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1. 24
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NEURAL CONTROL AND THE
SENSES
Chapter Outline
IMPACTS/ISSUES: IN PURSUIT OF Regions of the Human Brain
ECSTASY A Closer Look at the Cerebral Cortex
NEURONS—THE GREAT THE SENSES
COMMUNICATORS The Sensory Receptors
Neurons and Supporting Cells Somatic and Visceral Sensations
Organization of Nervous Tissue The Chemical Senses—Smell and Taste
The Action Potential Detecting Light
How Messages Flow from Cell to Cell The Human Eye
A Sampling of Signals At the Retina
Disrupted Signaling—Disorders to Drugs Hearing
ANIMAL NERVOUS SYSTEMS Sense of Balance
Invertebrate Nervous Systems IMPACTS/ISSUES REVISITED
The Vertebrate Nervous System SUMMARY
THE PERIPHERAL NERVOUS SYSTEM SELF-QUIZ
THE CENTRAL NERVOUS SYSTEM CRITICAL THINKING
The Spinal Cord
Objectives
• Explain the nature of the synthetic drug, ecstasy, its action, and typical and potential side effects.
• Describe the three types of neurons and their specific functions.
• Know the functional zones on a neuron.
• Explain the function of neuroglia.
• Define membrane potential.
• Compare and contrast resting potential, action potential, and threshold potential.
• Explain why action potentials move in only one direction.
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2. • Describe the role of chemical synapses and neurotransmitters in nerve transmission.
• List the major neurotransmitters and their effects.
• Compare and contrast the nerve net in a radially symmetric animal and neurons in a bilaterally
symmetric animal.
• Understand the distinction between the central nervous system and the peripheral nervous
system in vertebrates.
• Explain the importance of myelination of nerve axons.
• Compare and contrast the somatic, autonomic, sympathetic, and parasympathetic nervous
systems.
• Define meninges, cerebrospinal fluid, white and gray matter.
• Explain the importance of the blood-brain barrier.
• Know the function of the spinal cord.
• List the main regions of the brain and a function for each region.
• List the major lobes of the cerebral cortex and their function.
• Distinguish between thermoreceptors, mechanoreceptors, chemoreceptors, and photoreceptors.
• Briefly explain how sensory receptors can create differences in strength of stimulus.
• Describe sensory adaptation.
• Explain the difference between somatic sensations and visceral sensations.
• Define endorphins and briefly explain their function.
• Understand the chemical nature of taste and smell receptors.
• List the anatomical structures in the human eye.
• Explain the function of the lens.
• Understand the function of rods and cones, and their location in the retina.
• List the anatomical structures in the human ear.
• Explain the transmission of compressed air waves to sound receptors in the ear.
• Understand the function of the organ of Corti and the cochlea.
• Know the function of the vestibular apparatus.
Key Terms
axon nerve net white matter
dendrite peripheral nervous system chemoreceptor
interneurons autonomic nervous system mechanoreceptor
motor neurons parasympathetic neurons pain
neuroglia somatic nervous system photoreceptor
neurons sympathetic neurons somatic sensations
resting potential blood-brain barrier thermoreceptor
sensory neurons cerebrospinal fluid visceral sensations
action potential cerebellum choroid
threshold potential cerebrum conjunctiva
chemical synapse gray matter cornea
neurotransmitter hypothalamus endorphins
central nervous system medulla oblongata iris
ganglion meninges lens
myelin reflex pheromones
nerves spinal cord pupil
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3. retina fovea eardrum
sclera rod cell organ of Corti
cone cell cochlea
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4. Lecture Outline
24.1 Impacts/Issues: In Pursuit of Ecstasy
A. Ecstasy is MDMA (3,4-methylenedioxymethamphetamine)
1. psychoactive drug that alters brain function
2. releases an excess of serotonin
a. produces feelings of euphoria, energy, empathy
b. homeostatic mechanisms spiral out of control
B. Continued use alters structure of serotonin-secreting neurons.
1. Short-term use may be reversible.
2. Some ecstasy users have died.
24.2 Neurons – The Great Communicators
A. Neurons and supporting cells
1. A neuron is a cell that communicates with other cells by relaying chemical messages.
a. Sensory neurons detect stimuli.
b. Interneurons integrate signals between other neurons.
c. Motor neurons control muscles and glands.
