Fundamentals of the Nervous System and Nervous Tissue. Nervous System. Basic Divisions of the Nervous System C H A P T E R 12.

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1 C H A P T E R 12 Fundamentals of the Nervous System and Nervous Tissue Nervous System Sensory input Integration Motor output Figure 12.1 Basic Divisions of the Nervous System Brain CNS Spinal cord Nerves PNS Ganglia Figure

2 Sensory Input and Motor Output Sensory (afferent) signals picked up by sensory receptors Carried by nerve fibers of PNS to the CNS Motor (efferent) signals are carried away from the CNS Innervate muscles and glands Types of Sensory and Motor Information Central nervous system (CNS) Brain and spinal cord Integrative and control centers Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Somatic sensory fiber Skin Somatic nervous system Autonomic nervous system (ANS) Visceral sensory fiber Stomach Skeletal muscle Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Motor fiber of somatic nervous system Sympathetic division Mobilizes body systems during activity Parasympathetic division Conserves energy Promotes housekeeping functions during rest Structure Sympathetic motor fiber of ANS Heart Function Sensory (afferent) division of PNS Parasympathetic motor fiber of ANS Bladder Motor (efferent) division of PNS Figure 12.3 Nervous Tissue Two main cell types Neurons (nerve cells) transmit electrical signals - excitable Neuroglial cells (Support cells) nonexcitable Surround and wrap neurons 2

3 Structure of a Typical Large Neuron Dendrites Cell body (receptive (biosynthetic center regions) and receptive region) Dendrites Nucleus with nucleolus Neurofibril Neuron cell body Nucleus (b) Chromatophilic (Nissl) bodies Nuclei of neuroglial cells Nucleolus Nissl bodies Axon (impulse generating Impulse and conducting direction region) Node of Ranvier (a) Axon hillock Neurilemma (one internode) Terminal branches Axon terminals (secretory region) Figure 12.4 Two Neurons Communicating at a Synapse Presynaptic neuron Axon Axon terminal at synapse Postsynaptic neuron Synapse Dendrite (a) Two neurons connected by synapses Figure 12.5a Some Important Types of Synapses Presynaptic axon Neurofilament Axon terminal Vesicle releasing neurotransmitter Nerve impulses Microtubule Mitochondrion Synaptic vesicles Synaptic cleft Postsynaptic dendrite (b) Enlarged view of the synapse Figure 12.5b 3

4 Classification of Neurons Structural classification Multipolar possess more than two processes Numerous dendrites and one axon Bipolar possess two processes Rare neurons found in some special sensory organs Unipolar (pseudounipolar) possess one short, single process Start as bipolar neurons during development Neurons Classified by Structure Neurons Classified by Structure 4

5 Neurons Classified by Function Functional Classification of Neurons Sensory neurons deliver information from exteroceptors, interoceptors, or proprioceptors Motor neurons Form the efferent division of the PNS Interneurons (association neurons) Located entirely within the CNS Distribute sensory input and coordinate motor output Supporting Cells Six types of supporting cells Four in the CNS Two in the PNS Provide supportive functions for neurons Cover nonsynaptic regions of the neurons 5

6 Supporting Cells (Neuroglial Cells) in the CNS Four types of neuroglia in the CNS Ependymal cells Related to cerebrospinal fluid Astrocytes Largest and most numerous Oligodendrocytes Myelination of CNS axons Microglia Phagocytic cells Neuroglia in the CNS Capillary Neuron Astrocyte (a) Astrocytes are the most abundant CNS neuroglia. Figure 12.6a Neuroglia in the CNS Neuron Microglial cell (b) Microglial cells are defensive cells in the CNS. Figure 12.6b 6

