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Fundamentals of the Nervous System and Nervous Tissue Chapter 11 Marieb Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Nervous System The master controlling and communicating system of the body Works with the endocrine system to coordinate the organ systems of the body to maintain homeoastasis 3 modes of 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 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Nervous System Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.1 Organization of the Nervous System Central nervous system (CNS) brain spinal cord Peripheral nervous system (PNS) spinal and cranial nerves Ganglia – clusters of cell bodies in the PNS. sensory receptors Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Peripheral Nervous System (PNS): Two Functional Divisions Sensory (afferent) division Somatic afferent axons – carry impulses from skin, skeletal muscles, and joints to the brain Visceral afferent axons – transmit impulses from visceral organs to the brain Motor (efferent) division Transmits impulses from the CNS to effector organs 2 parts: Somatic nervous system- conscious control of skeletal muscles Autonomic nervous system (ANS)-regulates smooth muscle, cardiac muscle, and glands 2 Divisions – sympathetic, and parasympathetic. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Nervous 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 (neuroglia or glial cells): -Provide a supportive scaffolding for neurons -Segregate and insulate neurons -Protect neurons -Provide conditions for proper functioning of neurons, and promote the electrical activity. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Types of support cells: Astrocytes- most abundant, versatile, and highly branched glial cells They cling to neurons and their synaptic endings, and cover capillaries; functionally, they: -Support and brace neurons -Guide migration of young neurons -Control the chemical environment Microglia – small, ovoid cells with spiny processes; phagocytes that protect the health of neurons Ependymal cells – They line the central cavities of the brain and spinal column (epithelial-like) Oligodendrocytes – branched cells that wrap and myelinate CNS axons Schwann cells– surround and myelinate axons of the PNS Satellite cells- surround neuron cell bodies within ganglia Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurons (Nerve Cells) Long-lived, amitotic cells, with a high metabolic rate that function in communication through electrical signaling Neuron Cell Body (Soma) • Contains the nucleus and a nucleolus • Is the major biosynthetic center • Is the focal point for the outgrowth of neuronal processes • Has no centrioles (hence its amitotic nature) • Contains an axon hillock – cone-shaped area from which axons arise Neuron Processes • Armlike extensions from the soma • There are two types: Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings General Neuron structure Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neuron Processes Dendrites (afferent) • Short, tapering, and diffusely branched processes • They are the receptive, or input, processes of the neuron Most signals are from other neurons are received at synapses located on dendrites. • Electrical signals carried by dendrites are graded potentials (not action potentials) Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Axons • Processes of relatively uniform diameter arising from the hillock • They are the output processes of the neuron •electrical •secrete • signals carried by axons are usually action potentials neurotransmitters from the axonal terminals Long axons are called nerve fibers Usually there is usually only one axon per neuron; branches, if present, are called axon collaterals • • Axonal • terminal – branched terminus of an axon Bundled axons are called tracts in the CNS and nerves in the PNS Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Myelinated axons Axons may be surrounded by a myelin sheath, (neurilemma) • Whitish, fatty (protein-lipid), segmented sheath around most long axons • It functions to: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission Nodes of Ranvier- Gaps in the myelin sheath between adjacent Schwann cells; promotes faster conduction of an electrical signal Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Gray & White Matter in the Brain and Spinal Cord White matter – dense collections of myelinated axons. Gray matter – mostly somas and unmyelinated axons. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neuron Classification Structural: Multipolar — three or more processes Bipolar — two processes (axon and dendrite) Unipolar — single, short process Functional: Sensory (afferent) — transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS Interneurons (association neurons) — shuttle and integrate signals through CNS pathways Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurophysiology, Synapses, Neurotransmitters Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurophysiology Neurons are excitable cells, as are muscle cells Undergo a rapid change in membrane potential Changes in membrane potential are signals used by neurons (and muscle cells) to send, receive, or integrate information Changes in membrane potential that occur in neurons and other excitable cells are mainly produced by changes in membrane permeability to ions through channels that occur in response to: - neurotransmitter release - change in membrane potential - pharmacological agents/toxins Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Membrane Potentials as Signals Types of membrane potential changes that occur: Depolarization – the inside of the membrane becomes less negative Repolarization – the membrane returns to its resting membrane potential Hyperpolarization – the inside of the membrane becomes more negative than the resting potential Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Changes in Membrane Potential Figure 11.9 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Changes in Membrane Potential may be Graded Potentials or Action Potentials (APs) Graded potentials: Short-lived, local changes in membrane potential-can only travel over short distances Decrease in intensity with distance Their magnitude varies directly with the strength of the stimulus Sufficiently strong graded potentials can initiate action