Chapter 12 Nervous Tissue Copyright 2009 John Wiley & Sons, Inc. 1
Terms to Know CNS PNS Afferent division Efferent division Somatic nervous system Autonomic nervous system Sympathetic nervous system Parasympathetic nervous system Axons Dendrites Copyright 2009 John Wiley & Sons, Inc. 2
Overview of Major Structures Twelve pairs of cranial nerves. Thirty-one pairs of spinal nerves. Ganglia, small masses of nerve tissue, located outside the brain and spinal cord. Sensory receptors monitor changes in the internal or external environment. Copyright 2009 John Wiley & Sons, Inc. 3
Functions of Nervous System Sensory function: to sense changes in the internal and external environment through sensory receptors. Sensory (afferent) neurons serve this function. Copyright 2009 John Wiley & Sons, Inc. 4
Functions of Nervous System Integrative function: to analyze the sensory information, store some aspects, and make decisions regarding appropriate behaviors. Association or interneurons serve this function. Copyright 2009 John Wiley & Sons, Inc. 5
Functions of Nervous System Motor function is to respond to stimuli by initiating action. Motor(efferent) neurons serve this function. Copyright 2009 John Wiley & Sons, Inc. 6
Nervous System Divisions Central nervous system (CNS) consists of the brain and spinal cord Peripheral nervous system (PNS) connects CNS to muscles, glands & all sensory receptors Copyright 2009 John Wiley & Sons, Inc. 7
Subdivisions of the PNS Somatic (voluntary) nervous system (SNS) neurons from sensory receptors to the CNS motor neurons to skeletal muscle tissue Copyright 2009 John Wiley & Sons, Inc. 9
Subdivisions of the PNS Autonomic (involuntary) nervous systems motor neurons to smooth & cardiac muscle and glands sympathetic division (speeds up heart rate) parasympathetic division (slow down heart rate) Enteric nervous system (ENS) involuntary sensory & motor neurons control GI tract Copyright 2009 John Wiley & Sons, Inc. 10
Subdivisions of the PNS Speed: speed of neuron signal is affected by axon diameter, temperature, and myelin. Neurotransmitters: examples include; acetylcholine, glutamate, aspartate, gamma aminobutyric acid, glycine, norepinephrine, epinephrine, and dopamine. Copyright 2009 John Wiley & Sons, Inc. 11
Structural Classification of Neurons Multipolar Bipolar Unipolar Copyright 2009 John Wiley & Sons, Inc. 13
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Neuroglia of the CNS Most common glial cell type Each forms myelin sheath around more than one axons in CNS Analogous to Schwann cells of PNS Copyright 2009 John Wiley & Sons, Inc. 15
Neuroglia of the CNS Cells encircling PNS axons Each cell produces part of the myelin sheath surrounding an axon in the PNS Copyright 2009 John Wiley & Sons, Inc. 16
Neuroglia of the CNS Schwann cells myelinate (wrap around) axons in the PNS during fetal development Copyright 2009 John Wiley & Sons, Inc. 17
Myelinated and unmyelinated axons Copyright 2009 John Wiley & Sons, Inc. 18
Organization of the Nervous System Copyright 2009 John Wiley & Sons, Inc. 19
Electrical Signals in Neurons Communicate with 2 types of electric signals action potentials that can travel long distances graded potentials that are local membrane changes only In living cells, a flow of ions occurs through ion channels in the cell membrane Copyright 2009 John Wiley & Sons, Inc. 20
Electrical Signals in Neurons Communicate with 2 types of electric signals action potentials that can travel long distances graded potentials that are local membrane changes only In living cells, a flow of ions occurs through ion channels in the cell membrane Copyright 2009 John Wiley & Sons, Inc. 21
Types of Ion Channels Leakage (nongated) channels are always open nerve cells have more K+ than Na+ leakage channels Ligand-gated channels open and close in response to a stimulus results in neuron excitability Voltage-gated channels respond to a direct change in the membrane potential. Mechanically gated ion channels respond to mechanical vibration or pressure. Copyright 2009 John Wiley & Sons, Inc. 22
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Generation of Action Potentials An action potential (AP) or impulse is a sequence of rapidly occurring events that decrease and eventually reverse the membrane potential (depolarization) and then restore it to the resting state (repolarization). During an action potential, voltage-gated Na + and K + channels open in sequence According to the all-or-none principle, if a stimulus reaches threshold, the action potential is always the same. A stronger stimulus will not cause a larger impulse. Copyright 2009 John Wiley & Sons, Inc. 25
Action Potentials Copyright 2009 John Wiley & Sons, Inc. 26
Signal Transmission at the Synapse 2 Types of synapses electrical ionic current spreads to next cell through gap junctions chemical one-way information transfer from a presynaptic neuron to a postsynaptic neuron axodendritic -- from axon to dendrite axosomatic -- from axon to cell body axoaxonic -- from axon to axon Copyright 2009 John Wiley & Sons, Inc. 27
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Chemical Synapses Action potential reaches end and voltage-gated Ca+ 2 channels open Ca+2 flows inward triggering release of neurotransmitter Neurotransmitter crosses synaptic cleft Copyright 2009 John Wiley & Sons, Inc. 29
Chemical Synapses Diffusion move down concentration gradient Enzymatic degradation acetylcholinesterase Uptake by neurons or glia cells neurotransmitter transporters Prozac = serotonin reuptake inhibitor Copyright 2009 John Wiley & Sons, Inc. 30
Neurotransmitter Effects Agonist - anything that enhances a transmitters effects Antagonist - anything that blocks the action of a neurotranmitter 31