Nervous System.pdf

Nervous system Nervous system Two body systems control all the functions of the body : Nervous system - which works very rapidly Endocrine system - which works more or less slowly. Nervous system Nervous System The nervous system collect stimuli from the environment. Transforms such stimuli into ner...

Nervous system Nervous system Two body systems control all the functions of the body : Nervous system - which works very rapidly Endocrine system - which works more or less slowly. Nervous system Nervous System The nervous system collect stimuli from the environment. Transforms such stimuli into nervous impulses , and passes them to a large highly organized reception and correlation area where they are interpreted , and in turn, issued to effector organs to institute appropriate responses. Nervous system Major Structures of the Nervous System : Brain Spinal cord Cranial nerves Spinal nerves Ganglia Enteric plexuses Sensory receptors Nervous system The human nervous system : The major organs and nerves of the human nervous system. Spinal nerves are peripheral nerves that carry motor commands and sensory information into the spinal cord. Cranial nerves are the nerve fibers that carry information into and out of the brain stem. They include information related to smell, vision, eyes, eye muscles, the mouth, taste, ears, the neck, shoulders, and the tongue. Association nerves integrate sensory input and motor output; these nerves number in the thousands. Nervous system Sympathetic ( fight or flight ) Parasympathetic ( rest & digest ) Nervous system The autonomic nervous system (ANS) is the part of the peripheral nervous system that acts as a control system, functioning largely below the level of consciousness and controlling visceral functions. The ANS affects heart rate, digestion, respiratory rate, salivation, perspiration, pupillary dilation, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind. The ANS is classically divided into two subsystems: the parasympathetic nervous system (PSNS) and sympathetic nervous system (SNS). The enteric nervous system is sometimes considered part of the autonomic nervous system, and sometimes considered an independent system. Nervous system Nervous System The most complex system in the human body. Formed by network more than 100 million neuron. Each neuron has a thousand interconnection in a very complex system for communication. Nerve tissue is distribute throughout the body, anatomically divide into : CNS & PNS. Structurally consist : nerve cells & glial cells Nervous system The CNS Consist of : Cerebrum Cerebellum Spinal cord No connective tissue, soft, gel like When sectioned : White matter & Gray matter Covered by meninges Nervous system Cerebrum Gray Matter : White Matter Gray Matter Contains neuronal cell bodies, dendrites and glial cells Six layers composed of neuron White Matter: Contains myelinated axons and myelin-producing oligodendrocytes Cerebellum Gray Matter: White Matter Three layers: Outer molecular layer Central layer of large Purkinje cells & Inner granule layer White Matter: The same as cerebrum Nervous system Spinal Cord Gray Matter (central) Shape of “H” Central canal lined by Ependymal cells Legs of the “H” form : Anterior horns & Posterior horns Neurons : large and multipolar White Matter (peripheral) Nervous system Gray and White Matter White matter = myelinated processes (white in color) Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color) In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS Nervous system Classification of nervous system -Functionally/physiologically: Somatic nervous system (SNS) Autonomic nervous system (ANS) a- Sympathetic b- Parasympathetic Anatomically and histologically : Central nervous system (CNS) (brain and spinal cord) Peripheral nervous system (PNS) (cranial and spinal nerves with their ganglia ) The PNS Bundles of nerve fibers (axons) outside the CNS & surrounded by connective tissue. Main component: ( Peripheral nerves \ Ganglia \ Nerve endings ) Nervous system Subdivisions of the PNS Central nervous system (cns) Peripheral nervous system (pns) a) Somatic (voluntary) nervous system (sns) b) Autonomic (involuntary) nervous systems (ans) c) Enteric nervous system (ens) Nervous system Subdivisions of the PNS Autonomic Effectors: cardiac & smooth muscle, viscera & glands ƒefferent: Neuron chain neurotransmitter (NT) effects Stimulatory or inhibitory dependent on NT & receptor type Somatic Effectors: skeletal muscles efferent Efferent: heavily myelinated axons from CNS