Cecie Starr Christine Evers Lisa Starr www.cengage.com/biology/starr Chapter 29 Neural Control (Sections 29.6 - 29.8) Albia Dugger • Miami Dade College 29.6 Chemical Communication at Synapses • Action potentials cannot pass directly from a neuron to another cell • Chemicals relay signals from a neurons (presynaptic cell) to another neuron, muscle or gland (postsynaptic cell) across a fluid-filled synaptic cleft • synapse • Region where a neuron’s axon terminals transmit signals to another cell Sending Signals at Synapses • When an action potential arrives at the presynaptic cell’s axon terminals, it triggers release of a neurotransmitter • Neurotransmitter molecules diffuse across the synaptic cleft and bind to receptors on the postsynaptic cell • Example: At a neuromuscular junction, a motor neuron releases acetylcholine, which binds to receptors on a muscle fiber Key Terms • neurotransmitter • Chemical signal released by axon terminals • neuromuscular junction • Synapse between a neuron and a muscle • acetylcholine (ACh) • Neurotransmitter released at neuromuscular junctions, and at synapses in the heart and brain Communication at a Synapse Communication at a Synapse Fig. 29.10.1, p. 474 Communication at a Synapse Action potentials flow along the axon of a motor neuron to neuromuscular junctions, where an axon terminal forms a synapse with a muscle fiber. 1 axon of a motor neuron neuromuscular junction Fig. 29.10.1, p. 474 Communication at a Synapse Fig. 29.10.2,3, p. 474 Communication at a Synapse axon terminal of motor neuron The axon terminal stores chemical signaling molecules (green) called neurotransmitter inside synaptic vesicles. 2 plama membrane of muscle fiber synaptic vesicle 2 Arrival of an action potential causes exocytosis of synaptic vesicles, and neurotransmitter enters the synaptic cleft. 3 3 synaptic cleft Fig. 29.10.2,3, p. 474 Communication at a Synapse Fig. 29.10.4,5, p. 474 Communication at a Synapse The plasma membrane of the muscle fiber has receptors for neurotransmitter. 4 binding site for neurotransmitter (no neurotransmitter bound) ion channel closed Binding of neurotransmitter opens a channel through the receptor. The opening allows ions to flow into the postsynaptic cell. 5 neurotransmitter ion flows through now-open channel Fig. 29.10.4,5, p. 474 Animation: Synaptic Structure and Function Cleaning the Cleft • After neurotransmitter molecules do their work, they must be removed from synaptic clefts • Membrane pumps transport some neurotransmitter back into presynaptic cells or into nearby neuroglial cells • Postsynaptic cells have enzymes that break down neurotransmitter (e.g. acetylcholinesterase) Synaptic Integration • Neurotransmitter can have an inhibitory or excitatory effect on a postsynaptic cell • The postsynaptic cell’s response is determined by synaptic integration of messages arriving at the same time • synaptic integration • The summation of excitatory and inhibitory signals by a postsynaptic cell Synaptic Density • A typical neuron or effector cell receives messages from many neurons • An interneuron in the brain can have thousands of incoming synapses Neurotransmitter and Receptor Diversity • Different kinds of neurons release different neurotransmitters • Examples: norepinephrine, epinephrine, dopamine, serotonin, glutamate, GABA • Different kinds of postsynaptic cells have receptors that respond differently to the same neurotransmitter • Receptors may be stimulating or inhibiting Effects of Some Neurotransmitters Key Concepts • How Neurons Work • Messages flow along a neuron’s plasma membrane, from input to output zones • The messages are brief, self-propagating reversals in the distribution of electric charge across the membrane • At an output zone, chemical signals are sent to other neurons, muscles, or glands ANIMATION: Chemical synapse To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION: Neurotransmitters To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION: Events at a neuromuscular junction BBC Video: Exploring Neurotransmitters Animation: Synapse Function 29.7 Disrupted Signaling: Disorders and Drugs • Disorders of the nervous system often involve disruption of signaling at synapses • Symptoms of neurological disorders may arise from lowered levels of neurotransmitter; treatment with drugs raises the level of the appropriate neurotransmitter • Psychoactive drugs mimic neurotransmitters or disrupt their release or uptake Parkinson’s Disease • Damage to dopamine-secreting neurons in the part of the brain that governs motor control results in tremors, loss of balance, and involuntary movement • PET scans show high metabolic activity in dopaminesecreting neurons • Former heavyweight boxer Muhammad Ali and actor Michael J. Fox are among those affected Battling Parkinson’s Disease Battling Parkinson’s Disease Fig. 29.12b, p. 476 Battling Parkinson’s Disease Fig. 29.12c, p. 476 Attention Deficit Hyperactivity Disorder • A lower than normal dopamine level also plays a role in attention deficit hyperactivity disorder (ADHD) • Affected people have trouble concentrating, are unusually impulsive, and tend to fidget when required to remain seated • Drugs used to treat ADHD are stimulants that increase dopamine availability in the brain Alzheimer’s Disease • Alzheimer’s disease is the leading cause of dementia (loss of ability to think) • An affected person becomes increasingly confused, cannot communicate, and eventually is incapable of living independently • Affected people have a lower than normal level of ACh in the brain Mood Disorders • Interactions among several neurotransmitters, including serotonin, dopamine, and norepinephrine, affect mood • Antidepressants, including Prozac and Paxil, increase the level of serotonin by preventing its reuptake • Depression