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  • 1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 26 Coordination by Neural Signaling
  • 2. Most Animals Have a Nervous System That Allows Responses to Stimuli
  • 3. 26.1 Invertebrates reflect an evolutionary trend toward bilateral symmetry and cephalization
    • Invertebrate Nervous Organization
      • In simple animals, such as sponges, the most common observable response is closure of the osculum (central opening)
      • Hydras (cnidarians) have a nerve net that is composed of neurons
      • Planarians, (flatworms) have a ladderlike nervous system
      • In annelids (earthworm), arthropods (crab), and molluscs (squid) the nervous system shows further advances
    • Cephalization - concentration of ganglia and sensory receptors in a head region
      • Ganglion (pl. ganglia) - cluster of neurons
  • 4. Figure 26.1A Evolution of the nervous system
  • 5.
    • Figure 26.1A Evolution of the nervous system (continued)
  • 6. Vertebrate Nervous Organization
    • Cephalization, and bilateral symmetry, results in paired sensory receptors to gather information about environment
      • Eyes, ears, and olfactory structures
    • Central nervous system (CNS)
      • Spinal cord and brain and develops from an embryonic dorsal neural tube
        • Ascending tracts carry sensory information to the brain, and descending tracts carry motor commands to the neurons in the spinal cord that control the muscles
    • Vertebrate brain divided into three parts
      • Hindbrain - most ancient part and regulates motor activity below the level of consciousness
      • Midbrain - optic lobes are part of the midbrain and was a center for coordinating reflexes involving the eyes and ears
      • Forebrain - originally dealt mainly with smell. Later, the thalamus evolved to receive sensory input from the midbrain and the hindbrain and to pass it on to cerebrum
    • Cerebrum integrates sensory and motor input and is particularly associated with higher mental capabilities
  • 7. Figure 26.1B Organization of the vertebrate brain
  • 8. 26.2 Humans have well-developed central and peripheral nervous systems
    • Peripheral nervous system (PNS) consists of all the nerves and ganglia that lie outside the CNS
      • All signals that enter and leave the CNS travel through paired nerves
        • Those connected to spinal cord are spinal nerves
        • Those attached to brain are cranial nerves
      • Somatic sensory axons (fibers) send signals from the skin and special sense organs
      • Visceral sensory fibers convey information from the internal organs
    • CNS and PNS must work in harmony to carry out three primary functions
      • Receive sensory input
      • Perform integration
      • Generate motor output
  • 9. Figure 26.2 Organization of the nervous system in humans
  • 10. Neurons Process and Transmit Information
  • 11. 26.3 Neurons are the functional units of a nervous system
    • Neurons (nerve cells) receive sensory information and convey information to an integration center
      • Three major parts
        • Cell body - contains a nucleus and a variety of organelles
        • Dendrites - short, highly branched processes receive signals from sensory receptors or other neurons and transmit them to cell body
        • Axon - portion of the neuron that conveys information to another neuron or to other cells
      • Axons bundle together to form nerves and are often called nerve fibers
      • Axons are covered by a white insulating layer called the myelin sheath
    • Neuroglia - cells that provide support and nourishment to the neurons
      • Myelin sheath is formed from membranes of tightly spiraled neuroglia
      • In PNS, Schwann cells perform this function, leaving gaps called nodes of Ranvier , or neurofibril nodes
  • 12. Types of Neurons
    • Motor (efferent) neurons carry nerve impulses from CNS to muscles or glands
      • Have many dendrites and a single axon
      • Cause muscle to contract or glands to secrete
    • Sensory (afferent) neurons take nerve impulses from sensory receptors to the CNS
      • Sensory receptors may be the end of a sensory neuron itself (a pain or touch receptor), or may be a specialized cell that forms a synapse with a sensory neuron
    • Interneurons (association neurons) occur entirely within the CNS
      • Parallel the structure of motor neurons and convey nerve impulses between various parts of the CNS
  • 13. Figure 26.3A Motor neuron
  • 14. Figure 26.3B Sensory neuron
  • 15. Figure 26.3C Interneuron
  • 16. 26.4 Neurons have a resting potential across their membranes when they are not active
    • Voltage, in millivolts (mV), is a measure of the electrical potential difference between two points
      • In the case of a neuron, the two points are the inside and the outside of the axon
    • Membrane potential - w hen an electrical potential difference exists between the inside and outside of a cell
    • When a neuron is not conducting an impulse, its resting potential is about −65 mV
