The Nervous System The Nervous System 8 8. The Nervous System Two Organ Systems Control All the...

The Nervous

System

The Nervous

System88

The Nervous System

Two Organ Systems Control All the Other Organ Systems:•Nervous System characteristics

• Rapid response• Brief duration

•Endocrine System characteristics• Slower response• Long duration

The Nervous SystemTwo Anatomical Divisions:• Central Nervous System (CNS)

• Brain• Spinal cord

• Peripheral Nervous System (PNS)• All the neural tissue outside CNS• Afferent division (sensory input)• Efferent division (motor output)

• Somatic nervous system (controls skeletal muscle)

• Autonomic nervous system (controls smooth & cardiac muscle)

CENTRAL NERVOUS SYSTEM

PERIPHERALNERVOUSSYSTEM

Receptors

InformationProcessing

Sensory informationwithin

afferent division

Motor commandswithin

efferent division

includes

Somaticnervoussystem

Autonomicnervous system

Parasympatheticdivision

Sympatheticdivision

Skeletalmuscle

Effectors

Smoothmuscle

Cardiacmuscle

Adiposetissue

Glands

Somatic sensoryreceptors (monitorthe outside worldand our position

in it)

Visceral sensoryreceptors (monitorinternal conditions

and the statusof other organ

systems)

Neural Tissue Organization

Two Classes of Neural Cells:•Neurons

• For information transfer, processing, and storage

•Neuroglia• Supporting framework for neurons• Contain imbedded phagocytes


Three Classes of Neurons:•Sensory neurons

• Deliver information to CNS

•Motor neurons• Stimulate or inhibit peripheral tissues

• Interneurons (association neurons)• Located between sensory and motor

neurons• Analyze inputs, coordinate outputs


Neuron Anatomy:•Cell body

• Nucleus• Mitochondria, RoughER, other organelles

•Dendrites• Several branches• Signal receptors (inward)

•Axon• Signal propagation (outward)

Anatomy of a Typical Neuron


Structural Classes of Neurons:•Unipolar

• Dendrite, axon continuous, cell body off to one side

• Sensory neurons• Multipolar

• Many dendrites, one axon• Most common class of neuron• Motor Neurons

•Bipolar• One dendrite, one axon• Very rare, found in sense organs

Structural Classification of Neurons


Types of Neuroglia (glia):•Astrocytes

• Part of blood-brain barrier

•Oligodendrocytes• Responsible for myelination(protective

covering) around axons

•Microglia• Phagocytic defense cells

•Ependymal cells• Lining of brain, spinal cord cavities• Source of cerebrospinal fluid

Neuroglia in the CNS


Two Types of Neuroglia in the PNS:•Satellite cells

• Surround cell bodies

•Schwann cells• Surround all peripheral axons• Form myelin sheath on myelinated axons

Schwann Cells & Peripheral Axons


Key Note:

Neurons perform all of the communication, information processing, and control functions of the nervous system.

Neuroglia outnumber neurons and have functions essential to preserving the physical and biochemical structure of

neural tissue and the survival of neurons.


Anatomic Organization of CNS Neurons:•Center—Collection of neurons with a

shared function•Nucleus—A center with a discrete

anatomical boundary•Neural cortex—Gray matter covering of

brain portions•White matter—Bundles of axons (tracts)

that share origins, destinations, and functions


Anatomic Organization of PNS Neurons:•Ganglia—Groupings of neuron cell bodies•Nerve—Bundle of axons supported by

connective tissue• Spinal nerves

•To/from spinal cord• Cranial nerves

•To/from brain


Pathways in the CNS:•Ascending pathways

- Carry information from sensory receptors to processing centers in the brain

•Descending pathways

- Carry commands from specialized CNS

centers to skeletal muscles

Neuron Function

The Membrane Potential•Resting potential

• Excess negative charge inside the neuron

• Created and maintained by Na-K ion pump

• Negative voltage (potential) inside•-70 mV (0.07 Volts)

Cell Membrane at the Resting Potential

Neuron Function

A membrane potential exists across the cell membrane because:

(1) the cytosol and the extracellular fluid differ in their ionic composition, and

(2) the cell membrane is selectively permeable to these ions.

