Module 5: The Nervous System. Flashcards
How does nervous system work?
The nervous system detects changes in our environment (known as stimuli) through cells called receptors.
Receptors are sensitive to a number of different aspects of our environment, such as light, pressure (touch) and chemicals in the air (smell).
When receptors detect certain stimuli, they signal to the central nervous system (CNS) through initiating an electrical impulse through a neuron (nerve cell).
The neuron which sends an electrical impulse from the receptor within the sense organ and the coordination centre is called the sensory neuron.
The coordination centre receives impulses from various receptors around the body, processes the information and coordinates a response by signalling to other parts of the body. Coordination centres include the brain, spinal cord and pancreas.
These organs will signal to an effector (a muscle or gland) by releasing an electrical impulse along a motor neuron.
Stimulation of an effector will produce a response such as muscle contraction or hormonal release.
What is cell signalling?
To produce a response, receptors need to communicate with effectors and effectors may need to communicate with other cells, and this happens via cell signalling.
Cell signalling can occur between adjacent cells or between distant cells.
For example, cells in the nervous system communicate by secreting chemicals called neurotransmitters, which send signals to adjacent cells, such as other nerve cells or muscle cells.
The hormonal system works by cells releasing chemical messengers called hormones, which travel in the blood and act as signals to distant cells.
Cell- surface receptors allow cells to recognise the chemicals involved in cell signalling.
What are the 3 main neurones used in the nervous system?
- Sensory neurones
- Relay neurones
-Motor neurones
Describe the structure and function of Sensory neurones.
Transmits nerve impulses from receptors to the CNS.
- Have short dendrites and one long dendron to carry nerve impulses from receptor cells to the cell body, and one short axon that carries nerve impulses from the cell body to the CNS.
Describe the structure and function of Motor neurones.
- Transmits nerve impulses from the CNS to effectors
- Have many short dendrites that carry nerve impulses from the CNS to the cell body, and one long axon that carries nerve impulses from the cell body to effector cells.
Describe the structure and function of Relay neurones.
- Transmits nerve impulses between sensory neurones and motor neurones.
- Have many short dendrites that carry nerve impulses from sensory neurones to the cell body, and one axon that carries nerve impulses from the cell body to motor neurones.
What are transducers?
Something that converts one form of energy to another.
Sensory receptors convert the energy of a stimulus into electrical energy.
Describe what resting potential is.
When a neurone is not firing (i.e. it is not transmitting an action potential), there is a difference in charge between the inside and the outside of the membrane - the inside is negatively charged relative to the outside.
This means there is a voltage across the membrane - also known as potential difference.
The potential difference when a cell is at rest is called resting potential and is generated by ion pumps and ion channels.
Resting potential = -70mV
Describe what generator potential is.
It is the change in potential difference due to a stimulus.
When a stimulus is detected, the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell - altering the potential difference.
The size of the stimulus affects the size of the generator potential.
For example a bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference so a bigger generator potential is produced.
Describe what the action potential is.
If the generator potential is big enough, it will trigger an action potential - a nerve impulse along a neurone.
An action potential is only triggered if the generator potential reaches a certain level called the threshold level. If the stimulus is too weak the generator potential wont reach the threshold, so there is no action potential.
Describe the structure and function of the Pacinian Corpuscles.
Pacinian corpuscles are receptors which respond to changes in pressure - they are a type of mechanoreceptor. They are found deep in the skin and are abundant in the feet, fingers, external genitalia and in our joints. The Pacinian corpuscle consists of a single sensory neurone, surrounded by layers of tissue which are each separated by a gel, forming an onion-like structure.
The Pacinian corpuscle contains stretch-mediated sodium ion channels in the cell surface membrane.
Under normal conditions these channels are closed but when pressure is applied (e.g. tap on the arm) these channels become deformed and open, allowing a rapid influx of sodium ions.
This makes the membrane potential in the neurone less negative (depolarisation), producing a generator potential which can then produce an action potential.
Explain the movement of sodium and potassium ions to maintain resting potential.
The resting potential is created and maintained by sodium-potassium pumps and potassium ion channels in a neurones membrane.
- Sodium-potassium pumps use active transport to move 3 sodium ions out of the neurone for every 2 potassium moved in - ATP is needed to do this. The membrane is not permeable to sodium ions so they cannot diffuse back in. This creates a sodium ion electrochemical gradient as there are more positive sodium ions outside the cell than inside.
- when the cell is at rest, potassium ion channels allow facilitated diffusion of potassium ions out of the neurone, down their concentration gradient. This means that the membrane is permeable to potassium ions, so some diffuse back out.
In total, there are more positive ions moving out of the cell than in. This makes the outside of the cell positively charged compared to the inside.
Explain the stages of an action potential.
1) Stimulus - this excites the neurone cell membrane, causing sodium ion channels to open. The membrane becomes more permeable to sodium, sodium ions diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative.
2) Depolarisation - if the potential difference reaches the threshold, which is around -55mV, voltage-gated sodium ion channels open and more sodium ions diffuse into the neurone. This is positive feedback.
3) Repolarisation - at a potential difference of +30 mV, the sodium ion channels close and voltage-gated potassium ion channels open. The membrane is more permeable to potassium, so potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential. This is negative feedback.
4) Hyperpolarisation - potassium ion channels are slow to close so there is a slight ‘overshoot’ where too many potassium ions diffuse out of the neurone. The potential difference becomes more negative than the resting potential - less than -70mV.
5) Resting potential - the ions are rest. The sodium - potassium pump returns the membrane to its resting potential by pumping sodium ions out and potassium ions in, and maintains the resting potential until the membrane’s excited by another stimulus.
Explain what the refractory period is.
After an action potential, the neurone cell membrane cannot be excited again straight away and this is because the ion channels are recovering and they can’t be made to open.
-Sodium ion channels are closed during repolarisation and potassium ion channels are closed during hyperpolarisation.
This period of recovery is called the refractory period and it acts as a time delay between one action potential and the next. This makes sure that action potentials do not overlap but pass along as discrete, separate impulses.
This period also makes sure action potentials are unidirectional- can only travel in one direction.
Describe why a wave of depolarisation occurs.
Once an action potential occurs in one part of the neuron, it will stimulate an action potential in the adjacent part of the neuron, creating a kind of ‘Mexican wave’ of depolarisation.
This wave of depolarisation occurs because the sodium ions which diffuse into the neuron diffuse sideways, causing voltage-gated ion channels in the next portion of the neurone to open, so sodium ions move into the neurone further along the membrane.
The wave moves away from the part of the neurone which has just fired an action potential because that part of the neurone will be in the refractory period and cannot be stimulated.
