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Nervous System
Biology 30
Nervous System Function
• Did you know that 1 cm3 of your brain
contains about 50 million neurons (nerve
cells)?
• Each neuron may communicate with
thousands of other neurons forming intricate
networks that control our functions and store
our thoughts.
• The nervous system has 3 major functions.
– Sensory input moves signals from our various
sense organs to the brain.
– Integration is the interpretation of those signals
and the formation of an appropriate response.
– Motor output is the conduction of signals to the
body’s effectors – muscles and glands.
Nervous System Function
The Neuron
Dendrite – receives information from receptors or other neurons and
conducts nerve impulses toward the cell body.
Synaptic Knob – aids in nerve impulse transmission
Cell Body – contains nucleus and organelles
Nodes of Ranvier – gaps within the myelin sheath
Impulses jump from node-to-node therefore
speeding up the impulses
Neurillemma – delicate membrane that promotes regeneration of
damaged neurons
Only found in myelinated neurons
Myelin
– a fatty
protein
the axon
Axon – conducts nerve
impulses
away
fromthat
the covers
cell body
Composed of Schwann cells, which help regenerate
damaged neurons
Insulate the axon allowing nerve impulses to travel
faster
Myelination is only found outside the brain and spinal
cord
Types of Neurons
Sensory Neurons
(Afferent Neurons) –
conducts nerve
impulse from sense
organs to the brain
and spinal cord (CNS)
Interneuron
(Association Neuron) –
found within the CNS
No myelination
Intergrates and
interprets sensory
information and
relays information to
outgoing neurons
Motor Neuron
(Efferent Neurons) –
conducts nerve
impulses from CNS to
muscle fiber or glands
(effectors)
• The two major components of the nervous system are the central
nervous system (CNS) consisting of the brain and spinal column and
the peripheral nervous system (PNS) consisting of a vast network of
neurons held together by connective tissue in bundles we call nerves.
• The PNS also contains ganglia (s: ganglion) which are the cell bodies
of
• neurons found in the nerves.
• A quick summary of the parts of the nervous system can be seen in the
knee-jerk reflex.
• When the patellar tendon is tapped, sensory neurons detect the
stretching and send a signal to the CNS (in this case, the spinal
• cord).
• The information goes to a motor neuron and to an interneuron.
• The motor neuron stimulates the contraction of the quadriceps muscle
which extends the leg, while the interneuron sends an inhibitory signal
to the flexor muscle.
Threshold Levels
Threshold Level – minimum level of stimulus required to produce a
response (action potential)
Stimulus
Muscle Contraction
1 mV
--
2 mV
--
3 mV
5N
4 mV
5N
All-or-None Response – neuron either fires maximally or not at all.
Variation in response is due to the number of neurons firing or the
frequency in which they fire.
Neural Ion Transport
• Nerve impulses are initiated by the
movement of sodium (Na+) and potassium
(K+) and changes the polarity (+ and -) along
the cell membrane. The movement is
controlled in two ways:
– Protein Gates – voltage sensitive gates which
open when the neural membrane is stimulated.
Ion move in and out by diffusion.
– Sodium-Potassium Pumps – actively transports
Na+ and K+ ions across the neural membrane
Nerve Impulse
Nerve impulse animation
Resting
Potential
Resting Membrane
Action
Repolarization
Potentialor
(Depolarization)
Refractory Period
•Characterized
byby
~+ve
charge
ononthe
outside
•Characterized
•Characterized
by
~-ve
~+ve
charge
charge
on
the
the
•Time
takes toonrestore
the
~-veit charge
the inside
outside
outside
ion~Na+/K+
concentration.
No impulse
maintaining
~+ve
~-vecharge
charge
on
onthe
thepolarity
inside
inside
can
occur
during
this
time
(1 are open
•Na+ gates are closed,
some
of the
K+and
gates
~Na+
~K+
gates
open
Na+ rushes
to
10
milliseconds)
(allowing
to leak
of the K+
membrane)
in•OnceK+
polarity
is out
reversed
gates open and K+
•Na+/K+
pumps
pump
Na+ in 2 K+
•Na+/K+
pumps
pump
outpolarity
3has
Na+reached
and pump
•Occurs
rushes
once
out tothe
restore
neuron
the threshold
out
and
K+
in
to
restore
the
•Resting
level potential is about -70 mV (millivolts)
ion concentration.
