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Chapter 13
Nervous System
Points to Ponder
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What are the three types of neurons?
What are neuroglia?
What is the structure of a neuron?
What is the myelin sheath? Saltatory conduction? Scwhann
cell? Node of Ranvier?
Explain the resting and action potential as they relate to a
nerve impulse.
How does the nerve impulse traverse the synapse?
What are the two parts of the nervous system?
What 3 things protect the CNS?
What are the 4 parts of the brain and their functions?
What is the reticular activating system and the limbic system?
What are some higher mental functions of the brain?
What are the 2 parts of the peripheral nervous system?
Be able to explain the abuse of several drugs.
The Nervous System
• Allows for communication between cells
through sensory input, integration of data and
motor output
• Two major divisions
– Central Nervous System (CNS):
• brain and spinal cord
– Peripheral Nervous System (PNS):
• nerves outside of the CNS
• 2 cell types:
– Neurons:
• transmit nerve impulses in nervous system
– Neuroglia:
• support and nourish neurons
Functions of the Nervous System
1. Nervous System receives sensory input
-
Sensory receptors generate nerve impulses
that travel by way of the PNS to the CNS
2. CNS performs integrations
-
Sums up the input it receives from the body
3. CNS generates motor output
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Nerve impulses from the CNS go by way of the
PNS to the muscles and glands
13.1 Overview of the nervous system
Expanding on neurons
• 3 types of neurons:
1.Sensory – takes impulses from sensory receptor
to CNS
• Detect changes in the environment
2.Interneurons – receive information in the CNS
and send it to a motor neuron
• Sum up all the nerve impulses received from sensory
neurons and other interneurons before
communication with motor neurons
3.Motor – takes impulses from the CNS to an
effector (i.e. gland or muscle fiber)
• Effectors: carry out our responses to the
environmental changes
Neuron Structure (Ch 4 review)
• Cell body – main cell where organelles and
nuclei reside
• Dendrite – many, short extensions that carry
impulses to a cell body
• Receive signals from sensory receptors or other
neurons
• Signals result in nerve impulses that are conducted
by an axon
• Axon (nerve fiber) – single, long extension that
carries impulses away from the cell body
The Structure of Neurons
Figure 12–1
The Myelin Sheath
• A lipid covering on long axons
• Functions:
– Increase the speed of nerve impulse conduction
– Nerve insulation
– Nerve regeneration in the PNS only
• When severed, myelin sheath remains and serves as a
passageway for new fiber growth
• Neuroglia cells involved in Myelin Sheath formation
– Schwann cells – in the PNS
– Oligodendrocytes – in the CNS
• Nodes of Ranvier –
– gaps between myelination on the axons
• Saltatory conduction –
– conduction of the nerve impulse from node to node
Gray Vs. White Matter
• Gray matter in CNS:
– Contains no myelinated axons
• White matter in CNS:
– Contains myelinated axons
The Nerve Impulse: Resting Potential
• Voltmeter:
– Allows us to measure the potential difference between
two sides of the axonal membrane (plasma membrane of
the axon), expressed in term of voltage
• Resting potential – when the axon is not conducting a nerve
impulse
• More positive ions outside than inside the membrane
• There is a negative charge of -65mV inside the axon
• More Na+ outside than inside, More K+ inside than outside
• Unequal distribution due to sodium-potassium pump
• Active transport of 3Na+ out and 2K+ into the axon
• Membrane is permeable to K+ but not Na+
• More positive ions outside the membrane than inside
• Large, negatively charged organic ions in the axoplasm
contributes to negative charge inside the membrane
The Nerve Impulse: Action Potential
• Action potential – rapid change in the axon membrane that
allows a nerve impulse to occur
• If stimulus causes axonal membrane to depolarize to
certain level called threshold, action potential occurs