has a genetic component, and families predisposed to depression may be prone to anxiety disorders Effects of Psychoactive Drugs • People take psychoactive drugs, both legal and illegal, to alleviate pain, relieve stress, or feel pleasure • All major addictive drugs stimulate the release of dopamine • Habituation and tolerance can lead to drug addiction Warning Signs Of Drug Addiction Stimulants • Stimulants make users feel alert but also anxious, and they can interfere with fine motor control • Nicotine blocks brain receptors for ACh • Caffeine blocks receptors for adenosine • Cocaine prevents reuptake of dopamine, serotonin, and norepinephrine from synaptic clefts • Amphetamines increase secretion of serotonin, norepinephrine, and dopamine in the brain Analgesics • Narcotic analgesics, including morphine, codeine, heroin, fentanyl, and oxycodone, mimic the effects of endorphins • Ketamine and PCP (phencyclidine) numb the extremities by slowing the clearing of synapses • endorphins • Natural painkillers produced by the central nervous system • Promote feelings of pleasure Depressants • Depressants such as alcohol (ethyl alcohol) and barbiturates slow motor responses by inhibiting ACh output • Alcohol also stimulates the release of endorphins and GABA, so users typically experience a brief euphoria followed by depression Hallucinogens • Hallucinogens distort sensory perception and bring on a dreamlike state • LSD resembles serotonin and binds to receptors for it • Mescaline and psilocybin have weaker effects • THC in marijuana alters levels of dopamine, serotonin, norepinephrine, and GABA Key Concepts • Disrupted Signaling • Some common neurological disorders cause symptoms by interfering with the flow of signals through the nervous system • Psychoactive drugs also affect nervous system activity by raising or lowering the amount of signaling chemicals in the brain BBC Video: A New Genetic Link to Alzheimer’s Disease BBC Video: Targeting Alzheimer’s Disease BBC Video: Unnecessary Antidepressant Therapy 29.8 Peripheral Nervous System • Peripheral nerves are bundles of axons that run through your body, carrying signals to and from the spinal cord and brain • Myelin sheaths formed by neuroglial cells (Schwann cells) wrap around axons of most peripheral nerves • myelin • Insulating material that wraps most axons and increases the speed of signal transmission Nerve Structure Nerve Structure myelin sheath axon blood vessel nerve fascicle (a number of axons bundled inside connective tissue) A the nerve’s outer wrapping Fig. 29.13a, p. 478 Axons Bundled as Nerves • Action potentials occur only at nodes, where there are gated ion channels and no myelin • After an action potential occurs at a node, positive ions diffuse quickly through the cytoplasm to the next node because myelin prevents them from leaking out across the membrane • Arrival of positive ions at the next node pushes the region to threshold, and an action potential occurs • Jumping from node to node increases signal speed in myelinated axons Action Potential in a Myelinated Axon Action Potential in a Myelinated Axon unsheathed node axon B “Jellyrolled” Schwann cells of an axon’s myelin sheath Na+ ---- ++++ ++++ ++++ ++++ ------- ------- ---- ++++ ++++ action potential K+ resting potential resting potential Na+ ++++ ---- ++++ ------- ++++ ++++ ------- ++++ ---- ++++ resting potential restored action potential resting potential Fig. 29.13b-d, p. 478 ANIMATION: Ion flow in myelinated axons To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE Somatic and Autonomic Divisions • The vertebrate peripheral nervous system has two divisions: somatic and autonomic • somatic nervous system • Set of nerves that control skeletal muscle and relay signals from joints and skin • autonomic nervous system • Set of nerves that relay signals to and from internal organs (smooth and cardiac muscle) and to glands Sympathetic and Parasympathetic Nerves • Sympathetic and parasympathetic nerves work antagonistically in most organs – signals from one division oppose signals from the other • Sympathetic neurons of the autonomic system increase their output in times of stress or danger • During less stressful times, signals from parasympathetic neurons dominate Key Terms • sympathetic neurons • Neurons of the autonomic system that prepare the body for danger or excitement (“fight-or-flight”) • Sympathetic ganglia are close to the spinal cord • parasympathetic neurons • Neurons of the autonomic system that encourage housekeeping tasks • Parasympathetic ganglia are in or near the organs they affect Effects of Autonomic Nerves Effects of Autonomic Nerves Sympathetic Effects Widens pupils Organ Eyes Parasympathetic Effects optic nerve Narrows pupils Increases salivation Salivary glands Decreases salivation Increases heart rate Heart Decreases heart rate vagus nerve Widens airways Airways Constricts airways Increases Stomach secretions and movements Liver, Increases pancreas secretions to digestive tract Increases secretion Adrenal gland Decreases secretion Slows secretions and movements Slows secretions to digestive tract (most ganglia near spinal cord) thoraci c nerves (12 pairs) (all gangli Increases Slows Small intestine, a in secretions secretions andlarge intestine walls and movements of movements organ Inhibits urination Bladder Stimulates urination s) Promotes ejaculation Promotes erection, Genitals lubrication midbrai n medulla oblonga cervica ta l nerves (8 pairs) pelvic nerve lumbar nerves (5 pairs) sacral nerves (5 pairs) Fig. 29.14, p. 479 ANIMATION : Autonomic nerves To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE ANIMATION : Nerve structure To play movie you must be in Slide Show Mode PC Users: Please wait for content to load, then click to play Mac Users: CLICK HERE