12 PART 1
Fundamentals of the Nervous System and Nervous Tissue
Nervous System
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Master control and communication system
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Three overlapping functions
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Sensory receptors monitor changes inside and outside the body
• Stimulus—changes detected inside or outside the body
• Sensory input—information gathered by receptors
Nervous System
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Processes and interprets sensory input
• Integration—term for interpreting input
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Dictates a response by activating effector organs
• Motor output—is the response
Basic Divisions of the Nervous System
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Central nervous system (CNS)
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Composed of the brain and spinal cord
• Is the integrating and command center
Basic Divisions of the Nervous System
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Peripheral nervous system (PNS)
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Consists of nerves extending from brain and spinal cord
• Cranial nerves
• Spinal nerves
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Peripheral nerves link all regions of the body to the CNS
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Ganglia are clusters of neuronal cell bodies
Sensory Input and Motor Output
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Sensory (afferent) signals picked up by sensor receptors
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Carried by nerve fibers of PNS to the CNS
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Motor (efferent) signals are carried away from the CNS
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Innervate muscles and glands
Sensory Input and Motor Output
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Divided according to region they serve
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Somatic body region
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Visceral body region
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Results in four main subdivisions
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Somatic sensory
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Visceral sensory
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Somatic motor
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Visceral motor (autonomic nervous system)
Basic Divisions of the Nervous System
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Somatic sensory
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General somatic senses
• Receptors spread throughout outer tube of body
• Touch
• Pain
• Vibration
• Pressure
• Temperature
• General indicates “widespread”
Basic Divisions of the Nervous System
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Somatic sensory (continued)
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Proprioceptive senses—detect stretch in tendons and muscle
• Body sense—position and movement of body in space
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Special somatic senses (Chapter 16)
• Hearing
• Balance
• Vision
• Smell
Basic Divisions of the Nervous System
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Visceral sensory
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General visceral senses
• Stretch, pain, temperature, nausea, and hunger
• Widely felt in digestive and urinary tracts, and reproductive organs
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Special visceral senses
• Taste
• Smell
Basic Divisions of the Nervous System
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Somatic motor
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General somatic motor—signals contraction of skeletal muscles
• Under our voluntary control
• Often called “voluntary nervous system”
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Branchial motor
• Typical skeletal muscle derived from somitomeres
Basic Divisions of the Nervous System
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Visceral motor
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Regulates the contraction of smooth and cardiac muscle
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Makes up autonomic nervous system
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Controls function of visceral organs
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Often called “involuntary nervous system”
• Autonomic nervous system (Chapter 15)
Nervous Tissue
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Cells are densely packed and intertwined
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Two main cell types
• Neurons—transmit electrical signals
• Support cells (neuroglial cells in CNS)
• Nonexcitable
• Surround and wrap neurons
The Neuron
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Neurons are the basic structural unit of the nervous system
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Specialized cells conduct electrical impulses along the plasma membrane
• Nerve impulse (action potential)
The Neuron
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Special characteristics of neurons
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Longevity—can live and function for a lifetime
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Do not divide—fetal neurons lose their ability to undergo mitosis; neural stem cells
are an exception
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High metabolic rate—require abundant oxygen and glucose
• Neurons die after 5 minutes without oxygen
The Cell Body
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Cell body (soma)
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Size of cell body varies from 5 to 140 µm
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Contains usual organelles plus other structures
• Chromatophilic bodies (Nissl bodies)
• Clusters of rough ER and free ribosomes
• Function to renew membranes of the neuron
The Cell Body
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Neurofibrils
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Are bundles of intermediate filaments
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Form a network between chromatophilic bodies
The Cell Body
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Most neuronal cell bodies are located within the CNS
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Neuronal cell bodies in PNS
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Termed ganglia
Neuron Processes
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Dendrites
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Extensively branching from the cell body
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Transmit electrical signals toward the cell body
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Chromatophilic bodies—extend only into the basal part of dendrites and to the
base of the axon hillock
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Function as receptive sites for receiving signals from other neurons
Neuron Processes
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Axons
Neuron has only one
Impulse generator and conductor
Transmits impulses away from the cell body
Chromatophilic bodies are absent
No protein synthesis in axon
Neuron Processes
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Axons (continued)
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Neurofilaments, actin microfilaments, and microtubules
• Provide strength along length of axon
• Aid in transport of substances to and from the cell body
• This process is axonal transport
Neuron Processes
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Axons (continued)
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Branches along length are infrequent
• Axon collaterals
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Multiple branches at end of axon
• Terminal arboration (telodendria)
• End in knobs called terminal boutons (axon terminals)
Neuron Processes
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Nerve impulse
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Generated at the initial segment of the axon
