Transcript NERVOUS SYSTEMS – FUNCTION AT THE CELLULAR LEVEL

mean = 75.1
sd = 12.4
range = 52-99
LECTURE TEST 3
9
8
NUMBER OF STUDENTS
7
6
5
4
3
2
1
0
40s
50s
60s
70s
INTERVAL
80s
90s
50-59 = F – 2
60-67: D – 1 68-79: C – 10 80-89: B – 3 90-99: A - 2
2.5
SCORES - LECTURE TEST 3
NUMBER OF STUDENTS
2
1.5
1
0.5
0
SCORE
NERVOUS SYSTEMS – NEURON FUNCTION
Overview: neuron function
1) Electrical properties – excitability and resting potentials
2) Stimulation – the graded potential
3) impulse transmission – the action potential
4) Transmission of action potentials between neurons – the
synapse
The nervous system is responsible for
processing internal and external
information (sensory input) and making
decisions about how to respond
INTEGRATION
The nervous system coordinates the
activity of cells, tissues and organs
involved in responses
At the level of individual cells,
integration and decision-making is
done by neurons
Anatomy of a neuron:
a) dendrites - input
b) cell body – “decision making”; graded potential
c) axon hillock - initiation of action potential
d) axon - transmits action potential
e) axon terminals – multiple endings/neuron
e) synapses – communication with target cells
axon
hillock
dendrite
cell body
axon
terminals
All activity within the neuron is based on changes in electric charge
of the cytoplasm
- due to movement of positive or negative ions across the cell
membrane
changes in charge of the
changes in charge of the
dendrite/cell body region
axon are always
are variable in amount –
identical– action potential
graded potential
ELECTRICAL PROPERTIES OF NEURONS
Resting potential - electrical charge difference (membrane
potential) between inside and outside of cell when neuron is
inactive
- inside of cell negative
- outside of cell positive
more K
inside
+
more Na +
outside
more Cl outside
many anionic
(negative)
proteins
inside
Ions can move in
or out through
channels; proteins
can't
Charge difference in inactive neuron is ~70 mV (millivolts) - resting
potential
Membrane potential can change due to ion movement:
a) diffusion – ions move down concentration gradient, or towards
opposite electrical charge
b) active transport – against concentration gradient; uses ATP
resting
potential
stays constant
unless neuron
is stimulated
Ions diffuse based on two forces:
- concentration gradient: high  low concentration
- electrical charge:  region of opposite charge
These forces may oppose or reinforce each other
Na+ - “wants” to move in;
concentration gradient
and charge difference
both inwards
K+ - opposing forces:
concentration – out
charge - in
Unequal distribution of Na+ and K+ is maintained by active
transport (ATP): the Na/K pump
Membrane is more permeable to
K+ than to Na+ or other ions
- Na/K pump moves Na+ out, K+
in
diffusion
through "leaky" membrane
the Na/K pump:
active transport
Most diffusion of ions in or out occurs through transmembrane
proteins – ion channels
- facilitated diffusion – no ATP needed
- ion channels are selective and dynamic – only one type of ion
can pass; can be open or closed (gated)
At rest, some K+ channels are always open  free movement of
K+ in or out
Most gated channels open intermittently and briefly
2 types of channels in a typical neuron:
1) electrically gated (=voltage-gated) - membrane potential
changes  channel opens
2) chemically gated (=ligand-gated) - signaling molecule
(neurotransmitter ) binds to receptor  channel opens
A graded potential is any electrical change from resting potential
- opening of gated channels due to binding of neurotransmitters:
ion movement in or out of dendrites or cell body  change in
membrane potential of cell body
- amount of change varies (graded) depending on how many
channels open and number of ions moving in or out
If graded potential exceeds threshold (~10mV above resting
potential, or -60 mV) , an action potential is initiated in the
axon
Questions about resting or graded potentials?
Gated channels and their role in ion movement?
Where do graded potentials
occur in the neuron?
What causes graded potentials to
occur?
Which type of gated channels are
most often involved?
