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Transcript
BIO2305
Neurophysiology
The Neuron
The functional and structural unit of the nervous system
There are many, many different types of neurons but most have certain structural and functional
characteristics in common
Function
Neurons are excitable cells (responsive to stimuli) specialized to conduct information (communicate)
from one part of the body to another via electrical impulses (Action Potentials) conducted along the
length of axons
1
Electric Current
Nerve impulse: flow of ions across membrane
Basic Concepts Review
Ions – charged particles
Anions – Negatively charged particles
Cations – Positively charged particles
Electrostatic forces
Opposite charges attract, same charges repel
Ions flow along their electrochemical gradient when they move toward an area of opposite
charge
Concentration forces
Diffusion – movement of ions through semipermeable membrane
Ions flow along their chemical gradient when they move from an area of high concentration to
an area of low concentration
Together, the electrical and chemical gradients constitute the electrochemical gradient
Review: Passive and Active Transport
2
Sodium-Potassium Pump
Is one type of active transport system
It is an electrogenic pump that generates the voltage across a membrane
Ion Channels – Membrane Potential
Membrane potential is the voltage difference across a membrane
Resting membrane potential (when the cell is not depolarizing) is a -70mV difference between the
inside and the outside - the membrane is polarized
When voltage-gated ion channels open, ions diffuse across the membrane following their
electrochemical gradients
This movement of charge is an electrical current and can create voltage (measure of potential) energy
change across the membrane.
The sodium-potassium pump reestablishes the -70mv difference
This electrical charge across the membrane is the resting membrane potential
3
Resting Membrane Potential
The resting potential exists because ions are concentrated on different sides of the membrane:
Na+ and Cl- outside the cell
K+ and organic anions inside the cell
Due to different membrane permeabilities of the passive ion channels and operation of the sodiumpotassium pump
Electrical Signals: Ion Movement
Resting membrane potential determined by
K+ concentration gradients
Cell’s resting permeability to K+, Na+, and ClImpermeability of anions – amino acids, proteins
Gated channels control ion permeability
Mechanically gated
Chemical gated
Voltage gated
Threshold voltage varies from one channel type to another
Membrane Potentials: Signals
Neurons use changes in membrane potential to receive, integrate, and
send information
Two types of signals are produced by a change in membrane
potential:
Graded potentials (short-distance)
Action potentials (long-distance)
Signals Carried by Neurons
a) Resting neuron – membrane is polarized; cytoplasmic side is
negatively charged
b) Stimulation of the neuron  depolarization; Na+ enters the cell,
making the inside positive compared to the outside. The charges
switch.
c) Positive on the inside, negative on the outside. Impulse propagates
down the length of the axon
d) Membrane repolarizes with efflux of K+ and with the help of Na+/K+
pumps
4
Graded Potentials
Short-lived, local changes in membrane potential
Currents decrease in magnitude with distance
Their magnitude varies directly with the strength of the stimulus – the stronger the stimulus the more
the voltage changes and the farther the current goes
Sufficiently strong graded potentials can initiate action potentials if they maintain threshold by the time
reach the trigger zone
Action Potentials
Supra-threshold stimuli cause voltage-gated Na+ channels to open
Na+ to enters the cell down its electrochemical gradient to produce depolarizing currents that are
translated into action potentials
Threshold Voltage – membrane is depolarized by ~ 15 mV stimulus (from -70 to -55mV)
The AP is a brief reversal of membrane potential with a total amplitude of 100 mV (from -70mV to
+30mV)
APs do not decrease in strength with distance
All-or-None phenomenon – action potentials either happen completely, or not at all
Action Potentials
Action potentials consist of three main phases:
Depolarization
Repolarization
Hyperpolarization
Depolarization Phase
Na+ activation gates open quickly and Na+ enters causing
local depolarization which opens more activation gates and
cell interior becomes progressively less negative.
Threshold – a critical level of membrane potential (~ -55
mV) where depolarization becomes self-generating
5
Repolarization Phase
Positive intracellular charge reduces the driving force of Na+
to zero. Na+ inactivation gates of Na+ channels close.
After depolarization, the slower voltage-gated K+ channels
open and K+ rapidly leaves the cell following its
electrochemical gradient restoring resting membrane
potential
The neuron is insensitive to stimulus and depolarization
during this time (refractory period)
Hyperpolarization
The slow K+ gates remain open longer than needed to
restore the resting state.
This excessive efflux causes hyperpolarization of the
membrane
The neuron may depolarize during this time but it would
require a much greater stimulus.
