Organization and Division of the Nervous System & Cranial Nerves: Sensory, Motor, Mixed KayOnda Bayo

Figure 11.1 The nervous system’s functions. Sensory input Integration Motor output

Divisions of the Nervous System Central nervous system (CNS) Peripheral nervous system (PNS)

Peripheral Nervous System (PNS) Two functional divisions Sensory (afferent) division Motor (efferent) division Two divisions Somatic nervous system Autonomic nervous system

Motor Division of PNS: Somatic Nervous System Somatic motor nerve fibers Conducts impulses from CNS to skeletal muscle Voluntary nervous system

Motor Division of PNS: Autonomic Nervous System Visceral motor nerve fibers Regulates smooth muscle, cardiac muscle, and glands Involuntary nervous system Two functional subdivisions Sympathetic Parasympathetic

Figure 11.2 Levels of organization in the nervous system. Central nervous system (CNS) Brain and spinal cord Integrative and control centers Peripheral nervous system (PNS) Cranial nerves and spinal nerves Communication lines between the CNS and the rest of the body 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) Somatic sensory fiber Skin Somatic nervous system Somatic motor (voluntary) Conducts impulses from the CNS to skeletal muscles Autonomic nervous system (ANS) Visceral motor (involuntary) Conducts impulses from the CNS to cardiac muscles, smooth muscles, and glands Visceral sensory fiber Motor fiber of somatic nervous system Stomach Skeletal muscle Sympathetic division Mobilizes body systems during activity Parasympathetic division Conserves energy Promotes house- keeping functions during rest Sympathetic motor fiber of ANS Heart Parasympathetic motor fiber of ANS Bladder Structure Function Sensory (afferent) division of PNS Motor (efferent) division of PNS

Histology of Nervous Tissue Two principal cell types Neuroglia Neurons (nerve cells)

Neuron Cell Body (Perikaryon or Soma) Biosynthetic center of neuron Synthesizes proteins, membranes, and other chemicals Rough ER (chromatophilic substance or Nissl bodies) Most active and best developed in body Spherical nucleus with nucleolus Some contain pigments In most, plasma membrane part of receptive region Most neuron cell bodies in CNS Nuclei – clusters of neuron cell bodies in CNS Ganglia – lie along nerves in PNS

Figure 11.4a Structure of a motor neuron. Dendrites (receptive regions) Cell body (biosynthetic center and receptive region) Nucleus Nucleolus Axon hillock Chromatophilic substance (rough endoplasmic reticulum) Axon (impulse- generating and -conducting region) Impulse direction Schwann cell Myelin sheath gap (node of Ranvier) Terminal branches Axon terminals (secretory region)

Neuron cell body Dendritic spine Figure 11.4b Structure of a motor neuron.

Myelin Sheath Composed of myelin Segmented sheath around most long or large- diameter axons Function of myelin Nonmyelinated fibers conduct impulses more slowly

Table 11.1 Comparison of Structural Classes of Neurons (1 of 3)

Table 11.1 Comparison of Structural Classes of Neurons (2 of 3)

Functional Classification of Neurons Three types Sensory (afferent) Motor (efferent) Interneurons

Table 11.1 Comparison of Structural Classes of Neurons (3 of 3)

Role of Membrane Ion Channels Large proteins serve as selective membrane ion channels Two main types of ion channels Leakage (nongated) channels Gated

Role of Membrane Ion Channels: Gated Channels Three types Chemically gated (ligand-gated) channels Voltage-gated channels Mechanically gated channels

Figure 11.6 Operation of gated channels. Chemically gated ion channelsVoltage-gated ion channels Open in response to binding of the appropriate neurotransmitter Open in response to changes in membrane potential Receptor Closed Neurotransmitter chemical attached to receptor Open ClosedOpen Chemical binds Membrane voltage changes

Resting Membrane Potential: Differences in Ionic Composition ECF has higher concentration of Na + than ICF ICF has higher concentration of K + than ECF K + plays most important role in membrane potential

Action Potentials (AP) Principle way neurons send signals Principal means of long-distance neural communication Occur only in muscle cells and axons of neurons Brief reversal of membrane potential with a change in voltage of ~100 mV

Figure The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents. The big picture Resting state 1 2 Depolarization Membrane potential (mV) –55 –70 Action potential Threshold Time (ms) Repolarization Hyperpolarization 3 4 The AP is caused by permeability changes in the plasma membrane: Membrane potential (mV) –70 – Time (ms) Action potential Na + permeability K + permeability Relative membrane permeability Outside cell Inside cell Activation gate Inactivation gate ClosedOpenedInactivated The events The key players Voltage-gated Na + channels Closed Opened Outside cell Inside cell Voltage-gated K + channels Sodium channel Potassium channel Activation gates Inactivation gate Resting state Depolarization Repolarization Hyperpolarization

Figure The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents. (1 of 3) Resting state. No ions move through voltage-gated channels. Depolarization is caused by Na + flowing into the cell. Repolarization is caused by K + flowing out of the cell. Hyperpolarization is caused by K + continuing to leave the cell. Action potential Threshold Time (ms) Membrane potential (mV) – –

Action potential Threshold Time (ms) Membrane potential (mV) – – Resting state. No ions move through voltage-gated channels. 1 Figure The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents. (1 of 3)

Depolarization is caused by Na + flowing into the cell. Action potential Threshold Time (ms) – – Resting state. No ions move through voltage-gated channels. 1 Membrane potential (mV)

Figure The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents. (1 of 3) Depolarization is caused by Na + flowing into the cell. Repolarization is caused by K + flowing out of the cell. Action potential Threshold Time (ms) – – Resting state. No ions move through voltage-gated channels. 1 Membrane potential (mV)

Figure The action potential (AP) is a brief change in membrane potential in a “patch” of membrane that is depolarized by local currents. (1 of 3) Resting state. No ions move through voltage-gated channels. Depolarization is caused by Na + flowing into the cell. Repolarization is caused by K + flowing out of the cell. Hyperpolarization is caused by K + continuing to leave the cell. Action potential Threshold Time (ms) – – Membrane potential (mV)

Importance of Myelin Sheaths: Multiple Sclerosis (MS) Autoimmune disease affecting primarily young adults Myelin sheaths in CNS destroyed Treatment Drugs that modify immune system's activity improve lives Prevention? High blood levels of Vitamin D reduce risk of development

The Synapse Nervous system works because information flows from neuron to neuron Neurons functionally connected by synapses

Important Terminology Presynaptic neuron Postsynaptic neuron

Information Transfer Across Chemical Synapses AP arrives at axon terminal of presynaptic neuron Causes voltage-gated Ca 2+ channels to open Ca 2+ floods into cell Synaptotagmin protein binds Ca 2+ and promotes fusion of synaptic vesicles with axon membrane Exocytosis of neurotransmitter into synaptic cleft occurs Higher impulse frequency  more released

Information Transfer Across Chemical Synapses Neurotransmitter diffuses across synapse Binds to receptors on postsynaptic neuron Often chemically gated ion channels Ion channels are opened Causes an excitatory or inhibitory event (graded potential) Neurotransmitter effects terminated

Termination of Neurotransmitter Effects Within a few milliseconds neurotransmitter effect terminated in one of three ways Reuptake By astrocytes or axon terminal Degradation By enzymes Diffusion Away from synaptic cleft

Neurotransmitters 50 or more neurotransmitters have been identified Most neurons make two or more neurotransmitters Usually released at different stimulation frequencies