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The Nervous System
Components: Two types of Cells (Nerrons and Neuroglia) Brain, Spine and Peripheral Nerves
Functional Classification of Neurons
1. sensory (afferent) neuron - transmit impulses from sensory receptors TOWARD the CNS
a. almost all are unipolar and located just outside the spinal column
i. Dorsal Root Ganglion of the spinal cord (sensory info from body)
2. motor (efferent) neuron - transmit impulses AWAY FROM the CNS to the target tissue
a. almost all are multipolar, with cell bodies in the CNS
3. association neuron (interneuron) - between sensory and motor neurons. Occurs in brain and spinal cord.
Components of Nervous System : brain, spinal cord and peripheral nerves.
A. Central Nervous System (CNS) - brain and spinal cord
B. Peripheral Nervous System (PNS) - spinal/cranial nerves
1. Sensory (Afferent) Division - TO the CNS
a. somatic afferents - from skin, muscle, joints
b. visceral afferents - from membranes & organs
2. Motor (Efferent) Division - FROM the CNS
a. Somatic Nervous System (Voluntary) - to skeletal muscles
b. Autonomic Nervous System (Involuntary) - to organs & glands
i. Sympathetic Division
ii. Parasympathetic Division
Nerves are made of bundles of neurons and neuroglial cells.
Neurons are functional cell of Nervous System: Three parts Axon, Dendrites and Cell Body
1. amitotic - "not mitotic"; they cannot reproduce or regenerate after certain point in life
2. longevity - neurons can survive entire lifetime
3. high metabolic rate - require OXYGEN and GLUCOSE at all times
Dendrites are plasmamembrane extentions; this is the
sensing end
Cell body contains all organelles including nucleus with
genetic information that directs synthesis. Dendritic end
surrounds cell body.
Axon is elongated region terminates at the axon
terminus; the location of the synapse. Action potential
develops in the axon.
axon depends upon the cell body for everything:
organelles, proteins, and enzymes for synthesis of
neurotransmitter
In humans, neurons can grow up to a meter long. All
the functions of the nervous system involve neurons
communicating either with one another, or with cells of
other types.
Three types of neurons
Sensory Neurons, sense conditions both inside and outside the body include receptors for touch, temperature and
pressure and sight, smell, hearing and taste. They carry information from the sensory receptors to the CNS.
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Interneurons are located only in the brain and spinal cord. They are responsible for analysis, integration or
processing of sensory inputs and coordination of motor output. The CNS consists of the brain and spinal cord, they
are responsible for integrating and coordinating sensory data and issuing action or motor commands.
Motor neurons carry signals from the CNS to effectors, such as skeletal muscles or gland cells. Effectors
responding to the signal usually with a motor output
Sensory Neurons   
Interneurons
   Motor Neurons
Cells of Nervous Tissue
1. Neuron: Nerve cell. Structural and functional
unit of nervous tissue. Carry signals from one part
of the body to another. Note the three parts of the
neuron: Dendrite, Axon and Synapse
2. Supporting cells: Nourish, protect, and insulate
neurons. There are roughly 50 supporting cells for
every neuron. On this figure they are the Schwann
cells.
In humans, Schwann cells wrap around the
axons of neurons, forming a myelin sheaththat is
essential for transmission of nerve impulses.
Support Cells of the Nervous System
Neuroganglia Support Neurons. Perhaps there are 50 neuroglia to every 1 neuron. These cells protect, insulate
and reinforce the neuron.
1.
Support Cells of the CNS (Glial Cells
a. astrocytes - regulate environment around neurons and selective transport from capillaries
b. microglia - eat infectious microbes of CNS
c. ependymal cells - line cavities of brain and spinal cord, flushing cerebrospinal fluid
d. oligodendrocytes - form "myelin sheaths" around axons of CNS; increase speed of impulses
2. Support Cells of the PNS
a. Schwann cells - form "myelin sheaths" around axons; also assist in regeneration of axon
b. satellite cells - control chemical environment
myelin sheath - wrap of Scwhann cells (PNS) and
oligodendricytes (CNS) around the axon
a. increases speed of action potential signal [myelinated
(150 m/s); unmyelinated (1 m/s)]
b. nodes of Ranvier - gaps between myelin cells at
regular intervals on axon
c. white matter of brain - areas with myelinated axons
d. gray matter of brain - areas with cell bodies and
unmyelinated cell processes
More Vocabulary:
ganglion - a collection of cell bodies in the PNS compared to nucleus - a collection of cell bodies in the CNS
tract - a bundle of axons in the CNS compared to a nerve - a bundle of axons in the PNS
axolemma - plasma membrane of neuron
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axon hillock - the cone-shaped region of attachment of the axon to the cell body; site
where action potential is triggered
Communication between nerve cells is a result of changes in the electrical properties of the neuron’s plasma
membrane. It involved stationary Organic Molecules found inside and the movement of Na and K ions (in
opposite directions).
