Chapter 25

Neural Control and the Senses

Communication units of nervous systemsDetect information about internal and

external conditionsIssue commands for responsive actions

Neurons

stimulus(output)

receptors

integrators

motor neurons

effectors

response(output)

muscles,

glands

interneurons of brain, spinal

cord

sensory neurons

Sensory neurons Detect and relay information

Interneurons Receive and process information

Motor neuronsTransmit signals from interneurons to

effectors

Types of Neurons

Structure of a Neuron

dendrites

cell body

TRIGGER ZONE

INPUT ZONE

CONDUCTING ZONE

OUPUT ZONEaxon

axon endings

Fig. 25-1b, p.423

Cells that metabolically assist, structurally support, and protect neurons

Make up more than half the volume of the vertebrate nervous system

Neuroglia

Electrical gradient across membraneAbout -70 mVMaintained by sodium-potassium pump

Potassium (K+) higher insideSodium (Na+) higher outside

Resting Membrane Potential

neuron becomesmore positive inside

more gated channelsfor Na+ open

more Na+ flowsinto the neuron

outside

plasma membrane

inside

K+

K+

Na+

Na+

p.424a

How Ions Move across Membrane

passive transporters with open channels

passive transporters with voltage-sensitive gated channels

active transporters

lipid bilayer of neuron membrane

interstitial fluid

cytoplasm Na+/K+ pump

Brief reversal in membrane potentialVoltage change causes voltage-gated

channels in membrane to open Inside of neuron briefly becomes more

positive than outside

Action Potential

Action Potential

Na+Na

+Na

+

Na+ Na

+

Na+

K+

K+

K+

Na+Na+

K+ K+ K+

K+

Na+

1 2

3 4

Fig. 25-4a, p.425

interstitial fluid

cytoplasm

Fig. 25-4b, p.425

Na+

Na+

Na+

Fig. 25-4c, p.425

Na+

Na+

Na+

K+

K+

K+

Fig. 25-4d, p.425

Na+

Na+

K+K+

K+

K+

Na+

Na+/K+

pump

Positive Feedback

neuron becomesmore positive inside

more gated channelsfor Na+ open

more Na+ ionsflow into the neuron

All action potentials are the same sizeIf stimulation is below threshold level, no

action potential occursIf stimulation is above threshold level, cell

always depolarizes to same level

All or Nothing

Once action potential peak is reached, Na+ gates close and K+ gates open

Movement of K+ out of cell The inside of the cell once again becomes

more negative than the outside

Repolarization

Recording of Action Potential

action potential

threshold

resting membrane potential

Time (milliseconds)

Mem

bra

ne p

ote

nti

al

(mil

livo

lts)

-40

-70

-20

0

+20

0 1 2 3 4 5

Action potential in one part of an axon brings neighboring region to threshold

Action potential moves from one patch of membrane to another

Can only move one direction

Propagation of Action Potentials

Action potentials cannot jump from cell to cellSignal is transmitted from axon end, across a

synaptic cleft, by chemical signals called neurotransmitters

Chemical Synapses

Gap between the terminal ending of an axon and the input zone of another cell

Chemical Synapse

synaptic vesicle

plasma membrane of axon ending of presynaptic cell

plasma membrane of postsynaptic cell

synaptic cleft

membrane receptor

Action potential in axon ending triggers release of neurotransmitter from presynaptic cell into synaptic cleft

Synaptic Transmission

vesicle inside presynaptic cell

synaptic cleft

postsynaptic cell

Neurotransmitter diffuses across cleft and binds to receptors on membrane of postsynaptic cell

Binding of neurotransmitter to receptors opens ion gates in membrane of postsynaptic cell

Synaptic Transmission

Ion Gates Open

ions

neurotransmitter

receptor for neurotransmitter

gated channel protein

Many signals reach a neuron at the same time

Signals may suppress or reinforce one another

Whether or not an action potential occurs depends on the sum of the signals the neuron receives

Synaptic Integration

Synapse between motor neuron and skeletal muscle fiber

Neuron releases chemical neurotransmitter acetylcholine (ACh)

