The Nervous System
I.
Response to Stimuli
A. your body responds to changes in the environment and adjusts itself
B. stimulus: any change inside or outside the body that causes to
respond
C. Internal control systems respond to maintain homeostasis
D. The nervous system controls and coordinates function throughout
the body and responds to internal and external stimuli
II.
Neurons
A. definition: nerve cell; working unit of the nervous system
B. receptors: located in skin and sense organs, these transmit stimuli
(nerve impulses) to different types of neurons
C. types1.
sensory neuron: receive info from sense organs and send
impulses to the brain and spinal cord
2.
interneurons: relay the impulses from the sensory neurons
once they reach the brain or spinal cord to the motor
neurons (most numerous)
3.
motor neurons: conduct impulses from the brain or spinal
cord to muscles or glands throughout the body
D. structure:
1.
cell body: contains nucleus and cytoplasm
2.
dendrite:
i. receives messages and sends them to the cell body
ii. many branched extensions
3.
axon:
i. long fiber that carries messages away from the cell body
ii. single branched end have swellings called terminals
iii. may have a myelin sheath: insulating membrane which
increases the speed at which impulses can travel
E. nerve: clusters of dendrites and axons
III.
The Nerve Impulse:
A. Definition: electrical signals carried by a neuron depending on the
movement of negatively charged electrons (e-)
B. Resting Potential in a neuron: the difference in electrical charge
across a membrane
1.
there are more potassium ions (K+) inside the cell than
outside
2.
there are more sodium ions (Na+) outside of the cell than
inside
3.
these concentrations must be maintained by active
transport
4.
negative charge builds up inside the cell as more K+ ions
diffuse out than Na+ diffuse in
C. The Moving Impulse
1.
a nerve impulse is caused by the movement of ions across
a membrane and begins when a neuron is stimulated by
another neuron or its environment
2.
a neuron’s cell membrane has 1000’s of protein channels
that allow ions to pass through when the “gates” are
opened
3.
as an impulse moves down a neuron, the “gates” are
opened and Na+ flows into the cell
4.
the cell membrane gains a “+” charge inside and a “–“
charge outside creating an action potential
5.
as the impulse passes, the K+ gates open and these ions
move out of the cell and the resting potential is reestablished
D. Threshold
1.
the strength of the impulse is always the same (you
respond or you don’t)
2.
the stimulus must be of adequate strength to cause an
impulse = threshold
E. synapse: location at which a neuron can transfer an impulse to
another cell
1.
a small cleft (gap) separates the axon terminal from the
dendrites of the adjacent cell
2.
3.
4.
5.
6.
the terminals contain tiny sacs filled with
neurotransmitters (chemicals used by a neuron to transmit
impulses across a synapse)
when an action potential arrives at the axon terminal, the
sacs release the neurotransmitter into the cleft
neurotransmitter diffuses across the cleft and attach
themselves to receptors on the membrane of the adjacent
cell
this stimulus causes Na+ to rush across the membrane
if the threshold is met, then the impulse begins an action
potential in the next cell
IV. Central Nervous System (CNS)
A. made of the brain and spinal cord
B. job: relay messages, processes information, and analyzes
information
C. brain
1.
made up of 100 billion neurons (mainly interneurons)
2.
divided into 3 major sections
i. cerebrum (largest)
- responsible for conscious/voluntary activities of the
body
- divided into 2 sections called hemispheres,
controlling the opposite side of the body
- the right hemisphere may control creativity and
artistic ability
- the left hemisphere may control analytical and
mathematical ability
- site of learning, judgment, and intelligence
- the cerebral cortex (outer layer) consists of grey
matter with densely packed nerve cell bodies
(ganglia) which process info from the sense organs
and control body movements
- the inner surface of the cerebrum consists of white
matter made up of myelin sheaths of the axons
ii. cerebellum
- behind and under the cerebrum
- coordinates voluntary muscle movement
- maintains balance and muscle tone
iii. brain stem
- extends out from the cerebrum and connects the
brain to the spinal cord
- made up of the midbrain, the pons, and medulla
oblongata: these regulate the flow of info to the
brain from the rest of the body
- controls heartbeat, breathing, and blood pressure
by coordinating involuntary muscle movements
iv. thalamus:
- found between the brain stem and cerebrum
- receives messages from the sense organs and relay
info to the proper section of the cerebrum
v. hypothalamus
- found between the brain stem and cerebrum
- control center for recognition of thirst, hunger,
fatigue, anger, and body temp
D. spinal cord
1.
extension of the brain stem
2.
processes reflexes
3.
made of bundles of neurons that carry impulses from all
parts of the body to the brain and from the brain to all
parts of your body
E. protection
1.
the cranium protects the brain
2.
the vertebrae protect the spinal cord
3.
both are wrapped in three layers of connective tissue
called the meninges
4.
space between these layers is filled with cerebrospinal
fluid that acts as a shock absorber
V.
Peripheral Nervous System (PNS)
A. all of the nerves outside of the central nervous system including the
spinal and cranial nerves which connect the brain and spinal cord to
other body parts
B. made up of
1.
12 pairs of cranial nerves (from brain)
2.
