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The Nervous System
Functions of the Nervous System:
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monitors external environments and integrates sensory information
monitors the internal environments
coordinates the voluntary and involuntary responses of the body
Note: The nervous system works along with the endocrine system to coordinate activities in order to maintain
homeostasis.
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Nervous System – responds swiftly, yet briefly
Endocrine System – responds more slowly, but the effects are longer lasting
The Nervous System
Central Nervous
System (CNS)
S)(CNS)
Brain
Spinal Cord
Peripheral Nervous
System (PNS)
Afferent
Division
Somatic
Sensory
Receptors
Efferent
Division
Visceral
Sensory
Receptors
Somatic
Nervous
System
Skeletal
muscle
Autonomic
Nervous
System
Sympathetic
Nervous System
Parasympathetic
Nervous System
Smooth muscle, cardiac muscle, and glands
Cells within Neural Tissue:
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Neurons = communicate with other cells
Neuroglia or glial cells = regulate the environment around the neurons
Neuroglia / Glial cells
Neuroglia of the CNS:
1. Astrocytes – largest and most numerous
 Secretes chemicals to make the capillaries inside the brain impermeable to most substances
 This creates what is called the blood-brain barrier
2. Oligodendrocytes
 Create myelin which is rich in lipids and white in color creating “white matter”
 Myelin insulates the neurons and helps increase the speed of the impulse traveling down
the neuron
 Areas that are not myelinated are called the nodes of Ranvier
 Areas that are myelinated are called internodes
 Note: Not all neurons in the CNS are myelinated
 Unmyelinated neurons have a speed of impulse is around 1 meter per second
 Myelinated neurons have a speed of impulse is between 18 – 140 meters per second
3. Microglia – smallest and most rare
 white blood cells that have come from the capillaries
 engulf cell waste and pathogens
4. Ependymal cells
 lining of the ventricles (cavities) within the brain
 some areas produce cerebrospinal fluid (CSF) to bathe the brain and spinal cord
 also contain cilia to circulate the CSF
Neuroglia of the PNS:
5. Schwann cells
Similar in function to oligodendrocytes
All neurons of the PNS are myelinated
MS is a disorder in which demyelination of the myelin sheaths occurs followed by inflammation
and damage to the affected axons
Neurons
Neurons contain:
1. cell body
2. dendrites = receive incoming signals
3. axon = carries the outgoing signal or
impulse
4. Synaptic terminals (knobs) = end of the
axon where chemicals are released to
communicate with the next cell
5. no centioles so neurons cannot divide
and replace damaged cells
6. ribosomes and rough endoplasmic
reticulum that give the cell bodies of
neurons the gray color (“gray matter”)
Types of neurons (classified by structure)
1. multipolar neurons – most neurons of the CNS and motor neurons of the peripheral nervous system
2. unipolar neurons – sensory neurons of the peripheral nervous system
Types of neurons (classified by function)
1. sensory neurons
about 10 million
bring information to the brain about the internal or external environment
2. motor neurons
about ½ million
bring commands to the muscles or glands from the brain
3. interneurons or association neurons
about 20 billion
neurons in the brain and spinal cord
How neurons work:
1. The neuron is normally at rest. At this point in time, the difference in charge between the outside
and the inside of the cell is -70 mV. This difference exists because there are more positive ions
outside the cell and fewer positively charged ions inside the cell.
2. Part of the neuron received a stimulus. Stimulus could be chemical, mechanical pressure,
temperature change, or changes in ion concentrations
3. If the stimulus is strong enough or long enough to reach a certain level known as the threshold, the
neuron will trigger. Once the neuron is triggered it cannot be reversed. This is called the all-ornone principle.
4. The action potential (impulse) will travel down
the neuron as new areas of the cell membrane
open sodium gates and allow sodium ions to flow
into the cell. This step makes up what is called
the depolarization period. At this point in time,
the difference in charge between the outside and
the inside of the cell is +30 mV. If the neuron is
myelinated, the impulse will skip over these
insulated areas and travel instead from node to
node.
5. Once the Na+ ions have rushed in, the cell
membrane open K+ gates to allow K+ ions to
leave. This step makes up what is called the
repolarization period.
6. Along with the repolarization period, the last
step in the process makes up the rest of what is
called the refractory period. The refractory
period is the time it takes to get the neuron back
to a state in which it could fire again. This is
accomplished using sodium-potassium pumps
within the cell membrane. These pumps move 3
Na+ ions into the cell for every 2 K+ ions that are
moved out.
