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Functional Organization of Nervous Tissue the Nervous System

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Functional Organization of Nervous Tissue the Nervous System Nervous tissue , charachteristics, neurons, glial cells Functional Organization of Nervous Tissue The Nervous System • Components – Brain, spinal cord, nerves, sensory receptors • Responsible for – Sensory perceptions, mental activities, stimulating muscle movements, secretions of many glands • Subdivisions – Central nervous system (CNS) – Peripheral nervous system (PNS) The Nervous System • THE NERVOUS SYSTEM • THE RAPID COMMUNICATION SYSTEM OF THE BODY, (CARRYING ELECTROCHEMICAL IMPULSES). • Functions of the nervous system:- • SENSATION - in internal & external environment • INTEGRATION - of sensory information • CO-ORDINATION - of voluntary & involuntary actions • REGULATION - of peripheral structures & systems • HIGHER FUNCTIONS - CENTRAL NERVOUS SYSTEM (CNS) • = BRAIN, SPINAL CORD • (Approx 95% body's neural tissue) • Performs INTEGRATION & CO-ORDINATION of nervous activity PERIPHERAL NERVOUS SYSTEM (PNS) • ALL OF NEURAL TISSUE OUTSIDE CNS • Links CNS with all of body tissues & outside world Central Nervous System • Consists of – Brain • Located in cranial vault of skull – Spinal cord • Located in vertebral canal • Brain and spinal cord – Continuous with each other at foramen magnum Peripheral Nervous System • Two subcategories – Sensory or afferent – Motor or efferent • Divisions – Somatic nervous system – Autonomic nervous system (ANS) » Sympathetic » Parasympathetic » Enteric Nervous System Organization Another way to look at the Nervous System • The master controlling and communicating system of the body • Functions – Sensory input – monitoring stimuli occurring inside and outside the body – Integration – interpretation of sensory input – Motor output – response to stimuli by activating effector organs Nervous System Organization of the Nervous System • Central nervous system (CNS) – Brain and spinal cord – Integration and command center • Peripheral nervous system (PNS) – Paired spinal and cranial nerves – Carries messages to and from the spinal cord and brain Peripheral Nervous System (PNS): Two Functional Divisions • Sensory (afferent) division – Sensory afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain – Visceral afferent fibers – transmit impulses from visceral organs to the brain • Motor (efferent) division – Transmits impulses from the CNS to effector organs Motor Division: Two Main Parts • Somatic nervous system – Conscious control of skeletal muscles • Autonomic nervous system (ANS) – Regulates smooth muscle, cardiac muscle, and glands – Divisions – sympathetic and parasympathetic Histology of Nerve Tissue • The two principal cell types of the nervous system are: – Neurons – excitable cells that transmit electrical signals – Supporting cells – cells that surround and wrap neurons Cells of Nervous System • Neurons or nerve cells – Receive stimuli and transmit action potentials – Organization • Cell body or soma • Dendrites: Input • Axons: Output • Neuroglia or glial cells – Support and protect neurons Nerve Cells • Two distinct cell types form nervous tissue. • Neurons, which are excitable cells that initiate and transmit nerve impulses • Glial cells, which are nonexcitable cells that support and protect the neurons Characteristics of Neurons • Neurons have a high metabolic rate. • Neurons have extreme longevity. • Neurons typically are non-mitotic. Neuron Structure • Neurons come in all shapes and sizes, but all neurons share certain basic structural features. • A typical neuron has a cell body, dendrites, and axons. • Neuron Structure – Cell Body • The cell body serves as the neuron’s control center and is responsible for receiving, integrating, and sending nerve impulses. Neuron Structure – Dendrites • Dendrites tend to be shorter, smaller processes that branch off the cell body. • Some neurons have only one dendrite, while others have many. • Dendrites conduct nerve impulses toward the cell body; they receive input and then transfer it to the cell body for processing. • The more dendrites a neuron has, the more nerve impulses that neuron can receive from other cells. Neuron Structure – Axon • The larger, typically longer nerve cell process emanating from the cell body is the axon, sometimes called a nerve fiber. • Most neurons have only one axon. • The axon transmits a nerve impulse away from the cell body toward another cell. Types of Neurons • Functional classification – Sensory or afferent: Action potentials toward CNS – Motor or efferent: Action potentials away from CNS – Interneurons or association neurons: Within CNS from one neuron to another • Structural classification – Multipolar, bipolar, unipolar Classifications of Neurons • Neurons vary widely in