• Steps for Action Potential:
1. First, Sodium gates open letting Na+ in
• Depolarization occurs
• Interior of axon loses negative charge (+40mV)
2. Secondly, Potassium gates open letting K+ out
• Repolarization occurs
• Interior of axon regains negative charge (-65mV)
3. Resting potential is restored by moving potassium inside
and sodium outside
*Wave of depolarization/repolarization travels down axon*
Propagation of Action Potentials
Two Methods
1. Continuous propagation:
– unmyelinated axons
2. Saltatory propagation:
– myelinated axons
Propagation of Action Potential
1. Continuous Propagation:
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Unmyelinated axons
Whole membrane depolarizes and repolarizes
sequentially hillock to terminal
Continuous Propagation
Figure 12–14 (Step 2)
Propagation of Action Potential
2. Saltatory Propagation:
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Myelinated axons
Depolarization only on exposed membrane at nodes
Myelin insulates covered membrane from ion flow
Action potential jumps from node to node
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Faster and requires less energy to reset
Saltatory Propagation
Figure 12–15 (Steps 1, 2)
Saltatory Propagation
Figure 12–15 (Steps 3, 4)
13.1 Overview of the nervous system
The synapse
• A small gap between the sending neuron
(presynaptic membrane) and the receiving
neuron (postsynaptic membrane)
• Transmission is accomplished across this
gap by a neurotransmitter (e.g. ACh,
dopamine and serotonin)
• Neurotransmitters are stored in synaptic
vesicles in the axon terminals
Transmission across the synapse
1. Nerve impulse reaches the axon terminal
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Close to dendrite or cell body of another neuron
2. Calcium ions enter the axon terminal
- stimulate synaptic vesicles to fuse with presynaptic
membrane
3. Neurotransmitters are released
- diffuse across the synapse and bind with the
postsynaptic membrane via specific receptors that
inhibit or excite the neuron
- Excitation:
- neurotransmitters cause Na+ gates to open, and Na+
diffuses into the receiving neuron
- Inhibition:
- neurotransmitters cause K+ to enter the receiving
neuron
Transmission across the synapse
4. Neurotransmitter removal from the cleft,
either
1. Enzymes that rapidly inactivate the
neurotransmitter
2. Sending membrane rapidly reabsorbs the
neurotransmitter for
- repackaging in synaptic vesicles
- molecular breakdown
Neurotransmitter Molecules
• Acetylcholine
– act at neuromuscular junctions excites skeletal muscle
– Inhibits cardiac muscle
– excites or inhibits smooth muscles and glands
• Norepinephrine
– excites smooth muscle
– Important to dreaming, waking, and mood
• Serotonin
– Involved in thermoregulation, sleeping, emotions, and
perception
• Decreased levels of norepinephrine and serotonin
is linked to depression
Drugs
1.
2.
3.
4.
Block the release of neurotransmitters
Mimic the action of a neurotransmitter
Block the receptor
Interfere with the removal of a
neurotransmitter from a synaptic cleft
Neuromodulators
•
Block the release of a neurotransmitter or
modify a neuron’s response to a
neurotransmitter
1. Caffeine
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Interferes with effects of inhibitory
neurotransmitters in the brain
2. Substance P
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Released by sensory neurons during pain
3. Endorphins
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Block release of substance P, natural painkiller
Synaptic
Integration
• Integration is the
summation of the
inhibitory and
excitatory signals
received by a
postsynaptic neuron
• This occurs because
a neuron receives
many signals
The nervous divisions
• 2 divisions:
– Central nervous system (CNS):
• Brain and spinal cord
– Peripheral nervous system (PNS):
• Nerves and ganglia (cell bodies)
The central nervous system
• Consists of the brain and spinal cord
• Both are protected by:
• Bones – skull and vertebral column
• Meninges – 3 protective membranes wrap around CNS
• Cerebral spinal fluid (CSF) – space between meninges is
