Albia Dugger • Miami Dade College

Chapter 32Neural Control

Sections 1-6

32.1 In Pursuit of Ecstasy

• Ecstasy (MDMA) is a psychoactive drug, similar in structure to methamphetamine

• Drugs like MDMA flood the brain with signaling molecules and saturate receptors, disrupting neural controls

• Repeated doses of MDMA may alter and even kill neurons in the brain

• A bad reaction to MDMA can cause death

Meth and Ecstasy

methamphetamine Ecstasy (MDMA)

Effect of Ecstasy

32.2 Evolution of Nervous Systems

• Interacting neurons give animals a capacity 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

Nerve Nets

• 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

Cnidarian Nerve Net

A nerve net(highlighted in purple)controls thecontractilecells in theepithelium.

Bilateral, Cephalized Invertebrates

• 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, and a 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

Flatworm Cephalization

pair of nerve cords connectedby lateral nerves

pair of ganglia

Insect with a Simple Brain

brain

nerve cords with ganglia

ANIMATED FIGURE: Bilateral nervous systems

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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 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

Nerves

• A nerve consists of nerve fibers bundled inside a sheath of connective tissue

• Peripheral nerves are divided into two functional categories

• Autonomic nerves regulate the body’s internal state; they control smooth muscle, cardiac muscle, and glands

• Somatic nerves monitor body’s position and external conditions; they control skeletal muscle

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

Figure 32-3 p543

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

Temporary storage in the cerebral cortex

SHORT-TERM MEMORY

Input forgotten

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

Recall of stored input

Stepped Art

Figure 32-25 p559

lumbar nerves (five pairs)

coccygeal nerves (one pair)

sciatic nerve (one in each leg)

thoracic nerves (twelve pairs)

Spinal Cord

Brain

cranial nerves (twelve pairs)

cervical nerves (eight pairs)

ulnar nerve (one in each arm)

sacral nerves (five pairs)

Figure 32-4 p543

Take-Home Message: What are the features of animal nervous systems?

• Cnidarians and echinoderms have a simple nervous system, a nerve net with no central integrating organ.

• Bilateral animals have three types of neurons: sensory neurons, interneurons, and motor neurons.

• Flatworms have paired ganglia that serve as an integrating center. Other invertebrates have more complex brains.

• Bilateral invertebrates usually have a pair of ventral nerve cords. In contrast, the chordates have a dorsal nerve cord.

• The vertebrate nervous system includes a well-developed brain, a spinal cord, and peripheral nerves.

ANIMATION: Vertebrate nervous system divisions

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32.3 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

Three Types of Neurons

dendrites

receptor endings

peripheral axon

cell body

axon axon terminals

axon terminals

cell body

cell body

axon axon

dendrites

A Motor Neuron

axon terminalsaxon

dendrites

cell bodyInput zone

Trigger zone

Output zoneConducting zone

1

2

3 4

ANIMATED FIGURE: Neuron structure and function

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Properties of the Neuron Plasma Membrane

• Neurons have electrical and concentration gradients across their plasma membrane – their cytoplasm is more negatively charged than the interstitial fluid outside the cell

• Negatively charged proteins and active transport of Na+ and K+ ions maintain voltage difference across a cell membrane, called the membrane potential

• An unstimulated neuron has a resting membrane potential of about –70 mV

Resting Membrane Potential

neuron’s cytoplasm

150 K+

5 K+150 Na+

65

interstitial fluid

plasma membrane

15 Na+

Transport Proteins in a Neuron Membrane

interstitial fluid

cytoplasmADP + Pi

2 K+3 Na+

A Sodium–potassiumcotransporters activelytransport three Na+out of a neuron forevery two K+ theypump in.

B Passive transportersallow K+ ions to moveacross the plasmamembrane, downtheir concentrationgradient.

c Voltage-gatedchannels for Na+ orK+ are closed in aneuron at rest (left),but open when it isexcited (right).

Take-Home Message: How does a neuron’s structure affect its function?

• Sensory neurons have an axon with one end that responds to a specific stimulus and another that signals other cells.

• Interneurons and motor neurons have many signal-receiving dendrites and one signal-sending axon.

• Transport proteins in the neuron plasma membrane set up electrical and concentration gradients across the membrane of a resting neuron.

• A neuron’s axon has special voltage-gated channel proteins that function in the transmission of electrical signals along the axon.

ANIMATION: Measuring membrane potential

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32.4 The Action Potential

• When stimulated, neurons and muscle cells undergo an action potential – a brief reversal in the electric gradient across the plasma membrane

• During an action potential, membrane potential rises from its resting potential (–70 mV) to a peak of +30 mV, then declines to resting potential

Graded Potentials and Reaching Threshold

• Stimulation of a neuron’s input zone causes a local, graded potential – a slight shift in the voltage difference across the neuron’s membrane

• When stimulus in the neuron’s trigger zone reaches a threshold potential, gated sodium channels open

• Voltage difference decreases and starts the action potential

An All-or-Nothing Spike

• Diffusion of sodium into the neuron has a positive feedback effect – gated sodium channels open in an accelerating way after threshold is reached

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

• Outward diffusion of K+ causes membrane potential to decline to a bit below its resting value in a small area