•Na+ gates open and Na+ rushes in, reversing
polarity
Measuring Potential
Transmission Across A
Synapse
• Steps in neural transmission:
– Nerve impulse causes synaptic vesicles to fuse
with the presynaptic membrane and release
neurotransmitter into the synaptic cleft.
– Neurotransmitters diffuse across to the
postsynaptic membrane
– Neurotransmitters bind with postsynaptic
receptors in a lock-and-key fit.
– Neurotransmitters can result in excitation or
inhibition of next neuron. If excitatory the sodium
gates of the next neuron open and the action
potential continues
Neurotransmitters
Norepinephrine
excitatory
Acetylecholine
––mostly
excitatory
Dopamine
Serotonin
– excitatory/Inhibitory
- excitatory
Norepinephrine,
also
called
noradrenaline
Acetylcholine
released
where
nerves
Dopamine
Serotonin
affects
isisnormally
brain
processes
involved
inthat
is
atemperature
neurotransmitter
that doubles
part-time
meet
muscles
(neuromuscular
junction)
control
movement,
regulation,
emotional
sensory
response,
and
as
aability
As
amood
neurotransmitter,
and
ishormone.
therefore
responsible
for muscle
theperception,
to experience
and
pleasure
control.
However,
and pain.it
norepinephrine
helps
regulate
arousal,
contraction.
After
acetylcholine
Regulation
plays a major
of
dopamine
role
intoemotional
plays astimulates
crucial
disorders
role
dreaming,
moods.
As
ahealth.
hormone,
it
its
receptors,
itand
is quickly
inactivated
and
in our
such
mental
as and
depression,
physical
suicide,
impulsive
acts
to increase
pressure,
constrict
destroyed
byand
an blood
enzyme
known as
behavior,
aggression
blood vessels and increase heart rate cholinesterase
responses that occur when we feel stress.
Vertebrate Nervous System
• In all vertebrates, the CNS consists of the brain
and spinal cord, although the ability to integrate
stimuli varies greatly from fish to humans.
• The spinal cord, buried inside the vertebrae,
receives sensory information from the skin and
muscles and integrates simple responses such as
the knee-jerk reflex.
• The brain includes homeostatic centers that keep
the body functioning smoothly; sensory centers
that integrate data from sense organs; and centers
of emotion and intellect.
• The brain also sends out motor commands to
muscles.
Vertebrate Nervous System
• The CNS is serviced by a vast network of capillaries that
secrete cerebrospinal fluid into the fluid-filled spaces of the
brain (ventricles) and spinal cord (central canal).
• These capillaries are quite selective in allowing nutrients to pass
into the brain, but not waste products.
• This selective mechanism is called the blood-brain barrier and
helps maintain a stable environment for the brain.
• The brain and spinal cord are protected cerebrospinal fluid
under tough connective tissues called the meninges.
• Infection of these layers is called meningitis and is extremely
serious.
• The white matter of the CNS is mainly myelin-coated axons
and the gray matter is mostly neuron cell bodies and dendrites.
• Cranial nerves carry to or from the brain to our eyes, nose,
ears.
• Spinal nerves carry information to and from the muscles and
skin.
Reflex Arcs
• Reflexes are autonomic, involuntary
responses to changes occurring inside or
outside the body.
• Pathways of a reflex arc:
– Receptor generates nerve impulses due to
stimulation
– Sensory neurons carries impulse to interneurons
in the grey matter of the spinal cord
– Interneurons pass impulse to motor neurons
– Motor neurons stimulate effectors
– Effector receives impulse and reacts; glands
secrete or muscles contract.
Organization of the Nervous
System
Peripheral Nervous System
• The PNS consists of cranial (12) and spinal
nerves that extend outside the CNS
– Somatic System contains sensory neurons from
sense organs and motor neurons to the muscles
– voluntary control.
– Autonomic System contains sensory neurons
from viscera and motor neurons to control glands,
heart and smooth muscle of internal organs –
involuntary control.
