Bio 103
Lake Tahoe Community College
Winter Quarter
Instructor: Sue Kloss
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Chapter 48 - Nervous System
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Intro: Our gelatinous spinal cords are protected inside our bony vertebrae. The spinal cord acts as central communication
conduit between the brain and the rest of the body. Millions of motor nerve fibers carry information from the brain to the
muscles other fibers bring information from our senses (touch, vision, hearing taste etc.) back to our brains.
The human brain has 100 billion neurons (nerve cells); each may communicate with 1000s of others. MRIs are used to
detect which parts of the brain are responsible for various tasks (fig. 48.1).
I. Nervous system structure and function
A. n.s. (nervous system) receives sensory input, interprets it, and sends out response
1. Most ns have 2 main divisions
a. Central Nervous System (cns) - most integration occurs here. Consists of brain and spinal
cord. (48.2 c and d)
b. Peripheral Nervous System (pns) - made up of mostly communication lines called nerves
that carry signals in and out of cns (Fig. 48.2 e - h) - bundles of extensions of neurons
c. nerve nets - neurons arranged in this way in absence of CNS
2. a nerve is a cablelike bundle of neuron extensions tightly wrapped in connective tissue. A neuron
consists of a cell body and long, thin extensions called neuron fibers that convey signals
3. ganglia - clusters of neuron cell bodies in the nerves of the pns.
4. knee jerk reflex - (Fig. 48.4) colored balls are neuron cell bodies; lines are neuron fibers
5. 3 functional types of neurons
a. sensory neurons convey signals or info (blue arrows) to cns
b. interneurons (green) are entirely within the cns. They integrate data and relay signals
to other interneurons or motor neurons
c. motor neurons function in motor output, convey signals from cns to effectors (red)
6. When knee is tapped, 1. sense receptor detects a stretch in muscle; 2. sensory neuron conveys
info to cns (spinal cord). In cns, the information goes to 3. interneuron and 4. motor neuron.
7. Quad muscle responds by contracting. At same time, another motor neuron responds to signals
from interneuron and inhibits hamstrings to make them relax
B. Neurons are the functional units of nervous systems
1. Neurons vary widely in shape but share some common characteristics
2. Motor neuron, from spinal cord to skeletal muscles (Fig. 48.5)
3. cell body houses nucleus and most cytoplasmic organelles.
4. 2 types of fibers project from cell body
a. axons (Greek for axle) - on many neurons is a single fiber; where it joins cell body = axon hillock
1. conduct signals toward another neuron or toward an effector
a. many axons are long- may stretch from end of spinal cord to toes
b. giant fibers of a squid are axons
2. axons end in synaptic terminal, with another cell in an area called synapse
a. chemical messengers carrying synapse are neurotransmitters
b. dendrites (Greek for tree) - are often short, numerous, and highly branched
1. convey signals from their tips in, toward the rest of the neuron
2. signals come from sensory cell or interneuron
5. Lots of supporting cells in nervous system - called Glia, collectively. in mammal, 50x number of
neurons
a. astrocytes (Fig. 48.7) regulate extraccellular ions and neurotransmitters
1. may be involved in learning and memory by activating synapse to other neurons
2. also cause nearby blood vessels to dilate, increasing blood flow delivering O2, nutrients
3. astrocytes induce formation of blood/brain barrier - restricts passage of most
substances to brain, to t ightly control environment
b. other supporting cells are oligodendrites in cns and Schwann cells in pns (Fig. 48.8)
c. axons that convey signals very rapidly are enclosed along most of their length by a thick
insulating material
1. in vertebrates, = myelin sheath - resembles chain of long beads
2. each bead is a Schwann cell each wrapped many times around axon
3. spaces btn Schwann cells are called nodes of Ranvier- only points where signal is
transmitted on the axon (Fig. 48.15)
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a. voltage gated Na+ and K+ channels are concentrated at gaps in myelin sheath
b. extracellular fluid is in contact with axon membrane only at nodes
c. action potentials not generated in areas btn nodes
d. action potential appears to jump from node to node- saltatory conduction
4. rest of axon is insulated by myelin preventing signals from passing along it.
5. When a signal travels along a myelinated axon, it jumps from node to node
6. By jumping along the axon, signal travels much further than it would if it could
move along it smoothly - myelin sheath is largely lipid, poor conductor of electricity
7. in humans, sheath allows signals to move at 120 meters per second (300 mph.)
Unmyelinated fibers can move signals at 5 meters per second.