Ion Permeability during an AP
6
Propagation of an Action Potential
The action potential is self-propagating and moves away from the stimulus (point of origin)
Role of the Sodium-Potassium Pump
Repolarization restores the resting electrical conditions of the neuron, but does not restore the resting
ionic conditions
Ionic redistribution is accomplished by the sodium-potassium pump following repolarization
Refractory Periods
Absolute refractory period is the time from the opening of the Na+ activation gates until the closing of
inactivation gates; the neuron cannot respond to another stimulus
Relative refractory period follows the absolute refractory period. Na+ gates are closed (almost all Na+
gates are reset), K+ gates are open, and hyperpolarization is occurring. Only a strong stimulus can
generate an AP
7
Refractory Periods
Axon Conduction Velocities
Conduction velocities vary widely among neurons, determined mainly
by:
Axon Diameter – the larger the diameter, the faster the
impulse (less resistance)
Presence of a Myelin Sheath – myelination increases
impulse speed (Continuous vs. Saltatory Conduction)
Myelin Sheath
A Schwann cell envelopes and encloses the axon with its plasma
membrane.
The concentric layers of membrane wrapped around the axon are the
myelin sheath
Neurilemma – outermost cytoplasm and exposed membrane of a
Schwann cell
8
Saltatory Conduction
Gaps in the myelin sheath between adjacent Schwann cells are called nodes of
Ranvier (neurofibral nodes)
Voltage-gated Na+ channels are concentrated at these nodes
Action potentials are triggered only at the nodes and jump from one node to the
next
Much faster than conduction along unmyelinated axons
Trigger Zone: Cell Integration and Initiation of AP
A graded potential above threshold (above -55 mV) reaches the trigger zone and becomes an action
potential
a) A graded potential starts above
threshold (T) at its initiation point, but
decreases in strength as it travels through
the cell body. At the trigger zone, it is
below threshold and therefore does not
initiate an action potential.
b) A stronger stimulus at the same point on
the cell body creates a graded potential
that is still above threshold by the time it
reaches the trigger zone, so an action
potential results.
9
Synapse
As the AP reaches the axon terminals the signal is relayed to target cells at specialized junctions known
as synapses
Synapse is a junction that mediates information transfer from one neuron to another neuron or to an
effector cell
Synaptic Cleft: Information Transfer
An AP reaches the axon terminal of the presynaptic cell and causes V-gated Ca2+ channels to open
Ca2+ rushes in, binds to regulatory proteins on vesicles, & initiates NT exocytosis
NTs diffuse across the synaptic cleft and binds to receptors on the postsynaptic membrane and
initiate some sort of response on the postsynaptic cell
10
Synaptic Transmission
Neurotransmitter Removal
NTs are removed from the synaptic cleft via:
Enzymatic degradation
Diffusion
Reuptake
11
Effects of the Neurotransmitter
Different neurons can contain different NTs
Different postsynaptic cells may contain different receptors -thus, the effects of an NT can vary
Some NTs cause cation channels to open, which results in a graded depolarization (excitatory)
Some NTs cause anion channels to open, which results in a graded hyperpolarization (inhibitory)
EPSPs & IPSPs
Typically, a single synaptic interaction will not create a graded depolarization strong enough to migrate
to the axon hillock and induce the firing of an AP
However, a graded depolarization will bring the membrane potential closer to threshold. Thus,
it’s often referred to as an excitatory postsynaptic potential or EPSP.
Graded hyperpolarizations bring the membrane potential farther away from threshold and thus
are referred to as inhibitory postsynaptic potentials or IPSPs.
Excitatory And Inhibitory Neurotransmitters
Excitatory - a neurotransmitter depolarizes the post-synaptic neuron,
Inhibitory – a neurotransmitter hyperpolarizes the post-synaptic neuron
Whether a neurotransmitter is excitatory or inhibitory depends on its receptor
12
Excitatory And Inhibitory Neurotransmitters
KEY:
ICR = ion channel receptor
GPLR = G Protein Linked Receptor
Excitatory And Inhibitory Neurotransmitters
Acetylcholine is excitatory because its receptor is a ligand-gated Na+ channel
GABA is inhibitory because its receptor is a ligand-gated Cl- channel
Other transmitters (e.g. vasopressin, dopamine) have G-protein-linked receptors
Effects depend on the signal transduction pathway and cell type
13
Summation
One EPSP is usually not strong enough to cause an AP
However, EPSPs may be summed
Temporal summation
The same presynaptic neuron stimulates the postsynaptic neuron
multiple times in a brief period. The depolarization resulting from the combination of all the
EPSPs may be able to cause an AP
Spatial summation
Multiple neurons all stimulate a postsynaptic neuron resulting in a combination of EPSPs which
may yield an AP
Synaptic Organization
Communication between neurons is not typically a one-to-one event, but often a combination of
convergence and divergence
Convergence - as many as 10,000 neurons converge upon a single postsynaptic neuron, increasing its
sensitivity
Divergence – a single presynaptic neuron branches; its collaterals synapse on multiple target neurons,
allowing sensory info, for example, to reach multiple target nuclei in the brain and spinal cord
14