44.1.
Evolution of the Nervous System
A. Invertebrate Nervous Organization
1. Comparative study indicates evolutionary steps leading to centralized
nervous system of vertebrates.
2. Most primitive sponges, with only
a cellular level of organization, respond by closing the osculum.
3. Hydra (cnidarians) possess two
cell layers separated by mesoglea.
a. Hydra can
contract, extend, move tentacles to capture prey, and turn somersaults.
b. A simple
nerve net extends throughout the body within mesoglea. (Fig. 46.1a)
c. Nerve
net is composed of neurons in contact with one another
and with epitheliomuscular cells.
d. More
complex cnidaria (sea anemones and jellyfish) may have two nerve nets.
1) A fast-acting nerve net enables major responses, particularly in times of
danger.
2) Another nerve net coordinates slower and more delicate movements.
4. Planarian nervous system is bilaterally
symmetrical.
a. It has
two lateral nerve cords that allow rapid transfer of information from anterior
to posterior.
b. Nervous
system of planaria exhibits cephalization; at their anterior end,
planaria have a simple brain
composed of a cluster of neurons or ganglia.
c. Cerebral
ganglia receive input from photoreceptors in eyespots and sensory cells in
auricles.
d.
Transverse nerve fibers between nerve cords keep movement both sides of a
planarian body coordinated.
e. Bilateral
symmetry plus cephalization are important trends in nervous system development.
f.
Organization of planaria nervous system foreshadows central and peripheral
system.
5. Annelids, arthropods, and
molluscs are complex with true nervous systems. (Fig. 46.1c, d, e)
a. Central
nervous system (CNS) consists of a brain and a ventral solid nerve
cord.
1) Nerve cord has a ganglion in each segment of body that
controls muscles of that segment.
2) Brain still receives sensory information and controls ganglia so entire
animal is coordinated.
b. Presence
of a brain and other ganglia indicate increased number of neurons among
invertebrates.
C. Vertebrate Nervous Organization
1. Vertebrate nervous systems exhibit cephalization and bilateral symmetry.
(Fig. 46.1f)
a.
Vertebrate nervous system is composed of central and peripheral nervous
systems.
1) Central nervous system develop a brain and spinal cord from embryonic dorsal
nerve cord.
2) Peripheral nervous system consists of cranial and spinal nerves.
b. Paired
eyes, ears, and olfactory structures gather information from environment.
c. A vast
increase in number of neurons accompanied evolution of vertebrate nervous
system; an insect
may have one million neurons, vertebrates may contain a thousand to a billion
times more.
2. The Vertebrate Brain
a.
Vertebrate brain is at anterior end of the dorsal tubular nerve cord.
b.
Vertebrate brain is divided into hindbrain, midbrain, and forebrain.
1) A well-developed hindbrain regulates organs below a level of consciousness;
in humans it regulates
lung and heart function even when we sleep, and coordinates motor activity.
2) Optic lobes are part of a midbrain; originally a center for coordinating
reflex responses to visual input.
3) Forebrain receives sensory input from other two sections and regulates their
output; cerebrum is
highly developed in mammals and is associated with conscious control and outer
layer called
cerebral cortex is large and complex.
D. The Human Nervous System
1. Three specific functions of the nervous system are to:
a.
receive sensory input,
b.
perform integration, and
c.
stimulate motor output to muscles and glands.
2. Central nervous system
(CNS) is located in the midline of the body and integrates sensory
information
and controls
the body.
3. Peripheral nervous system
(PNS) lies outside the CNS and contains cranial and spinal nerves.
4. Peripheral nervous system is
divided into the somatic and autonomic systems.
5. CNS and PNS of the human nervous
system work together to perform functions of a nervous system.
44.2.
Nervous Tissue
A. Neurons In Motor Neurons
1. Neurons vary in size and shape; they all have three parts.
a. Dendrites
receive information and conduct impulses toward cell body.
b. Cell
body contains the nucleus and other organelles, and manufactures
neurotransmitters.
c. Single axon
conducts impulses away from cell body to stimulate or inhibit a neuron, muscle,
or gland.
2. Neurotransmitters are usually
stored at ends of axons in vesicles.
