Neural Control

Chapter 33 Part 1

Impacts, IssuesIn Pursuit of Ecstasy

Neural controls maintain life; drugs like Ecstasy flood the brain with signaling molecules and saturate receptors, disrupting these controls

Fig. 33-1a, p. 552

Fig. 33-1b, p. 552

Fig. 33-1c, p. 552

33.1 Evolution of Nervous Systems

Interacting neurons allow animals to respond to stimuli in the environment and inside their body

Neuron• A cell that can relay electrical signals along its

plasma membrane and can communicate with other cells by specific chemical messages

Neuroglia • Support neurons functionally and structurally

Three Types of Neurons

Sensory neurons detect stimuli and signal interneurons or motor neurons

Interneurons process information from sensory neurons and send signals to motor neurons

Motor neurons control muscles and glands

The Cnidarian Nerve Net

Cnidarians are the simplest animals that have neurons, which are arranged as a nerve net

Nerve net• A mesh of interconnecting neurons with no

centralized controlling organ

Bilateral, Cephalized Nervous System

Flatworms are the simplest animals with a bilateral, cephalized nervous system

Cephalization• The concentration of neurons that detect and

process information at the body’s head end

Ganglion• A cluster of neuron cell bodies that functions as

an integrating center

Nerve Cords

Annelids and arthropods have paired ventral nerve cords that connect to a simple brain• Pair of ganglia in each segment for local control

Chordates have a single, dorsal nerve cord; vertebrates have a brain at the anterior region of the nerve cord

Simple Nervous Systems

Fig. 33-2a, p. 554

a nerve net (highlighted in purple) controls the contractile cells in the epithelium

Hydra, a cnidarian

Fig. 33-2b, p. 554

pair of ganglia

pair of nerve cords cross-connected by lateral nerves

Planarian, a flatworm

Fig. 33-2c, p. 554

rudimentary brain

ventral nerve cord

ganglion

c Earthworm, an annelid

Fig. 33-2 (d-e), p. 554

brain brain

optic lobe (one pair, for visual stimuli)

branching nerves

paired ventral nerve cords

ganglion

Crayfish, a crustacean (a type of arthropod)

Grasshopper, an insect (a type of arthropod)

The Vertebrate Nervous System

Central nervous system (CNS)• Brain and spinal cord (mostly interneurons)

Peripheral nervous system (PNS)• Nerves from the CNS to the rest of the body

(efferent) and from the body to CNS (afferent)• Autonomic nerves and somatic nerves control

different organs of the body

Central Nervous System

Brain Spinal Cord

(cranial and spinal nerves)

Peripheral Nervous System

Autonomic Nerves

Nerves that carry signals to and from smooth muscle, cardiac muscle, and glands

Somatic Nerves

Nerves that carry signals to and from skeletal muscle,

tendons, and the skin

Sympathetic Division

Parasympathetic Division

Two sets of nerves that often signal the same effectors and

have opposing effects

Stepped Art

Functional Divisions of the Vertebrate Nervous System

Fig. 33-4, p. 555

Brain

cranial nerves (twelve pairs)

cervical nerves (eight pairs)

Spinal Cord

thoracic nerves (twelve pairs)

ulnar nerve (one in each arm)

sciatic nerve (one in each leg)

lumbar nerves (five pairs)

sacral nerves (five pairs) coccygeal

nerves (one pair)

Major Nerves of the Human Nervous System

33.1 Key Concepts How Animal Nervous Tissue is Organized

In radially symmetrical animals, excitable neurons interconnect as a nerve net

Most animals are bilaterally symmetrical with a nervous system that has a concentration of neurons at the anterior end and one or more nerve cords running the length of the body

33.2 Neurons—The Great Communicators

Neurons have special cytoplasmic extensions for receiving and sending messages• Dendrites receive information from other cells• Axons send chemical signals to other cells

Sensory neurons have an axon with one end that responds to stimuli; the other sends signals