Propagation of an Action Potential

• An action potential is self-propagating – sodium ions diffuse to the adjoining region of the axon, triggering sodium gates one after another

• The action potential can only move one way, toward axon terminals – a brief refractory period after sodium gates close prevents the signal from moving backwards

Action Potential Membrane Potential

Time (milliseconds)

resting level

threshold level

action potentialM

em

bra

ne

po

ten

tia

l (m

illi

vo

lts

)

5

+30

643210

-60

-70

1

2

3

4

Neuron at Rest

voltage-gatedion channels

Threshold

Na+ Na+ Na+

Na+

Na+

Na+

K+ Channels Open

K+

K+

K+

Na+

Na+

Na+

K+ Channels Close

K+ K+K+

Na+

Na+

ANIMATED FIGURE: Action potential propagation

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Take-Home Message: What happens during an action potential?

• An action potential begins in the neuron’s trigger zone. A strong stimulus decreases the voltage difference across the membrane. This causes gated sodium channels to open, and the voltage difference reverses.

• The action potential travels along an axon as consecutive patches of membrane undergo reversals in membrane potential.

Take-Home Message (cont.)

• At each patch of membrane, an action potential ends as sodium channels close and potassium channels open. Potassium ions flow out of the neuron and restore the voltage difference across the membrane.

• Action potentials can move in one direction, toward axon terminals, because gated sodium channels are briefly inactivated after they close

32.5 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

• A synapse is the region where an axon terminal (presynaptic cell) send chemical signals to a neuron, muscle fiber or gland cell (postsynaptic cell)

• The synapse between a motor neuron and a skeletal muscle fiber is called a neuromuscular junction

Chemical Synapses

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

• A motor neuron in a neuromuscular junction releases the neurotransmitter acetylcholine (ACh)

Chemical Synapses

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

• Postsynaptic membrane receptors bind the neurotransmitter and initiate the response

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

A Neuromuscular Junction

Figure 32-9a p548

neuromuscular junction

axon of a motor neuron

Figure 32-9b p548

synaptic cleft

plama membrane of muscle fiber

axon terminal of motor neuron

synaptic vesicle

2

4

3

Ca++

Figure 32-9d p548

ion channel closed

binding site for neurotransmitter (no neurotransmitter bound)

Figure 32-9d p548

ion flows through now-open channel

neurotransmitter

3D ANIMATION: Neurons: Synaptic Transmissions

Synaptic Integration

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

• Typically, a postsynaptic cell receives messages from many neurons at the same time

• Through synaptic integration a neuron sums all excitatory and inhibitory signals arriving at a postsynaptic cell at the same time

Synaptic Density

Synaptic Integration

Take-Home Message: How does information pass between cells at a synapse?

• Action potentials travel to a neuron’s output zone. There they stimulate release of neurotransmitters—chemical signals that affect another cell.

• Neurotransmitters are signaling molecules secreted into a synaptic cleft from a neuron’s output zone. They may have excitatory or inhibitory effects on a postsynaptic cell.

Take-Home Message (cont.)

• Synaptic integration is the summation of all excitatory and inhibitory signals arriving at a postsynaptic cell’s input zone at the same time.

• For a synapse to function properly, neurotransmitter must be cleared from the synaptic cleft after the chemical signal has served its purpose.

ANIMATION: Chemical synapse

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33.6 A Smorgasbord of Signals

• There are a variety of neurotransmitters

• Neurological disorders and psychoactive drugs interfere with their action

Neurotransmitter Discovery

• In the early 1920s, Otto Loewi discovered that the neurotransmitter ACh controls heart rate

• ACh also acts on skeletal muscle, smooth Muscle, many glands, and the brain

• Each type of tissue has a different kind of ACh receptor, so ACh elicits different responses in different cells

Neurotransmitter Diversity

• The body produces many kinds of neurotransmitters:• Norepinephrine and epinephrine (adrenaline) prepare the

body for stress or excitement• Dopamine influences reward-based learning and acts in

fine motor control• Serotonin influences mood and memory• Glutamate excitates the central nervous system• GABA (gamma aminobutyric acid) has a general inhibitory

effect on release of other neurotransmitters

Table 32-1 p550

Neuromodulators

• Neuromodulators • Neuropeptides made by some neurons that influence the

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

Disrupted Signaling

• Many disorders of the nervous system involve disruption of signaling at synapses:• Alzheimer’s disease (dementia) involves damage to

neurons and lowered levels of ACh in the brain• Parkinson’s disease involves dopamine-secreting neurons

in the motor-control part of the brain• Attention deficit hyperactivity disorder (ADHD) also

involves low levels of dopamine • Depression and anxiety disorders may involve low levels

of several neurotransmitters

Battling Parkinson’s Disease

Psychoactive Drugs

• 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)

Take-Home Message: How do disorders and drugs affect the nervous system?

• Neurological disorders lower the amount of a neurotransmitter or the balance among neurotransmitters.

• Psychoactive drugs act by stimulating release, inhibiting breakdown, or mimicking the action of natural neurotransmitters. Many are addicting, and using them can alter the body’s ability to produce neurotransmitter.

Video: Exploring Neurotransmitters