...that the human brain contains more than
100,000,000,000 (100 billion) neurons?
...that nerve impulses can travel at 120
...that Prozac acts by inhibiting reuptake of serotonin
in brain neurons?
...that some neurons are over a meter long?
...that the cerebellum (the region behind the
brainstem) contains half of the neurons in the brain?
...that when areas of the brain are damaged other
areas can actually take over the job of the damaged
Nervous system Facts
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the basic Nerve cell
Neurons are electrically
excitable cells that
function to process and
Neurons are the core
components of the
brain, spinal cord, and
Neurons are typically
composed of a
1. soma, or cell body, a
2. dendritic tree and an
Neurons receive input on the
cell body and dendritic
tree, and transmit output
via the axon.
Neurons communicate via
chemical and electrical
synapses. The fundamental
process that triggers synaptic
transmission is the action
potential, an electrical signal
that is generated by exploiting
the electrically excitable
membrane of the neuron.
Schwann cells and the Myelin sheath
mainly provide myelin insulation to
axons in the peripheral nervous
system and increase impulse speed.
Node ofNode of
the gapsthe gaps
(about 1(about 1
in diameter)in diameter)
sheath cellssheath cells
along axonsalong axons
or nerveor nerve
Chemical synapses are
specialized junctions through
which cells of the nervous
system signal to one another
and to non-neuronal cells such
as muscles or glands. A
chemical synapse between a
motor neuron and a muscle
cell is called a neuromuscular
Chemical synapses allow the
neurons of the CNS to form
interconnected neural circuits.
They are thus crucial to the
biological processes that
underlie perception and
thought. They also provide the
means through which the
nervous system connects to
and controls the other systems
of the body.
eveNts leadiNg to the coNductioN oF
a Nerve impulse (Neuron membrane
via ion channels)
1. Neuron membrane maintains resting
potential ( + outside, - inside = polarized,
No nerve impulse,)
2. Threshold stimulus is received. (Stimulus)
3. Na+ channels open (Rising Phase)
4. Na+ diffuses inward, depolarizing the
membrane (Peak = inside less -, outside
5. K+ channels in membrane open. (Falling
6. K+ ions diffuse outward, repolarizing the
membrane. (Undershoot )
7. Resulting action potential causes
bioelectric current that stimulates
adjacent portions of the membrane.
8. A wave of action potentials travel the
length of axon as a nerve impulse.
Action Potential Animation
Neurotransmitters are chemicals that are used to relay,
amplify and modulate electrical signals between a neuron and
Some examples of neurotransmitter action:
Acetylcholine - voluntary movement of the muscles
Norepinephrine - wakefulness or arousal
Dopamine - voluntary movement and emotional arousal
Serotonin - memory, emotions, wakefulness, sleep and
GABA (gamma aminobutyric acid) - motor behavior
Glycine - spinal reflexes and motor behaviour
Neuromodulators - sensory transmission-especially pain
Epinephrine (or adrenaline) - is a "fight or flight" hormone which
is released from the adrenal glands whenever danger threatens.
Oxytocin (Greek: "quick birth") In the brain, oxytocin is involved
in social recognition and bonding, and might be involved in the
formation of trust between people.
Endorphins work as "natural pain killers”.
Glucagon helps maintain the level of glucose in the blood
In the nervous system,
afferent nerves (sensory
nerves) carry nerve
impulses from receptors
or sense organs toward
the central nervous
Efferent nerves (motor
nerves) carry nerve
impulses away from the
central nervous system to
effectors such as muscles
The cell body of the
efferent neuron is found in
the central nervous
system where it is
connected to a single,
long axon and several
short dendrites projecting
out of the cell body itself.
The structure of an afferent
neuron contains a single
long dendrite and a short
axon; the shape of the cell
body of an afferent neuron
is smooth and rounded.
The nervous system is divided into the
1. central nervous system (CNS) The CNS consists of the brain and spinal cord.
2. peripheral nervous system (PNS). The PNS consists of all other nerves and
neurons that do not lie within the CNS. The large majority of what are
commonly called nerves are considered to be PNS. The peripheral nervous
system is divided into the
1. somatic nervous system and the
2. autonomic nervous system. The Autonomic is further divided
2. Parasympathetic, and
3. Enteric Nervous systems
OrganizatiOn Of the nervOus system
The brain has three main parts that interact with the nervous system: the
cerebrum, the cerebellum, and the medulla.
Central nervOus system (brain and spinal COrd)
Examples of the
learning, while the
functions, such as
The spine is the area where
reflexes are made. Split-
second decisions do not go
back to the brain and then
back to the organ or body part.
