brain and spinal cord. Neurokeratin = meshwork of protein material network around a nerve fiber. Nodes or Ranvier = gaps in the sheath of nerve fibers. Internodes = segment between nodes of ranvier. Period lines = regular mature myeline lamination of concentric, dense nature, 3 nm thick separated by light iintervals of 10 nm. Intraperiod lines = finer dense lines of 2 nm thickness bisecting the spaces between period lines. “ Jelly roll “ Hypothesis.

Schmidt-Lantermann Clefts (incisures)  

visible by light microscopy appear as small radial clefts or fissures extending thru the thickness of the myelin fixed in osmium tetroxide.

Peripheral nerves 

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composed of bundles of nerve fibers are held together by connective tissue. Appear as white due to myelinated fibers. Epineurium = capsule of connective tissue sheath around a nerve, composed of fibroblasts and collagenous fibers, longitudinal and elastic fibers, major blood vessels.

Peripheral nerves 

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Fascicles = bundles within an epineurium. Perineurium = connective tissue sheath around each fascicle, formed by concentric layers of flattened fibroblast-like cells one cell thick. It is continous with the pia arachnoid membrane of the CNS. Provides a barrier to the passage of materials into or out of the nerve fascicle. Endoneurium = strands of delicate connective tissue extending around and between individual nerve fibers.

Membranes of CNS Dura matter or pachymenninx = fibrous tough and inelastic, lines the cranium and attached firmly to the bone. Separated from the bone by an epidural or extradural space. Composed of the: Falx cerebelli= separate the 2 cerebral hemisphere Tentorium cerebelli= horizontal, between the occipital lobes of cerebrum above and below the cerebellum.

Membranes ….. Arachnoid membrane = the middle layer, composed of fine cobweb like strands of interlacing reticular fibers The thin, delicate, nonvascular membrane that lines the dura. Has a space” subarachnoid space”between trabeculae

Membranes …. Pia matter = the innermost layer, with blood vessels supplying the CNS, single layered” leptomeninnx or leptomeninges. The deeper layer “ Intima pia” close meshwork of fine reticular and elastic fibers adherent to the underlying nervous tissue. Contains branches of the internal carotid and vertebral artery into the substance of CNS.