8. multiple sclerosis leads to gradual destruction of myelin sheath by own immune
system
a. progressive loss of signal conduction, muscle control and brain function
b. MS is not yet curable but drugs suppress immune system can slow
progress of the disease
9. Axon ends in a cluster of branches with knobs at the end each
a. synaptic knobs relay signals to another neuron or to an effector such as a
muscle cell (Fig. 48.16)
II. Nerve signals and their transmission
A. Neurons maintain resting potential across membranes via ion pumps and channels
1. Resting neurons possess potential energy
2. potential energy is used to send signals across body, from one neuron to another
3. potential energy resides in electrical charge difference across neuron’s plasma membrane
4. cytoplasm inside membrane is negative, fluid outside membrane is positive
5. opposite charges tend to move toward each other, and membranes hold charges apart = potential
energy
6. strength of stored energy can be measured with a voltmeter. The voltage across the membrane is
called the resting potential
7. what causes charge difference? Membrane keeps dissolved proteins and other large organic
molecules inside the cell; most are negatively charged
8. membrane has channels and pumps to regulate large inorganic ions
a. resting membrane allows much more K+ than Na+ across
b. K+ can move out much more easily than Na+, so as more K moves out, inside becomes
more negatively charged
c. large organic negatively charged molecules and K+ movement out create most resting
potential
9. There are also membrane proteins called sodium-potassium pumps that actively transport Na+ out
of cell and K in, to help keep Na in cell low and K high. They move more Na+ out than K + in.
B. Nerve signal begins as change in membrane potential (fig. 48.13)
1. stimulating a neuron’s plasma membrane can trigger the release of potential energy
2. It uses the membrane’s potential energy to generate a nerve signal.
3. Stimulus - any factor that causes a nerve signal to be generated
4. stimuli can be - flashbulb, tap on the knee, electrical shock or temperature change
5. studying giant axons in squids helped Hodgkin and Huxley in 1940s figure out details of nerve
signal transmission
6. Action potential is the technical name for a nerve signal (aka nerve impulse)
a. all changes indicated by the graph occur at place where stimulus is applied.
b. graph records all changes in that place over time
1. resting potential (-70 mv) Na and K channels closed
2. stimulus is applied at time 0, and in 2-3 milliseconds, voltage rises to threshold
potential (-50 mv). Some Na channels open. Difference btn threshold potential and
resting potential is minimum change in voltage that must occur to generate action
potential (nerve signal). If threshold potential is reached, action potential is triggered.
3. Na+ channels open, K channels closed; interior of cell becomes more positive
4. voltage drops back down as K channels open and K+ rushes out; interior of cell more
negative than outside
5. K channels close slowly, so level overshoots resting potential and finally returns to it to
resting state
7. Ion movements coincide with charge changes
a. movements take place as Na and K channels (see numbered diagrams surrounding action
potential graph p. 1019
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C. Action potential regenerates itself along the neuron
1. action potential is a localized electrical event - change from resting potential at a specific point
2. to function as a signal, action potential (local event) must travel along neuron (Fig. 48.14)
3. nerve signal starts out as one action potential generated on the axon near the cell body of neuron
4. like dominoes - first domino does not travel, but the fall is relayed along the row, one at a time
5. As nerve signal passes from one area to another, resting potential immediately reestablished at
earlier points
6. local spreading of electrical charges (blue arrows) trigger opening of Na+ channels,
7. signal can’t travel backwards bc K channels are activated so Na pumps can’t work
8. action potentials are all or nothing events - they are the same no matter how weak or strong the
stimulus is that activated them.
9. If you rap your finger hard, a lot of signals get transmitted, if soft, a few signals. If hard, your cns
receives many more action potentials per millisecond than after a soft tap. The frequency of action
potentials changes with the intensity of the stimulus
III. Neurons communicate with other cells at synapses
A. When an action potential reaches the terminals of an axon, it generally stops there.
1. synapse- junction or relay point between 2 neurons or btn a neuron and an effector cell
2. When action potentials arrive at the end of one neuron’s axon, the info passes to a receiving cell
across the synapse
3. synapses are either electrical or chemical.
4. In electrical synapses, action potentials pass from one neuron to the next.
5. receiving neuron is stimulated with same frequency (intensity) as sending neuron
6. electrical synapses are lightning quick; lobsters and crayfish have electrical synapses in tail
7. humans have electrical synapses to heart and lungs (automatic processes), chemical synapses to
organs and muscles for varied and complex signaling information
8. chemical synapses have narrow gap called a synaptic cleft, separating a synaptic knob of a
sending neuron from a receiving neuron
9. cleft prevents direct transmission of the action potential. Instead, the electrical action potential is
converted to a chemical signal - consists of molecules of neurotransmitter that transmits signal
10. synapse - (Fig. 48.17) generally instituted at axon hillock
a. 1. action potential (electrical charge) arrives in synaptic knob (red arrow).
b. 2. action potential triggers chemical changes that make neurotransmitter vesicles fuse with
plasma membrane of sending cell
c. 3. fused vesicles release their neurotransmitter molecules (green dots) into synaptic cleft
d. 4. neurotransmitter molecules diffuse across cleft and bind to receptor molecules on
receiving cell’s plasma membrane
e. 5. binding of neurotransmitters to receptor opens ion channels in receiving cell’s membrane
f. ions can diffuse into receiving cell and trigger new action potentials
g. 6. neurotransmitter is broken down by an enzyme and channels close, ensuring brief and
precise response to neurotransmitter
h. only sending neurons have neurotransmitters to release at a synapse, only receiving neuron
has receptor molecules ensuring that signal only travels one way.