At the synapse
1. The action potential arrives and depolarizes the presynaptic membrane
2. Calcium channels are briefly opened to let in Ca++ ions
3. The Ca++ ions trigger exocytosis of synaptic vesicles containing neurotransmitters
4. Some neurotransmitters bind to receptors on the postsynaptic membrane and open sodium
channels. This depolarizes the postsynaptic membrane and transmits the impulse. Other
neurotransmitters inhibit the next neuron by opening the potassium channels instead. This
hyperpolarizes the postsynaptic membrane (to -80 mV) making it more difficult for the membrane
to reach the threshold of +30 mV and trigger the neuron.
5. The neurotransmitters are quickly broken down by enzymes within the synaptic cleft or gap
Neurotransmitters
Over 50 total, many whose functions are not well understood yet
1. Excitatory
 Acetylcholine
 Norepinephrine
2. Inhibitory
 Dopamine
 GABA = gamma aminobutyric acid
 Serotonin
Organization of Neurons
1. Divergence
One neuron spreads the impulse to many neurons
Example: the impulse created from a sound is brought to
multiple parts of the brain to be interpreted, compared to
sounds from the past, and reacted to
2. Convergence
A number of neurons come together to send the impulse to a
single neuron
Example: the impulse sent by the brain to the diaphragm to hold
your breath convergences with (and overrides) the usual
impulses that come from the midbrain and pons for breathing.
The Central Nervous System
The Meninges
Nervous Tissue is VERY delicate and has an extremely high rate of metabolism. (At rest, the 3 lb brain uses as
much oxygen as 61 lbs of skeletal muscle!) In order to properly protect it and supply it with oxygen and
nutrients that it needs, the CNS is surrounded by several important layers called the meninges.
Function of the meninges
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Protects brain from physical injury (shock absorption)
Protects against pathogens
Supplies oxygen and nutrients
3 layers of the meninges:
1. Dura Mater (means “hard
mother”)
 outermost layer
 made of 2 fibrous layers
 forms what is called the
dural folds in some areas of
the brain to help keep the
brain in position
2. Arachnoid
 thin middle layer but
contains collagen and
elastin fibers and
cerebrospinal fluid (CSF)
between the arachnoid and
the pia mater
 Important for shock
absorption
3. Pia Mater (means “delicate
mother”)
 highly vascularized
 supplies the underlying neural tissue with oxygen and nutrients
Associated terms:
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Epidural space: the space between the dura mater and the walls of the vertebrae (spinal column)which
contains loose connective tissue, adipose, and blood vessels
Meningitis = inflammation of the layers around the brain and/or spinal cord
The Spinal Cord
Serves as the “impulse highway” for incoming sensory messages and outgoing motor messages.
Also controls “spinal reflexes”
Characteristics of the Spinal Cord:
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About 18 inches long
Width generally decreases as the
spinal cord travels away from the
brain
Contains both white matter and
grey matter
The spinal cord exists as one unit
down to the 1st or 2nd lumbar
vertebrae where it then splits into
the cauda equine (“horse’s tail”)
Contains 31 segments total: C1 – C8,
T1 – T12, L1 – L5, S1 – S5, and Co1
Just outside of the spinal cord, each
segment contains a pair of dorsal
root ganglia = enlarged area of the
nerve bundle that contains the cell
bodies of the sensory neurons
Sensory axons (both somatic and
visceral) enter on the dorsal side of
the spinal cord (posterior grey
horns) while motor axons exit on
the ventral side of the spinal cord
(anterior grey horns)
The lateral grey horns of the spinal
cord contain the visceral motor
neurons
The spinal cord contains a central
canal filled with cerebrospinal fluid
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Posterior surface has the posterior
median sulcus (groove)
Anterior surface has the anterior
median fissure (deep crease)
Tracts are bundles of axons in the spinal
cord that share a common function,
origin, and destination
Columns are several tracts that run
together
Spinal Reflexes
Spinal reflexes are automatic motor responses (movements) to certain stimuli. These reflexes can be categorized as
simple reflexes or complex reflexes.
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Example of a reflex: knee jerk reflex. Sensory receptors sense that the muscle fibers have been stretched
suddenly and a signal is sent to the CNS.
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The wiring of a spinal reflex makes what is called a reflex arc.
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If it is a simple reflex, a signal will be sent directly from the spinal cord to a group of muscles to contract.
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If it is a complex reflex, a signal will be sent to one group of muscles to contract while inhibiting a signal to the
opposing set of muscles. There may also be signals sent to other sets of muscles to keep the body steady.
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As you can probably predict, complex reflexes take more time than simple reflexes.
The Ventricles of the Brain
1st, 2nd, 3rd, and 4th ventricles
The brain contains neural tissue, but it also has 4 internal cavities called ventricles that contain cerebrospinal
fluid (CSF). The four ventricles are connected with each other and with the central canal of the spinal cord.