B. Myelination
1. A long axon is called a nerve fiber.
2. Long-axons are covered by myelin
sheath.
3. Myelin sheath is
formed by membranes of neuroglial cells.
4. In PNS, neuroglial neurolemmocyte
cell performs this function, leaving gaps called neurofibril nodes.
C. Types of Neurons
1. Motor neurons have many dendrites and a single axon; conduct
impulses from CNS to muscle fibers or glands.
2. Sensory neurons are
unipolar.
a. Process
that extends from cell body divides into two processes, one to CNS and one to
periphery.
b. It
conducts impulses from the periphery toward the CNA.
3. Interneurons are
multipolar
a. They have
highly-branched dendrites within the CNS.
b. They
convey messages between various parts of the CNS.
c. They form
complex brain pathways accounting for thinking, memory, language, etc.
D. Transmission of the Nerve Impulses
1. Italian Luigi Galvani discovered in 1786 that a nerve is stimulated by an
electric current.
2. An impulse is too
slow to be due to movement of electrons as in electrical current.
3. Julius Bernstein proposed impulse
is movement of unequally distributed ions on either side of axomembrane.
4. 1963 Nobel Prize went to British
researchers A. L. Hodgkin and A. F. Huxley who confirmed this
a. They and
other researchers inserted a tiny electrode into giant axon of a squid.
b. Electrode
was attached to a voltmeter and oscilloscope to trace a change in voltage over
time.
c. Voltage
measured difference in electrical potential between inside and outside of the
membrane.
d.
Oscilloscope indicated polarity changes.
E. Resting Potential
1. When an axon is not conducting an impulse, oscilloscope records a membrane
potential equal to -65 mV,
indicating
that inside of the neuron is more negative than outside.
2. This is resting membrane
potential because axon is not conducting an impulse.
3. Polarity is due to
difference in electrical charge on either side of axomembrane.
a. Inside of
plasma membrane is more negatively charged than outside.
b. There is
a higher concentration of K+ ions inside the axon, and there is a higher
concentration of
Na+ ions outside the axon.
c. A sodium-potassium
pump maintains this unequal distribution of Na+ and K+ ions.
4. Sodium-potassium (Na+-K+)
pump is an active transport system that moves Na+ ions out
and K+
ions into axon.
5. Pump is always working because
membrane is permeable to these ions and they tend to diffuse toward
the lesser
concentration.
6. Since membrane is more permeable
to potassium ions than to sodium ions, there are always more
positive
ions outside; this accounts for some polarity.
7. Large negatively charged proteins
in cytoplasm of the axon also contribute to resting potential.
F. Action Potential
1. When an axon conducts a nerve impulse, rapid change in membrane potential is
the action potential.
2. Protein-lined channels in
axomembrane open to allow either sodium or potassium ions to pass; these
are sodium
and potassium gates.
3. Action potential is generated
only after occurrence of a threshold value.
4. The oscilloscope goes from -65 mV
to +40 mV in a depolarization phase indicating cytoplasm is now
more
positive than tissue fluid.
5. Trace returns to -65 mV again in repolarization
phase indicating inside of axon is negative again.
6. At completion, there are more
potassium ions outside and more sodium ions inside.
G. Propagation of Action Potentials
1. If axon is unmyelinated, an action potential stimulates an adjacent
axomembrane generating an impulse.
2. In myelinated fibers, action
potential at one neurofibril node causes action potential at next node.
a.
Myelinated sheath has neurofibril nodes, gaps where one
neurolemmocyte ends and next begins.
b. Action
potential "leaps" from one neurofibril node to another during saltatory
conduction.
c. Saltatory
conduction may reach rates of over 100 meters/second.
3. As each impulse passes, it
undergoes a short refractory period before it can open sodium
gates again.
4. This ensures a one-way direction
to the impulse.
H. Transmission Across a Synapse
1. The minute space between the axon bulb and the cell body of the next neuron
is the synapse.
2. Synapse consists of: presynaptic
membrane, synaptic cleft, and postsynaptic membrane.
a. Synaptic
vesicles store neurotransmitters that chemically carry across the synapse.
b. When
action potential arrives at presynaptic axon bulb, synaptic vesicles merge with
presynaptic membrane.
c. When
vesicles merge with membrane, neurotransmitters are discharged
into synaptic cleft.
d.
