Download Principles of Biology ______Lake Tahoe

Bio 103
Lake Tahoe Community College
Winter Quarter
Instructor: Sue Kloss
__________________________________________________________________________________________________________________________
Chapter 49 - Nervous System
__________________________________________________________________________________________________________________________
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. Nervous systems are the most intricate data processing systems on earth
a. nerve nets (Fig. 48.2a and b)-in cnidarians and sea star arms. nerves -sea stars
b. most animals are bilaterally symmetrical, with head and a tail and a tendency to move head first
through their environments.
c. The head - most often the first part of the animal to encounter new stimuli is usually equipped
with sense organs and a brain.
d. flatworms are the simplest animals to show:
1. cephalization - concentration of nervous system at head end
2. centralization- presence of cns, distinct from pns. Planaria has small brain composed
of ganglia and 2 parallel nerve cords (bundles of axons and dendrites). These elements
constitute the worms cns. Other, smaller nerves make up the pns.
e. many bilaterally symmetrical animals show much greater cephalization and centralization than
flatworms
1. still have brains and one or more nerve cords, but cns tends to be more complex and
distinctly set off from peripheral nerves.
2. leeches have much greater concentration of neurons than in the flatworm cns.
3. insects have a brain composed of several fused ganglia and a ventral nerve cord with a
ganglion in each body segment which control the muscles in that body segment
2. 1 cc of brain contains several million neurons (nerve cells) and supporting cells
3. neurons are specialized for carrying signals from 1 location to another
4. 1 neuron can communicate with several thousand others forming communication networks
5. we can learn, remember, perceive, and move
6. ns - 3 interconnected functions (Fig. 48.3)
a. sensory input - conduction of signals from sense receptors (e.g. light sensors in eyes)
to integration centers
b. integration is interpretation of sensory signals and formulation of responses
c. motor output is conduction of nerve signals from the integration centers to effectors,
such as muscle cells, which perform the responses.
7. 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
8. 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
9. ganglia - clusters of neuron cell bodies in the nerves of the pns.
10. knee jerk reflex - (Fig. 48.4) colored balls are neuron cell bodies; lines are neuron fibers
11. 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)
12. 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.
13. Quad muscle responds by contracting. At same time, another motor neuron responds to signals
1
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. oligodendrites in cns and
c. Schwann cells in pns (Fig. 48.8)
d. 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)
a. action potentials not generated in areas btn nodes
b. 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
2
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
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
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
3
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
(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
4
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
IV. Nervous systems
A. Nervous system organization usually correlates with body symmetry
1. there is remarkable uniformity throughout the animal kingdom in the way that nerve cells function
2. great variety in how nervous systems are organized
3. brain is the integrator of information and response,
a. brain - master control center;
1. includes homeostatic centers to keep body functioning smoothly.
2. sensory centers that integrate data from the sense organs
3. in humans, centers of emotion and intellect
4. exerts control over spinal cord and sends out its own motor commands to muscles
4. spinal cord transmits info to and from brain
and is responsible for reflex actions (Fig. 48.19)
a. vertebrate spinal cord is not ventral (as in invertebrates) and does not have segmental
ganglia, though there are ganglia adjacent to it
b. vertebrate cns is derived from embryonic nerve cord- hollow part is the ventricles of brain
and central canal of spinal cord (Fig. 48.20)
c. central canal and ventricles filled w/ cerebrospinal fluid formed in brain by filtration of blood
and circulates slowly then drains into veins- brings nutrients and hormones to brain and removes
waste
d. in mammals, cerebrospinal fluid protects brain by flowing between meninges, layers of
connective tissue surrounding cns
e. axons in the brain appear white bc of myelin sheaths (white matter) while cell bodies,
unmyelinated axons and dendrites appear as gray matter
f. spinal cord lies inside vertebral column and receives sensory info from skin and muscles and
sends out motor commands for movement
5. Vast network of blood vessels services the cns
6. in mammals, most grey matter is the cerebral cortex, center for higher brain function like problem
solving
7. ganglia and nerves of vertebrate pns are vast communication network
a. cranial nerves carry signals to or from brain. eyes, ears, and nose are all serviced by cranial
nerves
b. spinal nerves carry signals to or from spinal cord
c. all spinal and most cranial nerves carry sensory as well as motor neurons
B. PNS of vertebrates is a functional hierarchy (Fig. 48.21)
1. cranial nerves originate in brain and terminate mainly in head and upper body organs (12 pairs)
2. Spinal nerves originate in spinal cord and extend to all parts of body below head (31 prs) and contain
both sensory and motor neurons
3. Sensory nerves have 2 sets of neurons
a. sensing external environment - from eyes, ears, sense organs
b. sensing internal environment - eg. acidity of blood, etc.
c. both provide sensation of pain - warning of tissue damage
4. Motor nerves also have 2 sets of neurons
a. somatic nervous system carries signals to skeletal muscles, mainly in response to external
stimuli. somatic system is said to be voluntary but is often controlled by reflexes mediated by
spinal cord or brainstem
b. autonomic ns - generally involuntary, regulates smooth and cardiac muscles, and organs
of cv, digestive, excretory and endocrine system
C. autonomic system composed of sympathetic, parasympathetic and enteric neurons that regulate internal
environment
1. autonomic ns has 2 sets of neurons w/opposite effects on most body organs (fig. 48.22)
a. parasympathetic division - primes body for digestion and resting- conserves energy
“rest and digest”
b. sympathetic has opposite effect - primes body for intense activity “fight or flight”
2. these are extreme and opposite states; usually we are at some in-between state, w/ most organs
receiving signals from sympathetic and parasympathetic neurons
3. you receive signals from both, say for salivary glands, keeping your mouth moist but not wasting
metabolism, until you think about a snack then parasympathetic overpowers sympathetic signals
4. sympathetic and parasympathetic neurons emerge from different regions of the cns
5
5. enteric division controls digestive tract, pancreas, gall bladder and their secretions, as well as the
smooth muscle that produces peristalsis; normally its regulated by sympathetic and parasympathetic div.