potentials Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Action Potentials (APs) Action Potentials:: A rapid reversal of membrane potential above a threshold level with a total amplitude of ~100 mV Action potentials are only generated by muscle cells and neurons They do not decrease in strength over distance They are the principal means of neural communication An action potential in the axon of a neuron is often called a nerve impulse Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Conduction Velocities of Axons Conduction velocities vary widely among neurons Rate of impulse propagation is determined by: Axon diameter – the larger the diameter, the faster the impulse Presence of a myelin sheath – myelination dramatically increases impulse speed Current passes through a myelinated axon only at the nodes of Ranvier (salutatory conduction) Voltage-gated Na+ channels are concentrated at these nodessignal “jumps” Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Multiple Sclerosis (MS) An autoimmune disease that mainly affects young adults (onset 30-50 years old). Symptoms include visual disturbances, weakness, loss of muscular control, and urinary incontinence myelin sheaths in the CNS become nonfunctional scleroses and nerve fibers degenerate Shunting and short-circuiting of neuron electrical impulses occurs Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Synapses Junctions that mediate information transfer from one neuron: To another neuron To an effector cell Presynaptic neuron – conducts impulses toward the synapse Postsynaptic neuron – transmits impulses away from the synapse Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Synapses Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Figure 11.17 Types of Synapses (structural classification) Axodendritic – synapses between the axon of one neuron and the dendrite of another Axosomatic – synapses between the axon of one neuron and the soma of another Other types of synapses include: Axoaxonic (axon to axon) Dendrodendritic (dendrite to dendrite) Dendrosomatic (dendrites to soma) Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Types of Synapses (functional classification) Electrical synapses: Are less common than chemical synapses Made of gap junctions - ions pass directly from one neuron to another Chemical Synapses: Specialized for the release and reception of neurotransmitters Typically composed of two parts: Axonal terminal of the presynaptic neuron, which contains synaptic vesicles Receptor region on the dendrite(s) or soma of the postsynaptic neuron Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Synaptic Cleft of a Chemical Synapse Fluid-filled space separating the presynaptic and postsynaptic neurons Prevents neuron electrical impulses from directly passing from one neuron to the next Transmission across the synaptic cleft: Is a chemical event (as opposed to an electrical one) Ensures unidirectional communication between neurons Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Information Transfer at a Synapse Figure 11.17 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Termination of Neurotransmitter Effects Removal of neurotransmitters occurs when they: Are degraded by enzymes Are reabsorbed by astrocytes or the presynaptic terminals Diffuse away from the synaptic cleft This results in termination of the neurotransmitter effect Synaptic delay Time needed for neurotransmitter to be released, diffuse across the synapse, and bind to receptors is 0.3-5.0 ms This synaptic delay is the rate-limiting step of neuronal transmission Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters and their Receptors Chemicals used for neuronal communication with the body and the brain 50 different neurotransmitters have been identified Classified chemically and functionally Chemical Classification of Neurotransmitters _ Acetylcholine (ACh) _ Biogenic Amines _ Amino Acids _ Peptides _ Novel Messengers: ATP, Nitrous oxide. Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Acetylcholine First neurotransmitter identified, and best understood Released at the neuromuscular junction Synthesized and enclosed in synaptic vesicles Degraded by the enzyme acetylcholinesterase (AChE) Released by: All neurons that stimulate skeletal muscle NMJs Some neurons in the autonomic nervous system Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Biogenic Amines Include: Catecholamines – Dopamine, norepinephrine (NE), and epinephrine. Indolamines – serotonin and histamine. Broadly distributed in the brain Play roles in emotional behaviors and our biological clock Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Synthesis of Catecholamines * Enzymes present in the cell determine length of biosynthetic pathway Norepinephrine and dopamine are synthesized in axonal terminals Epinephrine is released by the adrenal medulla Figure 11.22 Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Amino Acids Include: - GABA (gamma amino butyric acid) - glycine - aspartate - glutamate Found only in the CNS Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitters: Peptides Include: - Substance P – mediator of pain signals. - Beta endorphin, enkephalin Act as natural opiates, reducing our perception of pain Bind to the same receptors as opiates and morphine Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Functional Classification of Neurotransmitters Two classifications: Excitatory neurotransmitters cause Depolarization. (e.g., glutamate) Inhibitory neurotransmitters cause hyperpolarization. (e.g., GABA and glycine) More negative than the resting membrane potential. Less likely to undergo AP. Some neurotransmitters have both excitatory and inhibitory effects Determined by the receptor type of the postsynaptic neuron Example: acetylcholine Excitatory at neuromuscular junctions with skeletal muscle Inhibitory in cardiac muscle Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings Neurotransmitter Receptor Mechanisms Direct: neurotransmitters that open ion channels Promote rapid responses Examples: ACh and amino acids Indirect: neurotransmitters that act through second messengers. (ANS). Slower onset of response; promote longer-lasting effects Examples: biogenic amines, peptides, and dissolved gases Many are G-protein-linked receptor mediated PLAY InterActive Physiology®: Nervous System II: Synaptic Transmission Copyright © 2004 Pearson Education, Inc., publishing as Benjamin Cummings