Nervous System

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

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


Structure of a Synapses

Figure 12.8a, b

PLAYPLAY 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


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


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

Nervous System

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