neural signalling sl & hl

Question 1

Neurons as cells within the nervous system that carry electrical impulses
- Students should understand that cytoplasm and a nucleus form the cell body of a neuron, with elongated nerve fibres of varying length projecting from it. An axon is a long single fibre. Dendrites are multiple shorter fibres. Electrical impulses are conducted along these fibres.

Answer 1

• Nervous systems is made up of neurons, which help internal communication by transmitting nerve impulses.
• Neurons transmit information along nerve fibers in the form of electrical impulses. The electrical impulse is not like an electrical current that flows along wires. An impulse is a momentary reversal in electrical potential difference in the membrane – a change in the position of charged ions between the inside and outside of the membrane of the nerve fibres.

Question 2

Neuron anatomy

Answer 2

• The neuron is the basic functional unit of the nervous system.
• Neurons have a cell body with cytoplasm and a nucleus, as well as a long narrow outgrowth called nerve fibers along which nerve impulses travel.
• Two kinds of fibers exist: Dendrites (short branches) and Axons (long fibers)

Question 3

nerve impulses result from…

Answer 3

A nerve impulse is a result of a change in concentration of sodium (Na+) and
potassium (K+) ions along the cell membrane.
Depending on the membrane potential (voltage), we can distinguish between a resting potential and an action potential.

Question 4

resting potential

Answer 4

occurs when the membrane potential across the nerve cell membrane is not stimulated

about -70mV

Question 5

action potential

Answer 5

involves the reversal (depolarization) and restoration (repolarization) of the electrical potential across the plasma membrane as a nerve impulse passes along the neuron

Question 6

Generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions
- They should understand the concept of a membrane polarization and a membrane potential and also reasons that the resting potential is negative.
- Students should understand how energy from ATP drives the pumping of sodium and potassium ions in opposite directions across the plasma membrane of neurons.

Answer 6

Sodium-potassium pumps
Leakage of ions back across the membrane by simple diffusion
Negatively charged proteins inside the nerve fibre.

Sodium-potassium pumps in the membrane transfer Na+ out of the neuron and K+ into the neuron. This is active transport and requires ATP. The number of ions pumped is inequal – three Na+ go out, two K+ ions go in. This causes an imbalance of ions and concentration gradients for both.

The axon membrane has a higher permeability of K+, which leaks out of the cell through its membrane channels faster than Na+ leaks in through its Na+ protein channels.

Question 7

NOTE: An action potential temporarily depolarizes the membrane to a positive value.

Question 8

Nerve impulses as action potentials that are propagated along nerve fibres
- Students should appreciate that a nerve impulse is electrical because it involves movement of positively charged ions.

AN ACTION POTENTIAL HAS @ STAGES….

Answer 8

Depolarization: The cell membrane’s charge becomes positive. This is caused by positive sodium ions going into the cell.

Repolarization: The cell membrane’s charge returns to negative. This is caused by positive potassium ions moving out of the cell.

Question 9

more

Answer 9

A stimulus causes sodium ions (Na+) to flow into the cytoplasm of the axon, reversing the polarity of the axon. This makes the membrane potential more positive (from -70mV to +40mV).

Towards the end of the action potential the flow of sodium ions stopps and potassium channels open up. This causes the flow of potassium ions (K+) out of the axon, bringen the membrane potential back down to -70mV.

Question 10

Depolarization and repolarization during action potentials
- Include the action of voltage-gated sodium and potassium channels and the need for a threshold potential to be reached for sodium channels to start to open.

Answer 10

Depolarisation starts with an electrical stimulus being carried along the neuron fibre. This acts on the voltage gated ion channels embedded within the membrane. The Na+ channel gates open, allowing a flow of Na+ ions following the concentration gradient, into the interior of the cell. This makes the membrane potential more positive inside compared to outside (ca. + 40 mV).

The Na+ gated channels close again, and the voltage gated K+ channels now open, allowing a K+ ions to diffuse out of the cell. This makes the inside of the cell more negative again. The resting potential is going to be restored.

Question 11

An Action potential is composed of two subsequent stages:

Answer 11

The action potential is initiated through the activation of voltage sensitive gates on ion channels which open when a threshold voltage across the membrane is exceeded.

