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THE NERVOUS SYSTEM Nervous system glossary Major anatomical and functional divisions: central nervous system (CNS) which contain control centers responsible for processing and integrating planning and responses to stimuli and providing short-term control over the activities of other systems peripheral nervous system (PNS) neural tissue outside the CNS whose function is to link the CNS with sense organs and other systems autonomic nervous system (ANS) components of the central nervous system and peripheral nervous system that are connected with the control of visceral functions Gross anatomy nucleus: a group of cell bodies with boundaries only in the CNS Center: a group of neuron cell bodies in the central nervous system that share a common function tract: a bundle of axons within the CNS that share a common origin, destination, and function Column: a group of tracks down within a specific region of the spinal cord pathways: centers and tracts that connect the brain with other organs and systems in the body ganglia: and anatomically distinct collection of sensory or motor neuron cell bodies within the peripheral nervous system nerve: a bundle of axons in the peripheral nervous system ____________________________________________________________________________________________________ Histology Grey matter: neural tissue dominated by neuronal cell bodies White matter: neural tissue dominated by myelinated axons Neural Cortex: a layer of gray matter at the surface of the brain neuron: the basic functional unit of the nervous system; a highly specialized cell; a nerve cell sensory neuron: a neuron whose acts on carries sensory information from the PNS towards the CNS motor neuron: a neuron whose acts on carries motor commands from the CNS towards the factors in the PNS Soma: the cell body of a neuron dendrite: neuronal processes that are specialized to respond to specific stimuli in the extracellular environment axon: a long slender cytoplasmic process of a neuron; axons are capable of conducting nerve impulses called action potentials myelin: a membranous wrapping, produced by glial cells, that coats axons and increases the speed of action potential propagation; axons coded with Mylan are said to be myelinated neuroglial or glial cells: supporting cells that interact with neurons and regulate the extracellular environment, provide defense against pathogens, and perform repairs with neural tissues FUNCTIONAL CATEGORIES: Receptor: effector: reflex: somatic: visceral: voluntary: a specialized cell, dendrite, or organ that responds to specific stimuli in the extracellular environment and who stimulation alters the level of activity in a sensory neuron a muscle, gland, or other specialized cell or organ that responds to neural stimulation by altering its activity and producing a specific effect a rapid stereotyped response to a specific stimulus pertaining to the control of skeletal muscle (somatic motor) activity or sensory information from skeletal muscles, tendons, and joints (somatic sensory) pertaining to the control of functions, such as digestion, circulation, etc. (visceral motor) or sensory information from visceral organs (visceral sensory) under direct conscious control involuntary: subconscious: action potential: not under direct conscious control, operations performed by ANS pertaining to centers in the brain that operate outside of persons conscious awareness sudden, transient changes in the membrane potential that are propagated along the surface of an axon or sarcolemma ARCHITECTURE OF THE NERVOUS SYSTEM The diagram below is a representation of the architecture of the nervous system. If you'll notice at the very top is the central nervous system. It's necessary to understand that the brain and spinal cord receive all sensory information from the outside world and the inside world called visceral. The brain is completely isolated from everything. It is dependent upon sensory receptors from our five senses and sensors in our organs and internal tissues. The two main divisions of the nervous system are sensory and motor. The function of the sensory system is to transmit information from the outside world and the viscera to the central nervous system. The function of the motor system is to send commands from the brain to the peripheral nervous system. The sensory system is comprised of special sensory receptors such as eyes ears nose and mouth and touch, as well as somatic sensory receptors which monitors skeletal muscles and joints skin surface; provide position sense and touch, pressure, pain and temperature sensations. Visceral sensory receptors monitor internal organs, including those of cardiovascular, respiratory, digestive, urinary, and reproductive systems. The motor commands also called efferent, include signals to somatic or skeletal muscles and the viscera via the autonomic nervous system. The autonomic nervous system has two divisions called sympathetic and parasympathetic. Both of these systems, sympathetic and parasympathetic have opposite actions on the viscera which include smooth muscle cardiac muscle and glands. The sympathetic system increases activity and the parasympathetic system decreases activity. The smooth muscle cardiac muscle and glands are called effect or organs. Skeletal muscles are also called effect or organs. Functions of the Nervous System 1) Sensory input • 2) Monitor/collecting stimuli occurring inside and outside the body – outside = somatic sensory (SS), inside = visceral sensory (VS) Integration (by interneurons) • To process and interpret sensory input and decide if action is needed (homeostasis) 3) Motor output • A response to integrated stimuli which only sends an AP • The response activates muscles or glands Structural Classification of the Nervous System Central nervous system (CNS) • Brain • Spinal cord Peripheral nervous system (PNS) • Nerves outside the brain and spinal cord including cranial nerves, nerves and ganglia Functional Classification of the Peripheral Nervous System • Sensory (afferent) division • • Nerve fibers that carry information to the central nervous system from the five senses and the internal viscera (guts) Motor (efferent) division • Nerve fibers that carry impulses away from the central nervous system to muscles and glands • Somatic nervous system = voluntary • Autonomic nervous system = involuntary (sympathetic and parasympathetic) Organization of the Nervous System Neurons and neuroglial cells (glial) Neuroglial cells: 1) Astrocytes • The most abundant cell type, star-shaped cells in CNS • They regulate neurotransmitter levels, signal increased blood flow through capillaries in active portions of the brain, control ionic environment of neurons and assist synaptic formation in developing neural tissue 2) Microglial Cells: • The smallest and least abundant glial cells with elongated cell bodies and cell processes with many ointed projections. They are phagocytes, keeping the area clear of debris and foreign materials 3) Ependymal Cells: These important cells line the ventricles and have cilia which circulate the cerebrospinal fluid (CSF) 4) Oligodendrocytes • Produce myelin sheath around nerve fibers in the central nervous system (CNS) One cell can myelinate many dendrites and axons (see figure) Neuroglia of PNS 1) Satellite cells • 2) Protect neuron cell bodies within ganglia (a group of cell bodies in the PNS) Schwann cells • Form myelin sheath around axons in the peripheral nervous system which increases the speed of the AP and insulates the impulse from neighboring axons Neuroglia vs. Neurons • • Neuroglia • Do not transmit electrical impulses • Undergo mitosis (brain tumors) Neurons (nerve cells) • Cells that are highly specialized to transmit messages by way of Action Potentials (AP) • Neurons live as long as you do. They do not reproduce! • Neurons are very highly metabolic requiring constant glucose and oxygen without which they can only live minutes Neuron Anatomy 1) Cell body • Nissl substance is specialized rough endoplasmic reticulum • Neurofibrils are intermediate cytoskeleton that maintains cell shape • Nucleus • Large nucleolus • Axon hillock – the area at the junction of the cell body and the axon 2) Dendrites are extensions that conduct impulses toward the cell body 3) Axons are extensions that conduct impulses away from the cell body 4) Neurons may have many shapes: multipolar – more than two processes, numerous dendrites and one axon, Bipolar: two processes that extend from opposite sides of the cell body and appear only in the inner ear, olfactory epithelium of the nose and the retina. Unipolar: one short process that emerges from the body and divides like an inverted T into two long branches. Axons and Nerve Impulses • Axons end in axonal terminals, also called bulb, button or bouton • Axonal terminals contain vesicles with neurotransmitters • Axonal terminals are separated from the next neuron by a gap • Synaptic cleft is the gap between adjacent neurons • Synapse is the junction between nerves Nerve Fiber Coverings • Schwann cells produce myelin sheaths (insulation) in jelly-roll like fashion • Nodes of Ranvier are gaps in myelin sheath along the axon caused by separation of different schwann cells • Multiple sclerosis is an autoimmune disease where myelin sheaths are destroyed causing an electrical short-circuits Functional Classification of Neurons • Sensory (afferent) neurons carry impulses towards the CNS • Cutaneous sense organs (pain/temperature, touch, pressure) • Proprioceptors – detect stretch or tension in the viscera Interneurons: integrate information between sensor and motor neurons (very abundant in the brain, less so in the spinal cord and only appear in the CNS) • Motor (efferent) neurons • Carry impulses from the central nervous system Functional Properties of Neurons 1) Irritability is the ability to respond to stimuli 2) Conductivity is the ability to transmit an impulse Continuation of the Nerve Impulse between Neurons • Transmission of a nerve impulse is an electrochemical event • Impulses are able to cross the synapse to another nerve (can be disrupted by analgesics or decreased blood flow) • Neurotransmitter is released from a nerve’s axon terminal • The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter • An action potential is started in the dendrite The Reflex Arc • Reflexes are rapid, predictable, and involuntary responses to stimuli • A reflex arc is direct route from a sensory neuron, to an interneuron, to an effector involving both PNS and CNS • Reflexes contain 4-5 components Types of Reflexes and Regulation • • Autonomic reflexes 1) Smooth muscle regulation 2) Pupillary response to light 3) Heart and blood pressure regulation 4) Regulation of glands 5) Digestive system regulation Somatic reflexes 1) Activation of skeletal muscles Gross Anatomy of Nerves 1) 2) 3) 4) 5) 6) a nerve is a cablelike organ in the PNS containing many axons and dendrites. Within a nerve, each axon is covered by Schwann cells (myelin) Schwann cells are covered in a thin layer of loose connective tissue called Endoneurium Groups of axons are bound into bundles called nerve fascicles (similar to muscles) with a covering call the Perineurium The whole nerve is covered by Epineurium A nerve fiber is a long axon