Graded potentials can be excitatory or inhibitory
Chemically-gated channels: opened by neurotransmitters
- positive ions in (Na+)= depolarization  less negative inside
- negative ions in (Cl-) = hyperpolarization  more negative
A depolarizing graded potential is excitatory – increases chance of an
action potential
- membrane potential moves closer to threshold (more positive)
A hyperpolarizing graded potential is inhibitory
- membrane potential moves farther from threshold (more negative)
threshold
A single receiving (post-synaptic) neuron can have 1000’s of
synapses with different incoming (pre-synaptic) axon terminals
- each synapse is either excitatory or inhibitory
simultaneous
stimulation at many
synapses is
"evaluated" by cell
body
= integration
Integration - cumulative effects of more than one graded
potential
Summation – 2 mechanisms
- temporal summation - multiple stimulation at one synapse in a
short period of time - additive
A graded potential that does not
reach threshold rapidly fades – cell
returns to resting potential
The same synapse releases
neurotransmitters twice in a short
period of time  larger graded
potential  action potential
Spatial summation – simultaneous stimulation from two or more nearby
synapses; each synapse is excitatory or inhibitory
- reinforcing or opposing effects
- equal excitatory and inhibitory graded potentials cancel each other
out
Two excitatory
synapses fire
simultaneously
Excitatory and
inhibitory synapse fire
simultaneously
The results of summation and integration in the dendrite/cell
body region are "evaluated" at the axon hillock
- if excitatory graded potential > threshold, then an action
potential is initiated due to the action of electrically-gated ion
channels in the axon
Questions about summation or integration?
What is the effect on the overall function of the neuron if the
graded potential doesn't reach threshold?
The action potential: rapid transmission of information
- “all or none”: does not vary in strength
- electrically-gated ion channels open in response to
depolarization  self-propagating
- the action potential passes undiminished as a wave down the
axon; a local, transient event
SELF-PROPAGATION
movement of Na+
ions in here
causes voltage-gated
Na+ channels to open
here
repolarization due to
K+ movement quickly
follows
Na+ channels inactivated: refractory
period – axon is temporarily "off"
- prevents movement of action
potential backwards  oneway propagation
A single axon can transmit many action potentials at the same time
- each is a brief local event
The action potential has 4 phases, each based on opening of voltagegated channels and ion movement
depolarization –
Na+ in
repolarization –
K+ out
refractory period hyperpolarized
return to resting
potential
Na+ channels
open
K+ channels
open
Na+ channels
inactive
Na+ channels
active, but closed
Questions about action potentials?
At one point on axon
Entire axon
What is the functional effect of an action potential on other cells or
tissues?
SPEED OF ACTION POTENTIAL CONDUCTION
Ranges from 1 to >100 m/s
increased by:
a) larger size - giant axons - invertebrates
b) increased body temperature  faster diffusion
c) fatty insulation - myelin sheaths - vertebrates
Myelin – fat produced by oligodendrocytes (Schwann cells)
- myelinated axons are white and shiny  “white matter” of
brain and spinal cord
Electrically gated Na+ channels are concentrated at gaps in the
myelin sheath - nodes of Ranvier
Saltatory conduction - AP “jumps” from one node to the next,
travels faster than in an uncovered axon
-maximum speed of conduction in myelinated vertebrate axons
- 120 m/s (= 270 mph)
- small, unmyelinated axon of human autonomic nervous system
– 1m/s (= 2.25 mph)
diffusion of Na+
ions inside the
axon causes
opening of Na+
channels at next
node
AP jumps
from node to
node
Information is transferred between neurons at the synapse
Synapse - junction between axon
terminals and target tissue
(nerve, muscle or gland)
Synaptic cleft - microscopic
separation between axon
and target - 10-20 nanometers
 Rapid (instantaneous) diffusion from pre-synaptic neuron to postsynaptic receptors
Neurotransmitter release at the axon terminal is caused by the
action potential
- neurotransmitters stored in synaptic vesicles
- AP opens voltage-gated Ca++ channels; Ca++ ions cause
vesicles to release contents at synapse (exocytosis)
Neurotransmitters diffuse
across synaptic cleft to
postsynaptic neuron
- neurotransmitters bind to
receptors on chemicallygated ion channels on
postsynaptic membrane
- channels open  ion
movement  graded
potential
- neurotransmitter removed
from receptor, inactivated