Transcript Nervous System

The Nervous System
AP Biology
The Nervous System
• Function: environment is
constantly changing – nervous
system detects those changes
and helps the organism
respond/adapt
• Irritability: ability to respond to a
stimulus
The Nervous System
• Nervous System detects
(sensory input), processes
(integration), and responds
(motor output)
• Peripheral Nervous System
detects and responds
• Central Nervous System
processes information
The Neuron (Nerve Cell)
• Three types of neurons:
–Sensory – carry impulses from
the sense organs (receptors) to
the CNS
–Motor – carry impulses from the
CNS to the muscles or glands
(effectors)
–Interneurons – connect and
carry impulses between
sensory and motor neurons
Neurons
1. Cell body – largest part; most
metabolic activities take place
here; contains nucleus
2. Dendrites – carry impulses from
the environment or other
neurons toward the cell body
Neurons
3. Axon – long fiber that carries
impulses away from the cell
body
• Terminal branches –
branching of axon
• Synaptic knobs – ends of
axon; contain vesicles with
neurotransmitters
The Nerve Impulse
• Resting potential – a nerve cell has
an electric potential because
OPPOSITELY charged ions are on
each side of the membrane
• Anions are mainly on the inside of
the the cell; cations on the outside
++++++++++++++++++++++++
-----------------------------------------
The Nerve Impulse
• Ungated ion channels allow ions to
diffuse across the plasma membrane
–These channels are always open
• This diffusion does not achieve an
equilibrium since sodium-potassium
pumps transport these ions against
their
gradients
Hyperpolarization
• Gated K+ channels
open  K+ diffuses
out of the cell 
the membrane
potential becomes
more negative
Depolarization
• Gated Na+
channels open 
Na+ diffuses into
the cell  the
membrane
potential becomes
less negative
The Nerve Impulse
• Action Potential – a rush of Na+ flow
into the membrane causing an
imbalance in the charge on each side
of the membrane
• This causes the POLARITY to shift
and a wave (impulse) moves down
the length of the neuron
+++++++++++++++++--------------------------------------+++++++
+
Step 1: Resting State
Step 2: Threshold
Step 3: Depolarization
Step 4: Repolarizing
Step 5: Undershoot
Na+ gates close & K+
gates open
Na+
gates open
Stimulus
K+ gates close
• During hyperpolarization or
undershoot, Na+ channels are closed
–Neuron cannot depolarize in
response to another stimulus:
refractory period
–The refractory period assures
impulse conduction is
unidirectional
Myelin
• Is composed of 80% lipid and 20%
protein
• Used for insulation and to help
speed up the nerve impulse
• Wraps around the axon of some
neurons
Myelin
• Gaps in myelin sheath cells
called Nodes of Ranvier – allow
impulses to move more quickly
down neurons
Myelin
• In Saltatory Conduction, only the
Nodes of Ranvier depolarize and
therefore conduct an impulse
faster
The Synapse
• SYNAPSE: the space between the
axon of one neuron and the dendrite
of another
• Axon terminals have vesicles
containing chemicals:
NEUROTRANSMITTERS
• These chemicals are secreted from
the axon of one neuron  stimulates
receptor sites on the effector or the
dendrite of the next neuron
Neurotransmitter Action at Synapse
1. Action potential arrives at axon
terminal of presynaptic neuron
2. Synaptic vesicles rupture, releasing
neurotransmitter into synapse
3. Neurotransmitter diffuses across
synapse & binds to receptor protein
on postsynaptic cell
4. Postsynaptic cell is excited or
inhibited
5. Neurotransmitter in synapse is
deactivated
Nervous System Organization
• Cnidaria - nerve net
–loose organization of
bi-directional neurons
–no centralization
• Flatworms - ladder
–2 anterior ganglia (rudimentary
brain) with paired,
longitudinal nerve cords
–paired sensory organs
(eyespots)
Nervous System Organization
• Segmented worms - advanced ladder
–prominent brain
–solid, fused, ventral
nerve cord
–segmentally arranged
ganglia
Nervous System Organization
• Arthropods
–prominent brain
–solid, fused, ventral
nerve cord
–extensive fusion of ganglia
–well-developed sensory organs
–exhibit complex behaviors
Organization of the NS
• The human nervous system is
divided into 2 major divisions:
–Central Nervous System (CNS)
• Control center of body, brain and
spinal cord
–Peripheral Nervous System (PNS)
• Nerves (bundles of axons)
CNS: Parts of the Brain
• Forebrain
• Midbrain
• Hindbrain
Hindbrain
• Cerebellum
–coordinates muscular movements
• Medulla oblongata
–regulates heart rate, blood
pressure and breathing
–contains reflex centers for
vomiting, swallowing, sneezing,
hiccupping, and coughing
• Pons
–helps regulate respiration
Forebrain
• Thalamus – switching station for
sensory input for all senses but
smell; relays sensory info to
cerebrum and motor info from the
cerebrum
• Hypothalamus – control hunger,
thirst, fatigue, anger, and body temp;
regulates pituitary gland
Forebrain
• Cerebrum – divided
into left and right
hemispheres
• Corpus callosum –
major connection