Circulation of the cerebrospinal fluid
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CSF circulates within the four ventricles, down the central canal, and then out into the subarachnoid space
(between the pia mater and the arachnoid layers).
Eventually the CSF will enter slender extensions of the arachnoid that project into the dura mater (of the
brain) and finally into the superior sagittal sinus, a large cerebral vessel that moves blood back towards the
heart.
Cerebrospinal fluid
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On average a person has 150 ml of CSF in the brain and spinal cord. The CSF is
important because it bathes and protects the delicate brain tissue. CSF
transports nutrients, chemical messengers, and waste products. It also holds
the 3 lb. (1400 g) brain in fluid, making the brain buoyant and therefore lighter
(only 50 g).
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The linings of the ventricles are permeable so that the CSF is able to move in
among the brain tissue as well.
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CSF samples are sometimes taken using a procedure called a lumbar puncture to see if the fluid around the
brain is functioning normally.
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Areas in the ventricles called the choroid plexus contain capillaries that secrete CSF. CSF is produced at a
rate of 500 ml / day which means that enough fluid is made so that the body will completely replace the CSF
volume every 8 hours or so.
Major Parts of the Brain:
1. Cerebrum
2. Cerebellum
3. Brain stem (Midbrain, Pons, and
Medulla Oblongata)
4. Diencephalon (Thalamus and
Hypothalamas)
Cerebrum: where conscious thoughts take place
as well as sensations, complex movements,
memory, and other intellectual functions
Cerebellum: balance and coordination; adjusts
voluntary and involuntary motor activities
Midbrain: maintains consciousness; also processes visual and auditory information and generates involuntary motor
responses
Pons: involved in motor control; connects the cerebellum to the brain stem
Medulla Oblongata: relays information to the thalamus and other brain centers; regulates heart rate, blood pressure,
respiration, and digestion
Thalamus: relay station for sensory information
Hypothalamas: where emotions originate along with hormone production and several other autonomic controls
Cerebrum
1.
2.
3.
4.
Largest part of the human brain
Overall size is larger in males, however the corpus callosum is larger in females
Processes conscious thoughts and actions
Contains:
a. the cerebral cortex = grey matter on the outside of the cerebrum
i. Brain surfaces are not identical, but have similar patterns
ii. ridges are called gyri
iii. depressions are called sulci
iv. deeper grooves are called fissures
b. white matter (myelinated axons) in the center of the cerebrum
corpus callosum
c. a few areas of grey matter called basal nuclei beneath the cortex (interspersed within the white matter)
Common landmarks of the cerebrum:
1.
2.
3.
4.
5.
6.
7.
longitudinal fissure
left & right cerebral hemispheres
central sulcus
frontal lobe
lateral sulcus
temporal lobe
insula (hidden beneath the temporal and
frontal lobes)
8. parietal lobe
9. parieto-occipital sulcus
10. occipital lobe
http://www.umanitoba.ca/
faculties/medicine/units/a
natomy/jvbmrgrbrsp.html
May 19, 2009
Important motor and sensory areas of the cerebral cortex:
a. primary motor cortex / precentral gyrus
control direct voluntary movements
b. primary sensory cortex / postcentral gyrus
relay feelings of touch, pain, temperature, and pressure from the sensory neurons coming from the skin
c. visual cortex
receives visual signals
d. gustatory cortex
receives messages about
taste
e. auditory cortex
receives signals for hearing
f.
olfactory cortex
receives signals about scent
Other important areas of the cerebral cortex:
g. somatic motor association area / premotor cortex
Coordinates movements that have already been learned in the past.
h. somatic sensory association area
Interprets the incoming signals from the skin
Examples: You are too hot or cold. You have a wood tick crawling on you. That punch hurt.
i.
visual association area
Interprets what you are seeing
j.
auditory association area
Interprets what you are hearing
k. Wernicke’s area (general interpretive area)
Interprets written and spoken language
l.
Broca’s area (speech center )
Helps you put together words and speak
m. prefrontal cortex
Performs abstract functions using the help of all of the association areas
Predicts consequences of actions – may cause anxiety, fear, frustration, tension
Estimates time and sequence of events
(This is the area that scientists say teens have not developed well. This area is developed by the age of
25.)
n. corpus callosum
Links the two hemispheres and
interconnects areas within the
hemispheres as well.
