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Axon (Nerve Fiber)— Chapter 12 *Lecture PowerPoint Nervous Tissue *See separate FlexArt PowerPoint slides for all figures and tables preinserted into PowerPoint without notes. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Introduction • The nervous system is one of great complexity • Nervous system is the foundation of our conscious experience, personality, and behavior • Neurobiology combines the behavioral and life sciences 11-2 Overview of the Nervous System • Expected Learning Outcomes – Describe the overall function of the nervous system. – Describe its major anatomical and functional subdivisions. 11-3 Overview of the Nervous System • Endocrine and nervous systems maintain internal coordination – Endocrine system: communicates by means of chemical messengers (hormones) secreted into to the blood – Nervous system: employs electrical and chemical means to send messages from cell to cell 12-4 Overview of the Nervous System • Nervous system carries out its task in three basic steps • Sense organs receive information about changes in the body and the external environment, and transmit coded messages to the spinal cord and the brain • Brain and spinal cord process this information, relate it to past experiences, and determine what response is appropriate to the circumstances • Brain and spinal cord issue commands to muscles and gland cells to carry out such a response 12-5 Overview of the Nervous System • Nervous system has two major anatomical subdivisions – Central nervous system (CNS) • Brain and spinal cord enclosed in bony coverings • Enclosed by cranium and vertebral column – Peripheral nervous system (PNS) • All the nervous system except the brain and spinal cord; composed of nerves and ganglia • Nerve—a bundle of nerve fibers (axons) wrapped in fibrous connective tissue • Ganglion—a knotlike swelling in a nerve where neuron cell bodies are concentrated 12-6 Overview of the Nervous System • Peripheral nervous system has two major functional subdivisions – Sensory (afferent) division: carries sensory signals from various receptors to the CNS • Informs the CNS of stimuli within or around the body – Somatic sensory division: carries signals from receptors in the skin, muscles, bones, and joints – Visceral sensory division: carries signals from the viscera of the thoracic and abdominal cavities • Heart, lungs, stomach, and urinary bladder 12-7 Overview of the Nervous System • Motor (efferent) division—carries signals from the CNS to gland and muscle cells that carry out the body’s response – Effectors: cells and organs that respond to commands from the CNS – Somatic motor division: carries signals to skeletal muscles • Output produces muscular contraction as well as somatic reflexes—involuntary muscle contractions 12-8 Overview of the Nervous System • Visceral motor division (autonomic nervous system) – Carries signals to glands, cardiac muscle, and smooth muscle – Involuntary, and responses of this system and its receptors are visceral reflexes – Sympathetic division • Tends to arouse body for action • Accelerating heart beat and respiration, while inhibiting digestive and urinary systems – Parasympathetic division • Tends to have calming effect • Slows heart rate and breathing • Stimulates digestive and urinary systems 12-9 Subdivisions of the Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central nervous Peripheral nervous system (CNS) system (PNS) Brain Spinal cord Nerves Ganglia Figure 12.1 12-10 Subdivisions of the Nervous System Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Central nervous system Peripheral nervous system Spinal Sensory Motor Brain cord division division Visceral Somatic Visceral Somatic sensory sensory motor motor division division division division Figure 12.2 Sympathetic Parasympathetic division division 12-11 Properties of Neurons • Expected Learning Outcomes – Describe three functional properties found in all neurons. – Define the three most basic functional categories of neurons. – Identify the parts of a neuron. – Explain how neurons transport materials between the cell body and tips of the axon. 12-12 Universal Properties • Excitability (irritability) – Respond to environmental changes called stimuli • Conductivity – Neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations • Secretion – When electrical signal reaches end of nerve fiber, a chemical neurotransmitter is secreted that crosses the gap and stimulates the next cell 12-13 Functional Classes • Three general classes of neurons (sensory, interneuron, motor) based