Neurotransmitter molecules diffuse across synaptic cleft to postsynaptic
membrane where they bind
with specific receptors.
e. The type
of neurotransmitter and/or receptor determines if the response is excitation or
inhibition.
f.
Excitatory neurotransmitters use gated ion channels and are fast acting.
g. Others
affect metabolism of the postsynaptic cells and are slower.
I. Neurotransmitter Molecules
1. At least 25 neurotransmitters have been identified.
2. Acetylcholine (Ach)
and norepinephrine (NE) are two well known neurotransmitters.
3. Once neurotransmitter is released
into a synaptic cleft, it initiates a response and is then removed.
4. In some synapses, postsynaptic
membrane contains enzymes that inactivate the neurotransmitter.
5. Acetylcholinesterase
breaks down acetylcholine.
6. In other synapses, presynaptic
membrane reabsorbs neurotransmitter for repackaging in synaptic vesicles
or for
chemical breakdown.
7. Short existence of
neurotransmitters in synapse prevents continuous stimulation (or inhibition) of
postsynaptic
membranes.
8. Many drugs that affect the
nervous system act by interfering with neurotransmitters.
J. Synaptic Integration
1. A neuron has many dendrites and may have one to ten thousand synapses with
other neurons.
2. A neuron receives many excitatory
and inhibitory signals.
3. Excitatory signals
have a depolarizing effect; inhibitory signals have a
hyperpolarizing effect.
4. Integration is the summing up of
excitatory and inhibitory signals.
5. If a neurons receives many
excitatory signals, or at a rapid rate from one synapse, the axon will
probably
transmit a nerve impulse.
6. If both positive and inhibitory
signals are received, the summing may prohibit the axon from firing.
44.3.
Peripheral Nervous System
A. Introduction to the PNS
1. Peripheral nervous system lies outside CNS.
a. Cranial
nerves connect to the brain.
b. Spinal
nerves lie on either side of the spinal cord.
2. Axons in nerves are nerve fibers.
3. Cell bodies of neurons are found
in CNS or in ganglia, collections of cell bodies in the PNS.
4. Humans have 31 pairs of spinal
nerves emerging from spinal cord. (Fig. 46.7)
a. Paired
spinal nerves leave spinal cord by two short branches (spinal
roots)
b. Dorsal
or sensory root contains fibers of sensory neurons conducting
nerve impulses to the cord.
c. Ventral
root contains axons of motor neurons that conduct nerve impulses away
from the cord.
d. All
spinal nerves are mixed nerves that conduct impulses to and from the spinal
cord.
e. Spinal
nerves are mixed nerves with sensory and motor fibers; each serves its own
region.
B. Somatic System
1. Somatic system has nerves to carry sensory information to CNS
and motor commands from CNS to
skeletal
muscles.
2. Voluntary control of muscles
involves the brain; reflexes involve brain or spinal cord.
3. Outside stimuli often initiate a
reflex action, some of which require the brain.
C. The Reflex Arc
1. Reflexes are automatic, involuntary responses.
2. A reflex arc involves the
following pathway.
a. Sensory
receptors generate an impulse in a sensory neuron that moves along
sensory axons toward spinal cord.
b. Sensory
neurons enter the cord dorsally and pass signals to interneurons.
c. Impulses
travel along motor axons to an effector, which brings about response to the
stimulus.
d. The
immediate response is that muscles contract to withdraw from source of pain.
3. Reflex response occurs because
the sensory neuron stimulates several interneurons.
4. Impulses extend to the cerebrum,
which makes a person conscious of stimulus and reaction.
D. Autonomic System
1. Autonomic system is a part of PNS and regulates cardiac and
smooth muscle and glands.
2. There are two divisions: sympathetic
and parasympathetic.
a. Both
function automatically and usually in an involuntary manner.
b. Both
innervate internal organs.
c. Both
utilize two neurons and one ganglion for each impulse.
1) First neuron has a cell body within CNS and a preganglionic fiber.
2) Second neuron has a cell body within ganglion and a postganglionic
fiber.
d. Breathing
rate and blood pressure are regulated by reflex actions to maintain
homeostasis.
