The Nervous System Flashcards
(16 cards)
What are the three types of neurons?
Motor
Sensory
Relay
What is the basic structure of all neurons
Cell body – Enlarged part containing the nucleus
Dendrites – Short, slender input channels extending from end of cell body
Axon – A single, long output channel extending from other end of cell body
What are the uses for schwann cells
envelop the axon with fatty material called myelin
What is the use of myelin in neurons
Acts as a electrical insulator
What are the nodes of Ranvier?
Uninsulated gaps between myelin sheaths, allowing for nerve impulses to jump from node to node.
What is the Formation of the Myelin Sheath like?
The Schwann cell spirals around, enclosing the axon in many layers of its cell surface membrane. This enclosing sheath, called the myelin sheath, is made largely of lipid, together with some proteins.
What are voltage gated ion channels?
Voltage-gated ion channels are membrane proteins that open or close in response to changes in membrane potential, allowing specific ions (e.g. Na⁺, K⁺, Ca²⁺) to enter or exit the cell, crucial for generating and propagating action potentials.
What is the resting potential of an unstimulated cell?
-70mV
What is the role of Ungated ion channels?
These channels are always open and allow ions to diffuse across the plasma membrane.
How is an electrochemical gradient formed>
When a neurone is “at rest”, sodium-potassium pumps in cell membranes move Na+ out and K+ into cells (by active transport)
Concentration of Na+ builds up outside the cell
K+ may diffuses in and out through some open channels
So the neuron’s outside is more positive than inside, a difference of about -70mV
This unequal distribution of ions is known as an electrochemical gradient
How do nerve impulses form?
A nerve impulse results from ion movements of in and out of voltage-gated channels
A sensory input causes Na+ channels to open at the site of stimulation
Ions move slowly at first, making the membrane potential less negative
At a threshold of about -50mV there is a sudden influx of Na+ into the cell causing “depolarization”
The inside is now positive – about +40mV
Local voltage change opens adjacent Na+ channels
How is resting potential restores after an action potential?
The change in voltage (amplitude) of the action
potential is the same at all points along the axon, it does not rise or fall
After a slight delay (about 1 millisecond), K+
voltage-gated channels open and K+ flows out of the cell
The negative charge in the cell is restored and
the Na+ channels snap close again
The resting potential is restored by the action of the sodium-potassium pump
How does Repolarization occur?
Restoration of the resting membrane potential, For this to happen, Na must stop pouring into the cell
First step closure, or inactivation, of the sodium channels
To quickly restore the charge on both sides of the membrane that changed during depolarization, potassium must go out of the cell
This works because K+ has the same charge as Na+
Also because K+ really wants to go out of the cell, so all we have to do is let it do what it wants to do naturally
To let potassium out, the potassium channel must open
It begins to open at depolarization, but only becomes fully open at about the time the sodium channel closes (inactivates)
The inside of the membrane becomes less positive as K+ ions pour out
Initial overshoot causing membrane potential to be more negative than before (-70mV) but quickly restored by sodium-potassium pumps
What is the refractory period?
It is when the resting potential is being re-established. During this time no new action potential can be generated.
What is saltatory conduction?
The Na+ and K+ cannot flow through the myelin sheath. This means that the ions can only flow through unprotected cell-surface membrane.
In the case of a myelinated neurone, the ions can only move in and out of the cytoplasm at the nodes of Ranvier.
Because of this, the action potential will ‘jump’ from one node to the next, a process called saltatory conduction, and so will travel much faster than in an unmyelinated neurone.
What factors effect the speed of conduction?
Myelinated neurons
Temperature (higher = faster)
Diameter of the axon
