Physio Lecture 10 PDF

[TRANS] LECTURE10: ORGANIZATION OF THE NERVOUS SYSTEM ORGANIZATION OF THE NERVOUS SYSTEM Distinct functional organization SPINAL CORD SUBDIVISIONS OF THE NERVOUS SYSTEM...

[TRANS] LECTURE10: ORGANIZATION OF THE NERVOUS SYSTEM ORGANIZATION OF THE NERVOUS SYSTEM Distinct functional organization SPINAL CORD SUBDIVISIONS OF THE NERVOUS SYSTEM  The most caudal part of the central nervous system  Receives and processes sensory information from the skin, joints, and muscles of the limbs and trunk and controls movements of the limbs and the trunk  Subdivided into cervical, thoracic, lumbar, and sacral regions CENTRAL NERVOUS SYSTEM BRAIN STEM  Bilateral  Essentially Symmetrical Structure  Consists of the medulla oblongata, pons,  Two main parts: Spinal Cord and Brain and midbrain  Receives sensory information from the skin and muscles of the head  Provides the motor control for the head’s musculature  Conveys information from the spinal cord to the brain and from the brain to the spinal cord  Contains several collections of cell bodies, the cranial nerve nuclei  Specialized to process information from three of the special senses: hearing, balance, and taste. 1 V. ANGELS NMD-1 1  LECTURE 10: ORGANIZATION OF NERVOUS SYSTEM Medulla Oblongata Cerebellum Directly rostral to the spinal cord Lies behind the pons and is connected to the Responsible for vital autonomic functions, brain stem by several major fiber tracts called such as digestion, breathing, and control peduncles heart rate Modulates the force and range of movement and is involved in the learning of motor skills Pons Diencephalon Rostral to the medulla Lies rostral to the midbrain Conveys information about movement from Contains two structures the cerebral hemispheres to the cerebellum The thalamus processes most of the information reaching the cerebral cortex from the rest of the central nervous system Hypothalamus regulates autonomic, endocrine, and visceral functions Broca’s- expressive Wernicke’s- receptive Conduction Dominant (aphasia) vs non dominant Midbrain (prosody) Rostral to the pons, controls many sensory PERIPHERAL NERVOUS SYSTEM and motor functions, including eye movement Coordination of visual and auditory reflexes Cerebrum AUTONOMIC NERVOUS SYSTEM (ANS) Comprises two cerebral hemispheres, each consisting of heavily wrinkled outer layer (the Portion of the nervous system that regulates cerebral cortex) and three deep-lying and controls visceral functions including heart structures (the basalganglia, the rate, blood pressure, digestion, hippocampus, and the amygdaloid nuclei). temperature regulation, and reproductive Cerebral cortex is divided into four distinct function lobes: frontal, parietal, occipital, and Anatomically composed of parts of the CNS temporal and PNS V. ANGELS NMD-1 2  LECTURE 10: ORGANIZATION OF NERVOUS SYSTEM NERVE CELLS AND ITS FUNCTIONS Cell Body or Soma The basic unit of the brain Metabolic center of the cell Contains the nucleus, which contains the genes of the cell, and the endoplasmic FOUR MORPHOLOGICALLY DEFINED REGIONS reticulum, an extension of the nucleus where OF THE NEURON: the cell’s proteins are synthesized Gives rise to two kinds of processes: Cell body ○ Several short dendrites Dendrites ○ One long, tubular axon Axon Presynaptic Terminals Subdivisions of the Nervous System SUBDIVISION COMPONENT SPECIAL FEATURES Central Brain (including CN II Oligodendrocytes AND retina) and provide myelin Dendrites spinal cord Axons cannot Branch out in tree-like fashion regenerate The main apparatus for receiving incoming Peripheral Peripheral ganglia Schwann cells signals from other nerve cells (including cell bodies) provide myelin sensory receptors; Axons can peripheral portions of regenerate spinal and cranial nerves (except CN II) both afferent and efferent. Autonomic Selected portions of Functionally the CNS and PNS distinct system Axon Extends some distance from the cell body and carries signals to other neurons Convey electrical signals over