filled with this fluid that cushions and protects the CNS
• Also contained in the ventricles of the brain
• Both made up of 2 types of nervous tissue:
• Gray matter – contains cell bodies and nonmyelinated
fibers
• White matter – contains myelinated axons
Ventricles
•
Interconnecting chambers that produce
and serve as a reservoir for CSF
A. Lateral Ventricle (2)
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Left and Right Cerebral hemisphere
B. Third Ventricle (1)
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Diencephalon (Hypothalamus and thalamus)
C. Fourth Ventricle (1)
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Brain stem and cerebellum
Connects to central canal of spinal cord
The CNS: Spinal cord
• Extends from the base of the brain through
foramen magnum and along the length of the
vertebral canal formed by the most vertebrae
• Functions
– Provide communication between the brain and
the body
– Center for reflex arcs
– Act as “gate” control flow of pain messages from
peripheral nerves to brain
• Pain message may pass to the brain to be perceived
• Pain message may be blocked from reaching the brain
The CNS: Spinal cord
• Nerves project from the cord between the vertebrae
• Central canal and meninges contains CSF
• Gray matter is in the center is a butterfly shape
– Portions of sensory neurons, motor neurons, and
interneurons are found here
– Dorsal root of spinal nerves contains sensory fibers
entering the gray matter
– Ventral root of a spinal nerve contains motor fibers exiting
the gray matter
• White matter surrounds the gray matter
– Ascending tracts  info to brain
– Descending tracts  info from brain to motor neurons
Reflex Arcs = Single Reflex
• Spinal cord is the center for reflex arcs
• Rapid, automatic nerve responses triggered
by specific stimuli
• Used to maintain homeostasis
• Simple reflex:
– Sensory perception in, motor response out
5 Steps in a Neural Reflex
Figure 13–14
Reflex Arcs for Internal Organs
• Blood Pressure, if low:
– Detected by carotid arteries and aorta
– Generate nerve impulses that pass through
sensory fibers to the cord
– Travels ascending tract to cardiovascular
center in the brain
– Nerve impulse passes down a descending
tract to spinal cord
– Motor impulses cause blood vessels to
constrict to rise blood pressure
13.2 The central nervous system
The CNS: Brain
4 major parts:
1.
2.
3.
4.
Cerebrum
Diencephalon
Cerebellum
Brain stem
13.2 The central nervous system
13.2 The central nervous system
The brain: Cerebrum
• Last center to receive sensory input and carry
out integration before commanding voluntary
motor responses
• Consists of:
– Cerebral hemisphere
– Cerebral cortex
– Primary motor and sensory areas of the cortex
– Association areas
– Processing centers
– Central white matter
1. The brain: Cerebrum – the lobes
• Cerebrum – largest portion of the brain
• Longitudinal fissure (deep grooves called sulci)
– divides the left and right cerebral hemispheres
• Corpus callosum:
– connects the two hemispheres via a bridge of tracts
• Sulci divide cerebrum into 4 lobes/hemispheres:
• Frontal lobe:
• primary motor area and conscious thought
• Temporal lobe:
• primary auditory, smell and speech area
• Parietal lobe:
• primary somatosensory and taste area
• Occipital lobe:
• primary visual area
The brain: Cerebrum – the cerebral hemispheres
1. The brain: Cerebrum – the cerebral cortex
• Cerebral cortex – thin, outer layer of gray matter
– Accounts for sensation, voluntary movement, and all the
thought processes we associate with consciousness
1. Primary motor area – voluntary control of skeletal muscle
- In the frontal lobe before the central sulcus
2. Primary somatosensory area – sensory information from
skeletal muscle and skin arrive here
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Dorsal to the central sulcus in the parietal lobe
Primary taste area  taste sensation
Primary visual area  occipital lobe receives info from eyes
Primary auditory area  temporal lobe receives info from ears
Primary olfactory area  temporal lobe receives info for smell
1.
