IB biology: topic 6: neurons and synapses

how many neurons are there in the human nervous system?

85 million

what do neurons consist of?

cell body and nerve fibres

what is the diameter of nerve fibres?

1 micrometre in diameter

what is the speed of condition of an impulse in an unmyelinated neuron?

around 1m/s

what does myelin consist of?

many layers of phospholipid bilayer

what cells deposit myelin?

Schwann cells

what is saltatory conduction?

when impulses can jump from one Node of Ranvier to another (across the gaps in the myelin sheath)

how fast is saltatory conduction?

around 100m/s

what is the potential difference across a membrane of a resting neurone?

\-70mV

what causes the resting potential in a neuron?

imbalance of positive and negative charges across membrane:

Na+/K+ pump pumps 3Na+ out but only 2K+ in.

Membrane is 50x more permeable to K+ than Na+ so K+ leak back faster.

Negatively charged proteins are found inside fibre.

describe an action potential? (7)

1. Pump stops, Na+ channels open, (K+ channels closed)
2. Na+ flow in down concentration gradient making inside less negative \[Depolarisation\]
3. As Na+ continue to flood in, positive charge builds up inside until gradient dissipates: inside now +30mV
4. Na+ channels close, K+ channels open
5. K+ flow out down concentration gradient making inside more negative \[Repolarisation\]
6. So many K+ flow out that inside becomes even more negative \[Hyperpolarisation\] (-90 mV)
7. K+ channels close, pump starts working again restoring resting potential.

what is depolarisation?

Na+ flow in down concentration gradient making inside

less negative

what is hyperpolarisation?

So many K+ flow out that inside becomes even more negative

what is repolarisation?

K+ flow out down concentration gradient making inside

more negative

describe the propagation of action potentials

Ion movements that depolarise one part of a neurone trigger depolarisation in neighbouring part.

Impulse can only be initiated at one terminal of neurone and take place in one direction.

Refractive period after depolarisation prevents propagation backwards.

what are local currents?

reduced Na+ concentration outside and increased Na+ concentration increased means that depolarised part now has different Na+ concentrations to neighbouring part.

So, Na+ diffuse between these regions both inside and
outside

how is the membrane potential affected by local currents?

increases membrane potential from -70mV to -50mV.

why is -50mV significant?

it is the threshold potential which opens the voltage-gated Na+ channels - causing a wave of depolarisation then repolarisation to occur along the axon

describe the process of synaptic transmission? (6)

1. nerve impulse reaches end of pre-synaptic neurone and the membrane depolarises
2. Ca2+ channels in membrane open causing influx of Ca2+, causing vesicles of neurotransmitter to migrate to and fuse with membrane
3. neurotransmitter released into synaptic cleft by exocytosis, diffuses across cleft and binds to receptors on post-synaptic membrane
4. Na+ channels in membrane open causing influx of Na+, causing membrane to reach the threshold potential, triggering action potential
5. action potential is propagated along neurone
6. neurotransmitter is broken down and removed from cleft.

what is acetylcholine? (ACh)

a neurotransmitter found many synapses

- ACh broken down into acetate and choline in cleft.

- The choline is reabsorbed into the pre-synaptic neurone.

how is acetylcholine produced?

produced in pre-synaptic neurone by combination of an acetyl group (from aerobic respiration) with choline (from diet).

what enzyme breaks down acetylcholine?

acetylcholinesterase (found in cleft)

what allows an action potential to be initiated?

once threshold potential has been reached (-50 microvolts)

what is an example of positive feedback in action potentials/synaptic transmission?

the opening of some Na+ channels and influx of Na+ increases membrane potential causing more channels to open

what is the importance of neurons having multiple synapses with many pre-synaptic neurons?

several presynaptic neurons may have to release neurotransmitters for threshold potential to be reached. This mechanism can be used to process info from different sources in body to help in decision making.

what has happened if a threshold potential is not reached?

quantity of neurotransmitter released may not be enough