chapter 48
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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
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