Chapter 48

Nervous Systems

Nerveswith giant axonsGanglia

MantleEye

Brain

Arm

Nerve

• Many animals have a complex nervous system which consists of:

– A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord

– A peripheral nervous system (PNS), which brings information into and out of the CNS

Sensor

Sensory input

Integration

Effector

Motor output

Peripheral nervoussystem (PNS)

Central nervoussystem (CNS)

Neuron Structure and Function

• Neurons- nerve cells that transfer information within the body

• Cell body- location of most of a neuron’s organelles

• Dendrites- highly branched extensions that receive signals from other neurons

• Axon- much longer extension that transmits signals to other cells at synapses

Neuron Structure and Function

• Synapse- is a junction between an axon and another cell

• Synaptic terminal- one axon passes information across the synapse in the form of chemical messengers called neurotransmitters

• Glia- cells that nourish or insulate neurons

DendritesStimulus

Nucleus

Cellbody

Axonhillock

Presynapticcell

Axon

Synaptic terminalsSynapse

Postsynaptic cellNeurotransmitter

Ion pumps and ion channels maintain the resting potential of a neuron

• Every cell has a voltage (difference in electrical charge) across its plasma membrane called a membrane potential

• Messages are transmitted as changes in membrane potential

Resting Potential- the membrane potential of a neuron not sending signals

• Concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside the cell

• Sodium-potassium pumps use the energy of ATP to maintain these K+ and Na+ gradients

• Ion channels- converts chemical potential to electrical potential

• A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell

Animation: Resting Potential

(b)

OUTSIDECELL

Na+Key

K+

Sodium-potassiumpump

Potassiumchannel

Sodiumchannel

INSIDECELL

Action potential

Fallingphase

Risingphase

Threshold (–55)

Restingpotential

Undershoot

Time (msec)

Depolarization–70–100

–50

0

+50

Mem

bran

e po

tent

ial (

mV)

• An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold

• An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane

• Action potentials are signals that carry information along axons

Generation of Action Potentials

• At resting potential

– 1. Most voltage-gated Na+ and K+ channels are closed, but some K+ channels (not voltage-gated) are open

• When an action potential is generated

– 2. Voltage-gated Na+ channels open first and Na+ flows into the cell

– 3. During the rising phase, the membrane potential increases

– 4. During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell

• 5. During the undershoot, membrane permeability to K+ is at first higher than at rest, then voltage-gated K+ channels close; resting potential is restored

KeyNa+

K+

+50Actionpotential

Threshold

0

1

4

51

–50

Resting potential

Mem

bran

e po

tent

ial

(mV)

–100Time

Extracellular fluid

Plasmamembrane

CytosolInactivation loop

Resting state

Sodiumchannel

Potassiumchannel

Depolarization

Rising phase of the action potential Falling phase of the action potential

5 Undershoot

2

3

2

1

3 4

• Refractory period after an action potential, a second action potential cannot be initiated.

• The refractory period is a result of a temporary inactivation of the Na+ channels

BioFlix: How Neurons Work

Animation: Action Potential

Axon

Plasmamembrane

Cytosol

Actionpotential

Na+

Actionpotential

Na+

K+

K+

ActionpotentialK+

K+

Na+

Conduction Speed

• Axons are insulated by a myelin sheath, which causes an action potential’s speed to increase

• Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS

Axon Myelin sheath

Schwanncell

Nodes ofRanvier

Schwanncell

Nucleus ofSchwann cell

Node of Ranvier

Layers of myelinAxon

Neurons communicate with other cells at synapses

• At electrical synapses, the electrical current flows from one neuron to another

• At chemical synapses, a chemical neurotransmitter carries information across the gap junction

• Most synapses are chemical synapses

Animation: Synapse

Voltage-gatedCa2+ channel

Ca2+12

3

4

Synapticcleft

Ligand-gatedion channels

Postsynapticmembrane

Presynapticmembrane

Synaptic vesiclescontainingneurotransmitter

5

6

K+Na+

Neurotransmitters

• The same neurotransmitter can produce different effects in different types of cells

• There are five major classes of neurotransmitters: acetylcholine, biogenic amines, amino acids, neuropeptides, and gases

Acetylcholine- common neurotransmitter, usually an excitatory transmitter

Biogenic Amines- include epinephrine, norepinephrine, dopamine, and serotonin

Amino Acid- gamma-aminobutyric acid (GABA) and glutamateNeuropeptides- relatively short chains of amino acids, include substance P and endorphinsGases- nitric oxide and carbon monoxide are local regulators in the PNS

• Central nervous system (CNS) where integration takes place (brain and nerve cord)

• Peripheral nervous system (PNS) brings information into and out of the CNS

Nervous systems consist of circuits of neurons and supporting cells

• Nerve net- a series of interconnected nerve cells

• Nerves- bundles that consist of the axons of multiple nerve cells

(a) Hydra (cnidarian)

Nerve net

Nervering

Radialnerve

(b) Sea star (echinoderm)

(c) Planarian (flatworm)

Nervecords

Transversenerve

Brain

EyespotBrain

(d) Leech (annelid)

Segmentalganglia

Ventralnervecord

(e) Insect (arthropod)

Segmentalganglia

Ventralnerve cord

BrainAnteriornerve ring

Longitudinalnerve cords

(f) Chiton (mollusc)

Ganglia

(g) Squid (mollusc)