11. a variety of factors can influence the amt of neurotransmitter that is released or the responsiveness of
the postsynaptic cell
a. modifications underlie animals ability to alter behavior in response to change and form basis for
learning and memory
B. Direct synaptic transmission
1. some synapses cause ligand gated ion channels to allow passage of K +and Na+, bringing membrane
close to action potential. EPSPs - excitatory postsynaptic potentials.
2. some synapses cause only certain ligand gate channels to allow passage of ions, eg. K+ only, making
the membrane of the neuron less likely to depolarize - IPSPs- inhibitory postsynaptic potentials
3. various mechanisms terminate the effect of neurotransmitters on postsynaptic cells
a. neurotransmitter diffuses out of synaptic cleft
b. neurotransmitter taken up by presynaptic neuron through active transport and repacked into
synaptic vesicles
c. enzymes break down neurotransmitters
d. Glia take up neurotransmitters and metabolize them as fuel
4. postsynaptic potentials are graded in their magnitude
a. how much neurotransmitter released
b. become smaller with distance from synapse
5. if 2 EPSPs occur in rapid succession at single synapse, second EPSP may begin before the
postsynaptic neurons membrane potential has returned to resting potential - temporal summation
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(Fig. 48.18)
6. if 2 EPSPs occur near each other at different synapses, spatial summation
7. IPSP can hyperpolarize membrane and prevent an action potential
8. Summed effect of EPSPs and IPSP determine whether an action potential is carried in a particular cell
C. Indirect synaptic transmission
1. neurotransmitter binds to a receptor that is not part of an ion channel
a. activates a signal transduction pathway involving a second messenger in postsynaptic cell
b. slower onset but last longer
2. eg. when norepinephrine binds to its receptor, a G protein is activated, which ultimately opens many
channels (review ch. 11)
D. Neurotransmitters - each bind to own receptor - some bind to different receptors which produce very different
effects in postsynaptic cells (Table 48.1)
1. Acetylcholine - one of the most common neurotransmitters in both vertebrates and invertebrates
a. can be inhibitory or excitatory, depending on the receptor
b. in vertebrates neuromuscular junction, synapse between a motor neuron and skeletal muscle
cell
c. acetylcholine released by neuron binds to ligand gated ion channels in muscle cell,
producing an EPSP via direct synaptic transmission
2. Biogenic Amines - derived from amino acids - catecholines are one group derived from tyrosine (an
amino acid) and includes epinephrine and norepinephrine, which also function as hormones
a. also includes dopamine
b. serotinin synthesized fromtryptophan
c. biogenic amines function mostly in indirect transmission, mostly in the cns
d. dopamine and serotonin affect mood, sleep, attention and learning.
e. prozac is a serotonin reuptake inhibitor
3. Amino Acids and peptides -neuropeptides, short chains of amino acids, serve as neurotransmitters
a. GABA - Gamma amino butyric acid - neurotransmitter at most inhibitory synapses in brain
b. glycine, glutamate and aspartate - amino acids known to function in CNS as neurotransmitters
c. endorphins - neuropeptides function as natural pain relievers decreasing pain perception
d. substance P - mediates perception of pain
4. Gases - NO- local regulator
a. NO causes smooth muscle tissue in penis to relax so blood can engorge and arouse
b. viagra inhibits enzyme which slows muscle relaxing effect of NO
Lesson Objectives Ch. 48
1. Define a membrane potential and a resting potential.
2. Describe the factors that contribute to a membrane potential.
3. Explain the role of the sodium-potassium pump in maintaining the resting potential.
4. Distinguish between gated and ungated ion channels and among ligand-gated ion channels, and voltage-gated ion
channels.
5. Describe the characteristics of an action potential. Explain the role of voltage-gated ion channels in this process.
6. Describe the two main factors that underlie the repolarizing phase of the action potential.
7. Explain how an action potential is propagated along an axon.
8. Describe the factors that affect the speed of action potentials along an axon and describe adaptations that increase the
speed of propagation.
9. Describe saltatory conduction.
10. Compare an electrical synapse and a chemical synapse.
11. Describe the structures of a chemical synapse and explain how they transmit an action potential from one cell to
another.
12. Explain how excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) affect the
postsynaptic membrane potential.
13. Define summation and distinguish between temporal and spatial summation. Explain how summation applies to EPSPs
and IPSPs.
14. Describe the role of signal transduction pathways in indirect synaptic transmission.
15. Describe the specific properties of the neurotransmitters acetylcholine and biogenic amines.
16. Identify and describe the functions of the four amino acids and several neuropeptides that work as neurotransmitters.
17. Explain how endorphins function as natural analgesics.
18. Describe the roles of nitric oxide and carbon monoxide as local regulators.
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