11.2 Nerve Signals

Question 1

Nerve Signals

Answer 1

• Nerve impulses are electrochemical signals
  created by movement of ions through the nerve
  cell membrane.
• When nerves are excited (in reaction to a
  pinprick or sight of someone’s face), a rapid
  change in electrical potential difference
  (commonly called just potential) is detected.

Question 2

Neural Communication via Synapses

Answer 2

• Synapse is a site where a neuron makes a connection w another neuron or effector
• Depending on the kind of neuron communication may occur chemically or electrically

Question 2

Chemical Synapses

Answer 2

• In a chemical synapse a chemical transmitter called a neurotransmitter is released
• The plasma membranes of presynaptic and postsynaptic cells are separated by a narrow gap called the synaptic cleft

Question 3

Electrical Synapse

Answer 3

• The membranes of the presynaptic and postsynaptic membranes are in direct contact
• Current flows in gap junctions between the cells
• Allow for rapid synchronous transmissions

Question 4

Conduction of Electrical Signals by Neurons

Answer 4

• All animal cells have a separation of positive and negative charges across the plasma membrane (Membrane potential)
• Membrane potential is caused by uneven distribution of Na+ and K+ inside and outside the cell
• Plasma membranes are selectively permeable – must travel through ion channels

Question 5

Resting Membrane Potential

Answer 5

A special ion channel, the Na+/K+ pump uses energy to pump 3Na+ out of the cell for every two K+ in, creating a net positive extracellular environment
• Resting Potential of a neuron is – 70 mV
• The concentration of anions within the cell results in the inside being negatively charged and outside being positively charged p.524

Question 6

Action Potential

Answer 6

• When a nerve becomes excited it conducts an impulse
• There is a quick temporary change in membrane potential called an action potential.
• An action potential begins as a stimulus that causes positive charges from outside the neuron to flow inward

Question 7

Phases of Action Potential

Answer 7

Phase 1 – Incoming positive ion raise membrane potential, this is called depolarization
• Depolarization will continue when the membrane potential reaches its threshold potential, about -50 to -55mv and Na+ channels ope

Question 8

Phases of AP (cont’d)

Answer 8

• Phase 2- Na+ channels continue to open, sodium rushes in along concentration gradient
• Phase 3 - Membrane becomes very positive reaching a peak +30mV or more
• Phases 4 –Na+ channels close and K+ channels open allowing potassium to exit
• Membrane potential falls again, called repolarization

Question 9

Phases of AP (cont’d 2)

Answer 9

• Phase 5- Potassium gates close slowly, membrane dips below resting (hyperpolarized)
• Phase 6 – membrane re-established resting membrane potential
• Stages 4-6 are considered the refractory period, the threshold that is required to generate another AP is much higher than normal (allows for one way communication)

Question 10

All or None

Answer 10

• APs occur either maximally or they do not occur at all.
• The intensity of the nerve impulse and speed of transmission remain the same.

Question 11

Conduction in Myelinated Neurons

Answer 11

Occurs in myelinated gaps called the Nodes of Ranvier
• Conduction is much more rapid, about 130 m/s vs 1m/s in unmyelinated nerve cells

Question 12

Chemical Synaptic
Transmission

Answer 12

Small spaces between neurons or neuron and effectors are known as synapses.
• At the end of axons, small vesicles are present that contain neurotransmitters.
• As impulse moves down the axon, neurotransmitters are released from pre synaptic neuron and diffuse along the synaptic cleft creating a depolarization of dendrites of the post synaptic neuron.

Question 13

Examples of Neurotransmitters (NT)

Answer 13

• Acetylcholine is an example of a NT found in pre synaptic neurons. It acts as a excitatory NT on post synaptic neurons by opening up Na+ ion channels.
• Cholinesterase from the postsynaptic membrane destroys acetylcholine so that the AP does not continue.
• Summation is the effect produced by the accumulation of NTs from two or more neurons.

Question 14

Examples of Neurotransmitters (NT)

Answer 14

• Acetylcholine is an example of a NT found in pre synaptic neurons. It acts as a excitatory NT on post synaptic neurons by opening up Na+ ion channels.
• Cholinesterase from the postsynaptic membrane destroys acetylcholine so that the AP does not continue.
• Summation is the effect produced by the accumulation of NTs from two or more neurons.

Question 15

Examples of Neurotransmitters (NT)

Answer 15

Acetylcholine is an example of a NT found in pre synaptic neurons. It acts as a excitatory NT on post synaptic neurons by opening up Na+ ion channels.
• Cholinesterase from the postsynaptic membrane destroys acetylcholine so that the AP does not continue.
• Summation is the effect produced by the accumulation of NTs from two or more neurons.