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BIOL 2401

BIOL 2401. Fundamentals of Anatomy and Physiology Mrs. Willie Grant wgrant4@alamo.edu (210) 486-2780. An Introduction to the Nervous System. Learning Outcomes 12-1 Describe the anatomical and functional divisions of the nervous system.

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BIOL 2401

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  1. BIOL 2401 Fundamentals of Anatomy and Physiology Mrs. Willie Grant wgrant4@alamo.edu (210) 486-2780

  2. An Introduction to the Nervous System Learning Outcomes 12-1 Describe the anatomical and functional divisions of the nervous system. 12-2Sketch and label the structure of a typical neuron, describe the functions of each component, and classify neurons on the basis of their structure and function. 12-3 Describe the locations and functions of the various types of neuroglia. 12-4 Explain how the resting potential is created and maintained. 12-5 Describe the events involved in the generation and propagation of an action potential. 12-6Discuss the factors that affect the speed with which action potentials are propagated. 12-7 Describe the structure of a synapse, and explain the mechanism involved in synaptic activity. 12-8 Describe the major types of neurotransmitters and neuromodulators, and discuss their effects on postsynaptic membranes. 12-9 Discuss the interactions that enable information processing to occur in neural tissue.

  3. An Introduction to the Nervous System The Nervous System Includes all neural tissue in the body Neural tissue contains two kinds of cells Neurons Cells that send and receive signals Neuroglia (glial cells) Cells that support and protect neurons Organs of the Nervous System Brain and spinal cord Sensory receptors of sense organs (eyes, ears, etc.) Nerves connect nervous system with other systems

  4. 12-1 DivisionsoftheNervousSystem The Central Nervous System (CNS) Consists of the spinal cord and brain Contains neural tissue, connective tissues, and blood vessels Functions of the CNS are to process and coordinate: Sensory data from inside and outside body ▪ Motor commands control activities of peripheral organs (e.g., skeletal muscles ▪ Higher functions of brain intelligence, memory, learning, emotion ThePeripheral Nervous System (PNS) Nerves(also called peripheral nerves) Bundles of axons with connective tissues and blood vessels ▪ Carry sensory information and motor commands in PNS Cranial nerves — connect to brain ▪ Spinal nerves — attach to spinal cord Includes all neural tissue outside the CNS Functions of the PNS Deliver sensory information to the CNS ▪ Carry motor commands to peripheral tissues and systems

  5. 12-1 DivisionsoftheNervousSystem Functional Divisions of the PNS Afferent division Carries sensory information from PNS sensosry receptors to CNS Efferent division Carries motor commands from CNS to PNS muscles and glands Receptors and effectors of afferent division Receptors Detect changes or respond to stimuli Neurons and specialized cells Complex sensory organs (e.g., eyes, ears) Effectors Respond to efferent signals Cells and organs

  6. 12-1 DivisionsoftheNervousSystem Functional Divisions of the PNS The efferent division Somatic nervous system(SNS) Controls voluntary and involuntary (reflexes) muscle skeletal contractions Autonomic nervous system (ANS) Controls subconscious actions, contractions of smooth muscle and cardiac muscle, and glandular secretions Sympathetic division has a stimulating effect Parasympathetic division has a relaxing effect

  7. 12-2 Neurons Neurons The basic functional units of the nervous system The structure of neurons: Multipolar neuron common in the CNS has a cell body (soma), short, branched dendrites, and a long, single axon. TheCell Body Large nucleus and nucleolus Perikaryon (cytoplasm) Mitochondria (produce energy) RER and ribosomes (produce neurotransmitters) Cytoskeleton Neurofilaments and neurotubules in place of microfilaments and microtubules Neurofibrils: bundles of neurofilaments that provide support for dendrites and axon Nissl bodies Dense areas of RER and ribosomes Make neural tissue appear gray (gray matter)

  8. 12-2 Neurons Dendrites Highly branched Many fine processes (dendritic spines) Receive information from other neurons 80–90% of neuron surface area Theaxon Is long and carries electrical signal (action potential) to target Axon structure is critical to function Structures of the Axon Axoplasm Cytoplasm of axon (Contains neurofibrils, neurotubules, enzymes, organelles) Axolemma—specialized cell membrane that covers the axoplasm Specialized cell membrane that covers the axoplasm

  9. 12-2 Neurons Structures of the Axon Axon hillock Thick section of cell body that attaches to initial segment Initial segment Attaches to axon hillock Collaterals Branches of a single axon Telodendria Fine extensions of distal axon Synaptic terminals Tips of telodendria

  10. 1 What roles do the dendrites, cell body, and axon play in communication of signals?