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Conducted along the axon to the terminal boutons
• Neurotransmitters are released from vesicles at the terminal boutons
Synapses
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Site at which neurons communicate
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Most information passed through chemical messengers
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Some information transmitted electrically through gap junctions
Synapses
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Presynaptic neuron
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Conducts signal toward a synapse
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Postsynaptic neuron
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Transmits electrical activity away from a synapse
Types of Synapses
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Axodendritic
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Between axon terminals of one neuron and dendrites of another
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Most common type of synapse
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Axosomatic
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Between axons and neuronal cell bodies
Synapses
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Elaborate cell junctions
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Synaptic vesicles on presynaptic side
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Membrane-bound sacs containing neurotransmitters
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Mitochondria abundant in axon terminals
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Synaptic cleft
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Separates the plasma membrane of the two neurons
12 PART 2
Fundamentals of the Nervous System and
Nervous Tissue
Classification of Neurons
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Structural classification
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Multipolar—possess more than two processes
• Numerous dendrites and one axon
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Bipolar—possess two processes
• Rare neurons
• Found in some special sensory organs
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Unipolar (pseudounipolar)—possess one short, single process
• Start as bipolar neurons during development
Functional Classification of Neurons
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Functional classification is
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According to the direction the nerve impulse travels relative to the CNS
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Types of neurons
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Sensory neurons
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Motor neurons
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Interneurons
Functional Classification of Neurons
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Sensory neurons
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Transmit impulses toward the CNS
• Virtually all are unipolar neurons
• Cell bodies in ganglia outside the CNS
• Short, single process divides into
• The central process—runs centrally into the CNS
• The peripheral process—extends peripherally to the receptors
Functional Classification of Neurons
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Motor (efferent) neurons
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Carry impulses away from the CNS to effector organs
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Most motor neurons are multipolar
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Cell bodies are within the CNS
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Form junctions with effector cells
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Interneurons (association neurons)—most are multipolar
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Lie between motor and sensory neurons
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Confined to the CNS
Neuroglia
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Six types of neuroglia
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Four in the CNS
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Two in the PNS
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Provide supportive functions for neurons
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Cover nonsynaptic regions of the neurons
Neuroglia in the CNS
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Most neuroglial cells have branching processes and a central cell body
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Outnumber neurons 10 to 1
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Make up half the mass of the brain
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Can divide throughout life
Neuroglia in the CNS
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Astrocytes are the most abundant glial cell type
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Sense when neurons release glutamate
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Extract blood sugar from capillaries for energy
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Take up and release ions to control environment around neurons
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Involved in synapse formation in developing neural tissue
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Produce molecules necessary for neuronal growth (BDTF)
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Propagate calcium signals involved with memory
Neuroglia in the CNS
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Microglia
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Are the smallest and least abundant glial cell
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Phagocytes—the macrophages
of the CNS
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Engulf invading microorganisms and dead neurons
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Derive from blood cells called monocytes
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Migrate to CNS during embryonic and fetal periods
Neuroglia in the CNS
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Ependymal cells
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Line the central cavity of the spinal cord and brain
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Bear cilia—help circulate the cerebrospinal fluid
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Oligodendrocytes—have few branches
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Wrap their cell processes around axons in CNS
• Produce myelin sheaths in the CNS
Neuroglia in the PNS
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Satellite cells—surround neuron cell bodies within ganglia
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Schwann cells (neurolemmocytes)—surround axons in the PNS
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Form myelin sheath around axons of the PNS
12 PART 3
Fundamentals of the Nervous System and
Nervous Tissue
Myelin Sheaths
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Segmented structures composed of the lipoprotein myelin
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Surround thicker axons
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Form an insulating layer
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Prevent leakage of electrical current
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Increase the speed of impulse conduction
Myelin Sheaths in the PNS
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Formed by Schwann cells
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Develop during fetal period and in the first year of postnatal life
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Schwann cells wrap in concentric layers around the axon
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Cover the axon in a tightly packed coil of membranes
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Outer collar of perinuclear cytoplasm (neurilemma)
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Material external to myelin layers
Myelin Sheaths in the PNS