Ganglia

Brain

Brain

Spinalcord(dorsalnervecord)

Sensoryganglia

(h) Salamander (vertebrate)

Organization of the Vertebrate Nervous System

• Central nervous system (CNS) where integration takes place (brain and nerve cord)

• Peripheral nervous system (PNS) brings information into and out of the CNS

Peripheral nervoussystem (PNS)

Cranialnerves

Brain

Central nervoussystem (CNS)

GangliaoutsideCNS

Spinalnerves

Spinal cord

Whitematter

Ventricles

Gray matter

The brain and spinal cord contain – Gray matter- neuron cell bodies, dendrites, and

unmyelinated axons – White matter- bundles of myelinated axons

Whitematter

Cell body ofsensory neuron indorsal rootganglion

Spinal cord(cross section)

Graymatter

Hamstringmuscle

Quadricepsmuscle

Sensory neuronMotor neuronInterneuron

The Peripheral Nervous System

• Afferent neurons transmit information to the CNS and efferent neurons transmit information away from the CNS

• Motor system- carries signals to skeletal muscles and is voluntary

• Autonomic nervous system- regulates the internal environment involuntarily

• Sympathetic division- correlates with the “fight-or-flight” response

• Parasympathetic division- promotes a return to “rest and digest”

• Enteric division- controls activity of the digestive tract, pancreas, and gallbladder

Efferentneurons

Locomotion

Motorsystem

Autonomicnervous system

Afferent(sensory) neurons

PNS

Hearing

CirculationGas exchange DigestionHormone

action

Entericdivision

Sympatheticdivision

Parasympatheticdivision

Stimulates glucoserelease from liver;inhibits gallbladder

Dilates pupilof eye

Parasympathetic division Sympathetic division

Action on target organs:

Inhibits salivary gland secretion

Accelerates heart

Relaxes bronchiin lungs

Inhibits activityof stomach and

intestines

Inhibits activityof pancreas

Stimulatesadrenal medulla

Inhibits emptyingof bladder

Promotes ejaculation and vaginal contractions

Constricts pupilof eye

Stimulates salivarygland secretion

Constrictsbronchi in lungs

Slows heart

Stimulates activityof stomach and

intestines

Stimulates activityof pancreas

Stimulatesgallbladder

Promotes emptyingof bladder

Promotes erectionof genitals

Action on target organs:

CervicalSympatheticganglia

Thoracic

Lumbar

SynapseSacral

Pons (part of brainstem), cerebellum

Forebrain

Midbrain

Hindbrain

Midbrain

Forebrain

Hindbrain

Telencephalon

Telencephalon

Diencephalon

Diencephalon

Mesencephalon

Mesencephalon

Metencephalon

Metencephalon

Myelencephalon

Myelencephalon

Spinal cord

Spinal cord

Cerebrum (includes cerebral cortex, white matter,basal nuclei)

Diencephalon (thalamus, hypothalamus, epithalamus)

Midbrain (part of brainstem)

Medulla oblongata (part of brainstem)

Pituitarygland

Cerebrum

Cerebellum

Central canal

Diencephalon:HypothalamusThalamus

Pineal gland(part of epithalamus)

Brainstem:MidbrainPonsMedullaoblongata

(c) Adult(b) Embryo at 5 weeks(a) Embryo at 1 month

CerebrumThalamusHypothalamus

Pituitary gland

Forebrain

Cerebralcortex

Midbrain

Hindbrain

PonsMedullaoblongataCerebellum

Spinalcord

The Brainstem

• Brainstem- coordinates and conducts information between brain centers

– Midbrain- contains centers for receipt and integration of sensory information

– Pons- regulates breathing centers in the medulla

– Medulla oblongata- controls breathing, cardiovascular activity, swallowing, vomiting, and digestion

The brainstem and cerebrum control arousal and sleep

The Cerebellum

• Cerebellum- coordination and error checking during motor, perceptual, and cognitive functions

• It is also involved in learning and remembering motor skills

• Corpus callosum- a thick band of axons provides communication between the right and left cerebral cortices

• The right half of the cerebral cortex controls the left side of the body, and vice versa

• Left hemisphere- language, math, logic, and processing of serial sequences

• Right hemisphere- pattern recognition, nonverbal thinking, and emotional processing

The Cerebrum

Corpuscallosum

Thalamus

Left cerebralhemisphere

Right cerebralhemisphere

Cerebralcortex

Basalnuclei

Cerebral cortex- controls voluntary movement and cognitive functions

• Each side of the cerebral cortex has four lobes: frontal, temporal, occipital, and parietal

Speech

Occipital lobe

Vision

Temporal lobe

Frontal lobe Parietal lobe

Somatosensoryassociationarea

Frontalassociationarea

Visualassociationarea

ReadingTaste

Hearing

Auditoryassociationarea

Speech

Smell

Mot

or c

orte

xSo

mat

osen

sory

cor

tex

Generating words

Max

Speaking words

Hearing words

Seeing words

Min

Memory and Learning

• Learning can occur when neurons make new connections or when the strength of existing neural connections changes

• Short-term memory is accessed via the hippocampus

• The hippocampus also plays a role in forming long-term memory, which is stored in the cerebral cortex

Fig. 49-18

ThalamusHypothalamus

Prefrontalcortex

Olfactorybulb

Amygdala Hippocampus

Stem Cell–Based Therapy

• The adult human brain contains stem cells that can differentiate into mature neurons