Neurones and nerve coordination Flashcards
1
Q
Describe the nervous system.
A
- It uses nerve cells to pass electrical impulses along their length.
- They stimulate target cells by secreting neurotransmitters directly on to them
- This results in rapid communication between specific parts of an organism
- These are short lived and restricted to localised areas of the body
2
Q
What is a neurone?
A
- A nerve cell that is specialised to rapidly carry electrochemical changes called nerve impulses from one part of the body to another
3
Q
What are the components of a motor neurone?
A
- A cell body = This contains the usual cell organelles, including a nucleus and large amounts of endoplasmic recticulum. This is associated with the production of neurotransmitters and proteins
- Dendrons = extensions of the cell body which subdivide into smaller branched fibres called dendrites. These carry impulses toward the cell body
- Axon = A long single fibre that carries impulses away from the cell body
- Schwann cells = these surround the axon, protecting it and providing it with electrical insulation. They also carry out phagocytosis (removal of cell debris) and carry out nerve regeneration. Schwann cells coil around the axon, forming layers around it.
- Myelin sheath = This is a covering of the axon that is made of schwann cells
- Nodes of ranvier =These are constrictions between adjacent schwann cells where there is no myelin sheath. These constrictions are 2-3um long and occur every 1-3um in humans
4
Q
What are the three types of neurone and what they do?
A
- sensory neurone = transmit nerve impulses from a receptor to an intermediate or a motor neurone. They have a long dendron which carries the impulse from the receptor to the cell body
- motor neurone = transmits nerve impulses from an intermediate or relay neurone to an effector such as a gland or muscle. They have long axons and many short dendrites.
- relay neurone = transmits impulses between neurones. For example from a sensory neurone to a motor neurone
5
Q
What is a nerve impulse?
A
- The temporary reversal of the electrical potential difference across the axon membrane
- The reversal between the two states is called the action potential and the resting potential
6
Q
How is resting potential maintained?
A
- Sodium potassium pumps actively transport 3 NA+ ions out of the axon membrane and 2K+ ions in to the axon membrane. This means that the active transport of sodium ions is greater than potassium ions and as a result there are more sodium ions in the tissue fluid surrounding the axon than inside the cytoplasm and the inside of the axon is more negative than the outside. This creates an electrochemical gradient and ensures that the axon is polarised
-Membrane protein channels in the axon membrane are more permeable to K+ than Na+, this means sodium ions do not diffuse across in to the axon,maintaining the inside of the axon as negative
- Most of the gates in the channels that allow K+ ions to diffuse are open whereas the Na+ gates are closed
7
Q
What is an action potential?
A
- When a stimulus is of a sufficient size is detected by receptors, it causes a temporary reversal of the charges either side of the axon membrane
- If the stimuli reaches a certain threshold, the inside of the axon membrane becomes positive
- This is called depolarisation
8
Q
What is the order of events of a nerve impulse?
A
- resting potential = the inside of the axon membrane is negative and the outside is positive
- action potential = the inside of the axon membrane is positive and the outside is negative
-Hyperpolarisation = The charges begin to reverse
9
Q
What is depolarisation?
A
Depolarisation is the process by which the membrane potential of a neuron becomes less negative, leading to an action potential. This is caused by the influx of positively charged ions, such as sodium ions, into the cell.
10
Q
Describe the steps of an action potential.
A
- At resting potential, some potassium voltage gated channels are open but all of the sodium voltage gated channels are closed
- If the energy of the stimuli meets the threshold, it will cause some of the sodium voltage gated channels to open. Therefore, sodium ions diffuse into the axon through these channels along the electrochemical gradient. Being positively charged, they trigger a reversal in the potential difference across the membrane
- As the sodium ions diffuse into the axon, they cause more sodium voltage gated channels to open which causes an even greater influx of sodium ions by diffusion into the axon
- Once the action potential of around +40 has been established, the voltage gated sodium ion channels close preventing the further influx of sodium ions and the voltage gated potassium ion channels begin to open
- With the potassium voltage gated channels open, the electrical gradient is now reversed, this causes more potassium ion channels to open. This means many potassium ion channels open and K+ diffuses out of the axon, starting repolarisation of the axon
- The outward diffusion of these potassium causes the inside of the axon to be more negative than the outside. Due to the positive charges of the K+ ions, they repel as they shoot out causing hyperpolarisation
- The potassium voltage gated channels then close and the activity of the sodium - potassium pump restarts, pumping 3 Na+ ions out of the axon and 2K+ ions in to the axon, restablishing the resting potential of -65. This means the axon has been repolarised.
11
Q
How does an action potential travel down the length of the axon?
A
- As one reigon of the axon produces an action potential and becomes depolarised, it acts as a stimuli for the depolarisation of the next reigon of the axon
12
Q
Describe the process in which an action potential passes across an axon?
A
- At resting potential, the inside of the axon is more negative than the outside
2.A stimuli then causes an influx of sodium ions and depolarisation begins and an action potential is produced - This causes the opening of voltage gated sodium channels, in which the influx of sodium ions diffuse across the axon and open more voltage gated sodium channels in the next reigons of the axon, propagating depolarisation
- Behind these areas of depolarisation, the voltage gated sodium channels close and the potassium voltage gated channels open, k+ ions begin to leave, starting a wave of repolarisation in the following reigons
- After this, the volatage gated potassium channels close, and the sodium potassium pump begins to work, propagating resting potential.
13
Q
How does an action potential pass along a myelinated neuron
A
- In a myelinated neuron, the fatty sheath acts as an electrical insulator, preventing action potentials from occuring
- However, in breaks of the myelination called nodes of ranvier, action potentials can occur
- Therefore, action potentials can only pass between adjacent nodes of ranvier, causing them to ‘jump’ from node to node.
- This is called saltatory conduction
- As a result, action potentials pass along a myelinated neuron faster than the non myelinated neuron
14
Q
What factors affect the speed of a nerve impulse?
A
- The myelin sheath
- The diameter of the axon = the greater the diameter of the axon, the faster the speed of conductance as there is less leakage of ions and a greater surface area of the membrane, also increasing the steepness of the electrochemical gradient
- Temperature = this increases the rate of diffusion and increases enzyme activity of enzymes involved in respiration, increasing the number of ATP produced for active transport. However, at high temperatures membrane proteins may become dentaured
15
Q
What is the all or nothing principle?
A
- the power of a stimulus is not dependent on the strength of a stimulus but are instead dependent on whether the initial threshold is met
- If this threshold value is not met, there is no action potential
- This means all action potentials are roughly the same size
