Chapter 12
An Overview of the Nervous System- Complex organ system that contains neurons,
neuroglia. Divided into two divisions, CNS and PNS
▪CNS
▪PNS
▫Afferent Division
▫Efferent Division
-Somatic Nervous System (SNS)
-Autonomic Nervous System (ANS)
Neurons
▪The Structure of Neurons- Use figures 12-2 and 12-3 to locate the following cell
structures
▫Cell Body (soma)
▫Perikaryon
▫Neurofiliments and neurotubules and neurofibrils
▫Nissl bodies
▫Dendrites
▫Axon
-axoplasm and axolemma
-axon hillock
-initial segment
-collaterals
-telodendria
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▫Axon terminals
▫Synapse
-neurotranmitters
-neuromuscular and neuroglandular junctions
-presynaptic and postsynaptic membranes
-axoplasmic transport
▪Classification of Neurons
▫Structural Classification
-Anaxonic
-Bipolar
-Unipolar
-Multipolar
▫Functional Classification of Neurons
-Sensory (Afferent)- deliver information from the sensory receptors
to the CNS.
-Cell bodies are located in peripheral sensory ganglia
-Somatic sensory neurons- monitor outside world
-Visceral sensory neurons- monitor internal conditions
- Interoceptors
-Exteroceptors
-Proprioceptors
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-Motor (Efferent)- carry instructions from the CNS to peripheral
effectors.
-Somatic motor neurons
-Visceral motor neurons
-Interneurons (association)
Neuroglia- supporting cells of the nervous system
▪Neuroglia of the Central Nervous System- CNS has 4 types of neuroglia
▫Ependymal cells
▫Astrocytes
1)
2)
3)
4)
5)
▫Oligodentrocytes
▫Microglia
▪Neuroglia of the Peripheral Nervous System
▫Cell bodies of neurons in the PNS are clustered in masses called ganglion
▫Satellite Cells
▫Schwann Cells
*use figure 12-7 to help distinguish between myelinated and
nonmyelinated axons*
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Neural Responses to Injuries
▪Neurons can respond to injury in a very limited, stereotyped way
▪Limited regeneration can occur in the CNS, but the situation is complicated.
Neurophysiology: Ions and Electrical Signals
▪There are five important membrane processes that will be discussed:
1) All living cells have a transmembrane potential that varies depending
on the activities of the cell.
2) A typical stimulus can produce a local graded potential, which
decreases with distance from the stimulus.
3) A large graded potential can produce an action potential in the axon
membrane. This does not diminish with distance from the stimulus.
4) Synaptic activity produces a graded potential on the postsynaptic cell
that is usually caused by neurotransmitters.
5) Response of postsynaptic cell can vary depending on the response of
the receptor that was stimulated
*see fig. 12-7 for an overview of these important processes*
▪Transmembrane Potential- three important concepts regarding the
transmembrane potential:
1)
2)
3)
▫Passive forces include:
-Chemical gradients
-Electrical gradients
-Electrochemical gradient
▫Active forces include the sodium/potassium exchange pump
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▪Changes in the transmembrane potential
▫Resting potential is the transmembrane potential of an “undisturbed” cell
▫Membrane channels control the movement of ions across the cell
membrane, we will focus on sodium and potassium channels.
-They can either be passive (leak) or active (gated), there are three
types of active (gated) channels
1)
2)
3)
▪Graded Potentials- also called local potentials
▫Changes in the transmembrane potential that cannot spread far from the
area surrounding the site of stimulation.
▫Can be depolarizing or hyperpolarizing, depending on stimulus
*Use figure 12-12 to visualize the process of a graded potential*
▪Action Potentials- propagated changes in the transmembrane potential that
affect an entire excitable membrane.
▫All-or-none principle
▫Generation of Action potentials- *use figure 12-14 to visualize the
process of an action potential*
▫The refractory period- the time from the initiation of an action potential
until the normal resting potential has returned.
-Absolute refractory period
-Relative refractory period
▫The role of the sodium-potassium exchange pump is to return cell to
pre-stimulation conditions of these ions. (Na out, K in)
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▫Propagation of Action Potentials- action potentials moving down axon.
-Continuous propagation-Saltatory propagation▫Axon diameter affects propagation speed.
-Type A fibers
-Type B fibers
-Type C fibers
Synaptic Activity
▪Electric events of messages moving from one place to another are called nerve
impulses.
▪General Properties of Synapses
▫Electrical synapse
-Extremely rare
-Presynaptic and postsynaptic cell are locked together at gap
junctions
-Action potentials are propagated quickly
▫Chemical synapse
-Most neural synapses, all between neurons and other cells
-Neurotransmitters released into synapse, picked up by receptors
on postsynaptic cell
-Can be excitatory or inhibitory neurotransmitters
▪Cholinergic synapses- release of ACh at synapse.
▫Each vesicle in synaptic knob contains thousands of molecules of ACh,
can be a million vesicles in one synaptic knob
▫Events at a cholinergic synapse; assume synapse is adjacent to initial
segment of axon.
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▫ACh is broken down into acetate and choline, reabsorbed and used again
by local cells.
*see figure 12-16 to see the events at a cholinergic synapse*
▫Synaptic Delay▫Synaptic Fatigue-
Information Processing
▪Postsynaptic potentials- graded potentials that develop in the postsynaptic
membrane in response to a neurotransmitter, two types.
▫Excitatory postsynaptic potential (EPSP)-
▫Inhibitory postsynaptic potential (IPSP)-
▫Summation- process of combining postsynaptic potentials that affect one
portion of the cell membrane. Can be EPSP’s, IPSP’s, or both. Two types:
-Temporal summation-Spatial summation▫Facilitation▪Presynaptic Inhibition and Facilitation
*use figure 12-20 to visualize examples of presynaptic inhibition and
facilitation.*
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