Nervous System Basics and Nervous System Tissues Chapter 11: Nervous System Basics and Nervous System Tissues

Santiago Ramon Y. Cajal (1852-1934) Founding Scientist in the Modern Approach to Neuroscience. Received Nobel Prize in 1906

Figure 11.1: The nervous system’s functions, p. 388. Sensory input Integration Motor output

Figure 11.2: Levels of organization in the nervous system, p. 389. Key: Brain Central nervous system (CNS) Brain and spinal cord Integrative and control centers = Sensory (afferent) division of PNS = Motor (efferent) Key: = Structure = Function Visceral sensory fiber Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body Central nervous system (CNS) Parasympathetic motor fiber of ANS Sympathetic motor fiber of ANS Visceral organ Spinal cord Skin Somatic sensory fiber Sensory (afferent) division Somatic and visceral sensory nerve fibers Conducts impulses from receptors to the CNS Motor (efferent) division Motor nerve fibers Conducts impulses from the CNS to effectors (muscles and glands) Motor fiber of somatic nervous system Skeletal muscle Sympathetic division Mobilizes body systems during activity Autonomic nervous system (ANS) Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Somatic nervous System Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles Peripheral nervous system (PNS) Parasympathetic division Conserves energy Promotes housekeeping functions during rest (b) (a)

Brain or spinal cord tissue Figure 11.3: Neuroglia, p. 390. Capillary Neuron (b) Microglial cell (a) Astrocyte Nerve fibers Myelin sheath Fluid-filled cavity Process of oligodendrocyte (c) Ependymal cells Brain or spinal cord tissue Cell body of neuron (d) Oligodendrocyte Schwann cells (forming myelin sheath) Satellite cells Nerve fiber (e) Sensory neuron with Schwann cells and satellite cells

Figure 11.4: Structure of a motor neuron, p. 392. Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Neuron cell body Nucleus Dendritic spine (a) Axon (impulse generating and conducting region) Nucleolus Impulse direction Nissl bodies Node of Ranvier Axon terminals (secretory component) Axon hillock Schwann cell (one inter- node) Neurilemma (sheath of Schwann) Terminal branches (telodendria) (b)

Schwann cell cytoplasm Schwann cell plasma membrane Axon Myelin sheath Figure 11.5: Relationship of Schwann cells to axons in the PNS, p. 394. Schwann cell cytoplasm Schwann cell plasma membrane Axon Myelin sheath Schwann cell nucleus (a) Schwann cell cytoplasm Axon Neurilemma (b) (d) Neurilemma Myelin sheath (c)

Figure 11.6: Operation of gated channels, p. 398. Neurotransmitter chemical attached to receptor Receptor Na+ Na+ Chemical binds K+ K+ Closed Open (a) Chemically gated ion channel Na+ Na+ Membrane voltage changes Closed Open (b) Voltage-gated ion channel

Figure 11.7: Measuring membrane potential in neurons, p. 399. Voltmeter Plasma membrane Ground electrode outside cell Microelectrode inside cell Axon Neuron

Figure 11.8: The basis of the resting membrane potential, p. 399. Cell exterior Na+ 15 mM Na+ Cell interior Na+ K+ 150 mM Na+ ion Diffusion us Na+–K+ pump Diff -70 mV Cl– 10 mM Na+ Na+ 150 mM A– 100 mM Na+ K+ Na+ A– 0.2 mM Na+ Plasma membrane K+ 5 mM K+ Cl– 120 mM K+ Cell interior Cell exterior K+ K+

Depolarizing stimulus Hyperpolarizing stimulus Figure 11.9: Depolarization and hyperpolarization of the membrane, p. 400. Depolarizing stimulus Hyperpolarizing stimulus +50 +50 Inside positive Inside negative Membrane potential (voltage, mV) Depolarization Membrane potential (voltage, mV) –50 –50 Resting potential –70 –70 Resting potential Hyper- polarization –100 –100 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Time (ms) Time (ms) (a) (b)