Neuromuscular Junction

neuromuscular junction

part of a skeletal muscle

motor neuron axons from spinal cord to skeletal muscle fibers

transverse slice of spinal cord

Fig. 25-6a, p.427

A Neuromuscular Junction

muscle fiber

axon ending

Fig. 25-6b, p.427

A Neuromuscular Junction

Acetylcholine (ACh)NorepinephrineEpinephrineDopamineSerotoninGABA

Neurotransmitters

After neurotransmitter has acted, it is quickly removed from synaptic cleft

Molecules diffuse away, are pumped out, or broken down

Cleaning Up

Information Flow

interneuron motor neuronsensory neuron

Neurons are bundled in nervesNerves are organized in circuits and reflex

pathwaysInformation from sensory neurons is relayed

to interneurons in spinal cord and brainMotor neurons carry signals to body

Organization

A bundle of axons enclosed within a connective tissue sheath

Nerve axon

myelin sheath

many neuronsinside a connective tissue sheath

Myelin Sheath

• Sheath blocks ion movements

• Action potential must “jump” from node to node

• Greatly enhances speed of transmission

A condition in which nerve fibers lose their myelin

Slows conduction Symptoms include visual problems,

numbness, muscle weakness, and fatigue

Multiple Sclerosis

Automatic movements in response to stimuliIn simplest reflex arcs, sensory neurons

synapse directly on motor neuronsMost reflexes involve an interneuron

Reflexes

Stretch Reflex

STIMULUSBiceps

stretches.

RESPONSE

Biceps contracts.

motor neuron

sensory neuron

All animals except sponges have some sort of nervous system

Nerve cells interact with one another in signal-conducting and information-processing highways

Invertebrate Nervous Systems

Bilateral Nervous System

ganglion (one in most body segments)

nerve cord

branching nerve

rudimentary brain

Earliest fishlike vertebrates had a hollow, tubular nerve cord

Modification and expansion of nerve cord produced spinal cord and brain

Nerve cord persists in vertebrate embryos as a neural tube

Vertebrate Development

Central nervous system (CNS)Brain

Spinal cord

Peripheral nervous systemNerves that thread through the body

Central and Peripheral Nervous Systems

Vertebrate Nervous Systems

Braincranial nerves(twelve pairs)

Spinal Cord

ulnar nerve(one in each arm)lumbar nerves(five pairs)sacral nerves(five pairs)

coccygeal nerves(one pair)

cervical nerves(eight pairs)

thoracic nerves(twelve pairs)

sciatic nerve(one in each leg)

Fig. 25-12, p.431

Major Nerves

Somatic nerves Motor functions(Shown in green)

Autonomic nervesVisceral functions(Shown in red)

Peripheral Nervous System

Sympathetic Parasympathetic

Most organs receive input from both Usually have opposite effects on organ

Two Types of Autonomic Nerves

cervical nerves (8pairs)

thoracic nerves (12 pairs)

lumbar nerves (five pairs)

sacral nerves (five pairs)

midbrain

(all ganglia in

walls of organs)

pelvic nerve

vagus nerve

optic nerve

medulla oblongata

(most ganglia

near spinal cord)

Fig. 25-13, p.432

eggs

salivary glands

heart

larynxbronchilungs

bladder

liverspleen

pancreas

kidneysadrenal glands

small intestineupper colonlower colon

rectum

uterus

genitals

stomach

Autonomic Nervous System

Originate in thoracic and lumbar regions of spinal cord

Ganglia are near the spinal cordRespond to stress or physical activity (fight-

or-flight response)

Sympathetic Nerves

Originate in brain and sacral region of spinal cord

Ganglia are in walls of organsPromote housekeeping responses such as

digestion

Parasympathetic Nerves

Most organs receive both sympathetic and parasympathetic signals

Example: Sympathetic nerves signal heart to speed up; parasympathetic stimulate it to slow down

Synaptic integration determines response

Opposing Systems

White matterTracts with myelin sheathsSensory and motor neurons

Gray matterUnmyelinatedCell bodies, dendrites, neuroglia

MeningesProtective coverings

Structure of CNS

Table 25-1, p.434

Expressway for signals between brain and peripheral nerves

Sensory and motor neurons make direct reflex connections in spinal cord

Spinal reflexes do not involve brain

Function of Spinal Cord

spinal cord

spinal nerve

vertebra

meninges(protectivecoverings)