31 pairs of spinal nerves (from cord) which are bundles of
sensory and motor neurons
C. 2 divisions
1.
sensory: transmits impulses from sense organs to the CNS
2.
motor:
i. transmits impulses from the CNS to muscles or glands
ii. further divided into the somatic and autonomic system
- somatic system: cranial and spinal nerves that go
from the CNS to your skeletal muscles; voluntary
movements and reflex arcs
- autonomic system: controls heart rate, breathing,
digestion, and gland functions; involuntary
movements; sympathetic and parasympathetic
systems
V. Reflexes
A. definition: involuntary and automatic response to a stimulus
B. involves a simple nerve pathway called a reflex arc
Example – touching a hot pan
i.
sensory receptors in fingers respond to the hot metal
ii. an impulse relaying this information is sent via sensory neuron
to the spinal cord
iii. the impulse passes to an interneuron in the spinal cord
iv. impulses are immediately sent to the motor neurons in your
arm causing you to move your hand
C. reflexes are controlled by your spinal cord
D. your brain acts after reflexes occur to help you figure out how to stop
the pain, calm heart beat, slow breathing, etc = maintain homeostasis
VI. The Senses
A. Your senses enable you to hear, see, taste, touch, and smell
whatever comes into your environment.
B. The energy that stimulates your sense organs may be in the form of
light rays, heat, sound waves, chemicals, or pressure.
C. Five categories of sensory receptors:
1.
2.
3.
4.
5.
pain receptors: throughout body (except brain) that
respond to chemicals released by damaged cells
thermoreceptors: in skin, body core, and hypothalamus
mechanoreceptors: in skin, skeletal muscles, and inner
ears
chemoreceptors: nose and taste buds
photoreceptors: in eyes
VII. Vision
A. Light travels in straight lines/rays and is bent by the cornea and
lens.
B. lens: convex lens that directs the rays onto the retina
C. retina:
1.
tissue at the back of the eye that is sensitive to light
energy
2.
contains rods: cells that respond to dim light used to detect
shapes and movement
3.
contains cones: cells that respond to bright light and color
D. How you see:
1.
Light rays are focused by the cornea and lens onto the
retina and a sharp image is formed
2.
Light energy stimulates impulses in the retinal cells which
pass impulses to the optic nerve then to the brain.
3.
The brain interprets theses impulses = you “see” the
image
E. Convex lens:
1.
thicker in the middle and thinner on the edges
2.
causes parallel light rays to come together at a focus point
F. Concave lens: lens that is thicker on the edges and thinner in the
middle
G. Correcting vision
1.
nearsightedness
i. if the eyeball is too long, light from distant objects is
focused just in front of the retina and a blurred image is
formed
ii. concave lenses are used to correct
2.
farsightedness
i. if the eyeball is too short, light from nearby objects is
focused behind the retina and a blurred image is formed
ii. convex lenses are used to correct
VIII. Hearing
A. How you hear:
1.
When objects vibrate, they cause the air around them to
vibrate creating energy in the form of sound waves.
2.
When the waves reach your ears they stimulate nerve cells
deep in the ear.
3.
impulses are sent from these cells to the brain
4.
the brain responds and you hear a sound
B. the ear is divided into 3 sections
1.
outer ear
i. traps sound waves and funnels them into the ear canal
and middle ear
ii. the sound waves cause the eardrum (membrane) to
vibrate like a drum
2.
middle ear
i. vibrations from the eardrum cause little bones (hammer,
anvil, stirrup) to vibrate
ii. the stirrup bone rests against a second membrane on the
opening of the inner ear
3.
inner ear
i. cochlea: fluid filled structure like a snail’s shell that
vibrates from the stirrup’s vibrations causing nerve
endings to send impulses to the brain via the auditory
nerve
ii. high-pitch sounds make the nerve endings move
differently than low-pitched sounds
iii. balance is also controlled here
- special structures and fluids in the semicircular
canals are constantly adjusting to the position of
your head
- this stimulates impulses to the brain which
interprets the position to help you adjust and
maintain your balance
IX. Smell
A. Food gives off gas molecules into the air which are taken into the
nasal passage
B. Olfactory cells are moistened; specialized nerve cells that are
stimulated by the gas molecules when these molecules dissolve in
the moisture
C. If enough molecules are present, am impulse starts in these cells and
travels to the brain and you “smell” the item
X.
Taste
A. Taste buds on your tongue are the major chemical sensory receptors
for taste
B. 10,000 taste buds in your mouth
C. In order to taste something, the food must be dissolved in saliva,
therefore the nervous system triggers salivation when it is time to
eat
D. The solution of saliva and food molecules wash over your taste buds
and an impulse is sent to the brain
E. The brain interprets the impulse and you can taste the food.
F. Four basic areas of taste buds: sweet, salty, sour, and bitter
G. When you are sick, you have trouble tasting food because the gas
molecules are blocked from dissolving in the moist membranes of
the nasal passage.
XI. Touch, Pressure, Pain and Temperature
A. Receptors in the skin (dermis) pick up changes in touch, pressure,
and temperature and transmit impulse to the brain or spinal cord
B. The body will respond to maintain homeostasis