morphology and location. • They can be classified according to either their structure or their function. • Neurons can be classified according to the number of processes extending from the cell body. • unipolar neuron has a single process • bipolar neurons have two processes • multipolar neurons have three or more processes • The neurons entirely within the CNS are upper motor neurons • • The neurons with cell bodies in the CNS , but whose axons pass into the peripheral nerves are lower motor neurons. • 11-27 • Interneurons • Interneurons, or association neurons, lie entirely within the CNS and are multipolar. • They receive nerve impulses from many other neurons and carry out the integrative function of the nervous system. • Thus, interneurons facilitate communication between sensory and motor neurons. Neuroglia of CNS • Astrocytes – Regulate extracellular brain fluid composition – Promote tight junctions to form blood-brain barrier • Ependymal Cells – Line brain ventricles and spinal cord central canal – Help form choroid plexuses that secrete CSF Neuroglia of CNS • Microglia – Specialized macrophages • Oligodendrocytes – Form myelin sheaths if surround axon Neuroglia of PNS • Schwann cells or neurolemmocytes – Wrap around portion of only one axon to form myelin sheath • Satellite cells – Surround neuron cell bodies in ganglia, provide support and nutrients • Satellite cells (syn: mantlecells or amphicytes) are flattened Schwann cells, a type of glial cell, lining the exterior surface of neurons in the peripheral nervous system. Satellite cells also surround neuron cell bodies within ganglia. They are thought to have a similar role to astrocytes in the central nervous system (CNS). They supply nutrients to the surrounding neurons and also have some structural function. Satellite cells also act as protective, cushioning cells. 11-33 • Sometimes referred to as neuroglia, occur withinboth the CNS and the PNS. • Glial cells are smaller and capable of mitosis. • Glial cells do not transmit nerve impulses. • Glial cells physically protect and help nourish neurons, and provide an organized, supporting framework for all the nervous tissue. • Glial cells far outnumber neurons. • Glial cells account for roughly half the volume of the central nervous system • They say that einstein’s brain did not contain more neurones but more glial cells 11-36 11-37 11-38 Glial Cells of the CNS • Astrocytes exhibit a starlike shape due to projections from their surface. • Astrocytes are the most abundant glial cells in the CNS, and they constitute over 90% of the tissue in some areas of the brain. • Help form the blood-brain barrier (BBB) that strictly controls substances entering the nervous tissue in the brain from the bloodstream. • Regulate tissue fluid composition. Functions of Glial Cells • Forming a structural network. • Replacing damaged neurons. • Assisting neuronal development Myelination • Neurolemmocytes also called Schwann cells, are associated with PNS axons and are responsible for myelinating PNS axons. • Myelination is the process by which part of an axon is wrapped with a myelin sheath, a protective fatty coating that gives it glossywhite appearance. • The myelin sheath supports, protects, and insulates an axon. • No change in voltage can occur across the membrane in the insulated portion of an axon. • In the PNS, myelin sheaths form from neurolemmocytes. • In the CNS, they form from oligodendrocytes 11-43 11-44 11-45 Mylenated vs. UnmylenatedAxon • In a myelinated axon, the nerve impulse “jumps” from neurofibril node to neurofibril node and is known as saltatory conduction. • In an unmyelinated axon, the nerve impulse must travel the entire length of the axon, a process called continuous conduction. • A myelinated axon produces a faster nerve impulse 11-47 Myelinated and Unmyelinated Axons • Myelinated axons – Myelin protects and insulates axons from one another – Not continuous • Nodes of Ranvier • Unmyelinated axons Electrical Signals • Cells produce electrical signals called action potentials • Transfer of information from one part of body to another • Electrical properties result from ionic concentration differences across plasma membrane and permeability of membrane Action Potential Propagation Saltatory Conduction 11-62 11-63 Regeneration of PNS Axons • PNS axons are vulnerable to cuts, crushing injuries, and other trauma. • A damaged axon can regenerate, however, if at least some neurilemma remains. • PNS axon regeneration depends upon three factors. • the amount of damage • neurolemmocyte secretion of nerve growth factors to stimulate outgrowth of severed axons • the distance between the site of the damaged axon and the effector organ 11-65 11-66 11-67 11-68 • A common neurotransmitter is acetylcholine, although there are many more used by different neurones.
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