For instance, if a ball was
thrown at an individual's head,
the reflex to move out of the
way would come from the
spine, not the brain, increasing
reaction time. The spine is also
the "highway" which passes
orders from the brain to motor
A diagram showing the CNS:
2. Central nervous system
(brain and spinal cord)
3. Spinal cord
develOpment Of the Cns
In the developing fetus, the CNS originates from the neural plate, a
specialized region of the ectoderm, the most external of the three
embryonic layers. During embryonic development, the neural plate folds
and forms the neural tube.
First, the whole neural tube will
differentiate into its two major
subdivisions: spinal cord
(caudal) and brain
Consecutively, the brain will
differentiate into the
brainstem (pons, medulla
oblongata, and midbrain) and
Later, the brainstem will subdivide into rhombencephalon and
mesencephalon, and the prosencephalon into diencephalon and
***The rhombencephalon gives rise to the pons, the cerebellum and the medulla
***The mesencephalon (midbrain) gives rise to the cerebral peduncle.
***The diencephalon give rise to the subthalamus, hypothalamus, thalamus, and
***Finally, the telencephalon (cerebrum) gives rise to the hippocampus and the
The CNS is covered by
the meninges. The brain
is protected by the skull
and the spinal cord by
rhOmbenCephalOn (pons, cerebellum and the medulla
pOns- is part of the CNS, and relays sensory
information between the cerebellum and
cerebrum.Some theories suspect that it has a
role in dreaming.
Cerebellum (“little brain") is a
region that plays an important role
in the integration of sensory
perception and motor output.
Because of this function, disorders
in fine movement, equilibrium,
posture, and motor learning are
associated with the cerebellum.
medulla OblOngata -controls autonomic
functions and relays nerve signals between the brain
and spinal cord.
The Medulla oblongata is responsible for controlling
several major autonomic functions of the body:
mesenCephalOn (midbrain, cerebral peduncle)
The mesencephalon is considered part of the brain stem. Its
substantia nigra (responsible for dopamine production in the brain, and
therefore plays a vital role in reward and addiction) is closely associated
with motor system pathways.
-this area contains
many nerve tracts
information to and
from the brain to the
rest of the body.
dienCephalOn (subthalamus, hypothalamus, thalamus,
The Thalamus also plays an important role
in regulating states of sleep and
wakefulness and consciousness. A human
with a severely damaged or severed
thalamus suffers permanent coma.
Many different functions are linked to the system to which thalamic parts
belong. The major function relates to sensory systems (except olfactory
function) auditory, somatic, gustatory and visual systems A major role of the
thalamus is devoted to "motor" systems.
The hypothalamus links the nervous system to the endocrine system via the
pituitary gland, also known as the "master gland," The major function is in
synthesizing and secreting neurohormones received from the pituitary gland.
Telencephalon Cerebrum (hippocampus and the neocortex)
hippocampus- plays a part in memory and spatial navigation.
The neocortex is the top layer
of the cerebral hemispheres.
The neocortex is part of the
cerebral cortex. It is involved
in higher functions such as
generation of motor
commands, spatial reasoning,
conscious thought, and
language. The neocortex
consists of gray matter
surrounding the deeper white
matter of the cerebrum.
The Telencephalon is the name for the large region of the brain (2 hemispheres-
Corpus Callosum communicates b/ 2 hemispheres) referred to as the cerebrum.
In Alzheimer’s disease, the hippocampus becomes one of the first
regions of the brain to suffer damage; memory problems and
disorientation appear among the first symptoms. Damage to the
hippocampus can also result from oxygen starvation (anoxia) and
The Frontal lobes have
been found to play a part in
impulse control, judgment,
language, memory, motor
function, problem solving,
spontaneity. The Frontal
lobes assist in planning,
and executing behavior.
People that have damaged frontal lobes may experience
problems with these aspects of cognitive function,
being at times impulsive; impaired in their ability to plan
and execute complex sequences of actions; perhaps
persisting with one course of action or pattern of
behavior when a change would be appropriate.
The Parietal lobe plays important roles in integrating
sensory information from various parts of the body, and in
the manipulation of objects. Portions of the parietal lobe are
involved with visuospatial processing. Much less is known
about this lobe than the other three in the cerebrum.
The Temporal lobes enclose the hippocampi and amygdala. The
posterior and lateral parts of the temporal lobes are involved in high-
level auditory processing (auditory cortex)
The amygdala perform primary roles in the
formation and storage of memories associated
with emotional events. Research indicates that
during fear conditioning, (including freezing or
immobility, tachycardia or rapid heartbeat,
increased respiration, and stress-hormone
release) sensory stimuli reach the amygdalae.
Damage to the
expression of fear
The Occipital lobe is the visual processing
center of the brain, containing most of the
anatomical region of the visual cortex.
through the optic
tracts to visual
cortex. Each visual
cortex receives raw
from the outside half
of the retina on the
same side of the
head and from the
inside half of the
retina on the other
side of the head.
White matter is one of the two main solid components of the
central nervous system.