Interneurons and motor neurons have many dendrites and one axon

Fig. 33-5, p. 556

dendritesinput zone

cell body

trigger zone

conducting zone output zone

axon axon terminals

A Motor Neuron

cell body

axon axon terminals

dendrites

c motor neuron

Fig. 33-6, p. 556

receptor endings

peripheral axon

cell body

axon axon terminal

a sensory neuron b

axoncell body

dendrites

interneuron

Stepped Art

Direction of Information Flow

STIMULI RESPONSE

p. 557

150 Na+ 5 K+ interstitial fluid

plasma membrane

15 Na+ 65 neuron’s cytoplasm

150 K+

33.3 Membrane Potentials Resting membrane potential• The interior of a resting neuron is more negative than

the fluid outside the cell (-70 mV)• Negatively charged proteins and active transport of Na+

and K+ ions maintain the resting potential

Action Potentials

Action potential• An abrupt reversal in the electric gradient across

the plasma membrane • When properly stimulated, voltage-gated

channels open, ions flow through, and the membrane potential briefly reverses

Fig. 33-7, p. 557

interstitial fluid

neuron cytoplasm

A Sodium–potassium pumps actively transport 3 Na+ out of a neuron for every 2 K+ they pump in.

B Passive transporters allow K+ ions to leak across the plasma membrane, down their concentration gradient.

C In a resting neuron, gates of voltage-sensitive channels are shut (left). During action potentials, the gates open (right), allowing Na+ or K+ to flow through them.

Membrane Proteins: Pumps, Transporters, and Gated Channels

33.4 A Closer Look at Action Potentials

An action potential begins• Stimulation of a neuron’s input zone causes a

local, graded potential• When stimulus in the neuron’s trigger zone

reaches threshold potential, gated sodium channels open

• Voltage difference decreases and starts the action potential

An All-or-Nothing Spike

Once threshold level is reached, membrane potential always rises to the same level as an action potential peak (all-or-nothing response)

Fig. 33-10, p. 559

action potential

threshold level

resting level

An All-or-Nothing Spike

Direction of Propagation

An action potential is self-propagating• Sodium ions diffuse to adjoining region of axon,

triggering sodium gates one after another

An action potential can only move one way, toward axon terminals• Brief refractory period after sodium gates close

Fig. 33-8a, p. 558

interstitial fluid with high Na+, low K+

Na+–K+ pump

voltage-gated ion channels

cytoplasm with low Na+, high K+

A Close-up of the trigger zone of a neuron. One sodium–potassium pump and some of the voltage-gated ion channels are shown. At this point, the membrane is at rest and the voltage-gated channels are closed. The cytoplasm’s charge is negative relative to interstitial fluid.

Propagation of an Action Potential

Fig. 33-8b, p. 558

Na+

Na+

Na+

B Arrival of a sufficiently large signal in the trigger zone raises the membrane potential to threshold level. Gated sodium channels open and sodium (Na+) flows down its concentration gradient into the cytoplasm. Sodium inflow reverses the voltage across the membrane.

Na+ Na+ Na+

Propagation of an Action Potential

Fig. 33-8c, p. 559

K+ K+

K+

Na+

Na+

Na+

C The charge reversal makes gated Na+ channels shut and gated K+ channels open. The K+ outflow restores the voltage difference across the membrane. The action potential is propagated along the axon as positive charges spreading from one region push the next region to threshold.

Propagation of an Action Potential

Fig. 33-8d, p. 559

Na+–K+ pump K+

Na+

Na+

Na+

K+

D After an action potential, gated Na+ channels are briefly inactivated, so the action potential moves one way only, toward axon terminals. Na+ and K+ gradients disrupted by action potentials are restored by diffusion of ions that were put into place by activity of sodium–potassium pumps.

