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Overview of Neurons, Synapses & Nervous System
Ch 48, 49 (8th ed.); Ch 48 (7th ed.)
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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
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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
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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
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Integration centers: analyze and interpret the sensation obtained from sensory input
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Motor neurons: exit the processing centers and trigger muscle or gland activity
Information processing
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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) Peripheral nervous system (PNS) Brain Cranial nerves Spinal cord Ganglia outside CNS Spinal nerves
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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 Dendrites Stimulus Nucleus Presynaptic cell Axon hillock Cell body Axon Synapse Synaptic terminals Postsynaptic cell Neurotransmitter
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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) Dendrites Stimulus Nucleus Presynaptic cell Axon hillock Cell body Axon Synapse Synaptic terminals Postsynaptic cell Neurotransmitter
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Transmission of electrical signal:
changing K+, Na+ and Cl- concentrations inside and outside the cell
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Resting potential: membrane potential of a resting neuron
negative inside the membrane positive outside the membrane. OUTSIDE CELL [Na+] [Cl–] [K+] INSIDE CELL [K+] [Na+] [Cl–]
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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
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Axon Plasma membrane Action potential Action potential is generated as Na+ ions flow in at one location: causes depolarization Na+ Cytosol
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Depolarization triggers action potential in the neighboring region and the previous region gets repolarized as K+ flows out Axon Plasma membrane Action potential Na+ Cytosol Action potential K+ Na+ K+
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Depolarization and repolarization continues down the axon; propagation of action potential down the length of the axon Axon Plasma membrane Action potential Na+ Cytosol Action potential K+ Na+ K+ Action potential K+ Na+ K+
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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 Schwann cell Nodes of Ranvier Nucleus of Schwann cell Axon Myelin sheath 0.1 µm
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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 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 6 Synaptic cleft 3 Ligand-gated ion channels
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4. vesicles release neurotransmitters into synaptic cleft
3. elevated Ca++ causes synaptic vesicles to fuse with presynaptic membrane 4. vesicles release neurotransmitters into synaptic cleft 5 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 6 Synaptic cleft 3 Ligand-gated ion channels
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5. neurotransmitters bind to ion channels and open them
6. neurotransmitters are released and ion channels close 5 Na+ K+ Synaptic vesicles containing neurotransmitter Presynaptic membrane Voltage-gated Ca2+ channel Postsynaptic membrane 1 Ca2+ 4 2 6 Synaptic cleft 3 Ligand-gated ion channels
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Neurotransmitters: Acetylcholine biogenic amines (serotonin, dopamine)
amino acids gases
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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
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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)
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Bilaterally symmetrical bodies show cephalization: arthropods, squids, salamanders
Brain Ventral nerve cord Brain Ganglia Segmental ganglia (e) Insect (arthropod) (g) Squid (mollusc)
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Brain Spinal cord (dorsal nerve cord) Sensory ganglia (h) Salamander (vertebrate)
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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
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Cerebrospinal fluid circulates through
central canal ventricles in the brain cushions the brain
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Brain and spinal cord had gray and white matter
Gray matter: neuron cell bodies, dendrites and unmyelinated axon White matter: myelinated axons
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Functional hierarchy in PNS
Efferent neurons Afferent (sensory) neurons Motor system Autonomic nervous system Hearing Sympathetic division Parasympathetic division Enteric division Locomotion Hormone action Gas exchange Circulation Digestion
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Vertebrate brain 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) Cerebrum Diencephalon: Hypothalamus Thalamus Pineal gland (part of epithalamus) Brainstem: Midbrain Pons Pituitary gland Medulla oblongata Spinal cord Cerebellum Central canal
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Regionalizations in the vertebrate brain
Frontal lobe Parietal lobe Motor cortex Somatosensory cortex Somatosensory association area Speech Frontal association area Taste Reading Speech Hearing Visual association area Smell Auditory association area Vision Temporal lobe Occipital lobe
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