Fundamentals of the Nervous System and Nervous Tissue

HUMAN ANATOMYfourth edition

MARIEB | MALLATT | WILHELM

Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings

PowerPoint® Lecture Slides prepared by Leslie Hendon,

University of Alabama, Birmingham

12

Fundamentals of theNervous System and

Nervous TissuePART 1


Nervous System

• Master control and communication system


Nervous System: Functions

• Three overlapping functions• Sensory receptors monitor changes inside and

outside the body• Change – a stimulus

• Gathered information – sensory input

• CNS Processes and interprets sensory input• Makes decisions – integration

• Dictates a response by activating effector organs• Response – motor output


Basic Divisions of the Nervous System: CNS

• Central nervous system (CNS)• Brain and spinal cord

• Integrating and command center


Basic Divisions of the Nervous System: PNS

• Peripheral nervous system (PNS)• Outside the CNS• Nerves extending

from brain and spinal cord• Cranial nerves• Spinal nerves

• Link all regions of the body to the CNS


Sensory Input and Motor Output

• Sensory signals picked up by sensory receptors• Carried by afferent nerve fibers of PNS to the CNS

• Motor signals are carried away from the CNS • Carried by efferent nerve fibers of PNS to effectors

• Innervate muscles and glands


Sensory Input and Motor Output

• Divided according to region they serve• Somatic body region

• Visceral body region

• Results in four main subdivisions• Somatic sensory

• Visceral sensory

• Somatic motor

• Visceral motor


Somatic Sensory

• Somatic sensory • General somatic senses – receptors are widely

spread • Touch, pain, vibration, pressure, and temperature

• Proprioceptive senses – detect stretch in tendons and muscle

• Body sense – position and movement of body in space

• Special somatic senses • Hearing, balance, vision, and smell


Visceral Sensory

• Visceral sensory• General visceral senses – stretch, pain,

temperature, nausea, and hunger• Widely felt in digestive and urinary tracts,

reproductive organs

• Special visceral senses – taste


Somatic Motor

• Somatic motor• General somatic motor – signals contraction of

skeletal muscles• Under voluntary control

• Often called “voluntary nervous system”


Visceral Motor

• Visceral motor• Regulates the contraction of smooth and cardiac

muscle and gland secretion

• Makes up autonomic nervous system

• Controls function of visceral organs

• Often called “involuntary nervous system”


Peripheral Nervous System Summary

Figure 12.3


Types of Sensory and Motor Information

Figure 12.3


Nervous Tissue

• Cells are densely packed and intertwined • Two main cell types

• Neurons – transmit electrical signals

• Support cells (neuroglial cells) – nonexcitable

• Surround and wrap neurons


The Neuron

• The human body contains billions of neurons• Basic structural unit of the nervous system

• Specialized cells conduct electrical impulses along the plasma membrane

• Graded potentials

• Action potentials


The Neuron: Special Characteristics

• Longevity – can live and function for a lifetime

• Do not divide – fetal neurons lose their ability to undergo mitosis; neural stem cells are an exception

• High metabolic rate – require abundant oxygen and glucose


Neuron Structure


The Cell Body or Soma (also called Perikaryon)

• Size varies from 5–140µm

• Contains nucleus, organelles plus other structures• Chromatophilic bodies (Nissl bodies)

• Clusters of rough ER and free ribosomes

• Stain darkly and renew membranes of the cell

• Neurofibrils – bundles of intermediate filaments

• Form a network between chromatophilic bodies


Nissl Body Staining


The Cell Body

• Most neuronal cell bodies• Located within the CNS (clustered in nuclei)

• Protected by bones of the skull and vertebral column

• Ganglia – clusters of cell bodies in PNS


Cell Body Structure

Figure 12.4


Neuron Processes: Dendrites

• Dendrites • Extensively branching from

the cell body

• Transmit electrical signals (graded potentials) toward the cell body

• Chromatophilic bodies – only extend into the basal part of dendrites

• Function as receptive sites


Dendritic Spines


Neuron Processes: Axons

• Axons (nerve fibers)• Neuron has only one, but it can

branch

• Impulse generator and conductor

• Transmits action potentials away from the cell body

• Chromatophilic bodies absent

• No protein synthesis in axon


Neuron Processes: Axons

• Axons• Neurofilaments, actin

microfilaments, and microtubules• Provide strength along

length of axon

• Aid in the transport of substances to and from the cell body

• Axonal transport


Neuron Processes

Neuron Structure

• Axons• Branches along length are

infrequent• Axon collaterals

• Multiple branches at end of axon• Terminal branches (telodendria)

• End in knobs called axon terminals (also called end bulbs or boutons)


Neuron Processes: Action Potentials

• Nerve impulse (action potential)• Generated at the initial segment of the

axon

• Conducted along the axon

• Releases neurotransmitters at axon terminals

• Neurotransmitters – excite or inhibit neurons

• Neuron receives and sends signals


Synapses

• Site at which neurons communicate

• Signals pass across synapse in one direction

• Presynaptic neuron• Conducts signal toward a synapse

• Postsynaptic neuron• Transmits electrical activity away from a synapse


Two Neurons Communicating at a Synapse

Figure 12.6


Types of Synapses

• Axodendritic• Between axon terminals of one neuron and

dendrites of another• Most common type of synapse

• Axosomatic • Between axons and neuronal cell bodies

• Axoaxonic, dendrodendritic, and dendrosomatic• Less common types of synapses• Function not as well understood


