nervous system

Question 1

function of neurons

Answer 1

Neurons are nerve cells that transfer information within the body.

Neurons use two types of signals to communicate:
Electrical signals – long distances
Chemical signals – short distances

Question 2

Information transfer

Answer 2

Neurons transfer different types of information as an electrical signal, via the movement of ions. Signals include:
Sensory information
Control of heart rate
Coordination of hand and eye movement
Recording memories
Generating dreams

Question 3

processing information

Answer 3

Interpreting and processing these signals occurs in groups of neurons organized into
- Ganglia – small clusters of neurons
- The Brain

Question 4

information processing

Answer 4

Three stages of information processing:
- Sensory input – by sensory (afferent) neurons
- Integration – by interneurons
- Motor output – by motor (efferent) neurons

Question 5

supporting cells

Answer 5

Cells found surrounding the neurons and help in neuronal function:
Glial cells
Found in CNS
Some form the Blood Brain Barrier
Others form the myelin sheath
Schwann cells
Found in PNS
Form the myelin sheath

Question 6

Resting Membrane Potential

Answer 6

• All living cells have a charge difference across their plasma membranes (inside is -ve compared to outside).
• This difference creates an electrical voltage called membrane potential – (between -50 mV and -100 mV where the outside of the cell is 0 mV).
• The resting potential is the membrane potential of a neuron not sending signals (typically -70 mV). when it is doing nothing (almost never)
• Messages are transmitted as changes in membrane potential – only neurons and muscle cells can conduct electrical impulses this way – called excitable cells.

Question 7

maintaining resting potential

Answer 7

To establish and maintain resting membrane potential you need:
1. A difference in ion concentration on either side of the cell’s membrane.
2. Selective permeability of the membrane.

action potential → getting the message from one synapse to another

Question 8

ions concentration

Answer 8

negative inside neuron, positive outside

Question 9

transport for anions and cations

Answer 9

• sodium needs the active transport, as we need energy to go against the gradient
• sodium potassium-pump
• potassium channel
• sodium channel

Question 10

changes to membrane potential

Answer 10

• Occur in response to stimuli ex. seeing or hearing.
• Stimuli at the dendrites affect specific ion channels called voltage gated channels.
• The effect the stimulus has on the neuron depends on the gated channel that is opened.

Question 11

hyperpolarized

Answer 11

• stimulus opening k channels
• k moves out of cell ALONG electrochemical gradient
• inside of cell becomes mroe negative
• stimulus opens Cl channels
• Cl moves into the cell along its electrochemical gradient
  -inside of cell becomes more negative

Question 12

depolarized

Answer 12

• stimulus opens Na Channels
• Na moves into the cell along its electrochemical gradient
• inside of the cell becomes less negative

Question 13

production of action potentials

Answer 13

• Voltage-gated Na+ and K+ channels respond to a change in membrane potential.
• When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell.
• The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open. (An example of positive feedback.)
• A strong stimulus results in a massive change in membrane voltage called an action potential.

Question 14

action potentials

Answer 14

• An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold (-55).
• An action potential is a brief all-or-none principle (it will fire or will not) depolarization of a neuron’s plasma membrane.
• Action potentials are signals that carry information along axons.

Question 15

generation of action potentials: a closer look

Answer 15

• A neuron can produce hundreds of action potentials per second.
• The frequency of action potentials can reflect the strength of a stimulus.
• An action potential can be broken down into a series of stages.

Question 16

After the action potential

Answer 16

The refractory period is important to ensure the signal does not travel ‘backwards’, i.e. can only travel in one direction.
For the resting potential to be reached again, Na+/K+ pumps are used.
Na+ ions are actively pumped out of the neuron and K+ ions are actively pumped into the neuron in order to achieve the resting potential.

• prevents it from going backwards
• use active transport to pump out the sodium with the pump

Question 17

Conduction of Action potentials

Answer 17

• An action potential can travel long distances by regenerating itself along the axon.
• At the site where the action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane.
• Because of the refractory period, action potentials travel only in one direction – towards the synaptic terminals.