to muscle Neurotransmitter effects: acetylcholine excitatory effect in nicotinic (N1) receptors Nervous system The PNS Bundles of nerve fibers (axons) outside the CNS & surrounded by connective tissue. Main component: Peripheral nerves Ganglia Nerve endings Nerve Fibers Consist of axons enveloped by a special sheath. Group of fibers constitute the peripheral nerve. Two types: Myelinated fiber Unmyelinated fiber Nervous system Peripheral Nerve Ganglia Ovoid structure containing neuronal cell bodies, glial cells supported by connective tissue. Function : Relay stations to transmit impulses. Types: Sensory ganglia & Autonomic ganglia Ganglia Cranial ganglia: Associated with the cranial nerve Sensory Ganglia (cell bodies of sensory neuron) Unipolar cell bodies enveloped by cuboidal capsule cells Spinal ganglia: Associated with the spinal nerve Autonomic Ganglia Cell bodies of postganglionic autonomic nerves Multipolar neuron enveloped by satellite cells. Some are located within certain organ (intramural). Nervous system Sensory ganglia Example : spinal ganglia Motor ganglia Example : sympathetic or parasympathetic ganglia Shape : large , rounded Shape : small angular Neuron : pseudo-unipolar Neuron : multipolar neuron Nucleus : centrally Nucleus : eccentric Found in groups Found in scattered Cells : well define satellite cell Cells : poorly define satellite cell present present Nervous system Somatic (voluntary) nervous system (SNS) Neurons from cutaneous and special sensory receptors to the CNS Motor neurons to skeletal muscle tissue Autonomic (involuntary) nervous systems (ANS) Sensory neurons from visceral organs to CNS Motor neurons to smooth & cardiac muscle and glands Sympathetic division (speeds up heart rate) Parasympathetic division (slow down heart rate) Enteric nervous system (ENS) Involuntary sensory & motor neurons control GI tract Neurons function independently of ANS & CNS Nervous system Connective Tissue Investments Epineurium: Dense collagenous Con. Tissue with thick elastic fiber Prevent damage by overstretching Perineurium : Dense con. Tissue Isolates neural environment (blood-nerve barrier) Endoneurium: Loose con. Tissue Regulation of microenvironment of nerve fiber Nervous system Structure of Neuron Neurons are the basic functional units of nervous tissue. They are highly specialized to transmit nerve impulses. Nervous tissue is made up of just 2 types of cells: Neurons Neuroglia (glial cells) (supporting cells) Nervous system The basic structure units in nervous system : Neurons (nerve cell) Supporting cells a-Neuroglia (in CNS) b-Schwann cell (in PNS) Nervous system Neuron-Structural & functional unit of nervous system consist of 2 parts Cell body/soma/perikaryon Process/neurite 2 types of neurite1. Axon 2. Dendrite (Axon & dendrite) is called nerve fiber. Nervous system Cell Body (Perikaryon) Central portion of the cell. Generally are polygonal. Different shape and size has characteristic regions of nervous system. Contain : Nucleus Perinuclear cytoplasm Nervous system Cell Body (Perikaryon) Ultrastructure of Neuron Nucleus : large, spherical to ovoid and centrally located single prominent nucleolus. Finely dispersed chromatin. Cytoplasm : Abundant of R.E.R Polyribosomes Basic dyes (a+b) Nissl Bodies lots of S.E.R Golgi bodies (perikaryon) protein secreting cell Many mitochondria, most abundant in axon terminal Extensive cytoskeleton Axonal transport One centriole Do not undergo cell divisions Nervous system Dendrite and Axon Axon : Single process up to 100 cm Originate from axon hillock Devoid ribosome Dilatation of distal portion ( axon terminal \ end bulbs \ synapse ) conducting impulse away from the soma Axonal transport Dendrites: Multiple elongated processes Receiving stimuli Nervous system Supporting cells of the CNS Glial cells of the CNS Astrocytes Oligodendrocytes…myelination Microglial Ependymal cells Nervous system Supporting cells (glial cells) of the PNS Schwan cells Satellite cells These supporting “glial” brace and protect the fragile neuron cells Act as phagocytes Control the chemical environment around the nerve cells. More about supporting cells later Nervous system All nerve cells have a cell body, also called the soma This is the control center of the cell. The cytoplasm contains mitochondria, lysosomes, a Golgi complex, numerous inclusions, and extensive rough endoplasmic reticulum(Nissl bodies) and cytoskeleton Cytoskeleton consists of dense mesh of microtubules and neurofibrils (bundles of actin filaments) Nervous system Structure of a Neuron