Nerves and Muscles 2

Question 1

What is the average resting potential of a neurone?

Answer 1

• 65 mV

Question 2

How do charged intracellular proteins contribute to resting potential?

Answer 2

Large negatively charged proteins cannot mass through the plasma membrane, and so keeping the potential negative.

Question 3

How does the Na+/K+ pump contribute to resting potential?

Answer 3

The Sodium-potassium pump is an ATPase pump that pumps out 3 Na+ ions for every 2 K+ ion pumped into the cell.

Question 4

How dos Sodium and Potassium ions contribute to the resting potential?

Answer 4

The plasma membrane is more permeable to K+ ions than Na+. As a result, K+ leaves the cell; more K+ leaves than Na+ enters. These ions move through ion channels.

Question 5

What is the Potassium equilibrium? How is it achieved?

Answer 5

• 70 mV. K+ moves out of the cell down a concentration gradient but moves into the cell by an electrochemical gradient. An equilibrium between these gradients are achieved. The potassium equilibrium is close to resting potential, the discrepancy is most likely due to Sodium ions.

Question 6

What is the Nernst Equation?

Answer 6

The equilibrium potential for any ion is worked out using this. It takes into account the temperature and the concentration of the ion both inside and outside the cell.

Question 7

How does Na+ add to the positivity of the cell?

Answer 7

The membrane is only slightly permeable to Sodium and so the net inward diffusion of Na+ slightly adds to the positivity of the cell.

Question 8

How is an action potential produced?

Answer 8

1. A stimulus causes Na+ ions to enter the cell. this causes membrane potential to rise.
2. If the potential reaches threshold potential, voltage gated ion channels are opened and an influx of sodium ions enters the cell.
3. This causes an action potential spike. Na+ voltage gated ion channel close when equilibrium potential is reached.
4. K+ voltage gated ion channels open and K+ rushes out of the cell, thereby reducing potential .
5. The K+ ions continue to flow out fo the cell while Na+ channels close.
6. Hyper-polarisation occurs as more K+ leaves even once resting potential is reached.

Question 9

What is the ‘all or nothing’ principle?

Answer 9

An action potential occurs when the membrane depolarises to a certain threshold, if this threshold is not reached the action potential will not be triggered. The size of the action potential is the same regardless of the size of the stimulus.

Question 10

What does a larger stimulus result in?

Answer 10

More frequent action potentials being sent.

Question 11

What is the refractory period?

Answer 11

Refractory period is a period immediately following a stimulus during which further stimulation has no effect.

Absolute refractory period: Where no other action potentials can be sent (usually during depolarisation). This determines the direction the action potential is sent.

Relative refractory period: If there stimulus is large enough, you can generate another action potential (usually during hyper polarisation)

Question 12

What is a local current?

Answer 12

The next section of the axon in which the action potential is sent, can be depolarised as sodium ions can diffuse across to the Nodes of Ranvier.

Local current flow following depolarisation results in depolarisation of the adjacent axonal membrane and where this reaches threshold, further action potentials are generated.

Action potentials cannot occur in regions behind as that region is in absolute refractory period.

Question 13

What is sensory transduction?

Answer 13

The conversion of environmental or internal signals into electrochemical energy.

Question 14

What is the muscle spindle?

Answer 14

A proprioceptor and mechanoreceptor. It is a modified muscle fibre enclosed in connective tissue that responds to stretch. They imitate reflexes which cause the muscle to contract to reduce the danger of over stretching. When a muscle is stretched passively the spindle is activated and so initiates a reflex. When the muscle contracts and shortens it is switched off.

Question 15

What is the Golgi tendon organ?

Answer 15

The Golgi tendon rogan lies in the tendon. A small bundle of collagen fibres enclosed in a layered capsule with a terminal branches of a large diameter afferent fibre intertwined with collagen bundles. Stimulated when the associated muscle contracts or is stretched. Sets up reflex that cause the muscle to relax an avoid stimulation. It is activated when the muscle is actively stretched.