Stimulation causes a neuron to change from a resting potential to go to action potential. A resting nerve cell has a
negative charge inside relative to outside. The negative charge inside is maintained by large organic molecules
inside (having a negative charge) and an excess of K+. Neurons change from Polarized to Depolarized to
Repolarized. Resting is Polarized, Action is Depolarized , return to rest is Repolarized
1. Polarized State when neuron has a Resting potential
(-70mV)
Na+-K+= ATPase Pump
+
[Na ]out > [Na+]in
[K+]out < [K+]in
+
K Leak Channels
K+ leaks out of the cell
Cl levels
[Cl-]out > [Cl-]in1.
2. Depolarization due to opening of Na+ channels
When the membrane at the axon hillock is depolarized
to a threshold level (-50 mV), voltage-gated Na+
channels are triggered to open, allowing Na+ to rush in,
causing further depolarization, and even more
Na+channels to open. This phenomenon spreads down
the axon like a series of falling dominos, in an "all-ornone" fashion.
2. immediate closure of the voltage-gated Na+ channels
so that Na+ can no longer enter the cell, and the resting
potential can be regenerated.
3. repolarization due to opening of K+ channels
As the Na+ channels close, voltage-dependent K+
channels open, allowing even more K+ to rush out of the
cell, until the resting membrane potential is restored.
Basic Principles of Electricity (A quick review)
A. voltage (potential difference / potential) - measure of the potential energy that results from the separation of
Positive and Negative charges
1. more charge separated = larger voltage; less charge separated = smaller voltage
2. volts - units of voltage Incells; millilvolt (mV) = 1/1000 volt (typical unit used for membrane voltages)
B. current - the flow of electrical charges from one area to another (eg. Na + into a cell)
1. currents in the body are usually the flow of ions (Na+, K+, Cl-, Ca++)
2. voltage - greater the separation of charge, the more "potential energy" for current to move
3. resistance - the hindrance to the flow of charge through which current must pass (plasmamembrane and Ion
Channels)
C. Regulation of Current/Voltage - Changing Resistance (Permeability) of Cell Membrane
1. leakage channels - channels that are always open (eg. K+ leakage channels)
2. chemical-gated (ligand-gated) channels - open or close when bound by a specific molecule (eg.
neurotransmitter: ACh, serotonin, etc.)
3. voltage-gated (dependent) channels - open or close depending on the voltage across
membrane
D. electrochemical gradient - net result of both the "electrical gradient" and "chemical gradient"
1. electrical gradient - positive charges move toward negative charges and vice versa
2. chemical gradient - diffusion from area of high concentration to low concentration
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SYNAPSE - the junction of a neuron that allows transfer of message to "postsynaptic cell" (eg.
another neuron, muscle fiber, gland, etc.)
1. Depolarization of the Axon terminal Opens Voltage-Gated Ca++ Channels - allowing Ca++ to rush INTO the cell
down its concentration gradient
2. Increased Ca++ In the Synapse Causes Neurotransmitter Release - As Ca++ increases in the axon terminal,
synaptic vesicles containing the neurotransmitter fuse with the plasma membrane, releasing contents into the
synaptic cleft
3. Neurotransmitter Binds Receptor - Opens Ion Channels - The released neurotransmitter crosses the synaptic cleft,
reversibly binds to receptors,
Inhibitory or Excitatory? - the action of a neurotransmitter can be either excitatory (allow Na + in) or
inhibitory (allow Cl- in), depending on what type of channel it opens
Neurotransmitters
 Acetylcholine: active in CNS and PNS; inhibits (slows) heart rate but excites skeletal muscle cells.
 Serotonin sleep, emotions, perception and thermoregulation
 Dopamine (affect sleep, mood, attention and learning)
 Epinephrine and (increases heart rate) Norepinephrine acts on smooth muscles
 GABA an inhibitory neurrotransmitter
Dopamine blockers - used to treat Schizophrenia (thorazine & haloperidol)
Amphetamines - activate Dopamine, Serotonin, and NE receptors (speed, crank)
NE and Serotonin reuptake inhibitors - used to treat depression (Prozac)
L-Dopa used to treat Parkinson's Disease
The Brain is the master control center, directing output through the spinal cord and including homeostatic centers,
sensory centers and centers of emotions and intellect. Information is carried to and from the brain by the cranial
nerve and to and from the spinal cord by the spinal nerve. In the interiors of the brain is a hollow space filled with
fluid called the cerebrospinal fluid. Neurons in the brain are surrounded by myelin.
Myelin is the fatty covering that insulates nerve cell fibers in the brain and spinal cord to assist the high-speed
transmission of electrochemical messages between the brain, the spinal cord, and the rest of the body. When myelin
is damaged, transmission of messages may be slowed or blocked completely.
The peripheral nervous system forms a vast intercommunicating network. Motor neurons are either under
voluntary control (somatic system carrying messages to skeletal muscles) or out of conscious control (autonomic
system carrying messages to glands and smooth muscles.