Dendrites – are receptive Regions : vast number of branches coming from a few thick branches from the soma Resemble bare branches of a tree in winter Primary site for receiving signals from other neurons The more dendrites the neuron has, the more information it can receive and incorporate into decision making Provide precise pathway for the reception and processing of neural information Nervous system Structure of a Neuron Axon (nerve fiber) cylindrical, relatively unbranched for most of its length Generate and conduct nerve impulses but branch into co-laterals Schwann cells and myelin sheath enclose axon The Axon ends in many small structures called Axon terminals or synaptic knob (terminal button) – little swelling that forms a junction (synapse) with the next cell Contains synaptic vesicles full of neurotransmitter Nervous system Structural classification of neurons Based on number of processes found on cell body Multipolar : several dendrites & one axon most common cell type Bipolar neurons : one main dendrite & one axon found in retina, inner ear & olfactory Unipolar neurons : one process only(develops from a bipolar) are always sensory neurons Nervous system Axons are covered with a fatty material called myelin. Axons in the PNS are heavily myelinated. This is done by the Schwann Cells These Schwann cells layer around the axions and squeeze their cytoplasm out creating many layers of plasma membrane tissues (proteins/lipids) surrounding the axion. This is the Myelin sheath. Areas of neuron not covered are called Nodes of Ranvier. Myelin insulates the nerve fibers and greatly increases the speed of neurotransmission by nerve fibers. Nervous system Nervous system Each axon terminal (synaptic knob) is separated from the cell body or dendrites of the next neuron by a tiny gap…synaptic cleft. Neurotransmitters are released into the synaptic cleft and diffuse across to bind to membrane receptors on the next neuron..initiating an electrical current or synaptic potential. Nervous system Axonal Transport Many proteins made in soma must be transported to axon and axon terminal to repair axolemma, serve as gated ion channel proteins, as enzymes or neurotransmitters Axonal transport – two-way passage of proteins, organelles, and other material along an axon Anterograde transport – movement down the axon away from soma Retrograde transport – movement up the axon toward the soma Nervous system Axonal Transport Microtubules guide materials along axon Motor proteins (kinesin and dynein) carry materials “on their backs” while they “crawl” along microtubules Kinesin – motor proteins in anterograde transport towards outside Dynein – motor proteins in retrograde transport towards center Nervous system Neuroglial Cells About a trillion (1012) neurons in the nervous system Neuroglia outnumber the neurons by as much as 50 to 1 neuroglia or glial cells Support and protect the neurons Bind neurons together and form framework for nervous tissue in fetus, guide migrating neurons to their destination If mature neuron is not in synaptic contact with another neuron is covered by glial cells Prevents neurons from touching each other gives precision to conduction pathways Nervous system Six Types of Neuroglial Cells Four types occur only in CNS Oligodendrocytes Form myelin sheaths in CNS each arm-like process wraps around a nerve fiber forming an insulating layer that speeds up signal conduction Ependymal cells Form epithelial membrane lining cerebral cavities & central canal cuboidal epithelium with cilia on apical surface secretes and circulates cerebrospinal fluid (CSF) clear liquid that bathes the CNS Nervous system Six Types of Neuroglial Cells Four types occur only in CNS Microglia Small, wandering macrophages formed white blood cell called monocytes thought to perform a complete checkup on the brain tissue several times a day wander in search of cellular debris to phagocytize Nervous system Astrocytes Most abundant glial cell in CNS Cover entire brain surface and most nonsynaptic regions of the neurons in the gray matter of the CNS Diverse functions : Form a supportive framework of nervous tissue Have extensions (perivascular feet) that contact blood capillaries that stimulate them to form a tight seal called the blood-brain barrier Convert blood glucose to lactate and supply this to the neurons for nourishment Nerve growth factors secreted by astrocytes promote neuron growth and synapse formation Nervous system Astrocytes Communicate electrically with neurons and may influence synaptic signaling Regulate