Types of Synapses

Figure 12.7


Synapses

• Axodendritic synapses – representative type

• Synaptic vesicles on presynaptic side• Membrane-bound sacs containing neurotransmitters

• Mitochondria abundant in axon terminals

• Synaptic cleft separates the plasma membrane of the two neurons


Structure of a Synapses

Figure 12.8a, b

PLAYPLAY Synapse


Synapse


Signals Carried by Neurons: Resting Membrane Potential

• Plasma membranes of neurons conduct electrical signals

• Resting neuron – membrane is polarized

• Inner, cytoplasmic side is negatively charged


Changes in Membrane Potential

• Signals occur as changes in membrane potential


Directional Signals

• Stimulation of the neuron depolarization

• Inhibition of the neuron hyperpolarization


Action Potentials

Figure 12.9a, b


Action Potentials on Axons

• Strong depolarizing stimulus applied to the axon hillock triggers• Action potential

• Membrane becomes positive internally

• Action potential travels the length of the axon

• Membrane repolarizes itself


Action Potentials on Axons

Figure 12.9c–e


Graded Potentials on Dendrites and the Cell Body

• Natural stimuli applied to dendrites and the cell body• Receptive zone of the neuron

• Membrane stimulation causes local depolarization• A graded potential – inner surface becomes less

negative• Depolarization spreads from receptive zone to the

axon hillock• Acts as the trigger that initiates an action potential

in the axon


Synaptic Potentials

• Excitatory synapses• Neurotransmitters alter the permeability of the

postsynaptic membrane

• Leads to an inflow of positive ions • Depolarizes the postsynaptic membrane

• Drives the postsynaptic neuron toward impulse generation


Synaptic Potentials

• Inhibitory synapses• The external surface of the postsynaptic membrane

becomes more positive• Reduces the ability of the postsynaptic neuron to

generate an action potential


Classification of Neurons

• Structural Classification

• Functional Classification


Structural Classification of Neurons

Classification based on number of processes• Multipolar

• Bipolar

• Unipolar (pseudounipolar)


Multipolar Neurons

Figure 12.10a–c

Possess more than two processes

Numerous dendrites and one axon


Bipolar Neurons

Figure 12.10a–c

Possess two processes Rare neurons – found in some special sensory organs


Unipolar (Pseudounipolar) Neurons

Figure 12.10a–c

Possess one single processStart as bipolar neurons during development


Functional Classification of Neurons

Classification based on direction of action potential propagation• Afferents – from CNS to periphery

• Efferents – from periphery to CNS

• Interneurons – within CNS


Afferent neurons

• Afferent (sensory) neurons – transmit impulses toward the CNS• Virtually all are pseudounipolar neurons (some true

bipolar)

• 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


Afferent Neurons

Sensory receptorsAxon terminals

Periphery CNS


Efferent Neurons

• Efferent (motor) neurons • Carry impulses away from the CNS to effector

organs

• Most efferent neurons are multipolar

• Cell bodies are within the CNS

• Form junctions with effector cells


Interneurons

• Interneurons (association neurons) – most are multipolar • Lie between afferent and efferent neurons

• Confined to the CNS


Neurons Classified by Function

Figure 12.11


Variety of Interneurons

• Purkinje cell, stellate cell, granule cell, and basket cell• Located in the cerebellum

• Pyramidal cell – located in the cerebral cortex


Variety of Interneurons


Glial Cells (Supporting Cells)

• Six types of glial cells• Four in the CNS

• Two in the PNS

• Provide supportive functions for neurons

• Cover nonsynaptic regions of the neurons


Supporting Cells (Neuroglial Cells) in the CNS

• Neuroglia – usually only refers to supporting cells in the CNS, but can be used for PNS• Glial cells have branching processes and a central

cell body

• Outnumber neurons 10 to 1

• Make up half the mass of the brain

• Can divide throughout life


Types of Glial Cells in the CNS

• Astrocytes

• Microglia

• Ependymal Cells

• Oligodendrocytes


Astrocytes

• Astrocytes – most abundant glial cell type• Take up and release ions to control the environment

around neurons

• Recapture and recycle neurotransmitters

• Involved with synapse formation in developing neural tissue

• Produce molecules necessary for neural growth (BDTF)

• Propagate calcium signals that may be involved in memory


Astrocytes

Figure 12.12a

Necessary for development and maintenance of theblood brain barrier


• Microglia – smallest and least abundant

• Phagocytes – the macrophages of the CNS

• Engulf invading microorganisms and dead neurons

• Derived from blood cells called monocytes

Microglia

Figure 12.12b


Ependymal Cells

• Ependymal cells• Line the central cavity of the spinal cord and brain

• Bear cilia – help circulate the cerebrospinal fluid


Oligodendrocytes

• Oligodendrocytes – have few branches• Wrap their cell processes around axons in CNS