distances ranging from 0.1 mm to 2m. V. ANGELS NMD-1 3  LECTURE 10: ORGANIZATION OF NERVOUS SYSTEM ACTION POTENTIALS Where will the presynaptic terminal terminate? Initiated at a specialized trigger region near a. Dendrite of the postsynaptic neuron the origin of the axon called the initial segment b. Axon of the postsynaptic neuron Speeds of 1 to 100 m/s c. Cell body of the postsynaptic neuron The amplitude of an action potential traveling down the axon remains constant at 100 mV Most presynaptic terminals end on the The action potential is an all-or-none impulse postsynaptic neuron’s dendrites but the terminals that is regenerated at regular intervals along may also terminal: the axon 1. Cell body 2. Beginning or end of the axon of the receiving cell SYNAPSES Types of Synapses Branches that contact other neurons at specialized zones of communication ○ Presynaptic Cell Nerve cell transmitting a signal Presynaptic cell transmits signals from (presynaptic terminals or nerve terminals) ○ Postsynaptic Cell The cell receiving the signal Synaptic Cleft ○ Narrow space that separates the presynaptic and postsynaptic MYELIN Increase the speed by which action potentials are conducted Insulating sheath of a lipid substance, myelin The sheath is interrupted at regular intervals by the nodes of Ranvier, uninsulated spots on the axon where the action potential is regenerated V. ANGELS NMD-1 4  LECTURE 10: ORGANIZATION OF NERVOUS SYSTEM Principle of Dynamic Polarization UNIPOLAR NEURONS Vary greatly in shape, length of their axons, and in the extent, dimensions, and intricacy of Electrical signals within a nerve cell “flow only The simplest neuron their dendritic branching. in one direction:from the receiving sites of the Have a single primary process which gives neurons, usually the dendrites and cell body, rise to many branches to the trigger region at the axon. One branch serves as the axon From there the action potential is propagated Other branches function as receiving along the entire length of the axon to its structures terminals. Nervous systems of invertebrates In vertebrates: autonomic nervous system Connectional Specificity Nerve cells do not connect randomly with one BIPOLAR NEURONS another in the formation of networks Rather each cell makes specific connections Have an OVAL soma – at particular contact points – with certain 2 distinct processes: Dendrites postsynaptic target cells but not with others. ○ DENDRITIC STRUCTURE Extent of branching correlates with the : received signals from the periphery number of synaptic contacts that other What distinguishes a neuron from one another? of the body. ○ AXON neurons make onto them A spinal motor neuron : dendrites receives The feature that most distinguishes one type : Carries information toward the about 10,000 contacts – 1,000 on the cell of neuron from another is form, specifically central nervous system body and 9,000 on dendrites Any sensory cells are bipolar the number of processes arising from the cell Dendritic tree of a purkinjie cell in the ○ Retina and in the olfactory body cerebellum is much larger and bushier, epithelium of the nose receiving as many as a million contacts BASIC TYPES OF NEURONS PSEUDO-UNIPOLAR CELLS FUNCTIONAL CATEGORIES OF NEURONS A variant of bipolar cells Develop initially as bipolar cells. SENSORY NEURONS Two cell processes fuse into a single continuous structure that emerges from a carry information from the body's peripheral single point in the cell body. sensors into the nervous system for the Axon splits into two branches, one running to purpose of both perception and motor the periphery (to sensory receptors in the coordination skin, joints, and muscle) and another to the spinal cord Afferent neurons Receptor neurons that convey touch, Applies to all information reaching the central pressure, and pain signals to the spinal cord. nervous system from the periphery, whether or not this information leads to sensation MULTIPOLAR NEURON MOTOR NEURONS Predominate in nervous