Nervous systems across phyla
The Nervous System: 1. Basics
Integration and Control:
Chapter 33, Starr et al.,
pages 554-563
Fig. 33-2, p. 554
Neurons: the functional unit of the nervous system
Impulse Transmission
•! Electrical
dendrites
cell body
–! Along the surface of the cell membrane
Input Zone
•! Fast
–! Up to 200 mph
–! In humans
axon
axon endings
Trigger Zone
•! ~ 200 meters per second
•! = 12 kph or 7.5 mph.
–! from brain to hand in milliseconds
Conducting Zone
Myelin sheath
Output Zone
http://en.wikipedia.org/wiki/Loligo_vulgaris
•! vs. seconds with the endocrine system.
–! Speed of transmission increases with increasing axonal diameter
•! Mediated by ion concentrations
Squid is up to 8” long
Neurons at rest
1. Concentrations of ions inside and outside the neuron differ.
K+
Na+/K+-pump illustrated!
Interstitial
fluid!
Na+ pumped
out
Na+
outside
K+ leaks
out
Plasma
membrane
plasma
membrane
K+
Na+
inside
2. Voltage difference is -70 mV inside of cell [relative to outside]
–!
–!
Maintained by Na+/K+ pump
Requires ATP for energy
Cytoplasm:
Low [Na+], !
High[K+]
Na+
leaks in
Membrane potential (millivolts)
action potential
+20
–! Causes graded depolarization of the cell body
–! Above a threshold level, this depolarization leads to generation of an
action potential at the trigger region of the neuron
•! All-or-nothing action potential
–! Moves from the trigger zone down the axon to the end
trigger zone
K+
leaks in
•! Selective ion channels regulate movement of Na+ and K+
•! Closed when neuron is at rest
•! Na+/K+ pump maintains differential ion concentrations
–! requires energy: ATP
Initiating an Action Potential
•! Graded depolarization of the dendrites
K+ pumped
in
0
-20
threshold
-40
resting
membrane
potential
-70
0
1
2
3
4
Time (milliseconds)
5
Events in an Action Potential
•! Trigger zone is rich in voltage-gated Na-channels
–! With enough stimulation, these voltage-gated Na-channels open
•! Na+ flows into cell, setting up a positive feedback cycle:
–! The more Na+ comes in, the more positive the cytoplasm becomes, the
more Na+ comes in …
•! Cell interior ---> positive ( up to +30mV)
•! Voltage-gated potassium channels open
–! K+ flows out of cell
•! Loss of positive ions restores electronegativity of cytoplasm
–! Na-channels close
•! Cell interior ---> very negative (-90 mV)
–! This patch of membrane is refractory to further stimulation for some
time
•! Ion imbalance triggers voltage-gated channels in adjacent membrane region
–! These channels open, propagating action potential down the axon
•! Backward propagation prevented by refractory period
After the Action Potential
Essentials of Action Potential
•! Until the threshold is reached, the trigger zone on the axon does
not respond.
•! Once the threshold is reached, a full response occurs.
•! Action potentials are all-or-nothing:
–! Each one is the same size.
–! Strength of a stimulus is measured by
•! frequency of action potentials
•! number of action potentials
•! Maintenance of ion concentrations requires active transport
–! Energy provided by ATP
•! Action potentials run only the length of a single axon.
–! They do not cross from neuron to neuron.
•! Speed of transmission depends on the diameter of the axon
Schwann cells and Saltatory Conduction
•! Neurons need to be fast!
–! Speed of impulse transmission increases with diameter of axon
–! Squid has giant axons; nerve impulses move very fast.
•! Duration of action potential is ~ 1 millisecond
•! Refractory period:
–! while cell interior is at -90 mV, no new action potential can be
generated
–! Sodium/potassium pump recreates differential ion concentrations
•! By pumping K+ into the cell
–! Then the cell membrane is ready for next depolarization
•! Alternative:
–! Saltatory conduction
•! Schwann cells secrete myelin -- lipid insulates axon
–! Ions can’t move in/out of axon except at junction of Schwann cells
•! Junction is node of Ranvier
–! Membrane is conductive and leaks charge as far as next node
–! So -- the electrical impulse jumps from one gap to the next
–! This is saltatory conduction.
