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The Nervous System Functions of the Nervous System Sensory input – gathering information To monitor changes occurring inside and outside the body Changes = stimuli Integration To process and interpret sensory input and decide if action is needed Motor output A response to integrated stimuli The response activates muscles or glands Structural Classification Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Nerves outside the brain and spinal cord Functional Classification: PNS Sensory (afferent) division Nerve fibers that carry information to the central nervous system Motor (efferent) division Nerve fibers that carry impulses away from the central nervous system Subdivisions of Motor division Somatic nervous system = voluntary Somatic motor division carries information to the skeletal muscles Autonomic nervous system = involuntary Efferent division of ANS carries information to the autonomic or visceral effectors Sympathetic division: prepares the body to deal with immediate threats to the internal environment; produces fight-or-flight response Parasympathetic division: coordinates the body’s normal resting activities; sometimes called the rest-and-repair division Visceral sensory division carries feedback information to autonomic integrating centers in the CNS Organization of the Nervous System Nervous Tissue: Support Cells (Neuroglia) Astrocytes Abundant, star-shaped cells Brace neurons Form barrier between capillaries and neurons Control the chemical environment of the brain Nervous Tissue: Support Cells Microglia Spider-like phagocytes Dispose of debris Ependymal cells Line cavities of the brain and spinal cord Circulate cerebrospinal fluid Some produce cerebrospinal fluid Nervous Tissue: Support Cells Oligodendrocytes Produce myelin sheath around nerve fibers in the central nervous system Nervous Tissue: Support Cells Satellite cells Protect neuron cell bodies Schwann cells Form myelin sheath in the peripheral nervous system Nervous Tissue: Neurons Neurons = nerve cells Cells specialized to transmit messages Major regions of neurons Cell body – nucleus and metabolic center of the cell Processes – fibers that extend from the cell body Neuron Anatomy Cell body Nissl bodies – specialized rough endoplasmic reticulum Produce proteins Neurofibrils – intermediate cytoskeleton that maintains cell shape Neuron Anatomy Cell body Nucleus Large nucleolus Neuron Anatomy Extensions outside the cell body Dendrites – conduct impulses toward the cell body Axons – conduct impulses away from the cell body Nerve Fiber Coverings Schwann cells – produce myelin sheaths in jelly-roll like fashion Nodes of Ranvier – gaps in myelin sheath along the axon Connective Tissue Layers Endoneurium- delicate layer of fibrous connective tissue surrounding each nerve Perineurium- connective tissue holding together bundles of fibers (fascicles) Epineurium- fibrous coat surrounding numerous fascicles and blood vessels to form a complete nerve Neuron Cell Body Location Most are found in the central nervous system Gray matter – cell bodies and unmyelinated fibers Nuclei – clusters of cell bodies within the white matter of the central nervous system Ganglia – collections of cell bodies outside the central nervous system Neuron Cell Body Location White matter PNS- myelinated nerves CNS- myelinated tracts In the CNS nerve fibers are referred to as tracts CYCLE OF LIFE: NERVOUS SYSTEM CELLS • Nerve tissue development • Begins in ectoderm • Occurs most rapidly in womb and in first 2 years • Nervous cells organize into body network • Synapses • Form and re-form until nervous system is intact • Formation of new synapses and strengthening or elimination of old synapses stimulate learning and memory • Aging causes degeneration of the nervous system, which may lead to senility Functional Classification of Neurons Sensory (afferent) neurons Carry impulses from the sensory receptors Cutaneous sense organs Proprioceptors – detect stretch or tension Motor (efferent) neurons Carry impulses from the central nervous system Functional Classification of Neurons Interneurons (relay neurons) Found in neural pathways in the central nervous system Connect sensory and motor neurons Mixed Nerves Contain both sensory and motor neurons Neuron Classification Structural Classification of Neurons Multipolar neurons – many extensions from the cell body One axon and many dendrites Most of the neurons in the brain and spinal cord Structural Classification of Neurons Bipolar