E. Sympathetic System:
1. Most preganglionic fibers of sympathetic system arise from
middle (thoracic-lumbar) portion of spinal
cord and
almost immediately terminate in ganglia that lie near cord (thoracic-lumbar
portion).
2. Preganglionic fiber is short, but
postganglionic fiber that contacts an organ is long.
3. Sympathetic system
is especially important during emergency situations ("fight or
flight" response).
4. To defend or flee, muscles need a
supply of glucose and oxygen; sympathetic system accelerates
heartbeat,
and dilates bronchi.
5. To divert energy from less
necessary digestive function, the sympathetic system inhibits digestion.
6. Neurotransmitter released by
postganglionic axon is mainly norepinephrine, similar to epinephrine
(adrenaline)
used as a heart stimulant.
F. Parasympathetic System
1. Parasympathetic system consists of a few cranial nerves,
including vagus nerve, and fibers that arise
from bottom
craniosacral portion of spinal cord.
2. Preganglionic fibers are long and
postganglionic fibers are short.
3. System is "housekeeper
system"; it promotes internal responses resulting in a relaxed
state.
4. Parasympathetic system causes eye
pupil to constrict, promotes digestion, and retards heartbeat.
5. Neurotransmitter released is
acetylcholine.
44.4.
Central Nervous System: Brain and Spinal Cord
A. Introduction to CNS
1. Central nervous system (spinal cord and brain) is where nerve
impulses are interpreted.
2. Both brain and spinal cord are
protected by bone.
3. Both are wrapped in three
connective tissue coverings called meninges.
4. Spaces between meninges are
filled with cerebrospinal fluid to cushion and protect the CNS.
5. Cerebrospinal fluid is contained
in central canal of spinal cord and within ventricles of the brain.
6. Interconnecting spaces that
produce and serve as reservoirs for cerebrospinal fluid.
B. The Spinal Cord
1. Spinal cord has two main functions.
a. It is the
center for many reflex actions.
b. It
provides the means of communication between the brain and the spinal nerves.
2. Spinal cord is composed of white
and gray matter.
a. Gray
Matter
1) Unmyelinated cell bodies and short fibers give gray
matter its color.
2) In cross section, gray area looks like a butterfly or letter H.
3) It contains portions of sensory neurons and motor neurons; short
interneurons connect them.
b. White
Matter
1) Myelinated long fibers of interneurons run together in tracts
and give white matter its color.
2) Tracts conduct impulses between brain and spinal nerves; ascending tracts
are dorsal and
descending tracts are ventral.
3) Tracts cross over near the brain-the side of the brain controls right side
of the body.
C. The Brain
1. Brain has four ventricles: two lateral ventricles and a third
and fourth ventricle.
2. Cerebrum is associated with two
lateral ventricles, diencephalon with third and brain stem with fourth.
D. The Brain Stem
1. Medulla oblongata, pons, and midbrain
all form the brain stem.
2. Medulla oblongata
lies between spinal cord and pons, anterior to cerebellum.
a. It
contains vital centers for regulating heartbeat, breathing, and
vasoconstriction.
b. It
contains reflex centers for vomiting, coughing, sneezing, hiccuping, and
swallowing.
c. It
contains nerve tracts that ascend or descend between spinal cord and the
brain's higher centers.
3. Pons contains
bundles of axons traveling between cerebellum and rest of CNS.
a. Pons
functions with medulla to regulate breathing rate.
b. It has
reflex centers concerned with head movements in response to visual or auditory
stimuli.
4. Besides acting as a relay station
for tracts passing between cerebrum and spinal cord or cerebellum,
midbrain
has reflex centers for visual, auditory, and tactile responses.
E. The Diencephalon
1. Hypothalamus and thalamus are in a portion of
brain known as the diencephalon, where the third
ventricle is
located.
2. Hypothalamus forms floor of third
ventricle.
3. Hypothalamus
maintains homeostasis.
a. Centers
regulate hunger, sleep, thirst, body temperature, water balance, and blood
pressure.
b. It
controls pituitary gland and thereby serves as a link between nervous and
endocrine systems.
4. Thalamus is in roof
of third ventricle.
a. It is the
last portion of brain for sensory input before cerebrum.
b. It is
central relay station for sensory impulses traveling up from body or from brain
to cerebrum.
c. Except
for smell, it channels sensory impulses to specific regions of cerebrum for
interpretation.
