CHAPTER 44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
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CHAPTER 44 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The Nervous System Chapter 44 Nervous System Organization • All animals must be able to respond to environmental stimuli • Sensory receptors – detect stimulus • Motor effectors – respond to it • Nervous system links the two – Consists of neurons and supporting cells 3 Nervous System Organization • Vertebrates have three types of neurons 1. Sensory neurons (afferent neurons) carry impulses to central nervous system (CNS) 2. Motor neurons (efferent neurons) carry impulses from CNS to effectors (muscles and glands) 3. Interneurons (association neurons) provide more complex reflexes and associative functions (learning and memory) 4 Nervous System Organization • Central nervous system (CNS ) – Brain and spinal cord • Peripheral nervous system (PNS) – Sensory and motor neurons – Somatic NS stimulates skeletal muscles – Autonomic NS stimulates smooth and cardiac muscles, as well as glands • Sympathetic and parasympathetic NS – Counterbalance each other 5 6 CNS Brain and Spinal Cord Motor Pathways PNS Sensory Pathways Sensory neurons registering external stimuli Sensory neurons registering external stimuli Somatic nervous system (voluntary) Sympathetic nervous system "fight or flight" Autonomic nervous system (involuntary) Parasympathetic nervous system "rest and repose" central nervous system (CNS) peripheral nervous system (PNS) 7 Nervous System Organization • Neurons have the same basic structure – Cell body • Enlarged part containing nucleus – Dendrites • Short, cytoplasmic extensions that receive stimuli – Axon • Single, long extension that conducts impulses away from cell body 8 Nervous System Organization 9 Nervous System Organization • Neuroglia – Support neurons both structurally and functionally – Schwann cells and oligodendrocytes produce myelin sheaths surrounding axons – In the CNS, myelinated axons form white matter • Dendrites/cell bodies form gray matter – In the PNS, myelinated axons are bundled to form nerves 10 Nervous System Organization 11 Nerve Impulse Transmission • A potential difference exists across every cell’s plasma membrane – Negative pole – cytoplasmic side – Positive pole – extracellular fluid side • When a neuron is not being stimulated, it maintains a resting potential – Ranges from –40 to –90 millivolts (mV) – Average about –70 mV 12 Nerve Impulse Transmission • The inside of the cell is more negatively charged than the outside 1. Sodium–potassium pump • Brings two K+ into cell for every three Na+ it pumps out 2. Ion leakage channels • Allow more K+ to diffuse out than Na+ to diffuse in 13 14 Nerve Impulse Transmission • Two major forces act on ions in establishing the resting membrane potential 1. Electrical potential produced by unequal distribution of charges 2. Concentration gradient produced by unequal concentrations of molecules from one side of the membrane to the other 15 Nerve Impulse Transmission • Sodium–potassium pump creates significant concentration gradient • Concentration of K+ is much higher inside the cell • Membrane not permeable to negative ions • Leads to buildup of positive charges outside and negative charges inside cell • Attractive force to bring K+ back inside cell • Equilibrium potential – balance between diffusional force and electrical force 16 Nerve Impulse Transmission 17 Nerve Impulse Transmission • Uniqueness of neurons compared with other cells is not the production and maintenance of the resting membrane potential • Rather the sudden temporary disruptions to the resting membrane potential that occur in response to stimuli • 2 types of changes – Graded potentials – Action potentials 18 Nerve Impulse Transmission • Graded potentials – Small transient changes in membrane potential due to activation of gated ion channels – Each gated channel is selective – Most are closed in the normal resting cell 19 Nerve Impulse Transmission • Chemically-gated or ligand-gated channels – Ligands are hormones or neurotransmitters – Induce opening and cause changes in cell membrane permeability 20 Nerve Impulse Transmission • Depolarization makes the membrane potential more positive • Hyperpolarization makes it more negative • These small changes result in graded potentials • Size depends on either the strength of the stimulus or the amount of ligand available to bind with their receptors • Can reinforce or negate each other • Summation is the ability of graded potentials to combine 21 Nerve Impulse Transmission 22 Nerve Impulse Transmission • Action potentials – Result when depolarization reaches the threshold potential (–55 mV) – Depolarizations bring a neuron closer to the threshold – Hyperpolarizations move the neuron further from the threshold – Caused by voltage-gated ion channels • Voltage-gated Na+ channels • Voltage-gated K+ channels 23 Nerve Impulse Transmission • Voltage-gated Na+ channels – Activation gate and inactivation gate – At rest, activation gate closed, inactivation gate open – Transient influx of Na+ causes the membrane to depolarize • Voltage-gated K+ channels – Single activation gate that is closed in the resting state – K+ channel opens slowly – Efflux of K+ repolarizes the membrane 24 Nerve Impulse Transmission • The action potential has three phases – Rising, falling, and undershoot • Action potentials are always separate, allor-none events with the same amplitude • Do not add up or interfere with each other • Intensity of a stimulus is coded by the frequency, not amplitude, of action potentials 25 26 Nerve Impulse Transmission • Propagation of action potentials – Each action potential, in its rising phase, reflects a reversal in membrane polarity – Positive charges due to influx of Na+ can depolarize the adjacent region to threshold – And so the next region produces its own action potential – Meanwhile, the previous region repolarizes back to the resting membrane potential • Signal does not go back toward cell body 27 28 Nerve Impulse Transmission • Two ways to increase velocity of conduction – Axon has a large diameter • Less resistance to current flow • Found primarily in invertebrates – Axon is myelinated • Action potential is only produced at the nodes of Ranvier • Impulse jumps from node to node • Saltatory conduction 29 Nerve Impulse Transmission 30 Synapses • Intercellular junctions with the dendrites of other neurons, with muscle cells, or with gland cells • Presynaptic cell transmits action potential • Postsynaptic cell receives it • Two basic types: electrical and chemical 31 • Electrical synapses – Involve direct cytoplasmic connections between the two cells formed by gap junctions – Relatively rare in vertebrates • Chemical synapses – Have a synaptic cleft between the two cells – End of presynaptic cell contains synaptic vesicles packed with neurotransmitters 32 Synapses • Chemical synapses – Action potential triggers influx of Ca2+ – Synaptic vesicles fuse with cell membrane – Neurotransmitter is released by exocytosis – Diffuses to other side of cleft and binds to chemical- or ligand-gated receptor proteins – Produces graded potentials in the postsynaptic membrane – Neurotransmitter action is terminated by enzymatic cleavage or cellular uptake 33 Synapses 34 Neurotransmitters • Acetylcholine (ACh) – Crosses the synapse between a motor neuron and a muscle fiber – Neuromuscular junction 35 Neurotransmitters • Acetylcholine (ACh) – Binds to receptor in the postsynaptic membrane – Causes ligand-gated ion channels to open – Produces a depolarization called an excitatory postsynaptic potential (EPSP) – Stimulates muscle contraction – Acetylcholinesterase (AChE) degrades ACh • Causes muscle relaxation 36 Neurotransmitters • Amino acids – Glutamate • Major excitatory neurotransmitter in the vertebrate CNS • Glycine and GABA (g-aminobutyric acid) are inhibitory neurotransmitters – Open ligand-gated channels for Cl– – Produce a hyperpolarization called an inhibitory postsynaptic potential (IPSP) 37 38 Neurotransmitters • Biogenic amines – Epinephrine (adrenaline) and norepinephrine are responsible for the “fight or flight” response – Dopamine is used in some areas of the brain that control body movements – Serotonin is involved in the regulation of sleep 39 Neurotransmitters • Neuropeptides – Substance P is released from sensory neurons activated by painful stimuli – Intensity of pain perception depends on enkephalins and endorphins – Nitric oxide (NO) • A gas – produced as needed from arginine • Causes smooth muscle relaxation 40 Synaptic Integration • Integration of EPSPs (depolarization) and ISPSs (hyperpolarization) occurs on the neuronal cell body – Small EPSPs add together to bring the membrane potential closer to the threshold – IPSPs subtract from the depolarizing effect of EPSPs • Deter the membrane potential from reaching threshold 41 Synaptic Integration 42 Synaptic Integration • There are two ways that the membrane can reach the threshold voltage 1. Spatial summation • Many different dendrites produce EPSPs 2. Temporal summation • One dendrite produces repeated EPSPs 43 Drug Addiction • Habituation – Prolonged exposure to a stimulus may cause cells to lose the ability to respond to it – Cell decreases the number of receptors because there is an abundance of neurotransmitters – In long-term drug use, means that more of the drug is needed to obtain the same effect 44 Drug Addiction • Cocaine – Affects neurons in the brain’s “pleasure pathways” (limbic system) – Binds dopamine transporters and prevents the reuptake of dopamine – Dopamine survives longer in the synapse and fires pleasure pathways more and more 45 46 Drug Addiction • Nicotine – Binds directly to a specific receptor on postsynaptic neurons of the brain – Binds to a receptor for acetylcholine 47 The Central Nervous System • Sponges are only major phylum without nerves • Cnidarians have the simplest nervous system – Neurons linked to each other in a nerve net – No associative activity • Free-living flatworms (phylum Platyhelminthes) are simplest animals with associative activity – Two nerve cords run down the body – Permit complex muscle control • All of the subsequent evolutionary changes in nervous systems can be viewed as a series of elaborations on the characteristics already present in flatworms 48 49 Vertebrate Brains • All vertebrate brains have three basic divisions: – Hindbrain – Midbrain – Forebrain • In fishes, – Hindbrain – largest