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Chapter 48

Chapter 48. Neurons, Synapses, and Signaling. Fig. 48-1. Concept 48.1: Neuron organization and structure reflect function in information transfer. Sensors detect external stimuli and internal conditions and transmit information along sensory neurons

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Chapter 48

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  1. Chapter 48 Neurons, Synapses, and Signaling

  2. Fig. 48-1

  3. Concept 48.1: Neuron organization and structure reflect function in information transfer • Sensors detect external stimuli and internal conditions and transmit information along sensory neurons • Sensory information is sent to the brain or ganglia, where interneurons integrate the information • Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity

  4. Many animals have a complex nervous system which consists of: • A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord • A peripheral nervous system (PNS), which brings information into and out of the CNS

  5. Fig. 48-3 Sensory input Integration Sensor Motor output Central nervous system (CNS) Effector Peripheral nervous system (PNS)

  6. Fig. 48-4 Neuron Structure and Function Dendrites Stimulus Presynaptic cell Nucleus Axon hillock Cell body Axon Synapse Synaptic terminals Postsynaptic cell Neurotransmitter

  7. Fig. 48-6 Concept 48.2: Ion pumps and ion channels maintain the resting potential of a neuron Key Sodium- potassium pump Na+ Potassium channel Sodium channel K+ OUTSIDE CELL [Na+] 150 mM [Cl–] 120 mM OUTSIDE CELL [K+] 5 mM [A–] 100 mM [K+] 140 mM INSIDE CELL [Na+] 15 mM [Cl–] 10 mM INSIDE CELL (a) (b) In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steady

  8. Concept 48.3: Action potentials are the signals conducted by axons • Neurons contain gated ion channels that open or close in response to stimuli

  9. Fig. 48-8 TECHNIQUE Microelectrode Voltage recorder Reference electrode

  10. Fig. 48-9 Stimuli Stimuli Strong depolarizing stimulus +50 +50 +50 Action potential 0 0 0 Membrane potential (mV) Membrane potential (mV) Membrane potential (mV) Threshold Threshold –50 –50 Threshold –50 Resting potential Resting potential Resting potential Depolarizations Hyperpolarizations –100 –100 –100 1 2 3 5 4 0 2 3 4 0 1 5 0 1 3 5 6 2 4 Time (msec) Time (msec) Time (msec) (b) Graded depolarizations (c) Action potential (a) Graded hyperpolarizations

  11. Production of Action Potentials • Voltage-gated Na+ and K+ channels respond to a change in membrane potential • When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell • The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open • A strong stimulus results in a massive change in membrane voltage called an action potential

  12. Fig. 48-9c Strong depolarizing stimulus +50 Action potential 0 Membrane potential (mV) –50 Threshold Resting potential –100 0 2 4 5 6 1 3 Time (msec) (c) Action potential

  13. Generation of Action Potentials: A Closer Look • A neuron can produce hundreds of action potentials per second • The frequency of action potentials can reflect the strength of a stimulus • An action potential can be broken down into a series of stages

  14. Fig. 48-10-5 Key Na+ K+ Falling phase of the action potential 4 Rising phase of the action potential 3 +50 Action potential 3 0 Membrane potential (mV) 2 4 Threshold –50 1 1 5 Resting potential Depolarization 2 –100 Time Extracellular fluid Sodium channel Potassium channel Plasma membrane Cytosol Inactivation loop Undershoot 5 Resting state 1

  15. Fig. 48-11-3 Axon Plasma membrane Action potential Cytosol Na+ Action potential K+ Na+ K+ Action potential K+ Na+ K+

  16. Conduction Speed • The speed of an action potential increases with the axon’s diameter • In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase • Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS

  17. Fig. 48-12 Node of Ranvier Layers of myelin Axon Schwann cell Schwann cell Nucleus of Schwann cell Nodes of Ranvier Axon Myelin sheath 0.1 µm

  18. Concept 48.4: Neurons communicate with other cells at synapses • At electrical synapses, the electrical current flows from one neuron to another • At chemical synapses, a chemical neurotransmitter carries information across the gap junction • Most synapses are chemical synapses

  19. Fig. 48-14 Postsynaptic neuron Synaptic terminals of pre- synaptic neurons 5 µm

  20. Fig. 48-15 5 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane Ca2+ 1 4 6 2 3 Synaptic cleft Ligand-gated ion channels

  21. Postsynaptic Potentials • Postsynaptic potentials fall into two categories: • Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold • Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold

  22. After release, the neurotransmitter • May diffuse out of the synaptic cleft • May be taken up by surrounding cells • May be degraded by enzymes

  23. Table 48-1

  24. Chapter 49 Nervous Systems

  25. Overview: Command and Control Center • The circuits in the brain are more complex than the most powerful computers • Functional magnetic resonance imaging (MRI) can be used to construct a 3-D map of brain activity • The vertebrate brain is organized into regions with different functions

  26. Fig. 49-1

  27. Fig. 49-2 Concept 49.1: Nervous systems consist of circuits of neurons and supporting cells Eyespot Brain Brain Radial nerve Nerve cords Ventral nerve cord Nerve ring Transverse nerve Nerve net Segmental ganglia (a) Hydra (cnidarian) (b) Sea star (echinoderm) (d) Leech (annelid) (c) Planarian (flatworm) Brain Brain Ganglia Anterior nerve ring Spinal cord (dorsal nerve cord) Ventral nerve cord Brain Sensory ganglia Longitudinal nerve cords Ganglia Segmental ganglia (e) Insect (arthropod) (h) Salamander (vertebrate) (f) Chiton (mollusc) (g) Squid (mollusc)