2. Neurons consist of dendrites, a cell body, and an axon.
a. Dendrites receive incoming messages.
b. Axons conduct impulses away from the cell body.
3. Neuroglia are cells that support and protect neurons.
B. Organization of nervous tissue
1. The difference in charges across a membrane causes a membrane potential.
2. Resting potential is the charge across a membrane of a neuron that is not being stimulated.
a. A higher concentration of sodium ions outside the cell membrane maintains the
membrane potential.
C. The action potential
1. An action potential is a brief reversal of the electrical gradient across the plasma
membrane.
a. A stimulus from another neuron shifts the membrane potential.
b. Threshold potential is a stimulus large enough to cause the shift.
c. Sodium flows into the cell, down its concentration gradient.
d. The membrane now has a net negative charge outside of the cell.
e. Gated potassium channels then open and allow potassium to diffuse outward and
restore the net negative charge outside of the plasma membrane.
f. Some of the sodium that diffused into the cell moves to an adjacent area along the
axon, moving the action potential along the axon.
g. Sodium gates swing open in response, moving the action potential along without
weakening.
h. The action potential moves in only one direction along the axon.
D. How messages flow from cell to cell
1. Action potentials cannot jump from the end of an axon to the next cell’s dendrite.
2. A chemical synapse exists as a communication point between neurons.
a. Vesicles in the axon end contain neurotransmitters (chemical signals).
b. Action potentials cause the vesicles to release the neurotransmitter by fusing their
membrane with the plasma membrane.
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5. c. The post-synaptic cell has receptors for the neurotransmitter.
d. Neurotransmitters may stimulate or inhibit the post-synaptic cell.
E. A sampling of signals
1. Acetylcholine (ACh) is a common neurotransmitter.
a. ACh stimulates skeletal muscles but inhibits the heart.
b. Different receptors enable different reactions to the same neurotransmitter.
c. Neurotransmitters must be broken down quickly after exerting their effects.
d. The enzyme acetylcholinesterase breaks down ACh.
F. Disrupted signaling – disorders and drugs
1. Alzheimer’s and Parkinson’s disease both disrupt signaling.
a. Alzheimer’s is caused by low ACh levels.
b. Parkinson’s disease is caused by death or impairment of dopamine-secreting neurons.
2. Mood disorder drugs act at synapses in the brain.
3. Addictive drugs stimulate the release of dopamine.
4. Stimulants make users feel alert, but anxious.
5. Depressants slow responses but also produce euphoria followed by depression.
6. Narcotic analgesics mimic the body’s natural pain-killers and produce euphoria.
a. They are highly addictive.
7. Hallucinogens distort perception.
24.3 Animal Nervous Systems
A. Invertebrate nervous systems
1. Radial, aquatic animals have a nerve net.
a. They have no centralized controlling organ.
b. Nerve net controls change of shape, mouth size, and position of tentacles.
2. Bilaterally symmetric animals have a concentration of nerves at their head.
a. Ganglion is a cluster of nerve cell bodies.
b. Ganglia connect to nerve cords that run the length of the body.
c. Arthropods have paired nerve cords connected to a simple brain.
B. The vertebrate nervous system
1. The central nervous system consists of the brain and spinal cord.
2. The peripheral nervous system extends through the body.
3. Sensory fibers convey information to the central nervous system.
4. Motor fibers convey information from the central nervous system.
5. Each nerve consists of bundles of axons enclosed in connective tissue.
a. Myelination of axons functions like insulation and speeds impulses.
b. Myelin is produced by neuroglial cells.
c. Lack of myelin is the cause of multiple sclerosis.
24.4 The Peripheral Nervous System
A. The somatic nervous system conveys impulses to skeletal muscles.
B. The autonomic nervous system conveys impulses to smooth muscles, glands, and cardiac
muscle.
1. The autonomic system also relays information about internal conditions to the central
nervous system.