Extensive system of
interconnecting axons (white
matter)
Hemispheric Lateralization
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When functions are particular to either the left
or the right side of the cerebrum
Left side:
 Analytical tasks: mathematics, logical
decision making
 Usually the Wernicke’s and Broca’s
areas are on the left side
Right side:
 Identification of familiar tastes, smells,
or touch
 Recognizing faces
 Understanding three-dimensional
relationships
Memory
A. Exists as fact memories (knowing information) or skill memories (knowing how to do something)
Skill memories involve both the cerebrum and the cerebellum to recall how you did something
B. Memory can be short-term (primary) memory or long-term memory
 Memory consolidation is the conversion of the short-term memory into long-term memory
 Hippocampus – a region of the cerebral cortex located in the temporal lobe and is associated with
learning and memory for processing spatial, visual, and verbal information. Also implicated in
converting short-term memories into long-term memories (the memories are then stored in either the
frontal or temporal lobes or both)
C. Memories are stored in the association areas of the part of the brain that was affected.
The Basal Nuclei
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Grey matter centers that are deep within the brain among the white matter
Regulate voluntary motor activities (planning, organizing, and coordinating voluntary movement) by
modifying instructions sent to the skeletal muscles by the primary motor cortex.
Involved in sensorimotor learning and expressions of emotional states such as smiling when happy, frowning
when sad, and running when afraid
Cerebellum
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Latin for “little brain” - smaller than the cerebrum
Also contains two hemispheres, but in contrast to the cerebrum, the right hemisphere of the cerebellum
receives sensations from, and controls movements of the right side of the body.
The cerebellum contains just as many neurons as both cerebral hemispheres combined.
The cerebellum is primarily a movement control center that has extensive connections with the cerebrum and
spinal cord.
Functions of the Cerebellum
1. Balance and posture
2. Cooperates with the basal nuclei and
cerebral cortex to coordinate refined
motor movement (smooth movements)
3. Also assists in sensorimotor learning
4. Smaller than normal cerebellum is linked
to autism
5. Physical trauma, strokes, and alcohol
strongly affect the functions of the
cerebellum (slurred speech, loss of
balance)
Diencephalon
Diencephalon– located on top of the brain stem and between cerebral hemispheres; composed of thalamus,
hypothalamus, and epithalamus
I.
Thalamus: relay station
 Acts as a “relay station” and transmits incoming sensory information to the appropriate areas of the
cortex for all senses except olfaction (olfactory signals go directly to the amygdala) and to the brain stem
and basal nuclei
 Only the signals that go to the cortex are the messages that we become aware of
 Also involved in motor activity
II.
Hypothalamus: below the thalamus
 Involved in hunger, thirst and water balance, sex, sleep, body temperature, metabolism, movement, and
emotional reactions
 Part of the hypothalamus is responsible for our circadian rhythms (periods of activity and rest)
 Damage to the hypothalamus can lead to uncontrollable laughter, intense rage, or aggression
 Also regulates the pituitary gland (an endocrine gland)
 Secretes hormones: ADH, oxytocin, and others
III.
Epithalamus: above the thalamus
 Contains the choroid plexus which is responsible for producing cerebral spinal fluid
 Contains pineal gland which secretes melatonin. Melatonin helps regulate the day/night cycles.
Brain Stem
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The brain stem is composed of the midbrain, pons, and medulla oblongata
The most primitive part of the mammalian brain
Relays information from the cerebrum to the spinal cord & cerebellum and vice versa.
Regulates vital functions such as breathing, blood pressure, consciousness, and the control of body temperature.
Damage to the brain stem most often leads to rapid death.
I. Midbrain – small part of the brain stem composed primarily of bulging fiber tracts
1) Superior colliculi – controls reflex movements having to do with visual stimulus (blinding light)
2) Inferior colliculi – controls reflex movements having to do with auditory stimulus (loud noise)
3) Cerebral peduncles –descending tracts from the eyes that go to the cortex and cerebellum
II. Pons – “bridge” – located just below the midbrain and connects the two halves of the cerebellum. Plays a role in
the integration of movements in the right and left sides of the body and is involved in the control of breathing
III. Medulla oblongata – most inferior part of the brain and merges into the spinal cord influencing flow of
information between spinal cord and brain. Regulates vital visceral activity including the coordination of
swallowing, coughing, sneezing, and vomiting. Also regulates heartbeat and blood pressure and the basic
breathing rate.
Reticular formation – extends from
the spinal cord through the hindbrain
and midbrain and into the
hypothalamus of the forebrain;
consists of over 90 nuclei (neurons).
Involved in respiration, coughing,
vomiting, posture, locomotion, and
REM sleep. Also vital to
consciousness, arousal, and
wakefulness. Damage to the reticular
formation disrupts the sleep-wake
cycle and can produce a permanent
coma-like state of sleep. Some
general anesthetics work by
deactivating the neurons of the
reticular formation stopping or
slowing sensory input that would
otherwise be experienced as pain.