F. The Cerebellum
1. Cerebellum lies below posterior portion of cerebrum separated
from brain stem by fourth ventricle.
2. Cerebellum integrates impulses
from higher centers to coordinate muscle actions, maintain equilibrium
and muscle
tone, and sustain normal posture.
3. Receiving information from inner
ear indicating body position, it sends impulses to muscles maintaining balance.
G. The Cerebrum
1. Cerebrum is largest and foremost part of brain.
2. It is the highest center
receiving sensory input and carry out integration to command motor responses.
3. Cerebrum carries out higher
thought processes for learning and memory, language and speech.
4. Reticular formation is network of
masses of cell bodies and fibers extending length of brain stem.
5. Reticular activating system (RAS)
arouses cerebrum via thalamus and causes alertness.
6. Inactive reticular formation
causes sleep; severe injury to RAS causes a person to be comatose.
H. The Cerebral Hemispheres
1. Right and left cerebral hemispheres are connected by a bridge
of nerve fibers, the corpus callosum.
2. Outer portion is highly
convoluted cerebral cortex consisting of gray matter containing
cell bodies and
short
unmyelinated fibers.
3. Cerebral cortex in each
hemisphere contains four surface lobes; different functions are associated
with
each lobe.
(Fig. 46.11)
4. Comparison of vertebrates shows
an evolutionary increase in relative size of the cerebrum.
a. Cerebral
cortex is most convoluted in humans.
b. In fishes
and amphibians, the cerebrum is largely olfactory.
c. In
reptiles, birds and mammals, it receives information from other parts and
coordinates sensory data
and motor functions.
5. Voluntary commands begin in the
primary motor area of the frontal lobe; we can map regions that receive
sensory
information and control various parts of the body.
a. Our hand
takes up a large proportion of primary motor area.
b. Ventral
to primary motor area is pre-motor area that organizes motor functions before
primary area
sends signals to cerebellum.
c. Left
frontal lobe has Broca's area for ability to speak.
d. Sensory
information from skin and skeletal muscles arrives at primary somatosensory
area.
e. Primary
visual area in occipital lobe receives information from eyes; a visual
association area associates
new visual information with old information.
f. Primary
auditory area in temporal lobe receives information from ears.
g. Primary
taste area is in parietal lobe.
h.
Somatosensory association area processes and analyzes sensory information from
skin and muscles.
i. General
interpretation area receives information from all sensory association areas and
allows us to
quickly integrate signals and send them to prefrontal area for immediate
response.
j.
Prefrontal area in frontal lobe receives input from other association areas and
reasons and plans.
I. The Limbic System
1. It is a complex network of tracts and nuclei that incorporate medial
portions of cerebral lobes,
subcortical
nuclei and diencephalon.
2. It blends higher mental functions
and primitive emotions.
3. Two major structures are hippocampus
and amygdala for learning and memory.
a. Hippocampus
makes prefrontal area aware of past experiences stored in association areas.
b. Amygdala
causes experiences to have emotional overtones.
c. Inclusion
of frontal lobe in limbic system allows reasoning to keep us from acting out
strong feelings.
4. Learning and Memory
a. Memory
is ability to hold thoughts in minds and recall past events.
b. Learning
takes place when be retain and utilize past memories.
c.
Prefrontal area in frontal lobe is active in short-term memory (e.g. telephone
numbers).
d. Long-term
memory is mix of semantic memory (numbers, words) and episodic memory (persons,
events).
e. Skill
memory is ability to perform motor activities.
f. The hippocampus
serves as a go-between to bring memories to mind.
g. Amygdala
is responsible for fear conditioning and associates danger with sensory
stimuli.
h. Long-term
memories are stored in bits throughout sensory association areas of cerebral
cortex.
i. Amygdala
adds emotional overtones to memories.
j. Long-term
potentiation (LTP) is an enhanced response at synapses within hippocampus.
k. LTP is essential
to memory storage; excited postsynaptic cells may die due to glutamate
neurotransmitter.
l.
Extinction of too many cells in hippocampus is underlying cause of Alzheimer
disease.