K+ K+

Propagation of an Action Potential

Fig. 33-9, p. 559

electrode inside

electrode outside

– – – – – – – – – – – –

unstimulated axon

++++ ++++++++

33.5 How Neurons Send Messages to Other Cells

An action potential travels along a neuron’s axon to a terminal at the tip

Terminal sends chemical signals to a neuron, muscle fiber, or gland cell across a synapse

Chemical Synapses

Synapse• The region where an axon terminal (presynaptic

cell) send chemical signals to a neuron, muscle fiber or gland cell (postsynaptic cell)

Action potentials trigger release of signaling molecules (neurotransmitters) from vesicles in the presynaptic terminal into the synaptic cleft

Neurotransmitter Action

Release of neurotransmitters from presynaptic vesicles requires an influx of calcium ions, Ca++

Postsynaptic membrane receptors bind the neurotransmitter and initiate the response

Example: A neuromuscular junction and the neurotransmitter acetylcholine (ACh)

A Neuromuscular Junction

Fig. 33-11 (a-b), p. 560

Neuromuscular junctionsA An action potential propagates along a motor neuron.

B The action potential reaches axon terminals that lie close to muscle fibers.

axon of a motor neuronmuscle

fiber

axon terminal

muscle fiber

Fig. 33-11 (c-d), p. 560

Close-up of a neuromuscular junction (a type of synapse)

C Arrival of the action potential causes calcium ions (Ca++) to enter an axon terminal.

one axon terminal of the presynaptic cell (motor neuron)

plasma membrane of the postsynaptic cell (muscle cell)

D Ca++ causes vesicles with signaling molecule (neurotransmitter) to move to the plasma membrane and release their contents by exocytosis.

synaptic vesicle

receptor protein in membrane of post-synaptic cell

synaptic cleft (gap between pre- and postsynaptic cells)

Fig. 33-11 (e-f), p. 560

Close-up of neurotransmitter receptor proteins in the plasma membrane of the postsynaptic cell

binding site for neurotransmitter is vacant

neurotransmitter in binding site

ion crossing plasma membrane through the now-open channelchannel through

interior is closed

E When neurotransmitter is not present, the channel through the receptor protein is shut, and ions cannot flow through it.

F Neurotransmitter diffuses across the synaptic cleft and binds to the receptor protein. The ion channel opens, and ions flow passively into the postsynaptic cell.

Receiving the Signal

A neurotransmitter may have excitatory or inhibitory effects on a postsynaptic cell

Synaptic integration • Summation of all excitatory and inhibitory signals

arriving at a postsynaptic cell at the same time

The neurotransmitter must be cleared from the synapse after the signal is transmitted

Fig. 33-12, p. 561

what action potential spiking would look like

excitatory signal

integrated potential

threshold

resting membrane potential

inhibitory signal

Synaptic Integration

Neural Control

Chapter 33 Part 2

33.6 A Smorgasbord of Signals

Different types of neurons release different neurotransmitters; Parkinson’s disease involves dopamine-secreting neurons and motor control

Battling Parkinson’s disease. (a) This neurological disorder affects former heavyweight champion Muhammad Ali, actor Michael J. Fox, and about half a million other people in the United States. (b) A normal PET scan and (c) one from an affected person. Red and yellow indicate high metabolic activity in dopamine-secreting neurons. Section 2.2 explains PET scans.

Major Neurotransmitters and Their Effects

The Neuropeptides

Neuromodulators • Neuropeptides made by some neurons that

influence the effects of neurotransmitters• Substance P enhances pain• Enkephalins and endorphins are pain killers

33.7 Drugs Disrupts Signaling

Psychoactive drugs exert their effects by interfering with the action of neurotransmitters• Stimulants (nicotine, caffeine, cocaine,

amphetamines)• Depressants (alcohol, barbiturates)• Analgesics (narcotics, ketamine, PCP)• Hallucinogens (LSD, THC)

PET Scan: Effects of Cocaine

Signs of Drug Addiction

33.2-33.7 Key Concepts How Neurons Work

Messages flow along a neuron’s plasma membrane, from input to output zones

Chemicals released at a neuron’s output zone may stimulate or inhibit activity in an adjacent cell