Cerebrum – the cerebral cortex
Primary Motor area and Primary
Somatosensory area
1. The brain: Cerebrum – the cerebral cortex
3. Association areas – integration occurs here
- Premotor area organizes motor functions for skilled
motor activities
- Primary motor area sends signals to the cerebellum,
which integrates them
- Somatosensory association area processes and
analyzes sensory information from the skin and
muscles
- Visual association area associates new visual
information with previously received visual info
- Auditory association area associates new auditory
information with previously received auditory info
1. The brain: Cerebrum – the cerebral cortex
4. Processing centers – perform higher level analytical
functions including Wernicke’s and Broca’s areas
both involved in speech
– Prefrontal areas receive info from other association areas
and uses this info to reason and plan our actions
– Wernicke’s area (dorsal part of left hemisphere)
• helps us understand both the written and spoken word
• sends the info to the Broca’s area
– Broca’s area (portion of primary motor area)
• adds grammatical refinements
• directs the primary motor area to stimulate the appropriate muscles
for speaking and writing
1. The brain: Cerebrum – the cerebral cortex
5. Central White Matter (mylinated axons)
– Develops as a child grows
• makes children more capable of speech
– Descending tracts:
• primary motor  lower brain centers
– Ascending tracts:
• Lower brain centers  primary somatosensory area
– Tracts cross over in the medulla
• Left controls right, right controls left
– Tracts take info between sensory, motor, and association
areas within the brain
– Corpus collosum:
• tract that joins the left and right hemispheres
2. The brain: Diencephalon
• Hypothalamus – helps maintain homeostasis
(hunger, sleep, thirst, body temperature and water
balance) and controls pituitary gland
• Link between the nervous and endocrine systems
• Thalamus – 2 masses of gray matter that receive all
sensory input except smell; involved in memory and
emotions
• Visual, auditory, and somatosensory info arrives at the
thalamus via the cranial nerves and tracts from the spinal
cord
• Integrates this info and sends it to the appropriate
portions of the cerebrum
• Involved in arousal of the cerebrum
• Pineal gland – secretes melatonin that controls our
daily rhythms
2. The brain: Diencephalon
13.2 The central nervous system
3. The brain: Cerebellum
• White matter = arbor vitae
– primary composition of cerebellum
• Gray matter = thin layer overlying white matter
• Receives and integrates sensory input from
the eyes, ears, joints and muscles about the
current position of the body
• Functions to:
• Maintains posture
• Coordinates voluntary movement
• Allows learning of new motor skills (i.e. playing
the piano or hitting a baseball)
13.2 The central nervous system
4. The brain: The brain stem
1. Midbrain –
- Relay station between cerebrum and spinal cord or
cerebellum
- Reflex centers for visual, auditory, and tactile
responses
2. Pons –
- a bridge between cerebellum and the CNS
- regulate breathing rate with the medulla oblongata
- reflex center for head movements in response to visual
and auditory stimuli
3. Medulla oblongata –
- reflex centers for regulating breathing, heartbeat and
blood pressure
- contains tracts that ascend or descend between the
spinal cord and higher brain centers
4. Reticular formation
4. The brain: The brain stem
• Reticular formation – major component of
the reticular activating system (RAS) that
regulates alertness
• Receives sensory signals and sends them up to
higher centers, and motor signals which it sends
to the spinal cord
• RAS arouses the cerebrum via the thalamus
• Can filter out unnecessary sensory stimuli
• Example: study with the TV on
• To inactivate RAS
• remove of visual and auditory stimuli
• Injury to RAS  coma
The reticular activating system
The limbic system
• Located between cerebrum and diencephalon
• Joins primitive emotions (i.e. fear, pleasure) with
higher functions such as reasoning
• Can cause strong emotional reactions to situations but
conscious thought can override and direct our
behavior
• Functions:
1. Establishes emotional states and drives
2. Links conscious functions of cerebrum to
autonomic functions of brainstem
3. Facilitates memory storage and retrieval
The limbic system
• Includes:
• Amygdala – has emotional overtones
• Creates sensation of fear, triggers the fight-or-flight
reaction
– Frontal cortex can override the limbic system and
cause us to rethink the situation
• Hippocampus – important to learning and memory
• Info gateway during learning process
• Link of hippocampus to Alzheimers
13.3 The limbic system and higher mental functions
The limbic system
13.3 The limbic system and higher mental functions
Higher mental functions
• Learning –
– what happens when we recall and use past memories
• Memory –
– ability to hold a thought or to recall past events
• Short-term memory –
– retention of information for only a few minutes
• Long-term memory – retention of information for more than
a few minutes and include the following:
• Episodic memory – persons and events
• Semantic memory – number and words
• Hippocampus serves as a bridge between the sensory
association areas, where memories are stored, and the
prefrontal area, where memories are utilized
• Long-term potentiation occurs after synapses have been used
intensively for a short period of time, they release more
neurotransmitters than before
• Causes memory storage
Higher mental functions
• Skill memory – performing skilled motor activities (i.e.
