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Overview of Neurons, Synapses & Nervous System Ch 48, 49 (8th ed.); Ch 48 (7th ed.) Neurons: nerve cells that transfer information within the body Two types of signals: long distance – electrical signals short distance chemical signals Glial cells: support nerve cells Neurons transfer different types of information: control heart rate, coordinate hand-eye movement, record memories, generate dreams Higher order processing is carried out by groups of neurons: ganglia, brain Connection by neurons specify the information transmitted Sensory neurons transmit information from sensors that detect stimuli: external stimuli – light, sound, touch, heat small, taste internal stimuli – blood pressure, carbon dioxide level, muscle tension Integration centers: analyze and interpret the sensation obtained from sensory input Motor neurons: exit the processing centers and trigger muscle or gland activity Information processing Central nervous system (CNS): Brain and longitudinal nerve cord (spinal cord) Peripheral nervous system (PNS): neurons that carry information in an out of the CNS Central nervous system (CNS) Brain Spinal cord Peripheral nervous system (PNS) Cranial nerves Ganglia outside CNS Spinal nerves Dendrites Stimulus Nucleus Cell body Axon hillock Presynaptic cell Axon Neuron structure: Cell body: contains nucleus and organelles Dendrites: branched extensions of the cell body that receive signals Axon: single extension that transmits signals to other cells Axon hillock: cone shaped extension where it joins the cell body Synapse Synaptic terminals Postsynaptic cell Neurotransmitter Dendrites Stimulus Nucleus Cell body Axon hillock Presynaptic cell Axon Synapse Synapse: junction where one neuron transmits information to another neuron or effector cell or organ Synaptic terminal: branch of the axon that forms the specialized connection Neurotransmitters: chemical messengers that send information from the transmitting neuron (presynaptic cell) to the receiving neuron (postsynaptic cell) Synaptic terminals Postsynaptic cell Neurotransmitter Transmission of electrical signal: changing K+, Na+ and Cl- concentrations inside and outside the cell Resting potential: membrane potential of a resting neuron negative inside the membrane positive outside the membrane. OUTSIDE [K+] CELL INSIDE CELL [K+] [Na+] [Cl–] [Na+] [Cl–] Action potential: rapid change in membrane potential of an excitable cell it is triggered by a stimulus, caused by opening and closing of voltage sensitive gates in sodium and potassium ion channels Axon Action potential is generated as Na+ ions flow in at one location: causes depolarization Action potential Na+ Plasma membrane Cytosol Axon Depolarization triggers action potential in the neighboring region and the previous region gets repolarized as K+ flows out Plasma membrane Action potential Cytosol Na+ K+ Action potential Na+ K+ Depolarization and repolarization continues down the axon; propagation of action potential down the length of the axon Axon Plasma membrane Action potential Cytosol Na+ K+ Action potential Na+ K+ K+ Action potential Na+ K+ Myelin sheath: Schwann cells wrap around the axon forming layers of myelin insulates, gaps are known as nodes of Ranvier Node of Ranvier Layers of myelin Axon Schwann cell Axon Nodes of Myelin sheath Ranvier Schwann cell Nucleus of Schwann cell 0.1 µm Chemical synapse: 1. action potential depolarizes the plasma membrane of the synaptic terminal 2. opens voltage-gated calcium channels in the membrane; influx of Ca++ + 5 Synaptic vesicles containing neurotransmitter Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 Synaptic cleft Presynaptic membrane 3 Ligand-gated ion channels 6 K+ Na 3. elevated Ca++ causes synaptic vesicles to fuse with presynaptic membrane 4. vesicles release neurotransmitters into synaptic cleft 5 Synaptic vesicles containing neurotransmitter Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 Synaptic cleft Presynaptic membrane 3 Ligand-gated ion channels 6 K+ Na+ 5. neurotransmitters bind to ion channels and open them 6. neurotransmitters are released and ion channels close 5 Synaptic vesicles containing neurotransmitter Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 Synaptic cleft Presynaptic membrane 3 Ligand-gated ion channels 6 K+ Na+ Neurotransmitters: Acetylcholine biogenic amines (serotonin, dopamine) amino acids gases Nervous system in animals: sessile and slow-moving – simple sense organs with no cephalization active and predatory – sophisticated nervous system with cephalization and corresponding well developed sense organs Cnidarians: diffused nerve net Sea stars: radial nerves connected to nerve ring Radial nerve Nerve ring Nerve net (a) Hydra (cnidarian) (b) Sea star (echinoderm) Bilaterally symmetrical bodies show cephalization: arthropods, squids, salamanders Brain Ventral nerve cord Brain Ganglia Segmental ganglia (e) Insect (arthropod) (g) Squid (mollusc) Brain Spinal cord (dorsal nerve cord) Sensory ganglia (h) Salamander (vertebrate) Spinal cord transmits information to and from the brain and also has nerve circuits that produce reflexes Cell body of sensory neuron in dorsal root ganglion Gray matter White matter Quadriceps muscle Hamstring muscle Spinal cord (cross section) Sensory neuron Motor neuron Interneuron Cerebrospinal fluid circulates through central canal ventricles in the brain cushions the brain Brain and spinal cord had gray and white matter Gray matter: neuron cell bodies, dendrites and unmyelinated axon White matter: myelinated axons Functional hierarchy in PNS PNS Afferent (sensory) neurons Efferent neurons Autonomic nervous system Motor system Locomotion Sympathetic division Parasympathetic division Hormone Gas exchange Circulation action Hearing Enteric division Digestion Cerebrum (includes cerebral cortex, white matter, basal nuclei) Diencephalon (thalamus, hypothalamus, epithalamus) Vertebrate brain Midbrain (part of brainstem) Pons (part of brainstem), cerebellum Medulla oblongata (part of brainstem) Diencephalon: Cerebrum Hypothalamus Thalamus Pineal gland (part of epithalamus) Brainstem: Midbrain Pons Pituitary gland Medulla oblongata Spinal cord Cerebellum Central canal Regionalizations in the vertebrate brain Frontal lobe Parietal lobe Speech Frontal association area Somatosensory association area Taste Reading Speech Hearing Smell Auditory association area Visual association area Vision Temporal lobe Occipital lobe