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Chapter 48 Nervous Systems Figure 48.2 Organization of some nervous systems Eyespot Nerve net Brain Brain Nerve cord Transverse nerve Radial nerve Nerve ring Ventral nerve cord Segmental ganglion (a) Hydra (cnidarian) (b) Sea star (echinoderm) (c) Planarian (flatworm) (d) Leech (annelid) Brain Brain Ventral nerve cord Segmental ganglia (e) Insect (arthropod) Anterior nerve ring Ganglia Longitudinal nerve cords (f) Chiton (mollusc) Brain Ganglia (g) Squid (mollusc) Spinal cord (dorsal nerve cord) Sensory ganglion (h) Salamander (chordate) Figure 48.3 Overview of information processing by nervous systems Sensory input Integration Sensor Motor output Effector Peripheral nervous system (PNS) Central nervous system (CNS) Figure 48.4 The knee-jerk reflex 2 Sensors detect a sudden stretch in the quadriceps. 3 Sensory neurons convey the information to the spinal cord. Cell body of sensory neuron in dorsal root ganglion 4 The sensory neurons communicate with motor neurons that supply the quadriceps. The motor neurons convey signals to the quadriceps, causing it to contract and jerking the lower leg forward. Gray matter 5 Sensory neurons from the quadriceps also communicate with interneurons in the spinal cord. Quadriceps muscle White matter Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron 1 The reflex is initiated by tapping the tendon connected to the quadriceps (extensor) muscle. Interneuron 6 The interneurons inhibit motor neurons that supply the hamstring (flexor) muscle. This inhibition prevents the hamstring from contracting, which would resist the action of the quadriceps. Figure 48.5 Structure of a vertebrate neuron Dendrites Cell body Nucleus Synapse Signal Axon direction Axon hillock Presynaptic cell Postsynaptic cell Myelin sheath Synaptic terminals Figure 48.6 Structural diversity of vertebrate neurons Dendrites Axon Cell body (a) Sensory neuron (b) Interneurons (c) Motor neuron Figure 48.8 Schwann cells and the myelin sheath Node of Ranvier Layers of myelin Axon Schwann cell Axon Myelin sheath Nodes of Ranvier Schwann cell Nucleus of Schwann cell 0.1 µm Figure 8.15 The sodium-potassium pump: a specific case of active transport Figure 48.12 Graded potentials and an action potential in a neuron Stimuli 0 Threshold 0 –50 0 1 2 3 4 5 Time (msec) (a) Graded hyperpolarizations produced by two stimuli that increase membrane permeability to K+. The larger stimulus produces a larger hyperpolarization. Threshold Action potential 0 –50 Resting Depolarizations potential Resting potential Hyperpolarizations –100 +50 Membrane potential (mV) +50 Membrane potential (mV) Membrane potential (mV) +50 –50 Stronger depolarizing stimulus Stimuli –100 Threshold Resting potential –100 0 1 2 3 4 5 Time (msec) (b) Graded depolarizations produced by two stimuli that increase membrane permeability to Na+. The larger stimulus produces a larger depolarization. 0 1 2 3 4 5 6 Time (msec) (c) Action potential triggered by a depolarization that reaches the threshold. Figure 48.14 Conduction of an action potential Axon Action potential – – + + ++ – Na – + + – – + K+ + + – – – – + + K+ + + – – + + + + + – – + – – + – – + – – + – – + Action potential – – + ++ Na + + – – K+ + – – + + + – – – – + + K+ + – – + + – – + Action potential – – + ++ Na + + – – + – – + – + + – 1 An action potential is generated as Na+ flows inward across the membrane at one location. 2 The depolarization of the action potential spreads to the neighboring region of the membrane, re-initiating the action potential there. To the left of this region, the membrane is repolarizing as K+ flows outward. 3 The depolarization-repolarization process is repeated in the next region of the membrane. In this way, local currents of ions across the plasma membrane cause the action potential to be propagated along the length of the axon. + – – + – + + – Figure 48.17 A chemical synapse Postsynaptic cell Presynaptic cell Synaptic vesicles containing neurotransmitter 5 Presynaptic membrane Neurotransmitter Postsynaptic membrane Ligandgated ion channel Voltage-gated Ca2+ channel 1 Ca2+ 4 2 Synaptic cleft Na+ K+ 3 Ligand-gated ion channels Postsynaptic membrane 6 Figure 48.19 The vertebrate nervous system Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Cranial nerves Ganglia outside CNS Spinal nerves Figure 48.21 Functional hierarchy of the vertebrate peripheral nervous system Peripheral nervous system Somatic nervous system Autonomic nervous system Sympathetic division Parasympathetic division Enteric division Figure 48.22 The parasympathetic and sympathetic divisions of the autonomic nervous system Parasympathetic division Sympathetic division Action on target organs: Location of preganglionic neurons: brainstem and sacral segments of spinal cord Neurotransmitter released by preganglionic neurons: acetylcholine Location of postganglionic neurons: in ganglia close to or within target organs Action on target organs: Dilates pupil of eye Constricts pupil of eye Inhibits salivary gland secretion Stimulates salivary gland secretion Constricts bronchi in lungs Sympathetic ganglia Cervical Accelerates heart Slows heart Stimulates activity of stomach and intestines Inhibits activity of stomach and intestines Thoracic Inhibits activity of pancreas Stimulates activity of pancreas Neurotransmitter released by postganglionic neurons: acetylcholine Stimulates gallbladder Stimulates glucose release from liver; inhibits gallbladder Lumbar Stimulates adrenal medulla Promotes emptying of bladder Promotes erection of genitalia Relaxes bronchi in lungs Inhibits emptying of bladder Synapse Sacral Promotes ejaculation and vaginal contractions Location of preganglionic neurons: thoracic and lumbar segments of spinal cord Neurotransmitter released by preganglionic neurons: acetylcholine Location of postganglionic neurons: some in ganglia close to target organs; others in a chain of ganglia near spinal cord Neurotransmitter released by postganglionic neurons: norepinephrine Unnumbered figure page 1029 Unnumbered figure page 1030 Unnumbered figure page 1031 Figure 48.27 The human cerebral cortex Frontal lobe Parietal lobe Speech Frontal association area Taste Speech Smell Somatosensory association area Reading Hearing Auditory association area Visual association area Vision Temporal lobe Occipital lobe