The membrane potential can quickly change, as the ionic permeability of the cell membrane changes, in response to chemical or physical stimuli.

+30

0

60_

70_

Tra

nsm

emb

ran

e p

ote

nti

al (

mV

)

Threshold

Restingpotential

1

2

3

4

REFRACTORY PERIOD

DEPOLARIZATION REPOLARIZATION

Localcurrent

Depolarization to threshold

Sodium ions

Activation of voltage-regulated sodium channels

and rapid depolarization

Potassium ions

Inactivation of sodiumchannels and activation of

voltage-regulatedpotassium channels

The return to normalpermeability and resting state

Time (msec)0 1 2 3

Neuron Function

Key Note:

“Information” travels within the nervous system primarily in the form of propagated electrical signals known as action potentials. The most important information (e.g., vision, balance, movement), is carried by myelinated axons.

Neural Communication

Synapse Basics• Intercellular communication

• Axon terminal receives electrical signal from cell body

• Input to next cell• Chemical signaling to cross

synapse•Neurotransmitter release


Structure of a Synapse:•Presynaptic components

• Axon terminal• Synaptic knob• Synaptic vesicles• Synaptic cleft

•Postsynaptic components• Neurotransmitter receptors


The Structure of a Typical Synapse


Synaptic Function and Neurotransmitters•Cholinergic synapses

• Release neurotransmitter acetylcholine• Enzyme in synaptic cleft

(acetylcholinesterase) breaks it down

•Adrenergic synapses• Release neurotransmitter norepinephrine

•Dopaminergic synapses• Release neurotransmitter dopamine

An action potential arrives anddepolarizes the synaptic knob

Action potential

ACh is removed by AChE(acetylcholinesterase)

Extracellular Ca2+ enters the synapticcleft triggering the exocytosis of ACh

ACh binds to receptors and depolarizesthe postsynaptic membrane

EXTRACELLULARFLUID

Synapticknob

PRESYNAPTICNEURON

Synaptic vesicles

ER

AChE

POSTSYNAPTICNEURONCYTOSOL

ACh

Ca2+

Ca2+Synaptic

cleft

Chemically regulatedsodium channels

Initiation ofaction potential

if thresholdis reached

ReceptorNa2+

Na2+Na2+Na2+Na2+

Propagation ofaction potential(if generated)



• Neuronal poolsGroups of interconnected neurons with specific functions

• DivergenceSpread of information from one neuron to several others

• ConvergenceSeveral neurons send information to one other

Neural CommunicationTwo Common Types of Neuronal Pools


Key Note:A synaptic terminal releases a neuro-transmitter that binds to the postsynaptic cell membrane. The result is a brief, local change in the permeability of the postsynaptic cell. Many drugs affect the nervous system by stimulating neurotransmitter receptors and thus produce complex effects on perception, motor control, and emotions.

The Central Nervous System

Meninges—Layers that surround and protect the brain and spinal cord (CNS)•Dura mater (“tough mother”)

• Tough, fibrous outer layer• Epidural space above dura of spinal cord

•Arachnoid (“spidery”)• Subarchnoid space• Cerebrospinal fluid

•Pia mater (“delicate mother”)• Thin inner layer

The Meninges


Spinal Cord Basics:•Relays information to/from brain•Processes some information on its

own (reflexes)•Divided into 31 segments•Each segment has a pair of:

• Dorsal root ganglia• Dorsal roots• Ventral roots

•Gray matter appears as horns•White matter organized into columns

Anatomy of the Spinal Cord

Sectional Anatomy of the Spinal Cord


Key Note:The sensory and motor nuclei (gray matter) of the spinal cord surround the central canal. Sensory nuclei are dorsal, motor nuclei are ventral. A thick layer of white matter consisting of ascending and descending axons covers the gray matter. These axons are organized into columns of axon bundles with specific functions. This highly organized structure often enables predicting the impact of particular injuries.