  11. 12-2 Neurons The Structure of Neurons Thesynapse—area where a neuron communicates with another cell The synapse Presynaptic cell (neuron that sends message) Postsynaptic cell (cell that receives message) The synaptic cleft (small gap that separates the presynaptic membrane and the postsynaptic) Types of Synapses Neuromuscular junction Synapse between neuron and muscle Neuroglandular junction Synapse between neuron and gland

  12. Figure 12-2 The Structure of a Typical Synapse Telodendrion Synaptic terminal Endoplasmic reticulum Mitochondrion Presynaptic Cell—sends a message Synaptic vesicles --Contains the neurotransmitter Presynaptic Membrane Synaptic cleft Postsynaptic membrane --separates the two cells Postsynaptic Cell—receives the message . http://www.youtube.com/watch?v=HXx9qlJetSU Neurotransmitter—chemical (Ach)

  13. Figure 12-3 A Structural Classification of Neurons Anaxonic neuron Bipolar neuron Unipolar neuron Multipolarneuron Dendrites Dendrites Initial segment Cell body Dendritic branches Axon Dendrite Cell body Cell body Axon Axon Cell body Axon Synaptic terminals Synaptic terminals Synaptic terminals 2 Which type of neuron is the most abundant type of neuron in the CNS?

  14. 12-2 Neurons Functions of Sensory Neurons Monitor internal environment (visceral sensory neurons) Monitor effects of external environment (somatic sensory neurons) Three Types of Sensory Receptors Interoceptors Monitor internal systems (digestive, respiratory, cardiovascular, urinary, reproductive) Internal senses (taste, deep pressure, pain) Exteroceptors External senses (touch, temperature, pressure) Distance senses (sight, smell, hearing) Proprioceptors Monitor position and movement (skeletal muscles and joints)

  15. 12-2 Neurons Motor Neurons Two major efferent systems Somatic nervous system (SNS) Includes all somatic motor neurons that innervate skeletal muscles Autonomic (visceral) nervous system (ANS) Visceral motor neurons innervate all other peripheral effectors Smooth muscle, cardiac muscle, glands, adipose tissue Two groups of efferent axons Signals from CNS motor neurons to visceral effectors pass synapses at autonomic ganglia dividing axons into: Preganglionic fibers Postganglionic fibers

  16. 12-2 Neurons Interneurons Most are located in brain, spinal cord, and autonomic ganglia Between sensory and motor neurons Are responsible for: Distribution of sensory information Coordination of motor activity Are involved in higher functions Memory, planning, learning

  17. Figure 12-6a Schwann Cells and Peripheral Axons Axon hillock Nucleus Axon Myelinated internode Initial segment (unmyelinated) Dendrite Nodes Schwann cell nucleus Axon Neurilemma Myelin covering internode Axon Axolemma A myelinated axon, showing the organization of Schwann cells along the length of the axon. Also shown are stages in the formation of a myelin sheath by a single Schwann cell along a portion of a single axon.

  18. 12-3 Neuroglia Neural Responses to Injuries (response is limited) Wallerian degeneration (Process of repairing damaged nerves) Axon distal to injury degenerates Schwann cells form path for new growth and wrap new axon in myelin Nerve Regeneration in CNS (limited) Limited by chemicals released by astrocytes that block growth and produce scar tissue

  19. 12-4 Transmembrane Potential Ion Movements and Electrical Signals All plasma (cell) membranes produce electrical signals by ion movements Transmembrane potential is particularly important to neurons Three important concepts The extracellular fluid (ECF) and intracellular fluid (cytosol) differ greatly in ionic composition Concentration gradient of ions (Na+, K+) Cells have selectively permeable membranes Membrane permeability varies by ion

  20. 12-4 Transmembrane Potential Five Main Membrane Processes in Neural Activities Resting potential is the transmembrane potential of a resting cell Graded potential is a temporary localized change in the resting potential caused by a stimulus Action potential is an electrical impulse produced by a graded potential that spreads along the surface of an axon to synapse. Synaptic activity releases neurotransmitters at presynaptic membrane and produces graded potentials in postsynaptic membrane Information processing is the response (integration of stimuli) of the postsynaptic cell

  21. Resting Membrane Potential

  22. Electrochemical Gradients for Potassium and Sodium Ions The electrochemical gradient for a specific ion is the sum of the chemical and electrical forces acting on the ion across the plasma membrane. The electrochemical gradients for K+ and Na+ are the primary factors affecting the resting membrane potential of most cells.

  23. Gated Channels Membrane channels control the movement of ions across the plasma membrane. There are Passive channels or leak channels (always open). There are Active channels or gated channels (open and close in response to specific stimuli. Gated channels can be chemically gated or ligand-gated channels. Gated channels can be voltage-gated channels. Gated channels can be mechanically gated channels.

  24. Graded Potentials Graded potentials, or local potentials are changes in the membrane potential that cannot spread Far from the site of stimulation.

  25. 12-4 Transmembrane Potential Graded Potentials (local potentials) Changes in transmembrane potential that cannot spread far from site of stimulation—local current. The resting state Opening sodium channel produces graded potential when resting membrane is exposed to chemical Sodium channel opens/Sodium ions enter the cell/Transmembrane potential rises Depolarization is a shift in the transmembrane potential toward 0mV, toward a more positive potential. Repolarizationis the restoration to the normal resting potential after depolarization. Hyperolarization is an increase in the negativity of the resting potential

  26. Figure 12-13 Depolarization, Repolarization, and Hyperpolarization Chemical stimulus removed Chemical stimulus applied Chemical stimulus removed Chemical stimulus applied Repolarization Transmembrane potential (mV) Resting potential Depolarization Hyperpolarization Return to resting potential

  27. 12-5 Action Potential Action Potentials (Propagated changes in transmembrane, that once initiated, affect the excitable membrane. These electrical events are known as nerve impulses. The membrane potential at which an action potential begins is called the threshold. This is between -60 mV and -55 mV. The All-or-None Principle This concept says that because a given stimulus either triggers a typical action potential, or none at all. The all-or-none principle applies to all excitable membranes.