Psychoactive drugs interfere with the information flow between cells

33.8 The Peripheral Nervous System

Peripheral nerves carry information to and from the central nervous system

Nerves are bundled axons of many neurons

Each axon is wrapped in a myelin sheath that increases transmission speed

Fig. 33-15a, p. 564

myelin sheath

axon

blood vessels

nerve fascicle (a number of axons bundled inside connective tissue)

Nerve Structure and Function

the nerve’s outer wrapping

Fig. 33-15b, p. 564

unsheathed node axon

b “Jellyrolled” Schwann cells of an axon’s myelin sheath

Nerve Structure and Function

(b–d) In axons with a myelin sheath, ions flow across the neural membrane at nodes, or small gaps between the cells that make up the sheath. Many gated channels for sodium ions are exposed to extracellular fluid at the nodes. When excitation caused by an action potential reaches a node, the gates open and sodium rushes in, starting a new action potential. Excitation spreads rapidly to the next node, where it triggers a new action potential, and so on down the axon to the output zone.

Fig. 33-15c, p. 564

Na+

action potential resting potential resting potential

Nerve Structure and Function

(b–d) In axons with a myelin sheath, ions flow across the neural membrane at nodes, or small gaps between the cells that make up the sheath. Many gated channels for sodium ions are exposed to extracellular fluid at the nodes. When excitation caused by an action potential reaches a node, the gates open and sodium rushes in, starting a new action potential. Excitation spreads rapidly to the next node, where it triggers a new action potential, and so on down the axon to the output zone.

Fig. 33-15d, p. 564

K+ Na+

resting potential restored action potential resting potential

Nerve Structure and Function

(b–d) In axons with a myelin sheath, ions flow across the neural membrane at nodes, or small gaps between the cells that make up the sheath. Many gated channels for sodium ions are exposed to extracellular fluid at the nodes. When excitation caused by an action potential reaches a node, the gates open and sodium rushes in, starting a new action potential. Excitation spreads rapidly to the next node, where it triggers a new action potential, and so on down the axon to the output zone.

Divisions of the Peripheral Nervous System

Somatic nervous system • Conducts information about the environment to

the central nervous system (involuntary)• Controls skeletal muscles (voluntary)

Autonomic nervous system• Conducts signals to and from internal organs and

glands

Divisions of the Autonomic Nervous System

The two divisions of the autonomic nervous system have opposing effects on effectors

Sympathetic neurons are most active in times of stress or danger (fight-flight response)

Parasympathetic neurons are most active in times of relaxation

Fig. 33-16, p. 565

eyesoptic nerve

midbrain

salivary glandsmedulla oblongata

cervical nerves (8 pairs)

heartvagus nerve

larynx bronchi lungs

stomach

liver spleen

pancreas

thoracic nerves (12 pairs)

kidneys adrenal glands

small intestine upper colon (all ganglia

in walls of organs)

lower colon rectum

lumbar nerves (5 pairs)

(most ganglia near spinal

cord)

bladder sacral nerves (5 pairs)uterus pelvic

nervegenitals

Divisions of the Autonomic Nervous System

33.9 The Spinal Cord

Spinal cord• Runs through the vertebral column and connects

peripheral nerves with the brain• Serves as a reflex center

Central nervous system (CNS)• The brain and spinal cord

Protective Features

Meninges• Three membranes that cover and protect the

CNS

Cerebrospinal fluid• Fills central canal and spaces between meninges• Cushions blows

White Matter and Gray Matter

White matter• Bundles of myelin-sheathed axons (tracts)• Outermost portion of spinal cord

Gray matter• Nonmyelinated structures (cell bodies, dendrites,

neuroglial cells)

Reflex Pathways

Reflex• An automatic response to a stimulus• Stretch reflex, knee-jerk reflex, withdrawal reflex

Spinal reflexes do not involve the brain• Signals from sensory neurons enter the cord

through the dorsal root of spinal nerves• Commands for responses go out on the ventral

root of spinal nerves

Fig. 33-18, p. 567

A Fruit being loaded into a bowl puts weight on an arm muscle and stretches it. Will the bowl drop? NO! Muscle spindles in the muscle’s sheath also are stretched.

