Transcript Nervous System

Nervous System
• All animals must respond to environmental
stimuli
– Sensory receptors – detect stimulus
– Motor effectors – respond to stimulus
• The nervous system links the two
• Consists of neurons (nerve cells) and supporting
cells (neuroglia)
Nervous System
• Vertebrates have three types of neurons
(nerve cell)
• Sensory – carry impulses from sensory receptors
to the central nervous system (CNS)
• Motor – carry impulses from the CNS to effectors
(muscles and glands)
• Interneurons (association neurons) – located in
the brain and spinal cord; provide more complex
reflexes and higher associative functions such as
learning and memory; “integrators”
Nervous System
• The central nervous system (CNS) consists of
the brain and the spinal cord
• The peripheral nervous system (PNS) consists
of sensory and motor neurons; a network of
nerves extending into different parts of the
body; carries sensory input to the CNS and
motor output away from the CNS
Nervous System: Neurons
• The basic structure of the neuron (consists of):
– Cell body – an enlarged region containing nucleus
– Dendrite – short cytoplasmic extensions extending
from the cell body; receive stimuli
– Axon – single, long extension that conducts
impulses away from the cell body
• The axons controlling the muscles in a person’s feet can
be more than 1 meter (3 ft) long!; axons from the skull
to the pelvis in a giraffe are 3 meters (9 ft) long!
Neuron
Nervous System: Neuroglia
• Neurons are supported structurally and
functionally by supporting cells called
neuroglia
– 1/10th size of a neuron, but 10x more abundant
– Schwann cells and Oligodendrocytes – produce
the myelin sheaths in the PNS and the CNS,
respectively
• Myelin sheaths surround and insulate the axon of many
types of neurons; “myelinated” axons
Nervous System: Neuroglia
• Other neuroglia provide neurons with
nutrients, remove wastes
• Small gaps known as nodes of Ranvier
interrupt the myelin sheath at intervals of 12μm; uninsulated, capable of generating
electrical activity (sites of action potential)
Conduction of the nerve impulse
• Upon stimulation of a nerve cell, electrical
changes spread or propagate from one part of
the cell to another
• Neuron function depends on a changeable
permeability to ions; an electrical difference
exists across the plasma membrane
• Membrane potential – voltage measured across a
membrane due to differences in electrical charge;
inside of cell is negative relative to outside
Conduction of the nerve impulse
• When a neuron is not being stimulated, it
maintains a resting potential; -70mV (average;
ranges from -40 to -90; “polarized”)
• The inside of the cell is negatively charged
relative to the outside
– Polarization is established by maintaining an
excess of Na+ ions on the outside, and an excess
of K+ ions on the inside
• Most animal cells have a low concentration of Na+ and
a high K+ relative to their surroundings
Conduction of the nerve impulse
• A certain amount of Na+ and K+ ions are
always leaking across the membrane through
leakage channels, but Na+/K+ pumps actively
restore the ions to their appropriate sides
– The Na+/K+ pump: brings in 2 K+ for every 3 Na+
pumped out
– Ion leakage channels: allow more K+ to diffuse out
than Na+ to diffuse in
Conduction of the nerve impulse
• Other ions, such as large, negatively-charged
proteins and amino acids, reside within the
cell
• It is these large, negatively-charged ions that
contribute to the overall negative charge on
the inside of the cell membrane relative to the
outside
• Negative pole: Cytoplasm (inside cell)
• Positive pole: Extracellular (outside cell)
Remember: Cells contain relatively high [K+] inside the cell, but low [Na+]
Conduction of the nerve impulse
• A nerve impulse is generated when the
difference in electrical charge disappears
– Occurs when a stimulus contacts the tip of a
dendrite and increases the permeability of the cell
membrane to Na+ ions
– Na+ ions rush into the cystoplasm, and the
difference in electrical charge across the
membrane disappears (depolarized)
– Remember, the concentration of Na+ inside the
cell is low relative to its surroundings
Conduction of the nerve impulse
• Some stimuli open K+ channels
– As a result, K+ leaves the cell (remember: high
[K+] inside the cell)
– Membrane potential becomes more negative
(more negative inside the cell)
– “hyperpolarization”
• Some stimuli open Na+ channels
– Causes Na+ to enter the cell
– Membrane potential becomes less positive
– “depolarization”
Conduction of the nerve impulse
• When the strength of stimuli determines how
many ion channels will open; graded response
• Caused by the acvtivation of a gated ion
channels which behave like a door that can
open or close, unlike ion leakage channels that
are always open
• Each gated channel is selective, opening only