• Produce myelin sheaths

Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings Figure 12.13

Supporting Cells in the PNS

• Satellite cells – surround neuron cell bodies within ganglia

• Schwann cells (neurolemmocytes) – surround axons in the PNS• Form myelin sheath around axons of the PNS


Myelin Sheaths

• Segmented structures composed of the lipoprotein myelin

• Surround thicker axons

• Form an insulating layer • Prevent leakage of electrical current

• Increase the speed of impulse conduction


Myelin Sheaths in the PNS

• Formed by Schwann cells

• Develop during fetal period and in the first year of postnatal life

• Schwann cells wrap in concentric layers around the axon• Cover the axon in a tightly packed coil of

membranes



• Nodes of Ranvier – gaps along axon

• Allow current exchange across axon membrane



• Thick axons are myelinated• Fast conduction velocity

• Thin axons are unmyelinated• Slow conduction velocity



Figure 12.14a


Myelin Sheaths in the PNS – myelinated axon

Figure 12.15b


Myelin Sheaths in the PNS – unmyelinated axons

Figure 12.15b


Myelin Sheaths in the CNS

• Oligodendrocytes form the myelin sheaths in the CNS• Have multiple processes

• Coil around several different axons


Oligodendrocytes


Nerves

• Nerves – cordlike organs in the PNS

• Consists of numerous axons wrapped in connective tissue

• Axon is surrounded by Schwann cells


Nerves

• Endoneurium – layer of delicate connective tissue surrounding the axon

• Nerve fascicles – groups of axons bound into bundles

• Perineurium – connective tissue wrapping surrounding a nerve fascicle

• Epineurium – whole nerve is surrounded by tough fibrous sheath


Simplified 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 • Provide communication between sensory and

motor neurons and between levels of the CNS


Example of Neuronal Organization: Reflexes

• Reflex arcs – simple neural pathways• Responsible for reflexes

• Rapid, autonomic motor responses

• Can be visceral or somatic


Five Essential Components to the Reflex Arc

• Receptor – detects the stimulus

• Afferent (sensory neuron) – transmits impulses to the CNS

• Integration center – consists of one or more synapses in the CNS

• Efferent (motor neuron) – conducts impulses from integration center to an effector

• Effector – muscle or gland cell• Responds to efferent impulses

• Contraction or secretion


Example of the Five Components to the Reflex Arc

Figure 12.17


Reflex Classification

• Monosynaptic or polysynaptic

• Spinal or cranial

• Somatic or autonomic

• Innate or learned


Types of Reflexes: Number of Classes

• 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


Monosynaptic Reflex

Figure 12.18a, b


Polysynaptic Reflex

Figure 12.18a, b


Spinal vs Cranial Reflexes

• Spinal = spinal cord integration center• Ex. Knee-jerk reflex

• Cranial = brain as integration center• Ex. Pupillary light reflex


Somatic vs Autonomic Reflexes

• Somatic = motor neurons to skeletal muscles• Ex. Knee-jerk reflex

• Autonomic = autonomic neurons to smooth muscle and glands• Ex. Pupillary light reflex


Innate vs Learned Reflexes

• Innate = born-with• Knee-jerk reflex, pupillary reflex

• Learned = develops based on experiences• Pavlov’s dogs salivation in response to bell


Neuronal Circuits

• Diverging circuit

• Converging circuit

• Reverberating circuit


Diverging Circuit

• Diverging circuit – one presynaptic neuron synapses with several other neurons (divergence)


Converging Circuit

• Converging circuit – many neurons synapse on a single postsynaptic neuron (convergence)


Reverberating Circuit

• Reverberating circuit – circuit that receives feedback via a collateral axon from a neuron in the circuit


Neural Processing

• Serial processing

• Parallel processing


Serial Processing

• Serial processing – neurons pass a signal to a specific destination along a single pathway from one to another


Parallel Processing

• Parallel processing – input is delivered along many pathways; a single sensory stimulus results in multiple perceptions


Gray versus White Matter in the Central Nervous System

• Gray matter

• Cell bodies

• Dendrites

• Synapses

•White matter•Axons (myelin)


Gray Matter in the Spinal Cord

• Gray matter in the spinal cord

• H-shaped (butterfly) region – surrounds central cavity

• Dorsal half contains cell bodies of interneurons

• Ventral half contains cell bodies of motor neurons

• Cell bodies are clustered in the gray matter


White Matter in the Spinal Cord

• White matter in the spinal cord

• Located externally to the gray matter

• Contains no neuronal cell bodies, but millions of axons

• Myelin sheath – white color

• Consists of axons running between different parts of the CNS

• Tracts – bundles of axons traveling to similar destinations


Gray Matter in Brain

• Cortex and nuclei


White Matter in Brain

• Pathways, tracts and commissures


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


Multiple Sclerosis Videos

• Symptoms of multiple sclerosis

http://medstat.med.utah.edu/kw/ms/video.html

Fundamentals of the Nervous System and Nervous Tissue

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