Figure 11.10: The mechanism of a graded potential, p. 401. Depolarized region Stimulus Plasma membrane (a) Depolarization (b) Spread of depolarization

Membrane potential (mV) Figure 11.11: Changes in membrane potential produced by a depolarizing graded potential, p. 402. Active area (site of initial depolarization) Membrane potential (mV) – 70 Resting potential Distance (a few mm)

Figure 11.12: Phases of the action potential and the role of voltage-gated ion channels, p. 403. Outside cell Na+ Outside cell Na+ Inside cell Inside cell K+ K+ Depolarizing phase: Na+ channels open Repolarizing phase: Na+ channels inactivating, K+ channels open 2 Action potential +30 3 Relative membrane permeability 2 Membrane potential (mV) PNa PK Threshold –55 1 1 4 –70 1 2 3 4 Time (ms) Outside cell Sodium channel Potassium channel Outside cell Na+ Na+ Inside cell Activation gates K+ Inside cell K+ Inactivation gate 4 Hyperpolarization: K+ channels remain open; Na+ channels resetting 1 Resting state: All gated Na+ and K+ channels closed (Na+ activation gates closed; inactivation gates open)

Figure 11.13: Propagation of an action potential (AP), p. 405. Voltage at 2 ms +30 Membrane potential (mV)) Voltage at 0 ms Voltage at 4 ms –70 (a) Time = 0 ms (b) Time = 2 ms (c) Time = 4 ms Resting potential Peak of action potential Hyperpolarization

Membrane potential (mV) Figure 11.14: Relationship between stimulus strength and action potential frequency, p. 406. Action potentials +30 Membrane potential (mV) – 70 Stimulus amplitude Threshold Voltage Time (ms)

Figure 11.15: Refractory periods in an AP, p. 406. Absolute refractory period Relative refractory period Depolarization (Na+ enters) +30 Repolarization (K+ leaves) Membrane potential (mV) After-hyperpolarization –70 Stimulus 1 2 3 4 5 Time (ms)

Figure 11.16: Saltatory conduction in a myelinated axon, p. 407. Node of Ranvier Cell body Myelin sheath Distal axon

Cell body Dendrites Axodendritic Axosomatic synapses synapses Figure 11.17: Synapses, p. 409. Cell body Dendrites Axodendritic synapses Axosomatic synapses Axoaxonic synapses Axon (a) Axon Axosomatic synapses Soma of postsynaptic neuron (b)

Figure 11.18: Events at a chemical synapse in response to depolarization, p. 410. Neurotransmitter Ca2+ Na+ Axon terminal of presynaptic neuron Action Potential Receptor 1 Postsynaptic membrane Mitochondrion Postsynaptic membrane Axon of presynaptic neuron Ion channel open Synaptic vesicles containing neurotransmitter molecules 5 Degraded neurotransmitter Na+ 2 Synaptic cleft 3 4 Ion channel closed Ion channel (closed) Ion channel (open)

Figure 11.19: Postsynaptic potentials, p. 412. +30 +30 Membrane potential (mV) Threshold Membrane potential (mV) Threshold –55 –55 –70 –70 10 20 Time (ms) 10 20 Time (ms) (a) Excitatory postsynaptic potential (EPSP) (b) Inhibitory postsynaptic potential (IPSP)

Figure 11.24: Types of circuits in neuronal pools, p. 422. Input Input Input Input 1 Input 2 Input 3 Output Output Output Output (a) Divergence in same pathway (b) Divergence to multiple pathways (c) Convergence, multiple sources (d) Convergence, single source Input Input Output Output (e) Reverberating circuit (f) Parallel after-discharge circuit

Figure 11.25: A simple reflex arc, p. 423. Integration center Sensory neuron Stimulus Receptor Interneuron Response Effector Motor neuron Spinal cord (CNS)