STIMULUSBiceps stretches.

B Stretching stimulates sensory receptor endings in this muscle spindle. Action potentials are propagated toward spinal cord.

C In the spinal cord, axon terminals of the sensory neuron release a neurotransmitter that diffuses across a synaptic cleft and stimulates a motor neuron.

D The stimulation is strong enough to generate action potentials that self-propagate along the motor neuron’s axon.

E Axon terminals of the motor neuron synapse with muscle fibers in the stretched muscle.

F ACh released from the motor neuron’s axon terminals stimulates muscle fibers.

RESPONSEBiceps contracts.

G Stimulation makes the stretched muscle contract. Ongoing stimulationsand contractions hold the bowl steady.muscle

spindleneuromuscular

junction

Stretch Reflex

33.8-33.9 Key Concepts Vertebrate Nervous System

The central nervous system consists of the brain and spinal cord

The peripheral nervous system includes many pairs of nerves that connect the brain and spinal cord to the rest of the body

The spinal cord and peripheral nerves interact in spinal reflexes

33.10 The Vertebrate Brain

The brain is the body’s main information integrating organ, part of the CNS

During development, the brain is organized as three functional regions: forebrain, midbrain and hindbrain

Hindbrain and Midbrain

The hindbrain includes the medulla oblongata, the pons, and the cerebellum

The midbrain in mammals is reduced

The brain stem (pons, medulla, and midbrain) is involved in reflex behaviors

The Forebrain

Cerebrum• Main processing center in humans• Evolved as an expansion of the olfactory lobe

Thalamus and hypothalamus• Important in thirst, temperature regulation, and

other responses related to homeostasis

Fig. 33-19 (a-c), p. 568

forebrain midbrain

hindbrain

Development of the Human Brain

Protection at the Blood-Brain Barrier

Blood-brain barrier• Protects the CNS from harmful substances• Tight junctions form a seal between adjoining

cells of capillary walls• Some toxins (nicotine, alcohol, caffeine, mercury)

are not blocked

The Human Brain

Cerebellum• Has more interneurons than other brain regions• Involved in balance, motor skills and language

Cerebrum• Divided into two hemispheres, coordinated by

signals across the corpus callosum• Each hemisphere deals with the opposite side of

the body

Major Brain Regions of Vertebrates

Fig. 33-20a, p. 569

olfactory lobe

forebrain

midbrain

hindbrain

FISH AMPHIBIAN REPTILE BIRD

shark frog alligator goose

(a) Major brain regions of five vertebrates, dorsal views. The sketches are not to the same scale.

Fig. 33-20b, p. 569

corpus callosum

hypothalamus thalamus pineal gland

locationpart of optic nerve

midbrain

cerebellum

pons

medulla oblongata

(b) Right half of a human brain in sagittal section, showing the locations of the major structures and regions. Meninges around the brain were removed for this photograph.

33.11 The Human Cerebrum

Each cerebral hemisphere is divided into frontal, temporal, occipital and parietal lobes

Cerebral cortex• Outermost gray matter of the cerebrum• Controls voluntary activity, sensory perception,

abstract thought, language and speech • Distinct areas receive and process signals

Fig. 33-21, p. 570

frontal lobe (planning of movements, aspects of memory, inhibition of unsuitable behaviors)

primary motor cortex

primary somatosensory cortex

parietal lobe

(visceral sensations)

Wernicke’s area

Broca’s area

temporal lobe (hearing, advanced visual processing)

occipital lobe (vision)

Lobes of the Brain

Functions of the Cerebral Cortex

Specific areas of the cerebral cortex correspond to specific body parts or functions

Examples: • The body is spatially mapped out in the primary

motor cortex of each frontal lobe• Association areas are scattered throughout the

cortex, but not in motor or sensory areas

The Primary Motor Cortex

Fig. 33-23, p. 570

Motor cortex activity when speaking

Prefrontal cortex activity when generating words

Visual cortex activity when seeing written words

Association Areas Integrate Inputs

Three PET scans that identify which brain areas were active when a person performed three kinds of tasks. Yellow and orange indicate high activity.