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Myelin sheath gaps (nodes of Ranvier)
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Are gaps along axon
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Speed up nerve transmission
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Thick axons are myelinated
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Thin axons are nonmyelinated
• Conduct impulses more slowly
Myelin Sheaths in the CNS
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Oligodendrocytes form the myelin sheaths in the CNS
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Have multiple processes
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Coil around several different axons
Gross Anatomy of the Nervous System
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Nerves—cablelike organs in the PNS
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Consist of numerous axons wrapped in connective tissue
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Most nerves contain myelinated and nonmyelinated sensory and motor axons
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Axon is surrounded by Schwann cells
Gross Anatomy of the Nervous System
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Endoneurium—layer of delicate connective tissue surrounding the axon
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Perineurium—connective tissue wrapping surrounding a nerve fascicle
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Nerve fascicles—groups of axons bound into bundles
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Epineurium—whole nerve is surrounded by tough fibrous sheath
Gray and White Matter in the CNS
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Gray matter
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Is gray-colored and surrounds hollow central cavities of the CNS
Forms H-shaped region in the spinal cord
• Dorsal half contains cell bodies of interneurons
• Ventral half contains cell bodies of motor neurons
Gray and White Matter in the CNS
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Gray matter
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Is composed primarily of neuronal cell bodies, dendrites, nonmyelinated axons
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Surrounds white matter of CNS in cerebral cortex and cerebellum
Gray and White Matter in the CNS
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White matter
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Lies external to the gray matter of the CNS
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Composed of myelinated axons
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Consists of axons passing between specific regions of the CNS
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Tracts are bundles of axons traveling to similar destinations
Neuronal Integration
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The CNS and PNS are functionally interrelated
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Nerves of the PNS
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Afferent PNS fibers respond to sensory stimuli
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Efferent PNS fibers transmit motor stimuli from CNS to muscles and glands
Neuronal Integration
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Nerves of the CNS
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Composed on interneurons that
• Receive sensory information
• Direct and transport information to specific CNS regions
• Initiate appropriate motor responses
Reflex Arcs
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Reflex arcs—simple chains of neurons
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Explain reflex behaviors
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Determine structural plan of the nervous system
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Responsible for reflexes
• Rapid, autonomic motor responses
• Can be visceral or somatic
Reflex Arcs
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Receptor—site where stimulus acts
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Sensory neuron—transmits afferent impulses to the CNS
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Integration center—consists of one or more synapses in the CNS
Reflex Arcs
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Motor neuron—conducts efferent impulses from integration center to an effector
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Effector—muscle or gland cell
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Responds to efferent impulses
• Contracting of a muscle
• Secretion from glands
Monosynaptic reflex
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Simplest of all reflexes
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Just one synapse
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The fastest of all reflexes
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Knee-jerk reflex
Polysynaptic reflex
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More common type of reflex
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One or more interneurons are part of the pathway
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Withdrawal reflexes
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Interneurons signal the motor neuron to contract muscle involved
Neuronal Circuits
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Diverging circuit—one presynaptic neuron synapses with several other neurons
(divergence)
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Converging circuit—many neurons synapse on a single postsynaptic neuron
(convergence)
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Reverberating circuit—circuit that receives feedback via a collateral axon from a
neuron in the circuit
Types of Processing
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Serial processing
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Neurons pass a signal to a specific destination along a single pathway from one to
another
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Parallel processing
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Input is delivered along many pathways; a single sensory stimulus results in multiple
perceptions
Integration Between PNS and CNS
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Neuronal circuits form networks of interneurons
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Example: painful stimulus
• Immediate response is spinal reflex
• Sensory information passed along to brain
• Pain is felt after reflexive withdrawal
Disorders of the Nervous System
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Multiple sclerosis
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An autoimmune disease
• Immune system attacks the myelin around axons in the CNS
• Varies widely in intensity among those affected
• More women than men are affected
• When men are affected, disease develops more quickly and is more devastating
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Cause is incompletely understood
Neuronal Regeneration
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Neural injuries may cause permanent dysfunction
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If axons alone are destroyed, cell bodies often survive, and the axons may regenerate
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PNS—macrophages invade and destroy axon distal to the injury
• Axon filaments grow peripherally from injured site
• Partial recovery is sometimes possible
Neuronal Regeneration
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CNS—neuroglia never form bands to guide regrowing axons and may hinder axon
growth with growth-inhibiting chemicals
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No effective regeneration after injury to the spinal cord and brain
Nervous Tissue Throughout Life
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Nervous system develops from the dorsal ectoderm
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Invaginates to form the neural tube and neural crest
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Neural tube walls begin as neuroepithelial cells
• These cells divide and become neuroblasts