6. somatic and autonomic systems cooperate to maintain homeostasis
a. body temp drops, hypothalamus signals blood vessels of autonomic ns to constrict surface
blood vessels, which reduce heat loss, also signals somatic ns to shiver, increase heat production
E. vertebrate brain regions (Fig. 48.23)
1. brain and spinal cord develop from dorsal hollow embryonic nerve cord
2. 3 ancestral regions are forebrain, midbrain, and hindbrain; still appear in embryos of all vertebrates
3. Brain evolution
a. birds and mammals have bigger brains relative to body size than reptiles, amphibs and fishes.
b. forebrain mid brain and hindbrain become subdivided into different regions that perform diff’t
functions
1. forebrain became diencephalon (thalamus, hypothalamus, epithalamus) and cerebrum
2. hindbrain became pons, medulla (both part of brainstem) and cerebellum
3. midbrain became becomes part of brainstem
c. as cerebrum developed in evolution, most complex vertebrate behavior developed
V. the Human Brain (Fig. 48.23)
A. the structure of a living supercomputer: the human brain
1. composed of 100 billion intricately organized neurons and much larger number of support cells
2. Human brain is more powerful than most complex supercomputer
3. 2 sections of hindbrain, medulla oblongata and pons, and midbrain make up functional unit called
brainstem
a. all sensory neurons carrying info to and from higher brain regions go thru brainstem
b. brainstem acts as a filter
c. regulates sleep and arousal
d. helps coordinate body movements
4. cerebellum is planning center for body movements
5. thalamus has all cell bodies for neurons that relay info to the cerebral cortex
a. sorts data into categories (eg. touch signals from hand)
b. suppresses some info and enhances others, so it controls somewhat the info going to the
cerebrum
6. hypothalamus controls pituitary and secretion of many hormones. Also regulate body temp., blood
pressure, sex drive, thirst, hunger and helps us experience emotions and pleasure
a. “pleasure center” in hypothalamus may also be called addiction center; strongly affected by
addictive drugs, eg. cocaine
b. biological clock also centered here, using visual data from eyes; controls biorhythms (Fig.
48.25)
7. cerebrum is largest and most sophisticated part of the brain (Fig. 48.27)
a. right and left cerebral hemispheres
b. thick band of nerve fibers called corpus callosum connects cerebral hemispheres, enabling
them to process info together
c. under corpus callosum are basal ganglia, important in motor coordination
d. motor and somatosensory cortex get info from hypothalamus, which directs the info to the
proper area of the brain. the motor and sensory cortex have orderly arrangement of neurons;
surface area of cortex involved depends amt. of neurons that innervate that part of the body
(Fig. 48.28)
8. Lateralization of cortical function
a. left hemisphere more adept at language, math, logic, serial processing of sequences of info;
more involved at detailed, speed optimized activities required for skeletal muscle control and fine
visual and auditory detail, perceptive focus
b. right hemisphere is stronger at pattern recognition, face recognition, spatial relations, nonverbal
thinking, emotional processing, simultaneous processing of many types of info, music and speech
patterns and info, perceiving relationship btn images and their context
c. two hemispheres work together trading info via corpus callosum - eg if cc is disrupted, reading
is impossible bc the visual field where the word exists cannot convey the word to the language ctr.
in other hemisphere
B. Limbic system involved in emotions, memory and learning
1. limbic system is a functional unit of several integrating centers and interconnecting neuron tracts in our
forebrains.
2. includes parts of thalamus and hypothalamus (Fig. 48.30) and partial rings around them formed by
6
portions of the cerebral cortex
3. sensory data converge in the amygdala- seems to act as a memory filter, labeling info to be
remembered by tying it to an event or emotion
4. hippocampus interacts closely with amygdala, hypothalamus, brainstem and prefrontal cortex involved in learning, reasoning, personality
5. memory is ability to store and retrieve information about a previous experience
a. short term memory - lasts only a few minutes - looking up and using a phone number
b. long term memory - recall after weeks, months or longer
6. Prefrontal cortex seems to play a major role in memory retrieval.
Lesson Objectives Ch. 49
1. Compare and contrast the nervous systems of the following animals and explain how variations in design and complexity
relate to their phylogeny, natural history, and habitat: hydra, sea star, planarian, insect, squid, and vertebrate.
2. Name the three stages in the processing of information by nervous systems.
3. Distinguish among sensory neurons, interneurons, and motor neurons.
4. List and describe the major parts of a neuron and explain the function of each.
5. Describe the function of astrocytes, radial glia, oligodendrocytes, and Schwann cells.
6. Compare the structures and functions of the central nervous system and the peripheral nervous system.
7. Distinguish between the functions of the autonomic nervous system and the somatic nervous system.
8. Draw and label the following structures of the following brain regions: medulla oblongata, pons, midbrain, cerebellum,
thalamus, hypothalamus, and cerebrum. State functions for each.
9. Describe the specific functions of the reticular system.
10. Distinguish between the functions of the left and right hemispheres of the cerebrum. Describe the function and actions
of the corpus callosum.
11. Describe the specific structures and functions of the limbic system.
7