Connections With the Limbic System

The cerebral cortex oversees the limbic system

Limbic system• Governs emotions, assists in memory, correlates

emotional-visceral responses • Includes the hypothalamus, hippocampus,

amygdala, and cingulate gyrus

Fig. 33-24, p. 571

(olfactory tract)

cingulate gyrus thalamus hypothalamus

amygdala

hippocampus

Limbic System Components

Making Memories

The cerebral cortex receives information and processes some of it into memories

Memory forms in stages• Short-term memory lasts seconds to hours• Long-term memory is stored permanently• Skill memory involves the cerebellum• Declarative memory stores facts and impressions

Fig. 33-25, p. 571

Sensory stimuli, as from the nose, eyes, and ears

Temporary storage in the cerebral cortex

SHORT-TERM MEMORY

Emotional state, having time to repeat (or rehearse) input, and associating the input with stored categories of memory influence transfer to long-term storage

LONG-TERM MEMORYInput irretrievable Stepped Art

Input forgotten

Recall of stored input

Stages in Memory Processing

33.12 The Split Brain

Investigations by Roger Sperry into the importance of information flow between the cerebral hemispheres showed that the two halves of the brains have a division of labor

Typically, math and language skills reside in the left hemisphere; the right hemisphere interprets music, spatial relationships, and visual inputs

Fig. 33-26, p. 572

Left Half of Visual Field

Right Half of Visual Field

COWBOY COW BOY

pupil

optic nerves retina

optic chiasm

corpus callosum

left visual cortex

right visual cortex

Visual Information and the Brain

Split-Brain Studies

33.13 Neuroglia—The Neurons’ Support Staff

Neuroglial cells make up the bulk of the brain

The adult brain has four types of neuroglial cells• Oligodendrocytes make myelin• Microglia have immune system functions• Astrocytes secrete various substances, take up

neurotransmitters, assist in immune defenses, and stimulate formation of the blood-brain barrier

• Ependymal cells line brain cavities

Astrocytes

About Brain Tumors

Unlike neurons, neuroglia continue to divide in adults, and can be a source of primary brain tumors (gliomas)

Exposure to ionizing radiation such as x-rays, or to chemical carcinogens, increases risk

33.10-33.13 Key Concepts About the Brain

The brain develops from the anterior part of the embryonic nerve cord

A human brain includes evolutionarily ancient tissues and newer regions that provide the capacity for analytical thought and language

Neuroglia make up the bulk of the brain

Animation: Neuron structure and function

Animation: Ion concentrations

Animation: Measuring membrane potential

Animation: Action potential propagation

Animation: Chemical synapse

Animation: Bilateral nervous systems

Animation: Comparison of nervous systems

Animation: Nerve net

Animation: Vertebrate nervous system divisions

Animation: Nerve structure

Animation: Ion flow in myelinated axons

Animation: Autonomic nerves

Animation: Stretch reflex

Animation: Sagittal view of a human brain

Animation: Receiving and integrating areas

Animation: Path to visual cortex

Animation: Action potential

Animation: Human brain development

Animation: Organization of the spinal cord

Animation: Primary motor cortex

Animation: Regions of the vertebrate brain

Animation: Structures involved in memory

Animation: Synapse function

Animation: Synaptic integration

ABC video: New Nerves

Video: In pursuit of ecstasy

Video: Brain stem

Video: Limbic system dissection