Brain Regions•Cerebrum•Diencephalon•Midbrain•Pons•Medulla oblongata•Cerebellum

The Brain


Brain—The four hollow chambers in the center of the brain filled with cerebrospinal fluid (CSF)•CSF produced by choroid plexus•CSF circulates

• From ventricles and central canal• To subarachoid space•Accessible by lumbar puncture

• To blood stream

Ventricles of the Brain

Circulation of Cerebrospinal Fluid


Functions of the Cerebrum:•Conscious thought• Intellectual activity•Memory•Origin of complex patterns of movement

Anatomy of Cerebral Cortex:•Highly folded surface

• Elevated ridges (gyri)• Shallow depressions (sulci)


Functions of the Cerebral Cortex•Hemispheres serve opposite body sides•Primary motor cortex (precentral gyrus)

• Directs voluntary movement

•Primary sensory cortex (postcentral gyrus)• Receives somatic sensation (touch, pain,

pressure, temperature)

•Association areas• Interpret sensation• Coordinate movement

Surface of Cerebral Hemispheres


Hemispheric Lateralization•Categorical hemisphere (usually left)

• General interpretative and speech centers• Language-based skills

•Representational Hemisphere (usually right)• Spatial relationships• Logical analysis

Hemispheric Lateralization

Brain Waves

(Electroencephalogram)


The Basal Nuclei•Lie deep within central white

matter of the brain•Responsible for muscle tone•Coordinate learned movements•Coordinate rhythmic movements

(e.g., walking)

The Basal Nuclei


Functions of the Limbic System•Establish emotions and related

drives•Link cerebral cortex intellectual

functions to brain stem autonomic functions

•Control reflexes associated with eating

•Store and retrieve long-term memories

The Limbic System


The Diencephalon•Switching and relay center• Integration of conscious and

unconscious motor and sensory pathways

•Components include:• Epithalamus

•Choroid plexus•Pineal body

• Thalamus• Hypothalamus

The Central Nervous SystemThe Diencephalon and Brain Stem

Figure 8-24(a)


Functions of the Thalamus•Relay and filter all ascending (sensory)

information• Relay a small proportion to cerebral

cortex (conscious perception)• Relay most to basal nuclei and brain

stem centers

•Coordinate voluntary and involuntary motor behavior


Functions of the Hypothalamus•Produce emotions and behavioral drives•Coordinate nervous and endocrine

systems•Secrete hormones•Coordinate voluntary and autonomic

functions•Regulate body temperature


Anatomy and Function of the Brain Stem•Midbrain

• Process visual, auditory information• Generate involuntary movements

•Pons• Links to cerebellum• Involved in control of movement

•Medulla oblongata• Relay sensory information• Regulate autonomic function


Functions of the Medulla Oblongata•Links brain and spinal cord•Relays ascending information to cerebral

cortex•Controls crucial organ systems by reflex

• Cardiovascular centers• Respiratory rhythmicity centers


Anatomy and Function of the Cerebellum•Oversees postural muscles•Stores patterns of movement•Fine tunes most movements•Links to brain stem, cerebrum, spinal cord

• Communicates over cerebellar peduncles


Key Note:The brain, a large mass of neural tissue, contains internal passageways and chambers filled with CSF. The six major regions of the brain have specific functions. As you ascend from the medulla oblongata to the cerebrum, those functions become more complex and variable. Conscious thought and intelligence are provided by the cerebral cortex.

The Peripheral Nervous System

PNS Basics•Links the CNS with the body•Carries all sensory information and motor

commands•Axons bundled in nerves•Cell bodies grouped into ganglia• Includes cranial and spinal nerves


The Cranial Nerves•12 Pairs

• Connect to brain not the cord

•Olfactory (CN I)• Sense of smell

•Optic (CN II)• Sense of vision

•Oculomotor (CN III)• Eye movement


The Cranial Nerves (continued)•Trochlear (CN IV)

• Eye movement•Trigeminal (CN V)

• Eye, jaws sensation/movement•Abducens (CN VI)

• Eye movement•Facial (CN VII)

• Face, scalp, tongue sensation/movement•Vestibulocochlear (CN VIII)

• Hearing, balance


The Cranial Nerves (continued)•Glossopharyngeal (CN IX)

• Taste, swallowing

•Vagus (CN X)• Autonomic control of viscera

•Accessory (CN XI)• Swallowing, pectoral girdle movement

•Hypoglossal (CN XII)• Tongue movement

The Cranial Nerves


Key Note:The 12 pairs of cranial nerves are responsible for the special senses of smell, sight, and hearing/balance, and control movement of the eye, jaw, face, tongue, and muscles of the neck, back, and shoulders. They also provide sensation from the face, neck, and upper chest and autonomic innervation to thoracic and abdominopelvic organs.