system of vertebrates Carry commands from the brain or spinal cord A single axon Many dendritic structures emerging from to muscles and glands (efferent information) various points around the cell body. V. ANGELS NMD-1 5  LECTURE 10: ORGANIZATION OF NERVOUS SYSTEM INTERNEURONS GLIAL CELLS SUPPORTS NERVE CELLS Most numerous GLIAL CELLS 2 classes: ○ RELAY OR PROJECTION Help support, connect, and protect the Have long axons and convey signals neurons of the central and peripheral nervous over a considerable distance, from systems one brain region to another. Outnumbered neuron (2 to 10 times more) ○ LOCAL INTERNEURONS Surround the cell bodies, axons, and Have short axons because they form dendrites of neurons. connections with nearby neurons in Morphology: do not form axon and dendrites local circuits. Sensory system interneurons can be Functionally: do not have the same classified according to: membrane properties as neurons EPENDYMAL CELLS ○ Type of sensory stimuli to which they Not electrically excitable Single layer of ciliated cuboidal cells, lines all respond Not directly involved in electrical signaling the ventricles of the brain ○ Location 2 MAJOR CLASSES OF GLIA Helping to move cerebrospinal fluid through ○ Density ○ MICROGLIA ventricular ○ Size ○ MACROGLIA (80%) Several places in the lateral and fourth ventricles the ependyma is continuous with 1. Sensory information is conveyed to the central Microglia cells of the choroid plexus, which covers thin nervous system (the spinal cord) from muscle. Immune system cells (PSEUDOUNIPOLAR NEURON) blood vessels that project intro the ventricles. Originate from mesoderm 2. Motor commands from the central nervous system Mobilized to present antigens and become are issued to the muscles that carry out the knee jerk. phagocytes during injury, infection, or degenerative diseases. 3. Inhibitory commands are issued ot motor neurons that innervate opposing muscles, providing coordination of muscle action. Macroglia (80%) 3 MAIN TYPES ○ OLIGODENDROCYTES (1/2) Found in CNS Produce myelin sheaths that insulate axons in the CNS. ○ SCHWANN CELLS Found in PNS Produce myelin sheaths in the PNS. ○ ASTROCYTES (1/2) Found in CNS V. ANGELS NMD-1 6  LECTURE 10: ORGANIZATION OF NERVOUS SYSTEM 3 MAIN TYPES OF MACROGLIA ○ FIBROUS ASTROCYTES MYELIN White matter and have Bimolecular layers of lipid interspersed Oligodendrocytes (½), long, fine processes that between protein layers Small cells with few processes contain large bundles of Composition is similar to that of the Form myelin in the CNS system tightly packed plasmalemma Each cell envelops from one to 30 axonal Consisting 70% lipid and 30% protein with intermediates filaments. segments called internodes high concentrations of cholesterol and Both types of astrocytes have end-feet phospholipid dilatations that contact and surround as an insulator for axons, enabling the rapid capillaries and arterioles throughout the brain transmission of electrical signals via saltatory Sheet-like processes of protoplasmic conduction where the impulse jumps between astrocytes envelope nerve cell bodies and nodes of Ranvier (gaps in the myelin sheath). synapses This helps increase the speed of End-feet of fibrous astrocytes contact axons communication between neurons at the nodes of Ranvier CNS: MOG Functions of Astrocytes PNS: MPZ/PMP22 Schwann CNC/MS: MBB and PLP ○ Astrocytes separate cells thereby Small cells with few processes GLIAL CELLS INVOLVED IN MYELIN insulating neuronal groups and Form myelin in the PNS PRODUCTION synaptic connections from each 1. Oligodendrocytes Each envelops a single segment of one axon other (In the central nervous system, CNS): (1:1) ○ Because astrocytes are highly ○ These glial cells produce myelin in permeable to K+ they help regulate the brain and spinal cord (CNS). A single oligodendrocyte can the K+ concentration in the space myelinate multiple axons. between neurons 2. Schwann Cells ○ Astrocytes perform other important (In the peripheral nervous