Chapter 25
 Which
characteristic
of life is an
organism’s
nervous
system
related to?

4 requirements before
nervous response can occur:
1. Detection of stimulus by
sensory receptors
2. Transmission of impulse
along neurons
3. Interpretation & analysis
of impulses in
coordination center
▪
brain or spinal cord
4. Response by effectors
▪
muscles or glands

Nerve
 many neurons bundled together

Neurons
 specialized cells that
transmit nerve impulses
through body
▪ different types
▪ sensory neurons
▪ interneurons
▪ motor neurons

Sensory neurons
 transmit impulses from
receptors to coordination
center

Interneurons
 transfer impulses from
sensory to motor neurons
▪ in brain & spinal cord

Motor neurons
 transmit outgoing
impulses to effectors

dendrites
 conduct impulses
toward cell body

cell body
 resembles other
types of cells
▪ contains nucleus &
cytoplasm

axon
 carries impulses
away from cell body
▪ often covered with
myelin sheath
end branch
dendrites
terminal buttons (end
branch)
nucleus
cytoplasm
cell body
(soma)
axon
myelin sheath
nodes of Ranvier

nerve impulse or
“action potential”
 results from
changes in ion
concentration
inside/outside
neuron plasma
membrane

between each
neuron is a
synapse
 small fluid-filled
space

neurotransmitters
(chemicals) help
impulse across
synapse
 In bilaterally
symmetrical animals
the nervous system
is divided into 2 parts
 central nervous
system (CNS)
 peripheral nervous
system (PNS)

receptors in PNS gather
info about animal’s
environment
 impulses along the PNS’s
sensory neurons travel to
CNS
▪ in CNS (brain & spinal cord)
impulses are interpreted &
analyzed
 impulses leaving CNS by
motor neurons of PNS are
carried to effectors
 which carryout response

cerebrum
 speech, reasoning,
emotions

cerebellum
 controls balance, posture,
muscle tone

medulla (oblongata)
 controls involuntary actions
of internal organs
▪ heart rate, breathing rate,
peristalsis, etc.

reflexes = involuntary/automatic responses to stimuli
 Involve CNS, but require no conscious control or decision
making b/c determined by fixed pathways
▪ some through brain (ex. blinking)
▪ others through spinal cord (ex. touch hot iron & respond)
Reflex Arc Animation
Reflex Arc Animation 2

Reflex arc
1. Receptors activated
2. Sensory neuron transmits
impulse towards spinal cord
3. Impulse crosses synapse to
interneuron in spinal cord
4. Impulse crosses synapse to motor
neuron & leaves spinal cord
5. Impulse activates effectors which
respond appropriately

Does NOT involve brain
1. Sensory
receptor activated
(receives stimulus)
2. Sensory
neuron transmits
impulse towards
spinal cord
4. motor neuron transmits
impulse away from spinal cord
5. Impulse received by
effector (which
responds
appropriately)
3. Impulse crosses
synapse into interneuron
where interpreted &
analyzed
Reflex
Arc
Interneuron = association nerve
 Since reflex actions (arcs) don’t involve the
brain, how does your brain become aware?
 Some spinal cord interneurons that receive
sensory impulses connect w/ other neuron
pathways leading to the brain
▪ Brain becomes aware of what has happened
▪ Secondary response occurs
 You yell, “Ouch!”

Euglena
 Light-sensitive structure
under the “eye-spot”
▪ Allows Euglena to swim in
direction of light

Paramecium
 Can respond to obstacles in path
▪ Turns 30 & moves forward again…
▪ Continues to do this until passes object
▪ Seems to be mainly under chemical
control

Radially symmetrical organisms
have no center for nervous control
 Many short neurons form nerve net
▪ No definite nerve pathways
 Ex. Hydra

Bilaterally symmetric animals
have a primitive control center(s)
(ganglia) & specific nerve
pathways
 Ex. Planaria

Other bilaterally
symmetric animals
also have brains
 Ex. earthworm
▪ cerebral ganglia (slightly
larger ganglia)
 Ex. grasshopper
▪ Brain divided into
specialized areas
▪ protocerebrum
▪ deutocerebrum

Lateral line system
 can sense the slightest
movements (vibrations)
in water

Electrical sense
 Ampullae of Lorenzini
▪ sense changes in
electrical fields

How do we see unity within diversity in relation to
nervous control???
How does form follow function???

Kid’s Health website

 http://kidshealth.org/PageManager.jsp?lic=1&article_set
=59295&cat_id=20607