The Spinal Nerves•31 Pairs

• 8 Cervical• 12 Thoracic• 5 Lumbar• 5 Sacral

•Dermatome—Region of the body surface monitored by a pair of spinal nerves


Nerve Plexus—A complex, interwoven network of nerves•Four Large Plexuses

• Cervical plexus• Brachial plexus• Lumbar plexus• Sacral plexus


Reflex—An automatic involuntary motor response to a specific stimulus•The 5 steps in a reflex arc

• Arrival of stimulus and activation of receptor

• Activation of sensory neuron• CNS processing of information• Activation of motor neuron• Response by effector (muscle or gland)

Arrival ofstimulus andactivation ofreceptor

Stimulus

Receptor

Activation of asensory neuron

Effector

REFLEXARC

Dorsalroot

Ventralroot

Sensationrelayed to

the brain bycollateral

Activation of amotor neuron

Informationprocessingin CNS

KEYSensory neuron(stimulated)ExcitatoryinterneuronMotor neuron(stimulated)

Responseby effector

Reflex Action


Examples of Reflexes

•Monosynaptic reflex—Simplest reflex arc;

sensory neuron synapses directly on motor

neuron

•Stretch reflex—Monosynaptic reflex to

regulate muscle length and tension

(example: patellar reflex)

•Muscle spindle—Sensory receptor in a

muscle that stimulates the stretch reflex

Copyright © 2007 Pearson Education, Inc., publishing as Benjamin Cummings

Stretching of muscle tendonstimulates muscle spindles

Stretch

Contraction

Muscle spindle(stretch receptor)

REFLEXARC

Spinalcord

Activation of motorneuron produces reflexmuscle contraction


Polysynaptic reflex—A reflex arc with at least one interneuron between the sensory afferent and motor efferent•Has a longer delay than a

monosynaptic reflex (more synapses)•Can produce more complex response•Example: flexor reflex, a withdrawal

reflex•Brain can modify spinal reflexes

The Flexor Reflex


Key Note:

Reflexes are rapid, automatic responses to stimuli that “buy time” for the planning and execution of more complex responses that are often consciously directed.

The Autonomic Nervous System

Autonomic Nervous System

Branch of nervous system that coordinates cardiovascular, digestive, excretory, and reproductive functions


Divisions of the ANS•Sympathetic division

• Preganglionic neurons in the thoracic and lumbar segments of the spinal cord

• “Fight or flight” system

•Parasympathetic division• Preganglionic neurons in the brain and

sacral segments• “Rest and digest” system


Key Note:

The two divisions of the ANS operate largely without our awareness. The sympathetic division increases alertness, metabolic rate, and muscular abilities;

The parasympathetic division reduces metabolic rate and promotes visceral activities such as digestion.

The Sympathetic Division


Effects of Sympathetic Activation•Generalized response in crises• Increased alertness•Feeling of euphoria and energy• Increased cardiovascular activity• Increased respiratory activity• Increased muscle tone

The Parasympathetic Division


Effects of Parasympathetic Activation•Relaxation•Food processing•Energy absorption•Brief effects at specific sites


Relationship between the Two Divisions:•Sympathetic division reaches visceral and

somatic structures throughout the body•Parasympathetic division reaches only

visceral structures via cranial nerves or in the abdominopelvic cavity

•Many organs receive dual innervation• In general, the two divisions produce

opposite effects on the their target organs

Aging and the Nervous System

Age-Related Changes•Reduction in brain size and weight•Loss of neurons•Decreased brain blood flow•Changes in synaptic organization of the

brain• Intracellular and extracellular changes in

CNS neurons

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