At the resting potential the voltage gated channel is closed. The flow of ions can only occur through leak channels or the sodium potassium pump.

An action potential starts once the treshold potential of the axon membrane reaches -50 mV. This causes the voltage gated channels to open up, allowing Na+ ions to enter the cell.

Question 12

NOTE: The action potential then progresses along the whole length of the axon fiber.

Question 13

Propagation of an action potential along a nerve fibre/axon as a result of local currents
- Students should understand how diffusion of sodium ions both inside and outside an axon can cause the threshold potential to be reached.

Answer 13

• A stimulus must be at or above a minimum intensity, known as the threshold of stimulation to initiate an action potential. Either the depolarization is sufficient to fully reverse the potential difference in the cytoplasm (from –70 mV to +40 mV), or it is not.
• The movement of an impulse in form of an action potential along an axon is due to the diffusion of sodium ions at the inside and outside of the axon fibre.
• The movements of ions inside and outside the axons are called local currents.

Question 14

importance of local currents

Answer 14

Local currents reduce the concentration gradient in the part of the neuron that has not yet depolarized. This makes the membrane potential rise from -70 to -50mV. The Sodium channels in the axon are voltage gated, which means they are triggered to open when the threshold potential of -50 mV has been reached. Opening the Sodium channels causes depolarisation. The local currents therefore cause a wave of depolarization followed by repolarization.

Question 15

Oscilloscope traces showing resting potentials and action potentials

Answer 15

• The cell potential (i.e. the voltage produced by the movement of ions) can be measured using microelectrodes impaled into cells. A minimal amount of stimulus is needed to fire an action potential (threshold minimum must be reached). An oscilloscope image showing the changes (in mV) can be obtained.
• The change in potential difference in the plasma membrane of a neurone can be shown using an oscilloscope which traces the changes in voltage over time. The action potential is transported along the axon fiber.

Question 16

Variation in the speed of nerve impulses
- Compare the speed of transmission in giant axons of squid and smaller non-myelinated nerve fibres.
- Also compare the speed in myelinated and non-myelinated fibres.

Answer 16

• A bigger diameter for nerve fibers reduces resistance and therefore increases conduction speed. (faster signal transmissions/protective reactions against danger)
• The presence of a myelin sheath surrounding the axon fibre increases the speed of transmission of the action potential. Only at the junctions in the sheath (nodes of Ranvier) the axon membrane exposed. Elsewhere along the fibre, the electrical resistance of the myelin sheath prevents depolarization of the nodes.
• The action potentials actually ‘jump’ from node to node (this is called saltatory conduction, meaning ‘to leap’)This greatly speeds up the rate of transmission.

Question 17

Saltatory conduction in myelinated fibres to achieve faster impulses
- Students should understand that ion pumps and channels are clustered at nodes of Ranvier and that an action potential is propagated from node to node.

Answer 17

• Ion pumps and channels are clustered at the nodes of Ranvier. An action potential is therefore generated only at these points, and from there propagated in “jumps” from node to node. This is called saltatory conduction.

Question 18

Synapses as junctions between neurons and between neurons and effector cells
- Limit to chemical synapses, not electrical, and these can simply be referred to as synapses. Students should understand that a signal can only pass in one direction across a typical synapse.

Answer 18

• A synapse is the link point between neurons. A synapse consists of the swollen tip (synaptic knob) of the axon of one neuron (pre-synaptic neuron) and the dendrite or cell body of another neuron (post-synaptic neuron). At the synapse, the neurons are extremely close but they have no direct contact. Instead there is a tiny gap, called a synaptic cleft, about 20 nm wide

Question 19

4 types of synapses

Answer 19

synapses between neurons
synapses between neurons and muscles/glands/sensory receptors of sense organs

Question 20

an action potential cannot cross the synaptic cleft btw. neurons

Answer 20

An action potential cannot cross the synaptic cleft between neurons – the nerve impulse is carried by chemical transmitter substances called neurotransmitters.

These chemicals are made in the Golgi apparatus of the cell that is sending the impulse (the pre-synaptic neuron) and stored in vesicles at the end of the axon.