  28. At the Resting Potential—the activation gates of the voltage-gated sodium channels are closed. At the Refractory Period—the membrane does not respond to additional depolarizing stimuli from the time an action potential begins until the normal resting membrane potential has stabilized. At the Absolute Refractory Period—the first part of the refractory period At the Relative Refractory Period—at the point when the sodium channels regain their normal resting condition, and continues until the membrane potential stabilizes at resting levels.

  29. Powering the Sodium–Potassium Exchange Pump To maintain concentration gradients of Na+ and K+ over time Requires energy (1 ATP for each 2 K+/3 Na+exchange) Without ATP Neurons stop functioning

  30. 12-5 Action Potential Propagation of Action Potentials Propagation—movement of action potentials generated in axon hillock along entire length of axon. It is either continuous or saltatory. Continuous propagation along an unmyelinated axons that affects one segment of axon at a time. In an unmyelinated axon, an action potential movesl along by continuous propagation. The action potential Spreads by depolarizing the adjacent region of the axon membrane. The process continues to spread as a Chain reaction down the axon.

  31. 12-5 Action Potential Saltatory Propagation along a myelinated axon. It is faster and uses less energy than a continuous propagation Myelin insulates axon, prevents continuous propagation Local current “jumps” from node to node Because myelin limits the movement of ions across the axon membrane, the action potential Must “jump” from node to node during propagation. This results in much faster propagation along the axon.

  32. 12-6 Axon Diameter and Speed Axon Diameter and Propagation Speed Ion movement is related to cytoplasm concentration Axon diameter affects action potential speed The larger the diameter, the lower the resistance Three Groups of Axons classed by diameter, myelination and speed: Type AType BType C 4-20 μm 2-4 μm 2 μm MyelinatedMyelinatedUnmyelinated 120 m/sec (268 mph) 18 m/sec (40 mph) 1m/sec (2 mph) 5 What is the functional advantage of myelination? 6 What factors determine the speed of propagation of an action potential?

  33. 12-6 Axon Diameter and Speed Information “Information” travels within the nervous system As propagated electrical signals (action potentials) The most important information (vision, balance, motor commands) Is carried by large-diameter, myelinated axons Synaptic Activity Action potentials (nerve impulses) Are transmitted from presynaptic neuron To postsynaptic neuron (or other postsynaptic cell) Across a synapse

  34. 12-7 Synapses Two Types of Synapses Electrical synapses (Direct physical contact between cells) Are locked together at gap junctions (connexons) Allow ions to pass between cells Produce continuous local current and action potential propagation Are found in areas of brain, eye, ciliary ganglia Chemical synapses (Signal transmitted across a gap by chemical neutrotransmitters) Are found in most synapses between neurons and all synapses between neurons and other cells Cells not in direct contact Action potential may or may not be propagated to postsynaptic cell, depending on Amount of neurotransmitter released Sensitivity of postsynaptic cell

  35. 12-7 Synapses Two Classes of Neurotransmitters Excitatory neurotransmitters Cause depolarization of postsynaptic membranes Promote action potentials Inhibitory neurotransmitters Cause hyperpolarization of postsynaptic membranes Suppress action potentials

  36. 12-7 Synapses Cholinergic Synapses Any synapse that releases ACh at: All neuromuscular junctions with skeletal muscle fibers Many synapses in CNS All neuron-to-neuron synapses in PNS All neuromuscular and neuroglandular junctions of ANS parasympathetic division

  37. 12-7 Synapses Synaptic Delay A synaptic delay of 0.2–0.5 msec occurs between: Arrival of action potential at synaptic terminal And effect on postsynaptic membrane Fewer synapses mean faster response Reflexes may involve only one synapse Synaptic Fatigue Occurs when neurotransmitter cannot recycle fast enough to meet demands of intense stimuli Synapse inactive until ACh is replenished

  38. 12-8 Neurotransmitters and Neuromodulators Neurotransmitters Other Than Acetylcholine Norepinephrine (NE) (Released by adrenergic synapses) Excitatory and depolarizing effect Widely distributed in brain and portions of ANS Dopamine (a CNS neurotransmitter) May be excitatory or inhibitory Involved in Parkinson’s disease and cocaine use Serotonin (a CNS neurotransmitter) Affects attention and emotional states Gamma Aminobutyric Acid (GABA) (a CNS neutrotransmitter) Inhibitory effect but is not well understood Nitric oxide and Carbon monoxide (gasses that are PNS and CNS neurotransmitters) Biogenic Amines Amino Acid Gases

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