Nervous System Part 1 Flashcards
Potential difference across membrane of resting cell is approximately…
The membrane is polarized and there is a ________ relative to outside
–70 mV in neurons
– Negative inside
The resting membrane potential is generated by:
A. Differences in ions concentration across the membrane (Concentration gradient)
B. Differential permeability of the plasma membrane
C. Ion pumps (lesser role)
Concentration Gradient
Na+ - higher concentration outside cell (in ECF)
Balanced chiefly by chloride ions (Cl-)
K+ - higher concentration inside cell (in ICF)
Balanced by negatively charged proteins
Ions diffuse quickly across membrane along electrochemical gradients
* From higher to lower concentration
* Along electrical gradients toward
opposite electrical charge
* Ion flow creates an electrical current
and voltage changes across membrane (membrane potential)
Equilibrium potential
The potential difference across a membrane at which (The concentration gradient = electrical gradient for an ion)
Ion movement ceases
The equilibrium potential for Potassium -90 mV= indicates
that the electrical force pulling K+ back into the cell is equal to
the force of the concentration gradient pushing K+ out
Nernst equation (Memorize the equation)
Equilibrium potential for any ion
Differences in Ion Permeability
More K+ diffuses out through leak channels than Na+
diffuses in
Cell more negative inside (-70 mV)
Establishes resting membrane potential
Goldman-Hodgkin-Katz equation: (Memorize the equation)
Expansion of Nernst equation that
adds permeability of ions
Ion Pumps
Sodium-potassium pump stabilizes resting membrane potential
Maintains concentration gradients for Na+ and K+
* 3 Na+ pumped out of cell
* 2 K+ pumped in
Two main types of ion channels
- Leakage (non-gated) channels: Always open
- Gated: Part of protein changes shape to open/close channel
Gated Channels
- Chemically gated (ligand-gated) channels
* Open with binding of a specific neurotransmitter - Voltage-gated channels
* Open and close in response to changes in membrane potential
* Give the membrane the ability to undergo Action potential (AP) - Mechanically gated channels
* Open and close in response to physical deformation of
receptors, as in sensory receptors
Properties of Gated Channels
- Each K+ channel has one voltage-sensitive gate
- Closed at rest
- Opens slowly with depolarization
Each Na+ channel has two voltage-sensitive gates
Activation gates
* Closed at rest; open with depolarization allowing Na+ to enter cell
Inactivation gates
* Open at rest; block channel once it is open to prevent more Na+ from entering cell. Each Na+ channel has two voltage-sensitive gates
- Graded potentials
- Action potentials
- Incoming signals operating over short distances
- Long-distance signals of axons
Graded Potentials
Graded potentials occur in the dendrites and cell body of neurons
They are used to transmit signals over short distances within the neuron.
Short-lived, localized changes in membrane potential
Triggered by stimulus that opens gated ion channels
- Types of graded potential:
– Receptor potential
– Synaptic potential
– Pacemaker potential
Either depolarization or hyperpolarization
Magnitude varies with stimulus strength
Stronger stimulus= more voltage changes= farther current flows
Threshold
Not all depolarization events produce APs
For axon to “fire”, depolarization must reach threshold
That voltage at which the AP is triggered (-55mV)
At threshold:
– Membrane has been depolarized by 15 to 20 mV
– Na+ permeability increases
– Na+ influx exceeds K+ leak out
– The positive feedback cycle begins
The All-or-None Phenomenon
An AP either happens completely, or it does not happen at all
Drugs and toxins can prevent APs
– Na+ channel blockers
* Procaine (Novocaine)
* Lidocaine (Xylocaine)
* Tetrodotoxin (TTX)
Aps signals, generated in sensory neurons in response to injury= can’t reach the brain to give rise to the sensation of pain
Propagation of an Action Potential
Propagation: allows AP to serve as a signaling device
2 factors that influence conduction velocity
1) Axon diameter
Larger diameters= less resistance= faster impulse conduction
2) Degree of myelination
What type of cells form the myelin sheaths that surround axons within the central nervous system?
Oligodendrocytes
The cells responsible for forming individual myelin sheaths around single axons in the peripheral nervous system are called ______.
Schwann cells
Unmyelinated regions of axon in between regions covered by myelin are known as the nodes of
Ranvier
Neurons entirely within the CNS that form links between other neurons and comprise the great majority of all neurons are called
interneurons
If axons are severed, they can repair themselves and restore function provided that ______.
the cell body is not destroyed
the damage occurs outside of the central nervous system
The permeability of a resting plasma membrane is greater for ______ than it is for ______; therefore, a net negative membrane potential develops.
potassium; sodium
Action Potentials (AP)
Principle way neurons send signals
Long-distance neural communication
Occur only in muscle cells and axons of neurons
Brief reversal of membrane
potential with a change in voltage
of ~100 mV
Generation of an Action Potential
1) Action potential begins when the neuron’s membrane potential reaches a threshold, causing sodium channels to open and Na+ to flow rapidly into the cell. This causes depolarization, a rapid change in membrane potential towards a positive value.
2) This is followed by the inactivation of sodium channels and the opening of potassium channels, causing K+ to flow out of the cell and make it less positive in the cell, leading to repolarization.
3) Before the potassium channels close, more K+ will leave the cell, causing hyperpolarization. The action potential then propagates along the axon with saltatory conduction in myelinated axons and continuous conduction in unmyelinated axons.
Role of the Sodium-Potassium Pump
Repolarization resets electrical conditions, not ionic conditions
After repolarization Na+/K+ pumps (thousands of them in an axon) restore ionic conditions