      • Negative sign indicates that the inside of the cell is more negative than the outside
    • Potential is created by an unequal distribution of ions
      • Due to the activity of the sodium-potassium pump, which moves three sodium ions (Na + ) out of the neuron for every two potassium ions (K + ), it moves into the neuron
  • 17. Figure 26.4 Resting potential: More Na + outside the axon and more K + inside the axon. Inside is −65 mV, relative to the outside
  • 18. 26.5 Neurons have an action potential across axon membranes when they are active
    • Action potential - rapid change in polarity across axonal membrane as nerve impulse occurs
    • An action potential uses two types of gated ion channels in the axon membrane
      • First, a gated ion channel allows sodium (Na + ) to pass into the axon
      • Another gated ion channel allows potassium (K + ) to pass out of the axon
    • If a stimulus causes the axon membrane to depolarize to threshold , an action potential occurs in an all-or-none manner
  • 19. Figure 26.5A An action potential can be visualized as voltage changes over time
  • 20. Depolarization and Repolarization
    • Sodium Gates Open
      • When an action potential begins, the gates of the sodium channels open, and Na + flows into the axon
        • Membrane potential changes from −65 mV to +40 mV
      • Called depolarization because inside axon changes from negative to positive
    • Potassium Gates Open
      • Gates of potassium channels open, and K + flows out of axon
        • Action potential changes from +40 mV back to −65 mV
      • Called repolarization because inside of axon becomes negative again as K + exits the axon
  • 21. Figure 26.5B Action potential begins: Depolarization to +40 mV as Na + gates open and Na + moves to inside the axon
  • 22. Figure 26.5C Action potential ends: Repolarization to −65 mV as K + gates open and K + moves to outside the axon
  • 23. 26.6 Propagation of an action potential is speedy
    • In nonmyelinated axons, action potential travels down an axon one section at a time, at a speed of about 1 m/second
      • As soon as an action potential has moved on, the previous section undergoes a refractory period , during which the Na + gates are unable to open
        • The action potential cannot move backward and instead always moves down an axon toward its terminals
      • After refractory period, the sodium potassium pump restores the previous ion distribution by pumping Na + to outside the axon and K + to inside the axon
    • In myelinated axons, gated ion channels that produce action potential are concentrated at the nodes of Ranvier
      • Ion exchange only at the nodes makes the action potential travel faster in nonmyelinated axons called saltatory conduction
  • 24. Figure 26.6 Saltatory conduction
  • 25. 26.7 Communication between neurons occurs at synapses
    • Every axon branches into many fine endings, tipped by a small swelling, called an axon terminal
      • Each terminal lies very close to the dendrite (or the cell body) of another neuron
    • Region of close proximity is called a synapse
      • At synapse, membrane of first neuron is pre synaptic
      • The membrane of the next neuron is the post synaptic
      • Small gap between the neurons is the synaptic cleft
    • A nerve impulse cannot cross a synaptic cleft
      • Transmission across a synapse is carried out by molecules called neurotransmitters , which are stored in synaptic vesicles
  • 26. Figure 26.7 Synapse structure and function
  • 27. 26.8 Neurotransmitters can be stimulatory or inhibitory
    • Acetylcholine (ACh) and norepinephrine (NE) are well-known neurotransmitters in both the CNS and the PNS
      • In the PNS, ACh excites skeletal muscle but inhibits cardiac muscle
      • ACh has either an excitatory or inhibitory effect on smooth muscle or glands
    • In the CNS, NE is important to dreaming, waking, and mood
      • Serotonin, another neurotransmitter, is involved in thermoregulation, sleeping, emotions, and perception
    • Clearing of Neurotransmitter from a Synapse
      • Once a neurotransmitter has been released into a synaptic cleft and has initiated a response, it is removed from the cleft
      • Short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of postsynaptic membranes
  • 28. 26.9 Integration is a summing up of stimulatory and inhibitory signals
    • A single neuron can have many synapses all over its dendrites and the cell body
      • A neuron is on the receiving end of many excitatory and inhibitory signals
        • An excitatory neurotransmitter produces a signal that drives the neuron closer to threshold
        • An inhibitory neurotransmitter produces a signal that drives the neuron further from threshold
    • Integration is the summing up of excitatory and inhibitory signals
      • If a neuron receives many excitatory signals the axon will transmit a nerve impulse
      • If a neuron receives both inhibitory and excitatory signals, the summing up of these signals may prohibit the axon from reaching threshold and firing
  • 29. Figure 26.9 Synaptic integration