Fig. 25-14, p.433

ventral dorsal

location of intervertebral disk

Spinal Cord

midbrain

thalamushypothalamus

pons

cerebellum

medulla oblongata

corpus callosum

part of optic nerve

pineal gland

location

Fig. 25-15, p.434

The Brain

Brain develops from a hollow neural tube

Forebrain, midbrain, and hindbrain form from three successive regions of tube

Most evolutionarily ancient nervous tissue persists as the brain stem

Development of the Brain

Divisions of Brain

Division Main Parts

Forebrain

Midbrain

Hindbrain

CerebrumOlfactory lobesThalamusHypothalamusLimbic systemPituitary glandPineal gland

Tectum

PonsCerebellumMedulla oblongata

Surrounds the spinal cord

Fills ventricles within the brain

Blood-brain barrier controls which solutes enter the cerebrospinal fluid

Cerebrospinal Fluid

Largest and most complex part of human brain

Outer layer (cerebral cortex) is highly foldedA longitudinal fissure divides cerebrum into

left and right hemispheres

Anatomy of the Cerebrum

Lobes of the Cerebrum

temporal

frontalparietal

occipital

primary motor cortex primary somatosensory cortex

Controls emotions and has role in memory

Limbic System

(olfactory tract) cingulate gyrus thalamus

amygdala

hippocampus

hypothalamus

Convert stimulus into action potentials

Sensory Receptors

Mechanoreceptors

Thermoreceptors

Pain receptors

Chemoreceptors

Osmoreceptors

Photoreceptors

Action potentials don’t vary in sizeBrain integrate information by

Which pathway carries the signalFrequency of action potentials

along each axonNumber of axons recruited

Stimulus Strength

TouchPressure

TemperaturePain

MotionPosition

Somatic Sensations

The Somatosensory Cortex

Free nerve ending

Ruffini endingPacinian

corpuscleBulb of KrauseMeissner’s

corpuscle

Receptors in Skin

A special senseOlfactory

receptorsReceptor axons

lead to olfactory lobe

Smell

olfactorybulb

receptor cell

A special senseChemoreceptors Five primary

sensations:sweet, sour, salty,

bitter, and umami

Taste

Sensitivity to light is not visionVision requires

Eyes Capacity for image formation in the

brain

Vision

Perceives visual fieldLens collects lightImage formed on retinaContains visual pigmentsStimulate photoreceptors

The Eye

Human Eye sclera

choroid

iris

lens

pupil

cornea

aqueoushumor

ciliary muscle

vitreous body

retina

foveaopticdisk

part ofopticnerve

Image on retina is upside down and reversed right to left compared with the stimulus

Brain corrects during processing

Pattern of Stimulation

Photoreceptors at back of retina, in front of pigmented epithelium

For light to reach photoreceptors, it must pass layers of neurons involved in visual processing

Organization of Retina

Signals from photoreceptors are passed to bipolar sensory neurons, then to ganglion cells

Axons of ganglion cells form the two optic nerves

Organization of Retina

Cone

Rod

Ganglion cell

Bipolar sensory neuron

Rods Contain the pigment rhodopsin Detect very dim light, changes in light

intensityCones

Three kinds; detect red, blue, or greenProvide color sense and daytime vision

The Photoreceptors

stacked, pigmented membrane

cone cell

rodcell

Fig. 25-28, p.443

Rods and Cones

Macular degenerationCataractGlaucoma

Eye Diseases

fovea

start of an optic nerve in back of the eyeball

Outer earMiddle earInner ear

Hearing

Ear detects pressure wavesAmplitude of waves corresponds to perceived

loudness Frequency of waves (number per second)

corresponds to perceived pitch

Properties of Sound

Anatomy of Human Ear

cochlea

auditory nerve

eardrumauditory canal

hammer

anvilstirrup

Sound waves make the eardrum vibrateVibrations are transmitted to the bones of the

middle earThe stirrup transmits force to the oval

window of the fluid-filled cochlea

Sound Reception

Sound Reception

hair cells in organ of Corti tectorial

membrane

lumen of cochlear duct

basilar membranelumen of scala tympani

to auditory nerve

Hair cells

Organ of Corti

Mechanoreceptors located in the inner ear

Maintains body position

Balance and Equilibrium

vestibular apparatus

semicircular canals