Question 21

Generation of an excitatory postsynaptic potential
- Include diffusion of neurotransmitters across the synaptic cleft and binding to transmembrane receptors. Use acetylcholine as an example. Students should appreciate that this neurotransmitter exists in many
types of synapse including neuromuscular junctions.

Answer 21

• The transmitter substance on the receptors is immediately inactivated by enzyme action to avoid overstimulation.
• The enzyme Acetylcholinesterase (AChE) hydrolyses ACh to choline and ethanoic/acetic acid, which are inactive as transmitters.
• As a consequence, the ion channel of the receptor protein closes, and the resting potential in the post-synaptic neuron is re-established.

Question 22

HL

Question 23

Effects of exogenous chemicals on synaptic transmission (including cocaine)
- Use neonicotinoids as an example of a pesticide that blocks synaptic transmission, and cocaine as an example of a drug that blocks reuptake of the neurotransmitter.

Answer 23

• Exogeneous chemicals are substances which enter the body from an outside source through ingestion, inhalation or absorption through the skin. The use of neonicotinoids as pesticides has resulted in effects on synaptic transmission in honey-bees.

Acetylcholine binds to acetylcholine receptors. There are two types of acetylcholine receptors (AChR) on the postsynaptic membrane and transmit its signal: muscarinic AChRs and nicotinic AChRs.

These receptors are functionally different. The muscarinic type mediates a slow metabolic response, while the nicotinic type mediate a fast synaptic transmission of the neurotransmitter.

Question 24

effects of exogeneous chemicals on synpatic transmision

Answer 24

Cocaine acts at synapses that use dopamine as a. neurotransmitter.
It blocks receptors on dopamine re-uptake pumps, which therefore causes it to remain in the synaptic cleft.
Therefore, dopamine builds up in the synaptic gap.
Increases post-synaptic transmission and continuous excitement.

Question 25

Inhibitory neurotransmitters and generation of inhibitory postsynaptic potentials
- Students should know that the postsynaptic membrane becomes hyperpolarized.
- alcohol example

Answer 25

Inhibitory:
Neurotransmitters used:
GABA
Dopamine

Consequence:
Incresead influx of Cl- ions into postsynaptic membrane and hyperpolarization. Membrane is more negative – more difficult to depolarize, impuls inhibited.

Alcohol binds to glutamate receptors in the brain and enhances the inhibitory effects of the neurotransmitter GABA which hyperpolarizes the postsynaptic neurone. It also helps to increase the release of dopamine. Alcohol particularly interacts with areas of the brain involved in decision making, memory formation and impulse control. It impairs reaction times and muscle coordination.

Question 26

Summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron
- Multiple presynaptic neurons interact with all-or-nothing consequences in terms of postsynaptic depolarization.

Answer 26

Usually, postsynaptic neurons have many synaptic junctions with presynaptic ones. The thing that decides if an impulse is passed on to create further neural activity is the overall summation of excitatory and inhibitory input to the post synaptic membrane.

The effect of each input from a pre-synaptic neuron is summative – and if the summative effect reaches threshold, an AP is propagated in the axon of the post-synaptic neuron.

If the sum of the inhibitory and excitatory signals are below the threshold, the no action potential will be triggered.

Question 27

Perception of pain by neurons with free nerve endings in the skin
- Students should know that these nerve endings have channels for positively charged ions, which open in response to a stimulus such as high temperature, acid, or certain chemicals such as capsaicin in chilli peppers.
- Entry of positively charged ions causes the threshold potential to be reached and nerve impulses then pass through the neurons to the brain, where pain is perceived.

Answer 27

• Pain receptors in the form of free nerve endings
• The nerve endings are receptors of sensory neurons and they are associated with channels for positively charged ions.

When ion channels are activated and the treshold potential is reached, a nerve impuls is passed through the sensory neuron to the spinal column and from there it is transducted to the cerebral cortex in the brain where pain is sensed and interpreted.

Question 28

Consciousness as a property that emerges from the interaction of individual neurons in the brain
- Emergent properties such as consciousness are another example of the consequences of interaction.