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Neurons, Synapses, and Signaling CHAPTER 48 Figure 48.1 Overview of a vertebrate nervous system NERVOUS SYSTEM • Central nervous system (CNS) – brain and spinal cord • Peripheral nervous system (PNS) – nerves that communicate motor and sensory signals between CNS and rest of body NEURON • Functional unit of nervous system • Relatively large cell body • Processes: – Dendrites – convey signals from tips to cell body; often branched – Axons – conduct signals away from body and toward tip; often single • Myelin sheath – protective, insulating layer that covers many axons • Axon ends at synaptic terminals – Synapse – site of contact between synaptic terminal and target cell (neuron or effector cell – for example a muscle cell) – Neurotransmitter – chemical messengers between neurons and other cells Figure 48.2 Structure of a vertebrate neuron Figure 48.0 A neuron on a microprocessor Figure 48.0x1 Aplysia neuron Figure 48.5 Schwann cells ORGANIZATION OF NEURONS • Sensory neurons – communicate sensory information from eyes and other senses and internal conditions – Senses, blood pressure, muscle tension, CO2 levels) • Interneurons – integrate sensory input and motor output; communicate only between neurons; make up vast majority of brain neurons • Motor neurons – convey impulses from CNS to effector cells (muscles and glands) • Ganglion – cluster of nerve cell bodies in PNS • Nuclei – clusters of nerve cell bodies in brain – Both allow activities without entire nervous system involved – Knee-jerk reflex • Reflex – automatic reaction to stimuli mediated by spinal cord and lower brain Figure 48.3 The knee-jerk reflex MEMBRANE POTENTIAL • Voltage measured across the membrane (like a battery) • Inside of cell more negative • Typically –50 to –80 mV (resting potential) • Sodium-potassium pump keeps ionic gradient (3Na+ out, 2K+ in) Figure 8.15 The sodium-potassium pump: a specific case of active transport Figure 48.6 Measuring membrane potentials Figure 48.7 The basis of the membrane potential Charges Across Membranes • Neurons have ability to generate changes in their membrane potential • Resting potential – membrane potential of cell at rest (-60mV to -80mV) • Gated ion channels control membrane potential – open to different stimuli – Hyperpolarization – increase in electrical gradient • Open K+ channel (K+ moves out) • Cell becomes more negative • No action potential because it makes it harder to depolarize – Depolarization – decrease in electrical gradient • Open Na+ channel (Na+ moves in) • Cell becomes more positive • Action potential generated if threshold is reached (-50mV to -55mV) –Massive change in voltage • Threshold causes all-or-none event Figure 48.8 Graded potentials and the action potential in a neuron Figure 48.9 The role of voltage-gated ion channels in the action potential ROLE OF GATED CHANNELS • Depolarizing – Na+ gates open rapidly so Na+ moves into cell • Repolarizing – K+ gates finally open and K+ moves out; Na+ gates close • Undershoot (Refractory Period) - K+ still open (they are slower to close) and Na+ still closed so cell becomes even more negative than resting and cannot be depolarized • Stronger stimuli result in greater frequency of action potentials and NOT from stronger action potentials • Propagation – Action potentials move in one direction due to refractory period Propagation of the action potential Na+ moves into cell starting action potential. Depolarization spreads and K+ repolarizes initial area. Prevents action potential on that side. Figure 48.11 Saltatory conduction • Voltage leaps from node to node SYNAPSES • Presynaptic cell – transmitting cell • Postsynaptic cell – receiving cell • Two types of synapses – Electrical • Need gap junctions (channels between neurons) • No delays – Chemical • Narrow gap, synaptic cleft, between cells • More common than electrical in vertebrates and most invertebrates • Require neurotransmitters (chemical intercellular messengers) • Depolarization of presynaptic membrane causes influx of Ca2+ • Increased Ca2+ in cell causes synaptic vesicles to fuse to cell membrane and release neurotransmitters via exocytosis • Neurotransmitters diffuse to postsynaptic cell • Postsynaptic membrane has gated channels that open when neurotransmitters bond to specific receptors Figure 48.12 A chemical synapse • A single neuron may receive many inputs simultaneously • Neurotransmitters cause 2 different responses depending on the gates that are opened – Inhibitory • (hyperpolarization) – Excitatory • (depolarization) • Neurotransmitters are quickly degraded • Excitatory postsynaptic potential (EPSP) – Na+ in and K+ out = depolarization • Inhibitory postsynaptic potential (IPSP) K+ out or CL- in = hyperpolarization Figure 48.13 Integration of multiple synaptic inputs Figure 48.14 Summation of postsynaptic potentials NEUROTRANSMITTERS • Acetylcholine – one of the most common – can excite skeletal muscle and inhibit cardiac muscle • Epinephrine and norepinephrine – also function as hormones • Dopamine – Usually excitatory – Excess dopamine can cause schizophrenia – Lack of dopamine can cause Parkinson’s • Sertonin – Usually inhibitory • Endorphins – Natural painkillers (morphine and opium mimic endorphins shape) • Nitric Oxide (NO) – Released during sexual arousal (increasing blood flow) – Nitroglycerin used to treat chest pain Nervous System Chapter 49 NERVOUS SYSTEM • Verebrate Nervous Systems – All have brain and spinal cord – Brain is integrative – White matter – axons with white meylin in CNS – Gray matter – dendrites and cell bodies in CNS CNS • The brain and spinal cord contain fluid filled spaces (called ventricles in the brain) • The spinal cord is hollow and has a central canal Figure 48.16 The nervous system of a vertebrate Figure 48.16x Spinal cord • Cerebrospinal fluid – Fills canal and ventricles – Brain filtered blood – Contains nutrients and WBC – Circulated and eventually empties into veins – Major function is as a shock absorber PNS – Cranial nerves – innervate organs of head and upper body – Spinal nerves – innervate entire body • Mammals have 12 pairs of cranial and 31 pairs of spinal nerves – Hierarchy of PNS • Afferent Division – convey info to CNS • Efferent Division – convey info from CNS Efferent Division • Motor –controls skeletal muscles (voluntary) • Autonomic – controls smooth and cardiac muscle (involuntary) • Sympathetic – arousal and energy generation (flight or fight) • Parasympathetic - calming and selfmaintenance (rest and digest) • Enteric – digestive tract, pancreas, and gall bladder Functional hierarchy of the peripheral nervous system Figure 48.18 The main roles of the parasympathetic and sympathetic nerves in regulating internal body functions BRAIN • Brainstem – Medulla oblongata – breathing, heart rate, swallowing, vomiting, digestion – Pons – breathing – Midbrain – receives sensory information Figure 48.19 Embryonic development of the brain • Cerebellum – Coordination of movement, hand-eye coordination, learning and remembering • Diencephalon – Hypothalamus, thalamus, and epithalamus • Hypothalamus - regulates hunger, thirst, sexual response, mating behaviors, fight or flight, biological clock –Contains the Suprachiasmatic nuclei – make proteins in response to light/dark (biological clock) • Cerebrum – Most complex integration – Controls learning, emotion, memory, and perception – Divided into right and left hemispheres – Cerebral cortex • Most complex, most evolved, and surface area is 0.5 m2 which is ~80% of total brain mass – Corpus callosum – connects hemispheres Figure 48.20 The main parts of the human brain Figure 48.20x1 Cerebral cortex, gray and white matter Neural Plasticity • The overall organization of the brain is developed in the embryo; however neural plasticity can change • Neural plasticity – ability of brain to be remodeled – Synaptic connections can increase or decrease – Formation of memory