Nervous System Topic 2

Nervous System Topic 2

Impulse Conduction

Impulse Conduction

• Caused by an electrical current 

• Impulse transmission results from the flow of charged particles from one point to another.

• In the body, whenever ions with opposite electrical charges are separated by a membrane, the potential exists for them to move toward one another. This is called membrane potential.

• •A membrane that exhibits membrane potential (an excess of positive ions on one side of the membrane and an excess of negative ions on the other side) is said to be polarized.

Multiple Choice

Impulse transmission results from the flow of

• membrane potential

• charged particles

• negative ions

Impulse Conduction: Step 1 Resting Potential

• When a neuron is not conducting an electrical signal, the interior has a negative electrical charge and the exterior has a positive charge. 

• The outside of the cell is rich with sodium ions (Na+), whereas the inside contains an abundance of potassium ions (K+). 

Impulse Conduction: Step 1 Resting Potential

• The interior also contains other large, negatively-charged proteins and nucleic acids. These additional particles give the cell’s interior its overall negative charge. 

Impulse Conduction: Step 1 Resting Potential

• Because of the membrane’s permeability, a certain amount of sodium and potassium ions leaks across the membrane. However, the sodium-potassium pump constantly works to restore the ions to the appropriate side. 

Impulse Conduction: Step 1 Resting Potential

• This state of being inactive and polarized is called resting potential. The neuron is resting, but it has the potential to react if a stimulus comes along. 

Multiple Choice

The state of being inactive and polarized

• Resting potential

• Impulse conduction

Multiple Choice

When a neuron is not conducting an electrical signal, the interior has a negative electrical charge and the exterior has a positive charge.

• True

• False

Impulse Conduction: Step 2 Depolarization

• A stimulus (such as chemicals, heat, or mechanical pressure) causes channels on the neuron’s membrane to open and Na+ from outside the membrane rushes into the cell. 

Impulse Conduction: Step 2 Depolarization

• •The addition of the positively charged ions changes the charge of a region of the cell’s interior from negative to positive. As the membrane becomes more positive, it is said to depolarize.

Multiple Choice

Depolarization is when the membrane becomes more ___________ charged

• Negatively

• positively

Impulse Conduction: Step 3 Action Potential

• If the depolarization goes above the threshold level, adjacent channels also open, which allows more Na+ to enter. This creates an action potential, which means that the neuron has become active as it conducts an impulse along the axon. (Another term for action potential is nerve impulse.)

Impulse Conduction: Step 3 Action Potential

• •The action potential continues down the axon as one segment stimulates the segment next to it.

Multiple Choice

Another name for acton potential is

• impulse conduction

• Polarization

• depolarization

• nerve impulse

Step 4 Repolarization

• The sudden influx of Na+ triggers other channels to open; this allows K+ to flow out of the cell.

  •Soon after K+ begins to exit, the Na+ channels shut to prevent any more Na+ from flowing into the cell. This repolarizes the cell; however, Na+ and K+ are now flip-flopped, with the outside containing more K+ and the inside containing more Na+.

Multiple Choice

When repolarization occurs Na+ rushes in and this allows ______ to leave the cell.

• $$O_2$$

• K+

• $$CO_2$$

Step 5 Refractory Period

• As long as Na+ and K+ are on the wrong sides of the membrane, the neuron won’t respond to a new stimulus even though the membrane is polarized. This is called the refractory period.

  
  

Multiple Choice

During the refractory period the neuron ________respond to a new stimulus

• will

• Won't

Step 5 Refractory Period

• The sodium-potassium pump returns Na+ to the outside and K+ to the inside. When this is completed, the nerve is again polarized and in resting potential until it receives another stimulus.

Multiple Choice

This returns the neuron to resting potential

• Oxygen carbon dioxide pump

• Sodium potassium pump

Impulse Conduction: Myelinated Fibers

• Myelin blocks the free movement of ions across the cell membrane; the only place ion exchange can occur is at the nodes of Ranvier. 

Multiple Choice

In a myelinated fiber the only place ion exchange can occur

• Nodes of Ranvier

• Axon

• Soma

• dendrite

Impulse Conduction: Myelinated Fibers

• 1.Electrical changes occur at the nodes of Ranvier, creating an action potential. The current flows under the myelin sheath to the next node, where it triggers another action potential.

Impulse Conduction: Myelinated Fibers

• 1.Because the action potentials only occur at the nodes, the impulse seems to “leap” from node to node. This type of signal conduction is called saltatory conduction. 

Multiple Choice

When the impulse seems to leap from node to node this is called

• refractory period

• action potential

• saltatory conduction

Synapses

• As impulses move from one neuron to the next, they pass through a synapse:

  1.When an action potential reaches a synaptic knob, the membrane depolarizes. Ion channels open and calcium ions enter the cell.

Synapses

• 2.The infusion of calcium causes vesicles to bind to the cell wall and release their store of a neurotransmitter into the synapse.

  3.The neurotransmitter binds to receptors on the postsynaptic membrane. Each neurotransmitter has a specific receptor. (For example, the neurotransmitter epinephrine can only bind to receptors specific to epinephrine.)

Synapses

• 4.The specific neurotransmitter determines whether the impulse continues (called excitation) or whether it is stopped (called inhibition). If the impulse is inhibitory, K+ channels open and the impulse stops. If the neurotransmitter is excitatory—as shown here—Na+ channels open, the membrane becomes depolarized, and the impulse continues.

Synapses

• 5.The receptor releases the neurotransmitter, after which it is reabsorbed by the synaptic knobs and recycled or destroyed by enzymes (as shown here).

Multiple Choice

Where the impulse passes from one neuron to the next

• Synapse

• neurotransmitter

Multiple Choice

chemical contained in the synaptic knob

• Sodium

• Neurotransmitter

• potassium