between hemispheres
–Left hemisphere primarily
responsible for right side of body;
right hemisphere primarily
responsible for left side
Forebrain
• Cerebral cortex – outer covering of
gray matter
–The more convoluted the surface,
the more surface area, the more
neurons
Forebrain
• Cerebrum – divided into frontal,
temporal, parietal, and occipital
lobes
• Frontal lobe
–Contains the
primary motor
cortex
(controls
actions of skeletal muscles) and
olfactory cortex (smell)
Forebrain
• Parietal lobe
–Contains the primary
somatosensory cortex and
gustatory cortex (taste)
• Temporal lobe
–Contains
auditory cortex
(sound)
• Occipital lobe
–Contains visual
cortex (sight)
PNS
• Sensory – transmits impulses
from the sense organs (such as
the ears and taste buds) to the
CNS
• Motor – transmits impulses from
the CNS to the muscles or
glands (somatic or autonomic)
• Somatic – conscious movement
of the body
• Autonomic – regulates activities
that are automatic or involuntary
–Sympathetic (stress, high
energy) and Parasympathic
(leisure, rest) are antagonistic
systems that turn an autonomic
response on or off
Sympathetic effects:
• dilates pupil
• accelerates heartbeat & respiration
• inhibits stomach & intestine activity
• relaxes urinary bladder
Parasympathetic effects:
• constricts pupil
• slows heartbeat & respiration
• stimulates stomach & intestine activity
• contracts urinary bladder
Reflex Arc
• Some actions don’t/can’t wait for
your brain to interpret the signal
• Reflexes are involuntary actions;
they travel from ____ to ____:
–Receptors (nerve “endings”)
–Sensory neurons
–Interneurons
–Motor neurons
–Effectors (muscles or glands)
Types of Sensory Receptors
• Thermoreceptors – detect heat and
cold
• Pain receptors (nocioceptors) –
detect chemicals released from
injured cells
• Mechanoreceptors –
detect mechanical
energy (touch,
pressure, vibration)
Types of Sensory Receptors
• Chemoreceptors – detect chemicals
• Photoreceptors – detect light energy
• Electroreceptors – detect electrical
fields
How are sounds sensed?
• The ear captures, transmits, and
converts sound into electrical
signals
• Ear has three basic parts:
1. Outer ear
2. Middle ear
3. Inner ear
How are sounds sensed?
• Outer ear: external ear (pinna) and
auditory canal
–Funnels sound
–Sound waves vibrate the
tympanic membrane
How are sounds sensed?
• Middle ear
–Tympanic membrane (ear drum)
–Three tiny bones: malleus
(hammer), incus (anvil), stapes
(stirrup); transfer vibrations to
the oval window on the cochlea
–Eustachian tube – equalize
pressure; connects middle ear to
pharynx
How are sounds sensed?
• Inner ear: cochlea
–converts vibrations into
electrical signals
–As the oval window vibrates, it
sets the cochlear fluid in motion
–Moving fluid brushes over hairs
–Bending of hairs is sensed by
mechanorecptors and sends the
signal to the brain (auditory
nerve)
Equilibrium
• Equilibrium is maintained by the
semicircular canals
Equilibrium
• The semicircular canals are
arranged in the X, Y, and Z planes
–Therefore, any movement in any
direction will be perceived
• Fluid in the canals brushes over
hairs
• Movement of hairs is sensed and
the signal is sent to the brain
Equilibrium
• Dizziness can be due to the
momentum of the fluid in the
canals
–You’ve stopped moving, but the
movement of the fluid in the
semicircular canals makes you
think you’re still moving
Fish “Hearing” – Lateral Lines
• Contains mechanoreceptors that
function similarly
to mammalian
inner ear
• Gives info about
direction and
velocity of water
flowing over fish’s
body
How is light sensed?
• Sclera – tough, white layer
• Conjunctiva – external cover of
sclera; keeps eye moist; conjuctivitis
(pink
eye)
How is light sensed?
• Cornea – transparent covering in
front of eye
• Choroid – thin, pigmented layer
lining interior surface of the sclera;
prevents light rays from scattering
and distorting the image
• Iris regulates size of pupil/amount of
light into eye
How is light sensed?
• Lens focuses light on retina
• Retina – Contains photoreceptors
(Except at the optic disk where the
optic nerve attaches)
–Rods: Black and White
–Cones: Color
• Optic nerve takes electric signals
from eye to brain
Rods and Cones
• ~ 125 million rod cells
–Rod cells are light sensitive but do
not distinguish colors
• ~ 6 million cone cells
–Not as light sensitive as rods but
provide color vision
–Most highly concentrated on the
fovea – area of retina lacking rods
How are scents sensed?
• Insects smell through their legs
and antennae
Male silkworm moth
Bombyx mori
Sensory hairs on
antennae detect
pheromones
released by female
How are scents sensed?
• Olfactory nerves are stimulated
when chemicals touch them
• Different chemicals create
different responses in the
olfactory nerves; hence we detect
different smells
How are tastes sensed?
• Taste buds on tongue act just like the
olfactory nerves
–Different chemicals stimulate the
nerves in the taste buds differently;
hence we detect different tastes
• Four “primary” tastes
are bitter, sour, salty,
and sweet