on function • Sensory (afferent) neurons – Specialized to detect stimuli – Transmit information about them to the CNS • Begin in almost every organ in the body and end in CNS • Afferent—conducting signals toward CNS 12-14 Functional Classes • Three general classes of neurons (cont.) • Interneurons (association neurons) – Lie entirely within the CNS – Receive signals from many neurons and carry out the integrative function • Process, store, and retrieve information and “make decisions” that determine how the body will respond to stimuli – 90% of all neurons are interneurons – Lie between and interconnect the incoming sensory pathways and the outgoing motor pathways of the CNS 12-15 Functional Classes • Three general classes of neurons (cont.) • Motor (efferent) neuron – Send signals out to muscles and gland cells (the effectors) • Motor because most of them lead to muscles • Efferent neurons conduct signals away from the CNS 12-16 Classes of Neurons Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Peripheral nervous system Central nervous system 1 Sensory (afferent) neurons conduct signals from receptors to the CNS. 2 Inter neurons 3 Motor (efferent) (association neurons conduct neurons) are signals from the CNS confined to to effectors such as the CNS. muscles and glands. Figure 12.3 12-17 Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Soma—the control center of the Dendrites neuron – Also called neurosoma, cell Soma Nucleus body, or perikaryon Nucleolus – Has a single, centrally located Trigger zone: Axon hillock nucleus with large nucleolus Initial segment – Cytoplasm contains Axon collateral Axon mitochondria, lysosomes, a Golgi complex, numerous inclusions, Direction of signal transmission and extensive rough endoplasmic Internodes reticulum and cytoskeleton Node of Ranvier – Cytoskeleton consists of dense Myelin sheath mesh of microtubules and Schwann cell neurofibrils (bundles of actin filaments) Terminal arborization • Compartmentalizes rough ER into dark-staining Nissl bodies Synaptic knobs (a) 12-18 Figure 12.4a Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Soma—the control center of the Dendrites neuron (cont.) Soma Nucleus – No centrioles: no further cell Nucleolus division Trigger zone: Axon hillock – Inclusions: glycogen granules, Initial segment lipid droplets, melanin, and Axon collateral lipofuscin (golden brown pigment Axon produced when lysosomes digest Direction of worn-out organelles) signal transmission • Lipofuscin accumulates with age Internodes • Wear-and-tear granules Node of Ranvier Myelin sheath • Most abundant in old neurons Schwann cell Terminal arborization Synaptic knobs (a) 12-19 Figure 12.4a Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites • Dendrites—vast number of branches coming from a few Soma Nucleus thick branches from the soma Nucleolus Trigger zone: – Resemble bare branches of a Axon hillock tree in winter Initial segment Axon collateral – Primary site for receiving Axon signals from other neurons Direction of – The more dendrites the signal transmission neuron has, the more Internodes information it can receive and Node of Ranvier Myelin sheath incorporate into decision Schwann cell making – Provide precise pathway for Terminal the reception and processing arborization of neural information Synaptic knobs (a) 12-20 Figure 12.4a Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Dendrites • Axon (nerve fiber)— originates from a mound on Soma Nucleus one side of the soma called Nucleolus the axon hillock Trigger zone: Axon hillock – Cylindrical, relatively Initial segment unbranched for most of its Axon collateral Axon length Direction of • Axon collaterals—branches signal transmission of axon Internodes – Branch extensively on distal Node of Ranvier Myelin sheath end Schwann cell – Specialized for rapid conduction of nerve signals to Terminal points remote to the soma arborization Synaptic knobs (a) 12-21 Figure 12.4a Structure of a Neuron Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. • Axon (nerve fiber) (cont.) Dendrites – Axoplasm: cytoplasm of axon Soma – Axolemma: plasma membrane Nucleus Nucleolus of axon Trigger zone: – Only one axon per neuron Axon hillock Initial segment – Schwann cells and myelin Axon collateral sheath enclose axon Axon – Distal end, axon has terminal Direction of signal transmission arborization: extensive complex Internodes of fine branches Node of Ranvier • Synaptic knob (terminal Myelin sheath button)—little swelling that forms Schwann cell
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