Motor neurones definition:
Neurones that carry an action potential from the CNS to effector
Relay neurones definition:
Join sensory and motor neurones
Sensory neurones definition:
Neurones that carry an action potential from the sensory receptor to CNS
Dendron/ dendrites definition:
The bit before the cell body that’s like the axon
Structure of neurones:
- Many are long
- Plasma membrane has many gated ion channels that control the entry or exit of sodium, potassium or calcium ions
- Sodium potassium pump actively transports sodium ions OUT of the cell and potassium ions INTO the cell
- Neurones maintain a potential difference across their plasma membrane
- The cell body contains the nucleus, and many mitochondria and ribosomes
- Neurone dendrites connect to other neurones and carry impules TOWARDS the cell body
- The axon carries imulses AWAY from the cell body
What is the myelin sheath made of?
Schwann cells
Lengths of dendron and axon in sensory neurone:
- Long dendron
- Cell body just outside of CNS
- Short axon
Lengths of dendron and axon in relay neurone:
- Many short denrites
- Short axon
Lengths of dendron and axon in motor neurone:
- Cell body inside CNS
- Long axon
Why do impulses travel faster in myelinated neurones?
- The myelin sheath is wrapped tightly around the neuron, preventing the movement of ions across the neurone membrane
- The movement of ions can only occur at the node of Ranvier
- This means the action potential jumps from one node to the next
Where are the Schwann cells in non-myelinated neurones?
Several neurones may be enshrouded in one loosely wrapped Schwann cell, meaning that the action potential moves in a wave
What are non -myelinated neurones often used in?
Coordinating body functions such as breathing and the action of the digestive system. They do not need to be so quick for this.
Pacinian corpuscle definition:
A pressure sensor found in the skin
Sensory receptors definition:
Cells/ sensory nerve endings that respond to a stimulus in the internal or external environment of an organism and can create action potentials
Structure of a Pacinian corpuscle:
- Oval shaped
- Series of concentric rings of connective tissue wrapped around the end of a nerve cell
What happens to the Pacinian corpuscle when there is pressure on the skin?
It deforms the rings of connective tissue, which push against the nerve ending. When pressure is constant, the corpuscle stops responding because the rings are not being deformed any more
What happens to sodium channels when the membrane of a nerve cell is deformed by changing pressure?
They open
What does the sodium/ potassium pump do on a nerve cell membrane
- Actively pump three sodium ions out of the cell for every two potassium ions actively pumped in
- This is when the channel proteins are closed and the neurone is at rest
Why do some potassium ions leak out of the nerve cell when it is at rest?
The membrane is more permeable to potassium ions
What is the negative charge inside nerve cells enhanced by?
The presence of negatively charged anions inside the cell
What is a nerve impulse created by?
- Altering the permeability of the nerve cell
- Sodium ion channels are opened and sodium ions rush in
- This creates a change in the potential difference and causes depolarisation
- If it is a small stimulus, only a few sodium channels will open, (generator potential)
- If more are open, the potential difference across the cell membrane will change significantly and will cause an action potential
Action potential definition:
A brief reversal of the potential across the membrane of a neurone, causing a peak of +40mV compared to the resting potential of -60mV
Resting potential definition:
The potential difference across the membrane while the neurone is at rest
What is the resting potential of a neurone at rest?
-60mV
What are voltage gated channels opened by?
- Changes in the potential difference across the membrane
- When there are sufficient generator potentials to reach the threshold, the voltage gated channels open
- Positive feedback
What happens when generating an action potential, once the sodium ions have rushed in and the depolarisation reaches +40V on the inside of the cell?
The neurone will start to transmit the action potential
Stages of an action potential:
- Starts off in resting state (-60mV)
- Sodium ion channels open and sodium diffuses into cell
- Membrane depolarises (threshold of -50mV)
- Positive feedback causes nearby voltage gated channels to open
- Potential difference across plasma membrane reaches +40mV
- Sodium ion channels close and potassium ion channels open
- Potasium ions diffuse out of the cell, bringing back the resting potential (repolarisation)
- Potential difference overshoots slightly, making the cell hyperpolarised
- Original potential difference restored
Why can another action potential not be stimulated straight after another one?
