F214 - Nerves

Question 1

Sensory Receptors

Definition

Answer 1

Specialised cells that can detect changes in the environment around them
They act as energy transducers, converting a stimulus into the electrical energy of a nerve impulse

Question 2

What are the sensory receptors for changes in light intensity and wavelength?

Answer 2

Rods and cones (light sensitive cells) in the retina of the eye

Question 3

What are the sensory receptors for the presence of volatile chemicals?

Answer 3

Olfactory cells lining the inner surface of the nasal cavity

Question 4

What are the sensory receptors for the presence of soluble chemicals?

Answer 4

Taste buds in the tongue, hard palate, epiglottis and the first part of the oesophagus

Question 5

What are the sensory receptors for pressure on the skin?

Answer 5

Pacinian corpuscles (pressure receptors) in the skin

Question 6

What are the sensory receptors for changes in the length of muscle fibres?

Answer 6

Proprioceptors (muscle spindles)

Question 7

How is the resting potential achieved and maintained?

Answer 7

• sodium and potassium ion pumps in the cell membrane of the neurone pump sodium ions out of the cell and potassium ion into the cell
• for every three sodium ions that are pumped out, two potassium ions are pumped in
• sodium ion channels are closed so sodium ions accumulate outside of the cell
• this creates a concentration gradient for potassium ions which exist in a higher concentration inside the cell
• potassium ions move out of the cell through potassium ion channels by facilitated diffusion
• there is an excess of positively charged ions outside of the sell which sets up an electrochemical gradient acting in the opposite direction to the potassium ion concentration gradient
• when there is no net movement of potassium ions the concentration gradient and electrochemcial gradient are said to be in equilibrium and a resting potential of -60mV across the membrane is acheived

Question 8

How is an action potential generated?

Answer 8

• a stimulus triggers the first in a sequence of sodium ion channels to open, sodium ions move into the neurone through sodium ion channels by facilitated diffusion
• this alters the potential difference across the membrane as the inside of the cell becomes less negative
• if a threshold potential of -55mV is reached then voltage gated sodium ion channels further along the axon open and more sodium ions diffuse in
• this continues like a wave down the neurone, at this point the process is unstoppable and the potential difference across the membrane rises to +40mV, this process is called depolarisation
• behind the front of the action potential ‘wave’ the depolarisation causes sodium ion channels to close and potassium ion channels to open
• potassium ions move out of the cell by facilitated diffusion through potassium ion channels along both a concentration and electrochemical gradient
• as a result the potential difference across the membrane becomes negative again, repolarisation
• but the potential difference overshoots slightly, too many potassium ions leave the cell, the potential difference is too negative and he neurone is hyperpolarised
• sodium potassium ion pumps and the facilitated diffusion of potassium ions restores a resting potential of -60mV across the membrane

Question 9

What is the myelin sheath?

Answer 9

An insulating layer of fatty material which surrounds nerones
It is made up of Schwann cells
The gaps between the Schwann cells are called the Nodes of Ranvier

Question 10

What is the difference between myelinated and non-myelinated neurones?

Answer 10

A myelinated neurone is a neurone that has a myelin sheath
In myelinated neurones the local currents are elongated as ionic exchange can only occur at the nodes of Ranvier, sodium ions diffuse along the axon from one node to the next and the impulse appears to jump this is called saltatory conduction
Because the local currents are elongated ionic exchange occurs at fewer points along the neurone so the speed of transmission of the impulse is faster than in non-myelinated neurones

Question 11

Function of Sensory Neurones

Answer 11

Carry impulses from receptors to the central nervous system

Question 12

Function of Motor Neurones

Answer 12

Carry impulses from the central nervous system to effectors (muscles or glands)

Question 13

Function of Relay Neurones

Answer 13

Connect sensory and motor neurones within the central nervous system

Question 14

How is an impulse transmitted across a synapse?

Answer 14

• an action potential reaches the end of the pre-synaptic neurone
• the action potential causes the voltage gated calcium ion channels to open and calcium ions move by facilitated diffusion into the pre-synaptic neurone
• the calcium ions cause vesicles containing neurotransmitters to fuse with the cell plasma membrane
• the neurotransmitters are released by exocytosis
• the neurotransmitters diffuse across the synaptic cleft and bind to receptor sites on sodium ion channels in the post-synaptic membrane
• the sodium ion channels open and sodium ions diffuse into the post-synaptic neurone, if the threshold potential is reached then an action potential will be triggered
• an enzyme breaks down any neurotransmitter molecules in the synaptic cleft, the break down products are reabsorbed in to the pre-synaptic neurone and recombined using ATP

Question 15

Cholinergenic Synapses

Answer 15

Neurotransmitter = acetyl choline

Enzyme = acetylcholinesterase
acetyl choline -> ethanoic acid + choline

Question 16

Summation

Definition

Answer 16

Several small potential changes can combine to produce one larger change in potential difference, one action potential

Question 17

Temporal Summation

Definition

Answer 17

1 action potential in the pre-synaptic neurone does not trigger a post-synaptic action potential
Multiple action potentials from the same pre-synaptic neurone are required to trigger a post-synaptic action potential

Question 18

Spacial Summation

Definition

Answer 18

Multiple pre-synaptic neurones can synapse with one post synaptic neurone
A combination of action potentials from multiple pre-synaptic neurones is required to trigger a post-synaptic action potential

Question 19

Acclimatisation

Definition

Answer 19

After repeated stimulation a neurone may run out of vesicles containing neurotransmitters, it is using the neurotransmitter molecules faster than it can produce them
The synapse is fatigued, this is why we get used to background noises and smells
This also prevents over stimulation

Question 20

Effect of a Higher Frequency of Transmission

Answer 20

When the stimulus is at a higher intensity more frequent action potentials will be triggered in the sensory neurone
When these action potentials arrive at the synapse more neurotransmitters will be released so more action potentials will be triggered in the post-synaptic neurone
The brain can identify the intensity of the stimulus from the frequency of the action potentials arriving

Question 21

What are the functions of synapses?

Answer 21

Ensure that impulses can only travel in one direction along any neurone as the vesicles containing the neurotransmitter molecules are only at one end of the neurone
Any one neurone can transmit to or receive impulses from multiple neurones
Allow summation
Filters out background/low level stimuli, multiple action potentials from multiple neurones required to generate a post-synaptic impulse which will reach the central nervous system