Im chapter24
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This document discusses the nervous system and senses. It begins by covering neurons, how they communicate via action potentials and chemical synapses, and examples of neurotransmitters. It then describes the central and peripheral nervous systems, including structures like the brain, spinal cord, and nerves. The senses are also outlined, including sensory receptors for sight, sound, smell, taste, touch, and balance. In summary, the document provides an overview of the nervous system, neuronal communication, and the different human senses.
Im chapter24
- 1. 24 ______________________________________________________________________________________________ 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. Chapter Twenty-Four 214
- 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 215 Chapter Twenty-Four
- 3. retina fovea eardrum sclera rod cell organ of Corti cone cell cochlea Neural Control and the Senses 216
- 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. 217 Chapter Twenty-Four
- 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. Neural Control and the Senses 218
- 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. 219 Chapter Twenty-Four
- 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. Neural Control and the Senses 220
- 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. 221 Chapter Twenty-Four
- 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. Neural Control and the Senses 222