  • 30. APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES 26.10 Drugs that interfere with neurotransmitter release or uptake may be abused
    • Alcohol
      • Acts as a depressant on many parts of the brain where it affects neurotransmitter release or uptake
        • Increases the action of GABA, which inhibits motor neurons, and increases the release of endorphins
    • Nicotine
      • Binds to neurons, causing the release of dopamine, neurotransmitter that promotes a sense of pleasure and is involved in motor control
      • In the PNS, nicotine is a stimulant by mimicking acetylcholine increasing heart rate, blood pressure, and muscle activity
    • Club and Date Rape Drugs
      • Structure of methamphetamine is similar to that of dopamine, and its stimulatory effect mimics that of cocaine
      • Ecstasy has an overstimulatory effect on neurons that produce serotonin, which, like dopamine, elevates our mood
    • Cocaine
      • Powerful stimulant in CNS, interferes with re-uptake of dopamine at synapses
      • Result is a rush of well-being that lasts from 5 to 30 minutes
  • 31. Heroin, Marijuana and Treatment for Addictive Drugs
    • Heroin
      • Highly addictive drug that acts as a depressant in the nervous system
      • Come from opium poppy plant, thus called opiates
        • Opiates depress breathing, block pain pathways, cloud mental function, and cause nausea
    • Marijuana (THC)
      • THC may mimic the actions of anandamide, a neurotransmitter
      • When THC reaches CNS, person experiences euphoria, with alterations in vision and judgment
        • In heavy users, hallucinations, anxiety, depression, body image distortions, paranoia, and psychotic symptoms can result
    • Treatment for Addictive Drugs
      • Mainly consists of behavior modification
      • Heroin addiction can be treated with synthetic opiate compounds, such as methadone that decrease withdrawal symptoms and block heroin’s effects
      • New treatment techniques include the administration of antibodies to block the effects of cocaine and methamphetamine
  • 32. Figure 26.10 Drug use
  • 33. The Vertebrate Central Nervous System (CNS) Consists of the Spinal Cord and Brain
  • 34. 26.11 The human spinal cord and brain function together
    • CNS consists of the spinal cord and the brain, where sensory information is received and motor control is initiated
      • Spinal cord and brain are wrapped in three protective membranes known as meninges
        • Spaces between meninges filled with cerebrospinal fluid that protects CNS
    • Spinal Cord - bundle of nervous tissue enclosed in vertebral column
      • Extends from base of the brain to the vertebrae just below the rib cage
      • Two main functions
        • Center for many reflex actions , automatic responses to external stimuli
        • Provides a means of communication between the brain and the spinal nerves, which leave the spinal cord
      • Central portion of gray matter and a peripheral region of white matter
        • Gray matter consists of cell bodies and unmyelinated fibers
      • Myelinated long fibers of interneurons that run together in bundles called tracts give white matter its color
        • Tracts connect the spinal cord to the brain
    • Brain Ventricles - brain contains four interconnected chambers called ventricles
      • Two lateral ventricles are inside the cerebrum
      • Third ventricle is surrounded by the diencephalon, and the fourth ventricle lies between the cerebellum and the pons
  • 35. Figure 26.11 The human brain
  • 36. 26.12 The cerebrum performs integrative activities
    • Cerebrum - largest portion of the brain in humans
      • Last center to receive sensory input and carry out integration before commanding voluntary motor responses
    • Cerebral Hemispheres - brain is divided into two halves
      • Each hemisphere receives information from and controls the opposite side of the body
      • The two are connected by a bridge of tracts within the corpus callosum
    • The Cerebral Cortex
      • A thin, but highly convoluted, outer layer of gray matter that covers the cerebral hemispheres
        • Primary motor area is in the frontal lobe and is where voluntary commands to skeletal muscles begin
        • Primary somatosensory area is in the parietal lobe and sensory information from the skin and skeletal muscles arrives here
    • Basal Nuclei
      • Integrate motor commands, ensure proper muscle groups activated
      • Huntington disease and Parkinson disease are believed to be due to malfunctioning basal nuclei
  • 37. Figure 26.12 The lobes of a cerebral hemisphere
  • 38. 26.13 The other parts of the brain have specialized functions
    • Hypothalamus and thalamus are in the diencephalon , a region that encircles the third ventricle
      • Hypothalamus forms the floor of the third ventricle
      • Thalamus consists of two masses of gray matter located in the sides and roof of the third ventricle
        • It is on the receiving end for all sensory input except smell