It is composed of myelinated nerve cell processes, or axons, which connect
various gray matter areas of the brain to each other and carry nerve
impulses between neurons.
Generally, white matter can be
understood as the parts of the
brain and spinal cord responsible
for information transmission
(axons); whereas, gray matter is
mainly responsible for information
processing (neuron bodies).
White matter forms the bulk
of the deep parts of the brain
and the superficial parts of the
Multiple Sclerosis is one of the most
common diseases which affects
white matter. In MS lesions, the
myelin shield around the axons has
been destroyed by inflammation.
Gray matter is a major component of the CNS,
consisting of nerve cell bodies, capillaries, and short
nerve cell extensions/processes (axons and dendrites).
Gray matter is composed
of unmyelinated neurons
as opposed to white
The function of gray matter is to route sensory or motor
stimulus to the CNS for creation of response to stimulus
through chemical synapse activity.
Spinal cord segments
The spinal cord is divided into 31
different segments, with motor nerve
roots exiting in the ventral aspects
and sensory nerve roots entering in
the dorsal aspects. The ventral and
dorsal roots later join to form paired
spinal nerves, one on each side of the
There are 31 spinal cord segments:
8 cervical segments8 cervical segments Supply muscles and skin of neck, motor impulses to
diaphragm, arm and hand.
12 thoracic segments12 thoracic segments Supply motor impulses to upper abdominal wall muscles and
receive impulses from skin and abdomen
5 lumbar segments Muscles and skin of lower abdominal wall, genitalia, thighs,
legs and feet (includes femoral and sciatic nerves)
5 sacral segments See lumbar
1 coccygeal segment See lumbar
part of the
Peripheral nervous system
The peripheral nervous system (or PNS) consists of the nerves
and neurons that reside or extend outside the CNS to serve the
limbs and organs. Unlike the central nervous system, however,
the PNS is not protected by bone or the blood-brain barrier,
leaving it exposed to toxins and mechanical injuries. The
peripheral nervous system is divided into the somatic nervous
system and the autonomic nervous system.
Purely Sensory; carries signals to the brain related to olfaction
Optic Purely Sensory; carries signals to the brain related to vision.
Mixed Nerves; chiefly motor nerves of the eye controlling not
only the eye muscles, but also the iris and ciliary muscles.
Trochlear Mixed Nerves; serves some somatic motor fibers.
Purely Sensory; Conveys impulses from the
skin of the scalp, upper eyelid, nose,
lacrimal gland (tear duct), teeth, cheeks,
upper lip, tongue, and chin.
Mixed Nerves; primarily supplies somatic
motor fibers to lateral rectus muscle (an eye
Mixed Nerves; They are the chief motor
nevers of the face. There are five major
branches - temporal, zygomatic, buccal,
mandibular, and cervical.
carries signals to the
brain related to
Mixed Nerves; The
tongue and pharynx
and contain sensory
receptors for taste and
Mixed Nerves; The
smooth muscle of the
esophagus, trachea, the
pharynx and the palate. It
is connected to the
external ear, eardrum,
and the epiglotus.
Mixed Nerves; The
as well as the trapezius
Mixed Nerves; motor nerves
of the tongue.
The autonomic nervous system (ANS) is the part of the peripheral nervous
system that controls homeostasis. It does so mostly by controlling
cardiovascular, digestive and respiratory functions, salivation,
perspiration, diameter of the pupils, and micturition - (the discharge of
urine). Many of the activities of the ANS are involuntary. However,
breathing, for example, can be in part consciously controlled.
Zygote Media Group,
Sympathetic and parasympathetic divisions typically
function in opposition to each other. But this opposition is
better termed complementary in nature rather than
For an analogy, one may think of the sympathetic division as the
accelerator and the parasympathetic division as the brake. The sympathetic
division typically functions in actions requiring quick responses. The
parasympathetic division functions with actions that do not require
immediate reaction. Consider sympathetic as "fight or flight" and
parasympathetic as "rest and digest". The purpose of these two
complementary systems is to maintain homeostasis.
energy as it slows the
heart rate, increases
intestinal and gland
activity, and relaxes
sphincter muscles in
The enteric nervous system (ENS) is the part of the nervous
system that directly controls the gastrointestinal system. It has as
many as one billion neurons. The ENS is sometimes referred to as
the “second brain”.
muscles, the motor
churning of intestinal
contents. The enteric
nervous system also
makes use of the same
the CNS, such as
There are several reasons
why the enteric nervous
system may be regarded as
a second brain. The enteric
nervous system can operate
autonomously. It normally
communicates with the CNS
parasympathetic (via the
vagus nerve) and
systems. Studies show that
when the vagus nerve is
severed, the enteric nervous
system continues to
nervous system is
also for receiving
It is the system
activities that are
Neuromuscular junction 1. Axon
2. Synaptical junction
3. Muscle fiber