Question 18

conduction speed

Answer 18

The speed of an action potential increases with the axon’s diameter. (i.e. less resistance to current)
- ex. giant axons in squid

• In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase.
• Myelin sheaths are made by glial cells— oligodendrocytes in the CNS and Schwann cells in the PNS.

Question 19

Myelinated Nerve cells

Answer 19

nodes needed to refresh as we travel the message through the axon
- undershoot

• Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found.
• Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction.
• The advantage of this is space efficiency. A myelinated axon 20 ㎛ in diameter has a conduction speed faster than a squid giant axon with a diameter 40 times greater.
• Na diffuses out of these gaps

Question 20

saltatory conduction

Answer 20

milenation → shorter nerves that travel faster, not losing charges

• depolarizing region are the nodes of ranvier
• space efficient!

Question 21

Neurons Communicate With Other Cells At Synapses

Answer 21

• At electrical synapses, the electrical current flows from one neuron to another.
• At chemical synapses, a chemical neurotransmitter carries information across the gap junction.
• Most synapses are chemical synapses.

Question 22

chemical synapses

Answer 22

The presynaptic neuron synthesizes and packages the neurotransmitter in synaptic vesicles located in the synaptic terminal.
Types of Neurotransmitters:
- Excitatory: speed up impulses by causing depolarization of postsynaptic membrane
- Inhibitory: slow impulses by causing hyperpolarization of postsynaptic membrane
ex. pain signals, as we do not want to feel this

Question 23

Chemical Synapses overview 2

Answer 23

1. An action potential depolarizes the plasma membrane of the synaptic terminal.
2. This opens voltage-gated calcium channels in the membrane, triggering an influx of Ca+2.
3. The elevated Ca+2 concentration in the terminal causes synaptic vesicles to fuse with the presynaptic membrane.
4. The vesicles release neurotransmitter into the synaptic cleft.
5. The neurotransmitter binds to the receptor portion of ligand-gated ion channels in the postsynaptic membrane, opening the channels. In the synapse illustrated here, both Na+ and K+ can diffuse through the channels.
6. The neurotransmitter is released from the receptors, and the channels close. Synaptic transmission ends when the neurotransmitter diffuses out of the synaptic cleft, is taken up by the postsynaptic cell, or is degraded by an enzyme.

Question 24

Generation of Postsynaptic Potentials

Answer 24

• Direct synaptic transmission involves binding of neurotransmitters to ligand-gated ion channels in the postsynaptic cell.
• Neurotransmitter binding causes ion channels to open, generating a postsynaptic potential.

Question 25

Postsynaptic Potentials

Answer 25

Postsynaptic potentials fall into two categories: - Excitatory postsynaptic potentials (EPSPs) are depolarizations that bring the membrane potential toward threshold. - Inhibitory postsynaptic potentials (IPSPs) are hyperpolarizations that move the membrane potential farther from threshold.

Question 26

Summation of Postsynaptic Potentials

Answer 26

- Unlike action potentials, postsynaptic potentials are graded and do not regenerate. - Most neurons have many synapses on their dendrites and cell body. - A single EPSP is usually too small to trigger an action potential in a postsynaptic neuron. - If two EPSPs are produced in rapid succession, an effect called temporal summation occurs. - In spatial summation, EPSPs produced nearly simultaneously by different synapses on the same postsynaptic neuron get added together. - The combination of EPSPs through spatial and temporal summation can trigger an action potential. - Through summation, an IPSP can counter the effect of an EPSP. - The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential.

Question 27

Temporal Summation

Answer 27

A stimulus at E1 alone is not enough for an action potential to occur. Multiple stimuli at E1 in rapid succession are added together and are enough for threshold to be reached causing an action potential in the postsynaptic cell.

Question 28

spatial summation

Answer 28

A stimulus at E1 alone, in addition to the inhibitory nature of I, is not enough for an action potential to occur. When both E1 and E2 are stimulated, they are added together and are enough for threshold to be reached causing an action potential in the postsynaptic cell.