chemical composition of tissue fluid by absorbing excess neurotransmitters and ions Astrocytosis or sclerosis – when neuron is damaged, astrocytes form hardened scar tissue and fill space formerly occupied by the neuron. Nervous system Neuroglial Cells (CNS): Astrocytes Star-shaped cells Form blood-brain barrier by covering blood capillaries Metabolize neurotransmitters Regulate K+ balance Provide structural support Nervous system Two types of Neuroglial Cells in the PNS Schwann cells Envelope nerve fibers in PNS Wind repeatedly around a nerve fiber produces a myelin sheath similar to the ones produced by oligodendrocytes in CNS assist in the regeneration of damaged fibers Satellite cells Surround the neurosomas in ganglia of the PNS Provide electrical insulation around the soma Regulate the chemical environment of the neurons Nervous system Glial Cells and Brain Tumors Tumors - masses of rapidly dividing cells mature neurons have little or no capacity for mitosis and seldom form tumors brain tumors arise from : Meninges (protective membranes of CNS ) By metastasis from non-neuronal tumors in other organs Most come from glial cells that are mitotically active throughout life Gliomas grow rapidly and are highly malignant Blood-brain barrier decreases effectiveness of chemotherapy Treatment consists of radiation or surgery Nervous system More facts about Myelin Myelin sheath – an insulating layer around a nerve fiber Formed by oligodendrocytes in CNS and Schwann cells in PNS Consists of the plasma membrane of glial cells , 20% protein and 80 % lipid Myelination – production of the myelin sheath Begins the 14th week of fetal development Proceeds rapidly during infancy Completed in late adolescence Dietary fat is important to nervous system development Nervous system Myelin In PNS, Schwann cell spirals repeatedly around a single nerve fiber lays down as many as a hundred layers of its own membrane No cytoplasm between the membranes Neurilemma – thick outermost coil of myelin sheath Contains nucleus and most of its cytoplasm External to neurilemma is basal lamina and a thin layer of fibrous connective tissue – endoneurium Nervous system Myelin In CNS – oligodendrocytes reaches out to myelinate several nerve fibers in its immediate vicinity Anchored to multiple nerve fibers Cannot migrate around any one of them like Schwann cells Must push newer layers of myelin under the older ones So myelination spirals inward toward nerve fiber Nerve fibers in CNS have no neurilemma or endoneurium Nervous system Myelin Many Schwann cells or oligodendrocytes are needed to cover one nerve fiber Myelin sheath is segmented Nodes of Ranvier – gap between segments Internodes – myelin covered segments from one gap to the next Initial segment – short section of nerve fiber between the axon hillock and the first glial cell Trigger zone – the axon hillock and the initial segment play an important role in initiating a nerve signal Nervous system Diseases of Myelin Sheath Degenerative disorders of the myelin sheath Multiple sclerosis Oligodendrocytes and myelin sheaths in the CNS deteriorate Myelin replaced by hardened scar tissue Nerve conduction disrupted (double vision, tremors, numbness, speech defects) Onset between 20 and 40 and fatal from 25 to 30 years after diagnosis Cause may be autoimmune triggered by virus Nervous system Diseases of Myelin Sheath Degenerative disorders of the myelin sheath Tay-Sachs disease A hereditary disorder of infants of Eastern European Jewish ancestry Abnormal accumulation of glycolipid called GM2 in the myelin sheath Normally decomposed by lysosomal enzyme Enzyme missing in individuals homozygous for Tay-Sachs allele Accumulation of ganglioside (GM2) disrupts conduction of nerve signals Blindness, loss of coordination, and dementia Fatal before age 4 Nervous system Regeneration & Repair Plasticity maintained throughout life Sprouting of new dendrites Synthesis of new proteins Changes in synaptic contacts with other neurons Limited ability for regeneration (repair) PNS can repair damaged dendrites or axons CNS no repairs are possible Nervous system Neurogenesis in the CNS Formation of new neurons from stem cells was not thought to occur in humans 1992 a growth factor was found that stimulates adult mice brain cells to multiply 1998 new neurons found to form within adult human hippocampus (area important for learning) Factors preventing neurogenesis in CNS Inhibition by neuroglial cells, absence of growth stimulating factors, lack of neurolemmas, and rapid formation of scar tissue