C. The autonomic system is two separate systems that are antagonistic.
1. The sympathetic nervous system controls bodily functions during stress, (flight or fight).
2. The parasympathetic system controls bodily functions during relaxation.
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6. 24.5 The Central Nervous System
A. The brain and spinal cord are the organs of the central nervous system (CNS).
1. The three meninges are protective membranes that surround them.
2. Cerebrospinal fluid (CSF) bathes the brain and spinal cord.
a. The blood-brain barrier prevents unwanted substances from entering the CSF.
3. White matter in the CNS consists of myelinated axons, and gray matter consists of cell
bodies of neurons.
B. The spinal cord
1. It contains nerves carrying signals between the brain and peripheral nervous system.
2. Spinal cord injuries lead to paralysis.
3. Reflex arcs connect through the spinal cord.
a. They are reactions to stimuli that do not require thought.
b. Action potentials travel along sensory neurons and synapse with motor neurons in the
spinal cord.
C. Regions of the human brain
1. Hindbrain
a. Medulla oblongata – influences heart beat, breathing, and other reflexive controls such
as swallowing, vomiting, coughing, and sneezing.
b. Pons – also affects breathing.
c. Cerebellum – controls posture and controls voluntary movements.
2. Midbrain
a. Plays an important role in reward-based learning.
3. Brainstem – the pons, medulla, and midbrain are collectively referred to as the brainstem.
4. Forebrain
a. Cerebrum
i. It is divided into right and left hemispheres connected by the corpus callosum.
ii. The thalamus sorts sensory signals.
iii. The hypothalamus controls homeostatic mechanisms.
D. A closer look at the cerebral cortex
1. Frontal lobe – planning of movements, aspects of memory, inhibition of unsuitable
behavior.
a. Frontal lobotomy was practiced in the 1950s to treat mental illness.
2. Primary motor cortex is at the rear of the frontal lobe – controls skeletal muscles.
3. Broca’s area is in the left frontal lobe – involves translations of speech, controls muscle
involved in speaking.
4. Primary somatosensory cortex is in the parietal lobe – receiving area for sensory input
from skin and joints.
5. Occipital lobe contains the primary visual cortex.
6. Perception of sound and odors is controlled in the primary sensory area of the temporal
lobe.
24.6 The Senses
A. The sensory receptors
1. Thermoreceptors are sensitive to heat or cold.
2. Mechanoreceptors detect changes in pressure, position, or acceleration.
3. Chemoreceptors detect substances in their surrounding fluid.
4. Photoreceptors contain light sensitive pigments that respond to light energy.
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7. 5. Sensory receptors convert stimulus energy into action potentials.
a. The brain is pre-wired for interpretation of certain stimuli, such as light.
b. Action potential frequency increases with stimulus intensity.
c. Stronger stimuli can recruit more receptors than weak stimuli.
6. Sensory adaptation occurs when a continuous stimulus does not maintain a
constant action potential.
a. Sensory adaptation occurs, for example, shortly after clothing touches the skin, so it
does not stimulate the brain all day.
B. Somatic and visceral sensation
1. Somatic sensations are localized to a specific body part.
2. Visceral sensations arise from internal soft organs and can be difficult to pinpoint.
3. Mechanoreceptors in muscles and joints detect limb motions.
4. Pain is the perception of tissue injury.
a. Pain is an adaptive response that alerts one to tissue damage.
b. Endorphins are natural pain relievers that are released to dampen the flow of pain-
related signals to the brain.
C. The chemical senses – smell and taste
1. Chemoreceptors for smell and taste respond to chemicals dissolved in the fluid
surrounding the receptors.
a. Receptors that connect to the olfactory bulbs in vertebrates relay chemical signals.
b. Invertebrates such as moths have chemoreceptors that are highly sensitive to
pheromones, enabling them to pinpoint the location of potential mates.
c. The vomeronasal organ is functional in reptiles and some mammals but reduced in
humans.
2. Taste buds are located inside the mouth, throat, and upper part of the tongue in humans.
a. Humans perceive five primary tastes:
Sweet – simple sugars
Salty – NaCl
Sour – acids
Bitter – alkaloids
Umami – amino acids in meat and cheese (savory)
D. Detecting light
1. Light detected by photoreceptors is converted to an action potential that is interpreted in
the brain as an image.