Na+
Multiple Sclerosis
•! A condition in which nerve fibers lose their myelin
–! Slows conduction
•! Neurons eventually die
action potential
resting potential
resting potential
•! Both CNS and peripheral neurons can be affected
•! Symptoms include
K+
Na+
–! Visual problems, numbness, muscle weakness, and fatigue
•! Symptoms vary
–! Between individuals
–! Temporally within individuals
•! Depending on which neurons are affected or die
resting potential restored
action potential
resting potential
•! Treatments include
–! Steroids, interferons, monoclonal antibodies, anti-cancer drugs, SSRIs
Saltatory conduction occurs because the action potential jumps from one
node of Ranvier to the next, greatly speeding transmission
Structure of a Nerve
A neuron is a single cell. A nerve consists
of many bundles of neurons, all running
in the same direction. Cell bodies are also
clustered together.
nerve’s
outer
wrapping
Transmission Across Synapses
axon
myelin sheath
•!
•!
•!
•!
There are small spaces between cells.
Between neurons, these intercellular spaces are called synapses.
Between a neuron and a muscle cell: the neuromuscular junction.
Electrical propagation of impulses occurs only along the axon of a
single cell.
•! To move between cells, the electrical impulse must become a
chemical signal.
•! Neurotransmitters transmit nerve signals across cell
•! Neurotransmitters trigger a response only in target cells
•! Receptors are required
•! Another class of signaling molecules
blood
vessels
nerve
fascicle
Synaptic cleft
Influx of Ca causes vesicles to
move to membrane, release
neurotransmitter.
Voltage-sensitive Cachannels open
Ca+ flows into cell
Neurotransmitters
•! Definition:
–! Chemicals that carry nerve impulses
•! Between nerve cells or
•! Between a neuron and another type of cell
–! Between sense organs and neurons
–! Between glands and neurons
–! Between neurons and muscles
•! Peripheral nervous system neurotransmitters:
–! acetylcholine, norepinephrine (adrenalin)
–! May be excitatory or inhibitory, depending on target cell
•! Central nervous system neurotransmitters
–! Many!
•! Each is intrinsically excitatory or inhibitory.
–!
–!
–!
–!
Acetylcholine (excitatory - depolarizes membrane)
GABA (inhibitory - polarizes)
Dopamine
Serotonin
–! The final strength of response to a stimulus is
•! The net effect of all of the inhibitory and excitatory impulses transmitted to a
nerve or muscle.
http://www.coolschool.ca/lor/BI12/unit12/U12L04.htm
Membrane potential (millivolts)
Other Elements of Neural Transmission
what action
potential spiking
would look like
-65
threshold
EPSP
integrated
potential
-70
IPSP
-75
resting
membrane
potential
•! Neuromodulators
–! alter sensitivity of the synapse
–! e.g.: endorphins
•! Enzymes
–! Remove neurotransmitters from the post-synaptic membrane
•! Transporters
–! Take up (scavenge) neurotransmitters
•! Pharmaceuticals
–! Valium enhances the inhibitory effect of GABA
–! SSRIs (selective serotonin reuptake inhibitors)
inhibit the removal of excitatory neurotransmitters
•! Prozac et alii
•! Pesticides
–! Parathion, carbaryl inhibit acetylcholinesterase
•! Facilitate transmission of neural impulses
–! DDT acts on neuronal membrane
•! Facilitates excitatory impulses
Stimulus
(input)
Reflex Arcs!
spinal cord
Stimulus:
biceps stretches
Receptors
(sensory neurons)
sensory
neuron
Integrators
(interneurons)
motor neurons
motor
neuron
Effectors
(muscles, glands)
Response
(output)
muscle spindle
Response:
biceps
contracts