neurons – one axon and one dendrite Least numerous Found in retina, inner ear and olfactory pathways Structural Classification of Neurons Unipolar neurons – have a short single process leaving the cell body Always sensory neurons, conduct towards the CNS Repair of Nerve Fibers Mature neurons can not undergo mitosis, damage is permanent If damage is not extensive, it can be repaired…only if Cell body and neurilemma intact and there is no scarring In the CNS, repair is unlikely Repair of Nerve Fibers Following injury, distal portion of axon and myelin sheath degenerates Macrophages remove debris The neurilemma and endoneurium form a tunnel from the point of injury to the effector Repair of Nerve Fibers New Schwann cells grow in the tunnel to maintain the path for the regrowth of the axon Cell body reorganizes its Nissl bodies to provide proteins Axon “sprouts” and begins to fill tunnel Skeletal muscle atrophies until nervous connection reestablished Axons and Nerve Impulses Axons end in axonal terminals Axonal terminals contain vesicles with neurotransmitters Acetylcholine, amines, amino acids, nitric oxide, neuropeptides, etc. Axonal terminals are separated from the next neuron by a gap Synaptic cleft – gap between adjacent neurons Synapse – junction between nerves The Reflex Arc Reflex – rapid, predictable, and involuntary responses to stimuli Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector Simple Reflex Arc Types of Reflexes and Regulation Autonomic reflexes Smooth muscle regulation Heart and blood pressure regulation Regulation of glands Digestive system regulation Somatic reflexes Activation of skeletal muscles Functional Properties of Neurons Irritability – ability to respond to stimuli Conductivity – ability to transmit an impulse Action potential refers to the membrane potential of a neuron that is conducting an impulse The plasma membrane at rest is polarized Fewer positive ions are inside the cell than outside the cell Starting a Nerve Impulse Depolarization – a stimulus depolarizes the neuron’s membrane A depolarized membrane allows sodium (Na+) to flow inside the membrane The exchange of ions initiates an action potential in the neuron The Action Potential If the action potential (nerve impulse) starts, it is propagated over the entire axon Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane The sodium-potassium pump restores the original configuration This action requires ATP The Action Potential The membrane potential of a nonconducting neuron’s plasma membrane is -70 mV Depolarization allows the membrane potential to move towards zero Nerve Impulse Propagation Impulses travel faster when fibers have a myelin sheath Continuation of the Nerve Impulse between Neurons Impulses are able to cross the synapse to another nerve 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 of the next neuron How Neurons Communicate at Synapses Synaptic Transmission Two types of synapses (junctions) Electrical synapse occurs where cells joined by gap junctions allow an action potential to simply continue along the postsynaptic membrane Chemical synapse occurs where presynaptic cells release neurotransmitters across a tiny gap to the postsynaptic cell, inducing an action potential Synaptic Transmission Action potential reaches a synaptic knob, causing release of calcium ions to diffuse into the knob Increased calcium concentrations trigger the release of neurotransmitters via exocytosis Neurotransmitters diffuse across the synaptic cleft and bind to receptor molecules causing ion channels to open This causes postsynaptic potential Synaptic Transmission Neurotransmitter’s action is quickly terminated by returning neurotransmitters back into synaptic knob, or metabolized into inactive compounds NEUROTRANSMITTERS • Neurotransmitters: means by which neurons communicate with one another; more than 30 compounds are known to be neurotransmitters, and dozens of others are suspected • Common classification of neurotransmitters: • Function • Chemical structure NEUROTRANSMITTERS • Acetylcholine • Present at various locations, sometimes in an excitatory role and other times inhibitory • Amines • Synthesized from amino acid molecules • Two categories: monoamines and catecholamines • Found in various regions of the brain, affecting learning, emotions, motor control • Amino acids • Believed to be among the most common neurotransmitters of the CNS • In the PNS, amino acids are stored in synaptic vesicles and used as neurotransmitters • Other small-molecule transmitters