portion – Midbrain – processes visual information – Forebrain – processes olfactory information 50 Vertebrate Brains 51 Vertebrate Brains • Relative sizes of different brain regions have changed as vertebrates evolved • Forebrain became the dominant feature 52 Vertebrate Brains – Cerebrum • The increase in brain size in mammals reflects the great enlargement of the cerebrum • Split into right and left cerebral hemispheres, which are connected by a tract called the corpus callosum • Each hemisphere receives sensory input from the opposite side • Hemispheres are divided into: frontal, parietal, temporal, and occipital lobes 53 Cerebrum 54 Cerebrum • Cerebral cortex – Outer layer of the cerebrum – Contains about 10% of all neurons in brain – Highly convoluted surface • Increases threefold the surface area of the human brain – Divided into three regions, each with a specific function 55 Cerebrum • Cerebral cortex • Primary motor cortex – movement control • Primary somatosensory cortex – sensory control • Association cortex – higher mental functions • Basal ganglia • Aggregates of neuron cell bodies – gray matter • Participate in the control of body movements 56 57 Each of these regions of the cerebral cortex is associated with a different region of the body 58 Other Brain Structures • Thalamus – Integrates visual, auditory, and somatosensory information • Hypothalamus – Integrates visceral activities – Controls pituitary gland • Limbic system – Hypothalamus, hippocampus, and amygdala – Responsible for emotional responses 59 Complex Functions of the Brain • Sleep and arousal – One section of reticular formation is the reticular-activating system • Controls consciousness and alertness – Brain state can be monitored by means of an electroencephalogram (EEG) • Records electrical activity 60 Complex Functions of the Brain • Language – Left hemisphere is “dominant” hemisphere • Different regions control various language activities • Adept at sequential reasoning – Right hemisphere is adept at spatial reasoning • Primarily involved in musical ability • Nondominant hemisphere is also important for the consolidation of memories of nonverbal experiences 61 62 Complex Functions of the Brain • Memory – Appears dispersed across the brain – Short-term memory is stored in the form of transient neural excitations – Long-term memory appears to involve structural changes in neural connections – Two parts of the temporal lobes, the hippocampus and the amygdala, are involved in both short-term memory and its consolidation into long-term memory 63 Complex Functions of the Brain • Alzheimers disease – Condition where memory and thought become dysfunctional – Two causes have been proposed 1. Nerve cells are killed from the outside in – External protein: b-amyloid 2. Nerve cells are killed from the inside out – Internal proteins: tau () 64 Spinal Cord • Cable of neurons extending from the brain down through the backbone • Enclosed and protected by the vertebral column and the meninges 65 Spinal Cord • 2 zones – Inner zone is gray matter • Primarily consists of the cell bodies of interneurons, motor neurons, and neuroglia – Outer zone is white matter • Contains cables of sensory axons in the dorsal columns and motor axons in the ventral columns 66 Spinal Cord • It serves as the body’s “information highway” – Relays messages between the body and the brain • It also functions in reflexes – The knee-jerk reflex is monosynaptic – However, most reflexes in vertebrates involve a single interneuron 67 Knee-jerk reflex is monosynaptic 68 Most reflexes in vertebrates involve a single interneuron 69 The Peripheral Nervous System • Consists of nerves and ganglia – Nerves are bundles of axons bound by connective tissue – Ganglia are aggregates of neuron cell bodies • Function is to receive info from the environment, convey it to the CNS, and to carry responses to effectors such as muscle cells 70 The Peripheral Nervous System • Sensory neurons – Axons enter the dorsal surface of the spinal cord and form dorsal root of spinal nerve – Cell bodies are grouped outside the spinal cord in dorsal root ganglia • Motor neurons – Axons leave from the ventral surface and form ventral root of spinal nerve – Cell bodies are located in the spinal cord 71 The Somatic Nervous System • Somatic motor neurons stimulate the skeletal muscles to contract – In response to conscious command or reflex actions – Antagonist of the muscle is inhibited by hyperpolarization (IPSPs) of spinal motor neurons 72 The Autonomic Nervous System • Composed of the sympathetic and parasympathetic divisions, plus the medulla oblongata • In both, efferent motor pathway has 2 neurons – Preganglionic neuron – exits the CNS and synapses at an autonomic ganglion – Postganglionic neuron – exits the ganglion and regulates visceral effectors • Smooth or cardiac muscle or glands 73 74 The Autonomic Nervous System • Sympathetic division – Preganglionic neurons originate in the thoracic and lumbar regions of spinal cord – Most axons synapse in two parallel chains of ganglia right outside the spinal cord • Parasympathetic division – Preganglionic neurons originate in the brain and sacral regions of spinal cord – Axons terminate in ganglia near or even within internal organs 75 76