Autonomic Nervous System
• Sympathetic Pathway: Fight or flight
– Neurons run from
the thoracic lumbar
Parasympathetic
Pathway
region of the spinal cord and into the
Neurons run from the sacral part of the
associated organs
spinal cord to the associated organs
– Used in emergency situations and
Release ACh and promotes a relaxed
associated with the fight or flight response
state reducing the effects of the
– Axons release norepinephrine as a
sympathetic
pathway
neurotransmitter
(inhibits digestion,
accelerates heart rate ad breathing rate)
CNS – Spinal Cord
• Spinal
cord
is surrounded by vertebrae;
Spinal
cord
communicates
brain is enclosed
inside
•Functions
a center for
reflexshell
arcs
brainare
andwrapped
spinal
• •Communicates
The brain and between
spinal cord
nerves
in three membranes (called meninges)
•Grey matter (letter H) – made from
that are filled with cerebrospinal fluid
unmyelinated neurons
that cushions
and protects.
•White
matter carries
information in tracts to
and from the brain
•Tracts cross over. Left side of the body is
controlled by the right side of the brain and the
right side of the body is controlled by the left
side of the brain.
The Brain
• The brain evolved from a set of three hollow
bulges at the anterior end of the neural tube
called the forebrain, midbrain, and hindbrain.
• This evolutionary progression is recapitulated (to
repeat the principal stages or phases) during
embryonic development, especially in mammals
and birds, as these structures become more
complex and specialized.
• Evolution of the most complex vertebrate
behavior correlates to the development of the
cerebrum.
• This portion of the forebrain undergoes the
greatest amount of development in humans who
have the largest brain surface area (gray matter)
relative to body size of all animals.
• With over 100 billion interconnected neurons the
human brain has more integrative power than any
computer.
• The ancestral hindbrain and midbrain, now called
the brain stem, has become the medulla
oblongata and the pons.
• All sensory and motor neurons carrying information
to and from the higher brain pass through here.
• Another development of the hindbrain, the
cerebellum, acts as the coordination centre for
body movement.
• Although you may consciously plan to go for a
walk, the cerebellum provides the complex
coordination required by integrating sensory input
about your movement with the necessary signals to
the motor neurons connected to the various
muscles used.
• The greatest sophistication occurs in the forebrain, consisting of
the thalamus, hypothalamus and cerebrum.
• The thalamus acts as a switchboard sending information to the
appropriate higher centers of the brain for further interpretation.
• The hypothalamus controls the pituitary gland and the secretion
of many hormones.
• The hypothalamus also regulates body temperature, blood
pressure, hunger, thirst, pleasure and the “fight or flight”
response. It also regulates our biological clock that maintains our
circadian rhythm.
• It is highly susceptible to the effects of drugs and its normal
functioning can be impaired making the person addicted.
• The cerebrum is the largest and most sophisticated part of our
brain.
• A thick band of nerve fibers called the corpus collosum
connects the left and right cerebral hemispheres.
• Under the corpus collosum are clusters of neuron cell bodies
called the basal ganglia (or basal nuclei) that are important in
• motor coordination.
• Degeneration of the basal ganglia occurs in Parkinson’s disease
and results in tremors eventually followed by paralysis.
• The cerebral cortex is a mosaic of specialized,
interactive regions.
• 5 mm thick, the cerebral cortex accounts for
80% of the brain’s total mass with some 10
billion neurons and hundreds of billions of
connections.
• This is where our reasoning, imagination,
artistry and personality exist.
• It integrates input from our senses and
regulates our voluntary movements.
• Oddly, each hemisphere receives information
from and controls movement on the opposite
side of the body.
• The corpus collosum communicates between
the two hemispheres.
• At the boundary between the frontal lobe and the parietal
lobe is the motor cortex and the somatosensory cortex.
• The somatosensory cortex receives touch, pain and
temperature from the body.
• Across the fissure, the corresponding motor cortex controls
body movements.
• Neurons associated with the head are located at the
bottom of the fissure and those of the toes are at the top.
• There are considerably more neurons associated with the
face and hands than others.
• Language results from some extremely complex
interactions among several association areas.
• Reading, writing, and speaking involve rapid interactions
between our visual association area in the occipital lobe
(to process the shape of words on a page), the auditory
association area in the temporal lobe, and the speech
areas of the parietal and frontal lobes.
• Lateralization refers to the specialization of the
hemispheres of the brain.
• The left hemisphere becomes adept at language,
logic, and mathematical operations.
• It has a bias for detailed skeletal motor control and
processing of fine visual and auditory details.
• The right hemisphere is stronger and spatial
relations, pattern and facial recognition, musical
ability, and emotional processing in general. In
about 10% of people, this lateralization may be
reversed or not observed.
Brain Stroke
Hemorrhage
Brain Cross Section (Vertical)