The autonomic system is either SYMPATHETIC or PARASYMPATHETIC. The two components act to
regulate the internal environment.
Parasympathetic division is associated with REST AND REPOSE it primes the body for digesting food
and resting, activities that gain and conserve the body’s energy supply. Including stimulation of all digestive
processes and slowing the heart and breathing rates.
Sympathetic division is associated with FIGHT OR FLIGHT. It prepares the body for intense, energyconsuming activities. Included is inhibition of digestion, increasing the heart and breathing rates, stimulation of liver
to release glucose and adrenal glands to release the fight or flight hormones (epinephrine and norepinephrine).
Neurotransmitters and Moods:
When your neurotransmitters are low, you are at a higher risk of developing mood-related medical problems.
When your body experiences stress, it releases neurotransmitters in response to the stress. The more stress, the more
neurotransmitters are released. Extreme cases, you can deplete your store of neurotransmitters. Many antidepressant
drugs are serotonin reuptake inhibitors.
What is Parkinson's disease?
Parkinson's disease is a slowly progressing, degenerative disease that is usually associated with the following
symptoms, all of which result from the loss of dopamine-producing brain cells: tremor or trembling of the arms, jaw,
legs, and face.
Brain
The adult brain weighs 1.3 to 1.4 kg (approximately 3 pounds).
The brain contains about 100 billion nerve cell called Neurons and trillons of "support cells" called glia .
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The Three Lobes of the Brain (HIND-, MID- AND FOREBRAIN)
Hindbrain: includes the Brain Stem and Cerebellum
1. The brain stem is the area of the brain between the thalamus and spinal cord and includes the medulla,
pons and reticular formation.. Functions of the Brain stem :Involved in involuntary activities such as
Breathing, Heart Rate and Blood Pressure
2. The cerebellum "little brain." is located behind the brain stem and like the cerebral cortex it is divided into
hemispheres and has a cortex that surrounds these hemispheres. Function: coordinated muscle movements
Midbrain:
Function:integrates auditory information, coordinates visual reflexes and relays sensory data as well as Eye and
Body Movement
Forebrain: includes Thalamus, Hypothalamus, Pituitary Gland, Cerebral Hemispheres and Limbic System
Functions: site of most sophisticated integration
 Thalamus: Contains cell bodies or neurons and relays information to the cerebral cortex. It receives
sensory information and relays it and vice versa.
 Hypothalamus: Regulates homeostasis, i.e. controls hormonal output of the pituitary gland. It controls the
pituitary gland, body temperature, blood pressure, hunger, thirst, biorhythms, sexual urges, emotions.
 Limbic System: is a group of structures that includes the amygdala , the hippocampus, mammillary bodies
and cingulate gyrus. These areas are important for controlling the emotional response to a given situation.
o The hippocampus is also important for learning and memory
o The amygdala is the seat of emotion
 The cerebrum has two hemispheres that divides it into left and right halves. Each hemisphere
communicates with the other through the corpus callosum , a bundle of nerve fibers.
The rational left (logic and mathematical ability)
The intuitive right (imagination, special perceptions artistic ability and emothions)
The four lobes of the cerebral hemispheres do specific tasks
Rich bed of neurons and synapses. It produces reasoning, language, imagination, artistic talent and personality
1. Temporal lobe: hearing, auditory association
2. Frontal lobe: smell; speech
3. Parietal lobe: taste, speech, reading
4. Occipital lobe: vision
 The basal NUCLEI are a group of structures important in coordinating movement. Parkinson's disease
believed to be caused by degeneration of neurons here.
SPINAL CORD
It is between 45-70 cm long and weighs about 35-40 grams and is protected by the bony vertebrae.
The vertebral column, the collection of bones (back bone) divided in cervical, thoracic, lumbar, sacrum and coccyx.
A. reflex - a rapid,"wired" motor response to a specific stimulus; involuntary, subconscious
1. somatic reflex - skeletal muscle
2. visceral (autonomic) reflex - smooth muscle, cardiac muscle, visceral organs, glands
B. Components of a Typical Reflex Arc; Sensory Receptor----Interneuron------Motor Neuron-Effector
The Effector - the muscle or organ that responds to signal of the motor neuron
Example:
1. The Patellar Tendon (Knee-Jerk) Reflex
a. reflex hammer hits patellar tendon, stretching the quadriceps muscle, fibers of muscle spindle are
stretched, sensory (afferent) fibers are activated, alpha motor neuron to quadriceps activated, resulting in quadriceps
contraction , alpha motor neuron to hamstrings inhibited (via interneuron), resulting in hamstring relaxation
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2. Plantar (Cutaneous) Reflex - gently stroking the lateral aspect of the bottom of the foot (test L4 - S2)
a. normal adult - the toes will flex (curl down)
b. Babinski sign – in infant (up to 6 months) and in adults with damaged corticospinal tract, toes will
fan out
3. Opposite of Stretch Reflex - the stretched or contracting muscle responds by relaxing
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