7 Neuroglia in the CNS Fluid-filled cavity Ependymal cells Brain or spinal cord tissue (c) Ependymal cells line cerebrospinal fluid filled cavities. Myelin sheath Process of oligodendrocyte Nerve fibers (d) Oligodendrocytes have processes that form myelin sheaths around CNS nerve fibers. Figure 12.6c, d Neuroglia in the PNS Satellite cells surround neuron cell bodies within ganglia s (neurolemmocytes) surround axons in the PNS Form myelin sheath around axons of the PNS Satellite cells Cell body of neuron s (forming myelin sheath) Nerve fiber (e) Satellite cells and s (which form myelin) surround neurons in the PNS. Figure 12.6e Myelin Sheaths in the PNS (a) Myelinated axon in PNS An axon wrapped with a fatty insulating sheath formed from s plasma membrane cytoplasm Axon 1 A envelops an axon. nucleus Myelin sheath cytoplasm Neurilemma Axon 2 The then rotates around the axon, wrapping its plasma membrane loosely around it in successive layers. Cross section of a myelinated axon (TEM 30,000 ) Neurilemma Myelin sheath 3 The cytoplasm is forced from between the membranes. The tight membrane wrappings surrounding the axon form the myelin sheath. Figure 12.7a 7

8 Unmyelinated Axons in the PNS (b) Unmyelinated axons in PNS Axons that are not covered with an insulating sheath Axons nucleus 1 A Schwann cell surrounds multiple axons. Neurilemma Axons Cross section of unmyelinated axons (TEM 11,000 ) 2 Each axon is encircled by the plasma membrane. Figure 12.7b Structure of a Nerve Axon Myelin sheath Perineurium Fascicle Epineurium Endoneurium Blood vessels Endoneurium Blood vessels (b) Perineurium Fascicle Nerve fibers (a) (c) nucleus Axon Myelin Node of Ranvier Figure 12.8 Gray and White Matter in the CNS PNS Sensory (afferent) fiber Spinal nerve Motor (efferent) fiber CNS Gray matter Short unmyelinated interneurons Cell bodies of interneurons and motor neurons Neuroglia White matter Fiber tracts of myelinated and unmyelinated axons Hollow central cavity Figure

9 Basic Neuronal Organization of the Nervous System Reflex arcs simple chains of neurons Explain reflex behaviors Determine structural plan of the nervous system Responsible for reflexes Rapid, autonomic motor responses Components of a Reflex Arc Stimulus Skin Receptor Sensory neuron Integration center Motor neuron Effector Interneuron Spinal cord (in cross section) Figure Types of Reflexes Monosynaptic reflex simplest of all reflexes Just one synapse The fastest of all reflexes Example knee-jerk reflex Polysynaptic reflex more common type of reflex Most have a single interneuron between the sensory and motor neuron Example withdrawal reflexes 9

10 Types of Reflexes 1 Sensory (stretch) receptor 2 Sensory (afferent) neuron 3 4 Motor (efferent) neuron 5 Effector organ (a) Monosynaptic stretch reflex Figure 12.11a Types of Reflexes 1 Sensory receptor 2 Sensory (afferent) neuron 3 Interneuron 4 Motor (efferent) neuron 5 Effector organ (b) Polysynaptic withdrawal reflex Figure 12.11b Neuronal Circuits Input Input 1 Input 2 Input 3 Output Output Output (a) Diverging circuit to multiple pathways (b) Converging circuit Input Output (c) Reverberating circuit Figure

11 Neural Processing Design of the Nervous System Sensory neurons located dorsally Cell bodies outside the CNS in sensory ganglia Central processes enter dorsal aspect of the spinal cord Motor neurons located ventrally Axons exit the ventral aspect of the spinal cord Interneurons located centrally Synapse with sensory neurons Simplified Design of the Nervous System Integration in gray matter. Multiple interneurons process the nerve impulses to localize the stimulus, identify its source, and plan a response. This complex processing is illustrated here in a simplified manner. Voluntary motor response. A nonreflexive motor response is initiated in the gray matter and transmitted down a descending fiber in the white matter to stimulate somatic motor neurons. Figure (continued) 11

12 Disorders of the Nervous System Multiple sclerosis common cause of neural disability Varies widely in intensity among those affected Cause is incompletely understood An autoimmune disease Immune system attacks the myelin around axons in the CNS Regeneration of the Peripheral Nerve Fiber Endoneurium s 1 The axon Aligning s 3 Axon sprouts, becomes form regeneration tube or filaments, Droplets fragmented at grow through a of myelin the injury site. regeneration tube formed by s. Fragmented axon Site of nerve damage Fine axon sprouts or filaments Macrophage 2 Macrophages clean out the dead axon distal to the injury. Site of new myelin sheath formation 4 The axon regenerates, and a new myelin sheath forms. Single enlarging axon filament Figure