riding a bike)
– When a skill is first learned
• more areas of the cerebral cortex are involved
– Skill memory involved all the motor areas of the cerebrum
below the level of consciousness
• Language – depends on semantic memory
– Any disruption can contribute to an inability to comprehend
our environment and use speech correctly
– Damage to
• Wernicke’s area  inability to comprehend speech
• Broca’s area  inability to speak and write
13.3 The limbic system and higher mental functions
What parts of the brain are active in
reading and speaking?
13.4 The peripheral nervous system
The peripheral nervous system (PNS)
• Includes cranial (12 pr) and spinal nerves
(31 pr) and ganglia outside the CNS
- Spinal nerves conduct impulses to and from the
spinal cord
- Cranial nerves conduct impulses to and from the
brain
• Divided into 2 systems:
- Somatic
- Autonomic
The peripheral nervous system
(PNS)
• Cell body and dendrites in CNS or ganglia
• Axons or neurons project from the CNS
and form the spinal cord
• Nerves = axons (long part of neurons)
The PNS
• Cranial Nerves
– Sensory nerves, motor nerves, or mixed nerves
– Controls the head, neck, and facial regions
– Example is the Vagus Nerve (X)
• Controls the pharynx, larynx, and internal organs
• Arise from medulla oblongata and communicates
with the hypothalamus to control internal organs
The PNS
• Spinal Nerves:
– Dorsal root of spinal nerve
• contains sensory fibers that conduct impulses toward the
spinal cord from sensory receptors
– Cell body of sensory neuron is in a dorsal root ganglion
– Ganglion
• collection of cell bodies outside of the CNS
– Ventral root of spinal nerve
• Contains motor fibers that conduct impulses away from the
cord to effectors
– All spinal nerves are mixed nerves
– Each spinal nerve serves the particular region of the body
The peripheral nervous system
The PNS: Somatic division
• Serves the skin, skeletal muscles and tendons
• Includes sensory receptors, sensory nerves,
and motor nerves
• Automatic responses are called reflexes
Features of the Autonomic System
• Function automatically and usually in an
involuntary manner
• They innervate all internal organs
• They utilize two neurons and one ganglion
for each impulse
– Preganglionic nerve fiber  ganglion 
postganglionic nerve fiber contact with organs
• Reflex actions of the ANS
– Regulate blood pressure and breathing rate
13.4 The peripheral nervous system
The PNS: Autonomic division
• Regulates the activity of involuntary muscles
(cardiac and smooth) and glands
• Divided into 2 divisions:
– Sympathetic: neurotransmitter  Norepinephrine
• coordinates the body for the “fight or flight” response
• speeds up metabolism, heart rate and breathing while
down regulating other functions
– Parasympathetic: neurotransmitter  acetylcholine
• brings a relaxed state
• slows down metabolism, heart rate and breathing and
returns other functions to normal
Degenerative brain disorders
• Alzheimer disease
– Usually seen in people after 65 yrs. old
– Starts with memory loss
– APOE4: 65% of AD persons have this gene
– Two histological causes:
• Abnormal neurons with plaques of beta amyloid
– Sticky B-amyloid forms when snipped by secretases
– Accumulation results in inflammation and neuronal death
• Neurofibrillary tangles in axons that extend around the
nucleus
– Protein tau losses it shape and grabs onto other tau
molecules resulting in tangles
Degenerative brain disorders
• Parkinson disease
– Usually begins between the ages of 50-60
– Characterized by loss of motor control
– Due to degeneration of dopamine-releasing
(inhibitory effect) neurons in the brain
• Dopamine is an inhibitory neurotransmitter