to allow diffusion of one type of ion
• Normally closed in a resting cell
Graded Potentials
Action Potentials
• Permeability changes are measurable as
depolarizations or hyperpolarizations of the
membrane potential
• Depolarization – makes membrane potential
less negative (more positive)
• Hyperpolarization – makes membrane
potential more negative
• Ex: -70mV  -65mV = Depolarization
-70mV  -75mV = Hyperpolarization
Action Potentials
• Action potentials are rapid, reversals in voltage
across the plasma membrane of axons
• Once a threshold of depolarization is reached
(-50 to -55 mV), an action potential will occur
• An ‘all or nothing’ response, not graded
• Magnitude of the action potential is
independent of strength of depolarizing stimuli
• Action potentials are the signals by which
neurons communicate and spread messages
Action Potentials
• An action potential is caused by a different
class of ion channels, voltage-gated ion
channels
• These channels open and close in response to
changes in membrane potential; only open at
certain membrane potentials; flow of ions
controlled by these channels creates the
action potential
Action Potentials
• Voltage-gated channels are very specific; each
ion has its own channel
– Voltage-gated Na+ channels
– Voltage-gated K+ channels
Action Potentials
• When the threshold voltage is reached, Na+
channels open rapidly
• Influx of Na+ causes the membrane to
depolarize
• K+ channels open slowly, eventually
repolarizing the membrane
• Action potential consists of three phases:
• Rising, falling, and undershoot
Action Potentials – the Spoiler
• At threshold, membrane is depolarized
enough that Na+ voltage-gated channels
open; Na+ moves into interior of cell,
becoming less negative; rapid depolarization
( 45mV), then stops
– Stops because channels will close after a specific
amount of time has elapsed
Action Potentials II – the Spoiler
• K+ voltage-gated channels will also open,
before membrane potential reaches zero; K+
moves out of cell, making cell become more
negative, returns cell to resting
• Na+/K+ pump is also activated, moving 3 Na+
out for every 2 K+ in, makes cell more negative
• Returns cell to rest (~-70mV)
Action Potentials III – the Spoiler
• Excess K+ diffuse out before K+ channel
closes, or over-activity of the Na+/K+ pump;
results in undershoot (hyperpolarization)
• Entire process occurs very rapidly: 2-4ms from
start to finish
Action Potentials
• Each action potential, in its rising, reflects a
reversal in membrane polarity
• Positive charges due to Na+ influx can
depolarize adjacent region to threshold,
causing the next region to produce an action
potential of its own
• The previous region then repolarizes back to
its resting membrane potential
3. Top curve
2. Rising Phase
Maximum voltage reached
Stimulus causes above threshold voltage
Potassium
gate opens
K+
Na+
1. Resting Phase
Equilibrium between diffusion of K+ out
of cell and voltage pulling K+ into cell
Voltage-gated
potassium channel
Membrane potential (mV)
Sodium channel
activation gate opens
Na+ channel
inactivation gate
closes
+50
0
–70
1
3
2
Time (ms)
4. Falling Phase
Undershoot occurs as excess potassium
diffuses out before potassium channel closes
Potassium channel
gate closes
Potassium
gate open
Equilibrium
restored
Potassium
channel
Voltage-gated
sodium channel
Sodium channel
activation gate closes.
Inactivation gate opens.
Na+ channel
inactivation gate
closed
Action Potentials
• Action potentials are localized events
• They DO NOT travel down the membrane
• They are generated anew in a sequence along
the neuron as they propogate along axon
• During undershoot, the membrane is less
likely to depolarize
• This keeps the action potential moving in one
direction
resting
repolarized
depolarized
+ + + + + + + + +
– – – – – – – – –
+ + + + + + + + +
– – – – – – – – –
– – + + + + + + +
+ + – – – – – – –
Na+
+ + + + + + + – –
– – – – – – – + +
K+
+ + – – + + + + +
– – + + – – – – –
Na+
+ + + + + – – + +
– – – – – + + – –
K+
K+
+ + + + – – – + +
– – – – + + + – –
Na+
+ + – – – + + + +
– – + + + – – – –
K+
K+
+ + + + + + + – –
– – – – – – – + +
Na+
– – + + + + + + +
+ + – – – – – – –
Cytoplasm
Cell
membrane
K+
Saltatory Conduction
• Two ways to increase velocity of conduction:
– Increase diameter of axon; reduces resistance to
current flow; found primarily in invertebrates
– Axon is myelinated; impulse jumps from node to
node (Nodes of Ranvier – the only site of action
potentials) = saltatory conduction
– one action potential still serves as stimulus for the
next one, but the impulse (depolarization at one
end) spreads quickly beneath the insulating
myelin to trigger the opening of voltage-gated ion
channels at the next node
Saltatory Conduction
http://www.flickr.com/photos/photoklick/2829645922/
Saltatory Conduction