Unit 6.5 - Neurons and Synapses

Question 1

Neurons

Answer 1

Nervous system composed of cells called neurons - carry messages at high speed in the form of electrical impulses. Many neurons are very elongated and carry impulses long distances in a short time.

Question 2

Myelinated fibres

Answer 2

have a myelin sheath with small gaps (Nodes of Ranvier) allowing nerve impulses to jump from node to node - saltatory conduction and speeds up transmission

Question 3

Synapses

Answer 3

junction between two neurons or a junction between a neuron and receptor / effector cells - plasma membranes of the neurons are separated by a narrow fluid filled gap called the synaptic cleft. Messages are passed across the synapse in the form of chemicals called neurotransmitters- allow pass from presynaptic neuron to postsynpatic neuron

Question 4

How do synapses work

Answer 4

1) Nerve impulse reaches the end of the pre-synpatic neuron
2) depolarisation of the pre-synaptic membrane causes vesicles of neurotransmitters to move to the pre-synpatic neuron membrane and fuse with it releasing the neurotransmitter into the synaptic cleft by exocytosis
3) Neurotransmitters diffuse across the synaptic cleft and binds to receptors in the post synaptic membrane
4) receptors are transmitter gated sodium channels which open when neurotransmitter binds . sodium ions diffuse into the post synaptic neuron causing depolarisation of the post synaptic membrane
5) depolarisation passes down as an action potential
6) neurotransmitters in the synaptic cleft is rapidly broken down to prevent continuous synaptic transmissions

Question 5

Cholinergic Synapses

Answer 5

synapses do not all use the same neurotransmitter but many use acetylcholine (cholinergic synapses) - pre synaptic neuron secretes acetylcholine into the synaptic cleft which diffuses across the synapse and the binds to receptors in the post synaptic membrane. acetylcholine broken down in the synaptic cleft by the enzyme cholinesterase producing acetyl groups and choline - choline reabsorbed by the pre-synaptic neuron

Question 6

Neonicotinoid Pesticides

Answer 6

bind to acetylcholine receptors in the post-synaptic membranes of cholinergic synapses in insects. cholinesterase does not break down these pesticides so they remain bound and block acetylcholine - block synaptic transmission which kills the insect
Honey bees also killed although not the target

Question 7

Resting potential

Answer 7

voltage across a plasma membrane of a neuron when it is not conducting a nerve impulse. There are sodium potassium pumps in the plasma membranes of axons pumping sodium out and potassium in. Inside of the neuron develops a net negative charge compared with the outside due to the presence of chloride and other negatively charges ions. Typical resting potential = -70mV

Question 8

Action Potentials (depolarisation)

Answer 8

depolarisation and repolarisation of a neuron due to facilitated diffusion or ions across the membrane through voltage gated ion channels. sodium ions diffuse in down the concentration gradient - entry causes the inside of the neuron to develop a net positive charge compared to the outside (depolarization)

Question 9

Action potentials (repolarisation)

Answer 9

reverse of membrane polarity causes potassium channels to open allowing potassium to diffuse out down conc gradient - causes the inside to develop a net negative charge compared to the outside (repolarisation)

Question 10

Propagation of Nerve impulses

Answer 10

nerve impulse = action potential that travels along the neuron from one end to the other. There is an action potential whenever part of the axon reaches -50 mV. action potential = part of the axon triggers an action potential in the next part = propagation of nerve impulse

Question 11

Propagation of Nerve Impulses

Answer 11

due to diffusion of sodium ions between a region with an action potential and the next region that is still at resting potential. diffusion of sodium ions (local currents). Local currents change the voltage from resting potential (-70mV) to threshold potential (-50mV) - causes an action potential as voltage-gated sodium channels opens

Question 12

Oscilloscope Traces

Answer 12

changes in membrane potentials in axons during an action potential can be measured using electrodes - results displayed on a oscilloscope

Question 13

Stages of an oscilloscope

Answer 13

1) axon membrane is at a resting potential of (-70mV) and then rises to threshold of (-50mV) either due to local currents or the binding of a neurotransmitter at a synapse
2) The membrane depolarises due to voltage - gated sodium channels opening and potassium ions diffusing out
3) membrane repolarises due to voltage dates potassium channels opening and potassium diffusing out
4)the membrane returns to the resting potential due to pumping of sodium out and potassium in to the axon. This rebuilds the conc gradient so that another action potential could occur

Question 14

Memory and Learning

Answer 14

higher functions of the brain including memory and learning are only partly understood.
Excellent example of cooperation and collaboration between groups of scientists (important in nature of science). - scientists with different disciplines meet and exchange ideas within universities and also at conferences.