• Without dopamine, excessive excitatory signals form
the motor cortex and brain result in symptoms of
Parkinsons disease
• Treatment
– I-dopa, chemical that can be changed into dopamine
Drugs and drug abuse
• Drugs have two general effects on the nervous
system:
– affect the limbic system
– Promote or decrease the action of a certain
neurotransmitter (stimulants or depressants)
• Most drug abusers take drugs that affect dopamine
– Dopamine involved in reward circuit, regulates mood
– Drugs artificially affect the reward circuit to the point
they ignore basic physical needs in favor of the drug
• Drug abusers show physiological and psychological
effect
• Once a person is physically dependent
– They usually need more of the drug for the same
effect because their body has become tolerant
13.5 Drug abuse
Drug abuse: Alcohol
• Alcohol
– depressant directly absorbed from the stomach and small
intestine
– Increases action of GABA and increases the release of
beta-endorphins in the hypothalamus
• Most socially accepted form of drug use
• About 80% of college-aged people drink
• Effects on the body
– Denatures proteins, causes damage to tissues such as
the brain and liver
– Chronic consumption can damage the frontal lobe,
decrease brain size, and increase the size of the
ventricles
• High blood alcohol levels can lead to
– poor judgment, loss of coordination or even coma and
death
Drug abuse: Nicotine and Cocaine
• Nicotine – stimulant derived from tobacco plant
– Causes neurons to release dopamine
– Mimics acetylcholine in PNS
• increases skeletal muscle activity, heart rate, blood pressure, and
digestive tract motility
– Adversely affects a developing embryo or fetus
– Psychological and physiological dependency
– “immunize” the brain against nicotine, prevent passage
through BBB
• Cocaine – stimulant derived from a plant
– Interferes with the re-uptake of dopamine at synapses
– Results in a rush sensation (5-30 minutes) and an
increased sex drive
– Results in hyperactivity and little desire for food and sleep
– Extreme physical dependence with this drug
– Continued use  body makes less dopamine to
compensate for the excess at synapses
• result is withdrawal symptoms
13.5 Drug abuse
Drug abuse: methamphetamine
•
•
•
•
Powder form is called speed
Crystal form is called meth or ice
Stimulatory effect mimics cocaine
Reverses the effects of fatigue and is a mood
elevator
• High agitation is common after the rush and can
lead to violent behavior
• Causes psychological dependency and
hallucinations
• “Ecstasy” is the street name for a drug
– has the same effects as meth without the hallucinations
13.5 Drug abuse
Drug abuse: Heroin
• Depressant from the sap of the opium poppy plant
• Leads to a feeling of euphoria and no pain
– it is delivered to the brain and is converted into morphine
– Depresses breathing, activates the reward circuit, and
blocks pain pathway
• Side effects
– nausea, vomiting and depression of the respiratory and
circulatory systems
• Can lead to
– HIV, hepatitis and other infections due to shared needles
• Extreme dependency
Drug abuse and its use: Marijuana
• Psychoactive drug derived from a hemp plant
called Cannabis
• Binds to receptors located in the hippocampus,
cerebellum, basal ganglia, and cerebral cortex
– Brain areas important for memory, orientation,
balance, motor coordination, and perception
• Causes
– Mild euphoria and brain damage
– Alterations to vision and judgment as well as impaired
motor coordination with slurred speech
• Heavy users may experience
– depression, anxiety, hallucinations, paranoia and
psychotic symptoms
• Banned in the US in 1937
– recently has been legalized in a few states for medical
use in seriously ill patients