2. Some animals can only detect light changes but no distinct image.
3. The most effective eyes contain a lens that bends (refracts) light, so it becomes focused on
photoreceptors.
4. Camera eyes (containing a lens) are found in cephalopod mollusks and vertebrates
(convergent evolution).
E. The human eye
1. The eyeball sits in the bony orbit, which protects the eye.
a. Skeletal muscles attach to the orbit and eye to move it.
b. The conjunctiva is a mucous membrane that folds inside the eyelid and over the front
of the eyeball.
2. Under the conjunctiva, are the cornea (clear) and the sclera, which forms the white of the
eye.
a. The cornea is a crystalline protein.
b. The sclera is fibrous connective tissue.
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8. 3. The middle layer of the eye is the choroid (vascular), and the iris (muscular).
a. The iris contains pigment that gives eyes their characteristic color seen from the front
of the body.
b. The empty hole formed by the iris is the pupil, the space that light passes through.
4. The anterior interior chamber is filled with aqueous humor.
5. The posterior chamber is filled with vitreous humor.
6. Both the cornea and lens bend light rays coming into the eye.
a. When the ciliary muscle contracts, the lens thickens and can bend light rays for close
vision.
b. When the ciliary muscle relaxes, the lens is stretched out, which allows for distance
vision.
7. About 150 million Americans have structural problems with their eyes that cause vision
problems.
a. Astigmatism is an uneven cornea that cannot bend light rays properly, (distance vision
is poor).
b. Nearsightedness occurs when the distance from the front to the back of the eyeball is
too long, (distance vision is poor).
c. In farsightedness, the distance from the front to the back of the eyeball is too short,
(close vision is poor).
d. Glasses, contact lenses, or surgery can correct most vision problems.
F. At the retina
1. The retina consists of several cell layers.
a. Rods and cones (photoreceptors) lie underneath several layers of interneurons that
process visual signals.
b. Rods detect dim light, and cones detect color.
c. Cones react to specific colors (red, blue, or green).
d. Color blindness results from lack of one or more type of cone.
2. The fovea contains the highest concentration of photoreceptors in the retina.
3. Light signals flow from the rods and cones to the ganglion cells.
a. Ganglion cells are bundled into what becomes the optic nerve.
b. The bundled ganglions form an area without photoreceptors called the blind spot.
G. Hearing
1. Compressed air forms sound waves, the type of energy that stimulates mechanoreceptors
in the ear.
a. Sound wave intensity is measured in decibels.
b. Some common decibel levels are:
Normal conversation – 60 dB
Chain Saw – 100 dB
Rock concert – 120 dB
2. Ear anatomy
a. The outer ear consists of the skin-covered flap of cartilage called the pinna, and the
auditory canal that leads to the middle ear.
b. The inner ear consists of the thin, membranous eardrum, and the three middle ear
bones: malleus, incus, and stapes.
c. The inner ear consists of the oval window membrane, the vestibular apparatus (for
balance), and the cochlea which contains the Organ of Corti.
d. Hair cells in the Organ of Corti are displaced when fluid in the cochlea moves in
response to air waves conducted from the inner ear.
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9. e. Hair cell movement generates action potentials that send auditory stimuli to the brain.
f. Pitch and volume of sound are determined by the location within the cochlea where
hair cells bend and the extent to which they bend.
H. Sense of balance
1. In humans, the vestibular apparatus provides information about balance and acceleration.
a. Fluid inside the vestibular apparatus shifts and bends receptors that convey
information about head position and acceleration.
b. Receptors in muscles and skin, and visual input combine with mechanoreceptor
information from the vestibular apparatus to provide information about balance.
24.7 Impacts/Issues Revisited: In Pursuit of Ecstasy
A. Brain cells that are damaged by the use of MDMA cannot be replaced.
B. The blood-brain barrier may also be damaged by the use of MDMA.
1. Loss of the blood-brain barrier allows harmful molecules to contact brain cells.
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