Question 16

What is the receptor potential?

Answer 16

The transmembrane potential difference produced by activation of a sensory receptor. A receptor potential is often produced by sensory transduction.

Question 17

What is the knee-jerk reflex?

Answer 17

A reflex initiated by the muscle spindle. When the knee is tapped, it causes a muscle contraction as if the muscle has been stretched.
There are no interneurones involved in this response. This reflex is a reflex of proprioception which helps maintain posture and balance, allowing to keep one’s balance with little effort or conscious thought.

Question 18

What are electrical synapses?

Answer 18

Direct passages of current via ions flowing through gap junctions. These gap junctions formed by interlocking connexions - made up of connexin proteins.

These synapses occupe in parts of the CNS such as the retina and hypothalamus where fast burst firing occurs. Current flow is unidirectional but can occur in both directions in invertebrates.

Question 19

What are chemical synapses?

Answer 19

These cells synapses use neurotransmitters to deliver the information from the presynaptic neurone to the postsynaptic neurone.

Question 20

How is a neurotransmitter released?

Answer 20

1. The action potential reaches the axon terminal of the presynaptic cell.
2. The action potential triggers calcium entry causing release of neurotransmitter from vesicles which fuse with the synaptic membrane.

Vesicles are docked and primed before action potential arrives.

3. Upon calcium entry, the vesicle fuses with the plasma membrane and the contents of the vesicles are related into the synaptic cleft.

Question 21

What are three groups of neurotransmitters?

Answer 21

Amines, amino acids and betides

Question 22

What is an ionotrophic receptor?

Answer 22

Ionotropic receptors are ion channels and comprise mostly 4 or 5 similar protein subunits arranged around a central pore that is normally closed to ion movements. When the transmitter (ligand) binds it causes a conformational change that briefly opens the pore and ions pass through to cause a rapid change in the resting potential of the underlying cytoplasm.

Question 23

What is a metabotrophic receptor?

Answer 23

Metabotropic receptors are single, long protein molecules, mostly crossing the cell membrane 7 times (7 trans-membrane domains). There is no ion pore. When the ligand binds there is a conformational change in the molecule that causes the intracellular part to interact with a G-protein that then sets off a chain of intracellular events, that may include opening of ion channels. This is a slower response.

Question 24

What is an IPSP?

Answer 24

Binding of GABA or glycine to their receptors causes an influx of Cl- ions giving rise to an inhibitory post-synaptic potential (IPSP) in postsynaptic cell. Makes it longer to reach threshold and so makes it harder to longer for an AP to be sent. IPSPs tend to hyperpolarise cell and make may initiation of an AP less likely i.e. inhibition

Question 25

What is an EPSP?

Answer 25

Excitatory post-synaptic potential. The EPSPs can have a summative effect and so lead to an action potential as it can result in the potential reaching threshold. Binding of glutamate or acetylcholine to their receptors causes an influx Na+ ions giving rise to an EPSP.

Question 26

What is a neuromodulator?

Answer 26

A substance that is released and modifies the action of a transmitter, but doesn’t have a direct action itself - such as some peptide neurotransmitters.

Question 27

What is a neuroactive substance?

Answer 27

A neutral term if a substance is known to have an effect in the CNS but its precise action is not known

Question 28

How can neurotransmitter be recycled?

Answer 28

1. Can be taken back up directly into neurons by transporters on the presynaptic membrane (or even postsynaptic).
2. Can be broken down by cell surface enzymes into constituent parts, which are then taken back up into neurons by transporters.
3. Can be taken up into glial cell processes lining the peri-synaptic zone by glial cell transporters, then “shuttled” back into neurons by a process involving other transporters.
4. Can be taken up into glial cell processes by glial cell transporters, broken down or converted by enzymes in the glial cells, then the resulting metabolites “shuttled” back into neurons by a process involving other transporters.