  28. Fig. 49-3 Organization of the Vertebrate Nervous System Cell body of sensory neuron in dorsal root ganglion Quadriceps muscle Gray matter White matter Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Interneuron

  29. Fig. 49-4 Peripheral nervous system (PNS) Central nervous system (CNS) Brain Cranial nerves Spinal cord Ganglia outside CNS Spinal nerves

  30. Fig. 49-5 Gray matter White matter Ventricles

  31. The central canal of the spinal cord and the ventricles of the brain are hollow and filled with cerebrospinal fluid • The cerebrospinal fluid is filtered from blood and functions to cushion the brain and spinal cord

  32. The brain and spinal cord contain • Gray matter, whichconsists of neuron cell bodies, dendrites, and unmyelinated axons • White matter, whichconsists of bundles of myelinated axons

  33. Glia in the CNS • Glia have numerous functions • Ependymal cells promote circulation of cerebrospinal fluid • Microglia protect the nervous system from microorganisms • Oligodendrocytes and Schwann cells form the myelin sheaths around axons

  34. Glia have numerous functions • Astrocytes provide structural support for neurons, regulate extracellular ions and neurotransmitters, and induce the formation of a blood-brain barrier that regulates the chemical environment of the CNS • Radial glia play a role in the embryonic development of the nervous system

  35. Fig. 49-6 PNS CNS Neuron VENTRICLE Astrocyte Ependy- mal cell Oligodendrocyte Schwann cells Microglial cell Capillary (a) Glia in vertebrates 50 µm (b) Astrocytes (LM)

  36. The Peripheral Nervous System • The PNS transmits information to and from the CNS and regulates movement and the internal environment • In the PNS, afferent neurons transmit information to the CNS and efferent neurons transmit information away from the CNS • Cranial nerves originate in the brain and mostly terminate in organs of the head and upper body • Spinal nerves originate in the spinal cord and extend to parts of the body below the head

  37. Fig. 49-7-2 PNS Afferent (sensory) neurons Efferent neurons Autonomic nervous system Motor system Hearing Sympathetic division Parasympathetic division Enteric division Locomotion Hormone action Gas exchange Circulation Digestion

  38. The PNS has two functional components: the motor system and the autonomic nervous system • The motor system carries signals to skeletal muscles and is voluntary • The autonomic nervous system regulates the internal environment in an involuntary manner

  39. The autonomic nervous system has sympathetic, parasympathetic, and enteric divisions • The sympathetic and parasympathetic divisions have antagonistic effects on target organs

  40. The sympathetic division correlates with the “fight-or-flight” response • The parasympathetic division promotes a return to “rest and digest” • The enteric division controls activity of the digestive tract, pancreas, and gallbladder

  41. Fig. 49-8 Sympathetic division Parasympathetic division Action on target organs: Action on target organs: Dilates pupil of eye Constricts pupil of eye Inhibits salivary gland secretion Stimulates salivary gland secretion Sympathetic ganglia Constricts bronchi in lungs Relaxes bronchi in lungs Cervical Slows heart Accelerates heart Stimulates activity of stomach and intestines Inhibits activity of stomach and intestines Thoracic Stimulates activity of pancreas Inhibits activity of pancreas Stimulates glucose release from liver; inhibits gallbladder Stimulates gallbladder Lumbar Stimulates adrenal medulla Promotes emptying of bladder Inhibits emptying of bladder Sacral Promotes erection of genitals Promotes ejaculation and vaginal contractions Synapse

  42. Table 49-1

  43. Concept 49.2: The vertebrate brain is regionally specialized • All vertebrate brains develop from three embryonic regions: forebrain, midbrain, and hindbrain • By the fifth week of human embryonic development, five brain regions have formed from the three embryonic regions

  44. Fig. 49-9 Cerebrum (includes cerebral cortex, white matter, basal nuclei) Telencephalon Forebrain Diencephalon Diencephalon (thalamus, hypothalamus, epithalamus) Midbrain Mesencephalon Midbrain (part of brainstem) Metencephalon Pons (part of brainstem), cerebellum Hindbrain Myelencephalon Medulla oblongata (part of brainstem) Diencephalon: Cerebrum Mesencephalon Hypothalamus Metencephalon Thalamus Midbrain Pineal gland (part of epithalamus) Myelencephalon Hindbrain Diencephalon Brainstem: Midbrain Pons Spinal cord Pituitary gland Forebrain Medulla oblongata Telencephalon Spinal cord Cerebellum Central canal (c) Adult (a) Embryo at 1 month (b) Embryo at 5 weeks

  45. As a human brain develops further, the most profound change occurs in the forebrain, which gives rise to the cerebrum • The outer portion of the cerebrum called the cerebral cortex surrounds much of the brain

  46. Fig. 49-UN1

  47. The Brainstem • The brainstem coordinates and conducts information between brain centers • The brainstem has three parts: the midbrain, the pons, and the medulla oblongata

  48. The midbrain contains centers for receipt and integration of sensory information • The pons regulates breathing centers in the medulla • The medulla oblongata contains centers that control several functions including breathing, cardiovascular activity, swallowing, vomiting, and digestion

  49. Arousal and Sleep • The brainstem and cerebrum control arousal and sleep • The core of the brainstem has a diffuse network of neurons called the reticular formation • This regulates the amount and type of information that reaches the cerebral cortex and affects alertness • The hormone melatonin is released by the pineal gland and plays a role in bird and mammal sleep cycles

  50. Fig. 49-10 Eye Input from nerves of ears Reticular formation Input from touch, pain, and temperature receptors

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