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Nervous System
CNS: coordinating center for
Mechanical and chemical reactions
Made up of brain and spinal cord
PNS: all parts of nervous system EXCEPT for
brain and spinal cord relays information btw CNS
and other parts of body
Handout LSM 9.1-1
Types of Nerve Cells
• Types of nerve cells
– Glial cells – non-conducting, provide structure
– Neurons – functional units
– Sensory neurons - carry impulses from sensory
receptors to CNS, (aka afferent neurons)
– Motor neurons –carry impulses from CNS to effectors
(muscles, organs and glands) (aka efferent neurons)
– Interneurons – link neurons in the body
Anatomy of a Nerve Cell
Handout LSM 9.1-3
nodes of Ranvier –gaps
between myelin sheath
along the axon
Dendrites
carry impulses
Towards cell body
Myelin sheath – insulating covering over
axon, prevents loss of charge, composed
of Schwann cells
Axon - Carries nerve
Impulses away from cell body
Neural Circuits
Example - Reflex Arc
Stimulus – touching a hot
stove or touching a tack
Response – arm pulls away
from the stimulus quickly
Handout LSM 9.1-5
Electrochemical Impulse
SBI 4U
Past Research
• Luigi Galvani (late 18th c.) – leg muscle of dead
frog twitched from electrical stimulation
• Emil DuBois-Reymond (1840) – refined
instruments to detect currents in nerves and
muscles
• Julius Bernstein (1900) – suggested nerve
impulses are electrochemical messages created
by ion movement through nerve cells
• Willem Einthoven (1906) – recorded transmission
of electrical impulses in heart muscle (ECG)
• Hans Berger (1929) – measured electrical activity
of brain with electrodes (EEG)
K.S. Cole & H.J. Curtis (1939)
• resting potential =
voltage difference across
a nerve cell membrane
during the resting stage
(-70 mV).
• action potential = the
voltage difference across
a nerve cell membrane
when the nerve is excited
(+40 mV).
Resting Potential
• resting membrane is about 50x more
permeable to K+
• more K+ diffuse out of nerve cell than Na+
diffuse in
• exterior becomes +ve relative to interior
• https://www.youtube.com/watch?v=9euDb4
TN3b0
Terminology
• polarized membrane = membrane charged
by unequal distribution of positively charged
ions inside and outside the nerve cell.
• depolarization = diffusion of sodium ions
into the nerve cell resulting in a charge
reversal.
• sodium-potassium pump = an active
transport mechanism that moves sodium
ions out of and potassium ions into a cell
against their concentration gradients.
• repolarization = process of restoring the
original polarity of the nerve membrane.
The Action Potential
a) K+ diffuses out faster than
Na+ into nerve cell. Outside
= +ve, inside = -ve
b) Electrical disturbance opens
Na+ gates and Na+ rush into
nerve cell. Membrane
becomes depolarized.
c) Depolarization closes Na+
gates, while K+ gates open. K+
follows concentration gradient
and moves out of nerve cell.
Action potential moves forward.
d) Na+-K+ pump restores
polarization (3 Na+ out to 2 K+ in).
refractory period = recovery time required before a
neuron can produce another action potential.
Movement of the Action
Potential
Movement of a Nerve Impulse
Threshold Levels and the
All-or-None Response
•
•
stimuli < 2 mV do not produce muscle contraction
increasing intensity will not produce an increased
response
Threshold Levels and the
All-or-None Response
•
•
•
•
threshold level = minimum level of a stimulus
required to produce a response.
threshold levels are different for each neuron
increasing the intensity of the stimuli above the
critical threshold value will not produce an
increased response – known as an all-ornone response.
Intensity of stimuli is detected in 2 ways:
– difference in frequency
– the number of neurons that fire
(different threshold levels)
https://www.youtube.com/watch?v=SdUUP2pMmQ4
The Synapse
Synaptic Transmission
• synapse = regions between neurons, or
between neurons and effectors (a.k.a.
synaptic cleft).
• neurotransmitters = chemicals released
from vesicles into synapses.
• presynaptic neuron – neuron that
carries impulses to the synapse.
• postsynaptic neuron – neuron that
carries impulses away from the synapse.
Synaptic Transmission
1.
2.
3.
4.
5.
action potential moves along axon of the
presynaptic neuron toward the synapse
vesicles release neurotransmitter from end
plate into synaptic cleft
neurotransmitters diffuse across cleft and
bind with receptors on postsynaptic neuron
enzymes remove neurotransmitters from
receptors and are either reabsorbed or
broken down
dendrites of postsynaptic neuron are
depolarized and action potential continues
Acetylcholine
• a.k.a. “ACh”
• a neurotransmitter found in the end
plates of many neurons
• can act as excitatory by opening Na+
channels
• broken down by cholinesterase so that
the neuron can recover
– insecticides block action of cholinesterase
so that the insects heart contracts but never
relaxes
An Excitatory Synapse
Acetylcholine
• can also act as inhibitory:
– ACh opens K+ gates on some postsynaptic
neurons
– rush of K+ outside cell increases number of
positive ions outside relative to inside
– membrane becomes hyperpolarized, i.e.
inside of the nerve cell membrane becomes
more negative than the resting potential
Summation
• = effect produced by the accumulation of
neurotransmitters from two or more
neurons
Other Neurotransmitters:
•
•
•
•
•
serotonin
dopamine
gamma-aminobutyric acid (GABA)
glutamic acid
norepinephrine
… interactions of excitatory and
inhibitory neurotransmitters is what
allows us to react to our
environment!
Neurotransmitter Disorders
• Parkinson’s disease – inadequate
production of dopamine resulting in
involuntary muscle contractions and
tremours
• Alzheimer’s disease – decreased
production of ACh has been associated
with the deterioration of memory and
mental capacity
Homework
• Section 9.1
– Copy table 1 on page 416
– Page 416 Question # 1, 2 4-6
• Section 9.2
– Page 426 Question # 2-13