- The sodium and potassium ions are in the wrong place and have to be moved back to their correct places by the sodium/ potassium pump
- Known as the refractory period
- Ensures that action potentials are only stimulated in one direction
What causes local currents?
Sodium ions flooding into the neurone, causing depolarisation
Formation of local currents:
- Sodium ion channels open
- Localised increase in concentration of sodium ions inside neurone (the action potential)
- Sodium ions diffuse along axon/ dendron
- Sodium gates further along will open because of movement of sodium ions.
- Action potential moves further along neurone as more sodium ions enter and set up another action potential
What happens with local currents in myelinated neurones?
- The ion exchanges can only happen at the nodes of Ranvier
- The local currents are elongated and sodium ions diffuse from one node of Ranvier to the next
- This means they appear to jump from one node to the next
- This is saltatory conduction
How do our brains determine the intensity of a stimulus?
From the frequency of action potentials arriving at the sensory region of the brain
Cholinergic synapse definition:
A synapse that used acetylcholine as its neurotransmitter
Neurotransmitter definition:
A chemical used as a signalling molecule between two neurones in a synapse
What is the small gap between two synapses called?
The synaptic cleft
How big is a synaptic cleft?
20nm
What does the action potential do in order to continue on the other side of the synaptic cleft?
Causes the release of neurotransmitters in the pre-synaptic neurone. This defuses across the cleft and causes a new action potential in the post synaptic neurone
What is the swelling at the end of the pre-synaptic neurone called?
pre-synaptic bulb (knob)
What specialised features does the pre-synaoptic bulb contain?
- Many mitochondria
- Large amounts of SER, which packages the neurotransmitters into vesicles
- Large number of vesicles containing acetylcholine
- Large number of voltage gated calcium ion channels on the cell surface membrane
What does the post-synaptic membrane contain?
Specialised sodium ion channels
What do the sodium ion channels on the post-synaptic membrane contain?
- Five polypeptide molecules
- Two of these have a special receptor site that is specific to acetylcholine
- When acetylcholine is present in the synaptic cleft, it binds to the two receptor sites and causes the sodium ion channels to open
How does transmission across a synapse work?
- Action potential arrives at the synaptic bulb
- Voltage-gates calcium ion channels open
- Calcium ions diffuse into synaptic bulb
- Calcium ions cause the vesicles to move to and fuse with the pre-synaptic membrane
- Acetylcholine is released by exocytosis and diffuses across the cleft
- Acetylcholine molecules bind to the receptor sites on the sodium ion channels in the post-synaptic membrane, causing the sodium ion channels to open
- Sodium ions diffuse across the post synaptic membrane into the post-synaptic neurone
- Generator potential or excitatory post-synaptic potential (EPSP) is created
- If sufficient generator potentials combine to reach the threshold, it creates an action potential in the post-synaptic neurone
What does acetylcholinesterase do?
- Found in the synaptic cleft
- Hydrolyses the acetylcholine to ethanoic acid and choline
- This stops the transmission of signals
- The ethanoic acid and choline are recycled. They re-enter the synaptic bulb by diffusion and are recombined to form acetylcholine using ATP
Summation definition:
Occurs when the effects of several excitatory post-synaptic potentials (EPSP) are added together
What is temporal summation?
When there is a series of action potentials travelling along a neurone so that they can creating EPSPs that come together to create an action potential
What is spatial summation?
When there are several pre-synaptic neurones that each bring action potentials, creating EPSPs that come together to create an action potential
What prevents the combination of several EPSPs producing an action potential?
An IPSP
How is one pre-synaptic neurone diverging into several post synaptic neurones beneficial?
One action potential can be transmitted to several parts of the nervous system. Useful in a reflex arc where one post-synaptic neurone elicits the response and the other informs the brain
How do synapses ensure the action potential travels in the correct direction?
Only the pre-synaptic bulb contains the vesicles of acetylcholine
What does it mean if we have become habituated to a stimulus?
After repeated stimulation, a synapse may run out of vesicles containing the neurotransmitter. We no longer notice the stimulus
