Neurons & Synapses (6.5)

Question 1

What are the main components of the Human Nervous System?

Answer 1

• The Central Nervous System (CNS)
  Composed of the brain and spinal cord. Relay neurons.
• The Peripheral Nervous System (PNS)
  Composed of all the nerves and sensory receptors outside of the CNS. Motor/sensory neurons.

Question 2

Give information on motor neurons and sensory neurons

Answer 2

Motor neurons transmit impulses from the CNS to the muscles and glands. Sensory neurons transmit impulse from the sensory receptors to the CNS.

Question 3

Define neurons

Answer 3

Neurons are specialised cells that function to transmit electrical impulses within the nervous system.

Question 4

What are the main components of neurons?

Answer 4

Cell body (soma)
Dendrites
Axon
Axon branches (w axon terminals at the end)
Nodes of Ranvier
Myelin sheath

Question 5

What is myelin?

Answer 5

Myelin is a fatty white substance that covers the axon of some neurons and functions as an insulating layer. It increases the speed of electrical transmissions via saltatory conduction. Its disadvantage is that it takes up significant space within an enclosed environment.

Question 6

Describe the movement of action potentials along unmyelinated neurons compared to myelinated neurons

Answer 6

In an unmyelinated neuron action potentials move along the axon in a continuous wave of depolarisation. In a myelinated neuron action potentials jump between the nodes of Ranvier, increasing the speed of electrical transmission by a factor of up to 100 fold.

Question 7

How do myelinated neurons compare to unmyelinated neurons in terms of appearance?

Answer 7

Myelinated axon tracts appear as white matter. Unmyelinated axon tracts appear as grey matter.

Question 8

Define nerve impulses

Answer 8

Action potentials that move along the length of an axon as a wave of depolarisation.

Question 9

Define resting potential

Answer 9

The electrical potential difference across the plasma membrane of a neuron that is not conducting an impulse i.e. is at rest.

Question 10

Define action potential

Answer 10

The rapid rise (depolarisation) and fall (repolarisation) in the electrical potential difference across the plasma membrane of a neuron that is excited.

Question 11

Describe the action potential impulse

Answer 11

When a neuron is stimulated the sodium (Na+) channels open and there is an influx of Na+ ions into the cell. This is called depolarisation and it creates a positive internal charge. It causes a burst of electrical activity to pass along the axon of the neuron.

With no further stimulation, the sodium channels close and the potassium (K+) channels open. There is an efflux of K+ ions from the cell. A negative internal charge is restored. This is called repolarisation.

However, potassium channels open and close very slowly so too much potassium can diffuse out leading to hyperpolarisation.

Question 12

Define the threshold potential

Answer 12

The level to which a membrane potential must be depolarized to initiate an action potential.

Question 13

Define synapses

Answer 13

The junctions between the end of one axon and the dendrites/cell body of a receiving neuron.

Question 14

Discuss synaptic transfer

Answer 14

Synaptic transfer is the process by which one neuron communicates with another. An electrical impulse (action potential) passes down the axon. When it reaches the axon terminal, it triggers the opening of voltage-gated calcium (Ca2+) channels. Calcium ions diffuse into the cell and signal to the vesicles containing neurotransmitters. The vesicles move to the membrane and release the neurotransmitters into the synaptic cleft by exocytosis. They bind to receptors on the post-synaptic membrane and open sodium ion channels. This causes depolarisation of the post synaptic membrane. The depolarisation passes down the post synaptic neuron as an action potential. The neurotransmitters are then broken down by enzymes on the membrane to prevent continued stimulation of the postsynaptic neuron.

Question 15

Define neurotransmitters

Answer 15

Chemical messengers released from neurons that function to transmit signals across the synaptic cleft.

Question 16

Name four neurotransmitters in the brain

Answer 16

Glutamate, adrenaline - both excitory
GABA - inhibitory
Dopamine - excitory and inhibitory

Question 17

Describe the secretion and absorption of acetylcholine

Answer 17

Acetylcholine is a neurotransmitter used by both the CNS and PNS. It is created in the axon terminal of neurons from choline and acetyl CoA. It is then stored in vesicles within the axon terminal until it is released by exocytosis into the synaptic cleft in response to a nerve impulse. It diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane. There it is broken down into choline and acetate by the enzyme acetylcholinesterase. The choline is transported back to the axon terminal and used to make acetylcholine. Acetylcholine must be continually removed from the synapse as overstimulation can lead to fatal convulsions and death.

Question 18

Describe neonicotinoid pesticides

Answer 18

Neonicotinoids are synthetic chemicals that bind irreversibly to the acetylcholine receptor. Acteylcholinesterase breaks down acetylcholine and prevents overstimulation. However it is unable to do so when the neonicotinoids bind to the receptors. Insects have a different composition of acetylcholine receptors which bind to neonicotinoids much more strongly than in humans so neonicotinoids lead to paralysis and death in insects.

Question 19

Advantages and disadvantages of neonicotinoid pesticides

Answer 19

They have been successfully used to protect crops from pest species.

However, their use has been linked to a reduction in honey bee populations and in bird populations (due to the loss of insects as a food source).