system, PNS): housekeeping chores that promote ○ These cells produce myelin for the neurons outside the brain and spinal efficient signaling between neurons cord (PNS). Unlike ○ Take up neurotransmitters from oligodendrocytes, each Schwann synaptic zones after release cell myelinates only one segment of ○ Astrocytes help nourish surrounding an axon. neurons by releasing growth factors Astrocytes Third main class of glial cells Irregular, roughly star-shaped cell bodies and large numbers of process 2 major types ○ PROTOPLASMIC ASTROCYTES Found in gray matter V. ANGELS NMD-1 7  LECTURE 10: ORGANIZATION OF NERVOUS SYSTEM MEMBRANOUS ORGANELLES Mitochondria Second main component of cytoplasm Generate adenosine triphosphate (ATP), the Mitochondria and peroxisomes-process major molecule by which cellular energy is molecule oxygen transferred or spent Tubular systems The cytoplasm of the nerve cell body extends Peroxisomes into the dendritic tree without functional Prevent accumulation of the strong oxidizing NEURON VERSUS GLIAL CELLS differentiation agent hydrogen peroxide Neuron Generally, all organelles in the cytoplasm of the cell body are also present in dendrites, Cytoskeleton Have ability to communicate precisely and although the densities of the rough rapidly with other cells at distant sites in the endoplasmic reticulum, Golgi complex and Determines the shape of cell body lysosoms rapidly diminish with distance from Responsible for the asymmetric distribution of Have receptive dendrites at one end and a the cell body organelles within the cytoplasm transmitting axon at the other In dendrites the smooth endoplasmic 3 filamentous structures Both electrically and chemically excitable reticulum is prominent at the base of thin ○ Microtubules Cell membrane of neurons contains processes called spines the receptive ○ Nuerofilament specialized proteins-ion channel and portion of excitatory synapses ○ microfilaments Ribosomes, rough endoplasmic reticulum and receptors-that facilitate the few of specific Account for approximately a quarter of the the golgi complex, the organelles that total protein in the cell inorganic ions comprise the main biosynthetic machinery for proteins in the neuron-are generally excluded Most proteins are made in the cell body from Glia from axons mRNAs in the cell body Less excitable Lysosomes and certain proteins are excluded Axons and terminals often lie at great Membranes contain transporter proteins that However, axons are rich in synaptic vesicles distances from the cell body, transport facilitate the uptake of ions as well as proteins and their precursor membranes mechanisms are crucial for sustaining the that remove neurotransmitter molecules from function of these remote regions the extracellular space, thus regulating neuronal function V. ANGELS NMD-1 8  LECTURE 10: ORGANIZATION OF NERVOUS SYSTEM FAST AXONAL TRANSPORT Membranous organelles move toward terminals (anterograde direction) and back toward the cell body (retrograde direction) Faster that 400mm per day in warm-blooded animals Organelles:synaptic vesicles precursors, large dense-core vesicles, mitochondria, elements of the smooth endoplasmic reticulum, particles carrying RNA Organelles moved by retrograde fast transport: primarily endosomes generated by endocyticactivity at nerve ending, mitochondria and elements of the endoplasmic reticulum NOTE: some pathogens are transported retrograde- (herpes simples, rabies and polio viruses) SLOW AXONAL TRANSPORT Cytosolic and cytoskeletal proteins move only in the anterograde direction Much slower form of transport: 0.2 to 2.5mm per day Carries the proteins that make up the fibrillary elements of the cytoskeleton  Concentric isotonic contractions – the muscle shortens as it moves the load  Eccentric isotonic contractions – the muscle lengthens as it resists the load  Isometric contractions result in increases in muscle tension, but no lengthening or shortening of the muscle occurs. 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