      • Pineal gland , which secretes the hormone melatonin, is located in the diencephalon
      • Cerebellum lies under the cerebrum and is separated from the brain stem by the fourth ventricle
        • Receives sensory input from the eyes, ears, joints, and muscles about the present position of body parts, and it also receives motor output from the cerebral cortex
    • Brain stem contains the midbrain, the pons, and the medulla oblongata
      • Midbrain acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum
        • Pons contains bundles of axons traveling between cerebellum and rest of CNS
        • Medulla oblongata contains a number of reflex centers for regulating heartbeat, breathing, and blood pressure
  • 39. Figure 26.13 The reticular activating system
  • 40. 26.14 The limbic system is involved in memory and learning as well as in emotions
    • Limbic system - complex network of tracts and nuclei that incorporates portions of the cerebral lobes, the basal nuclei, and diencephalon
      • Blends higher mental functions and primitive emotions
      • Two significant structures
        • Hippocampus - well situated in brain to make the frontal lobe aware of past experiences stored in various sensory areas
        • Amygdala - in particular, adds emotional overtones
    • Learning and Memory
      • Memory is the ability to hold a thought in mind or recall events from the past
        • Frontal lobe is active during short-term memory
      • A gradual extinction of brain cells, particularly in the hippocampus, appears to be the underlying cause of Alzheimer disease (AD)
  • 41. Figure 26.14 The limbic system (in purple)
  • 42. The Vertebrate Peripheral Nervous System (PNS) Consists of Nerves
  • 43. 26.15 The peripheral nervous system contains cranial and spinal nerves
    • The peripheral nervous system (PNS) lies outside the central nervous system and contains nerves, bundles of axons
      • Cranial nerves are attached to the brain
        • Some are motor nerves that contain only motor fibers, and others are mixed nerves that contain both sensory and motor fibers
      • Spinal nerves are attached to spinal cord
        • A spinal nerve separates the axons of sensory neurons from the axons of motor neurons
        • Cell body of a sensory neuron is in the dorsal root ganglion
        • Each spinal nerve serves the particular region of the body in which it is located
  • 44. Figure 26.15A Anatomy of a nerve
  • 45. Figure 26.15B Ventral surface of brain showing the attachment of the cranial nerves (yellow)
  • 46. 26.16 In the somatic system, reflexes allow us to respond quickly to stimuli
    • Somatic system nerves serve the skin, joints, and skeletal muscles
      • Includes nerves that take
        • Sensory information from external sensory receptors in the skin and joints to the CNS
        • Motor commands away from the CNS to the skeletal muscles
      • Acetylcholine (ACh) active in somatic system
    • Reflexes - involuntary responses to stimuli
      • Involve either the brain or just the spinal cord
      • Enable the body to react swiftly to stimuli that could disrupt homeostasis
  • 47. Figure 26.16 A reflex arc showing the path of a spinal reflex
  • 48. 26.17 In the autonomic system, the parasympathetic and sympathetic divisions control the actions of internal organs
    • Autonomic system - automatically and involuntarily regulates the activity of glands and cardiac and smooth muscle
    • Divided into parasympathetic and sympathetic
      • Parasympathetic Division
        • Includes a few cranial nerves as well as axons that arise from the last portion of the spinal cord
        • Promotes all the internal responses we associate with a relaxed state
          • Example: causes the pupil of the eye to constrict, promotes digestion of food, and retards the heartbeat
      • Sympathetic Division
        • Axons arise from portions of the spinal cord
        • Important during emergency situations and is associated with fight or flight
          • Example: accelerates the heartbeat and dilates the bronchi, while at the same time it inhibits the digestive tract
  • 49. Figure 26.17 Autonomic system
  • 50. Connecting the Concepts: Chapter 26
    • The human nervous system has just three functions: sensory input, integration, and motor output
    • The central nervous system (CNS) carries out the function of integrating incoming data
      • The brain allows us to perceive our environment, to reason, and to remember
      • After sensory data have been processed by the CNS, motor output occurs
      • Muscles and glands are the effectors that allow us to respond to the original stimuli
    • The human peripheral nervous system (PNS) contains nerves that carry sensory input to the CNS and motor output to the muscles and glands
    • There is a division of labor among the nerves
      • The cranial nerves serve the face, teeth, and mouth; below the head, there is only one cranial nerve, the vagus nerve
      • All body movements are controlled by spinal nerves, and this is why paralysis may follow a spinal injury