Module 5: Section 3 - Neuronal Communication

Sensory neurones

• Short dendrites 
• One long dendron (carrys impulses from receptor cells to the cell body)
• One long axon (carries impulses from the cell body to the CNS)
• Cell body outside the CNS

Motor neurones

• Many short dendrites (carry impulses from the CNS to the cell body)
• No dendron
• One long axon (carries nerve impulses from the cell body to effector cells)
• Cell body is in the CNS

Relay neurones

• Many short dendtrites (carry nerve impulses from sensory neurones to the cell body)
• Many short axons (carry nerve impulses from the cell body to motor neurones)

1. Stimulus is detected by receptor cells and a nerve impulse is sent along a sensory neurone

2. When a nerve impulse reaches the end of a neurone, neurotransmitters transmit the information across the synapse to the next neurone, which then sends a nerve impulse

3. The CNS processes the information, desides what to do about it and sends impulses along motor neurones to an effector, thus producing a response

Sensory receptors act as transducers - convert energy of a stimulus (eg. light energy or chemical energy) into electrical energy (nerve impulses)

1. When a receptor is at rest, there is a difference between the inside and the outside of the cell, maintained by the action of ion pumps and ion channels. This means there is a voltage across the membrane, known as the potential difference

2. The potential difference when the cell is at rest is called its resting potential. When a stimulus is detected, the cell membrane becomes more excited and is more permeable, allowing more ions to move in and out of the cell - altering the potential difference. The change in the potential difference is called the generator potential

4. A bigger stimulus excites the membrane more, causing a bigger movement of ions and a bigger change in potential difference - bigger generator potential is produced

5. If the generator potential is big enough it'll trigger an action potential (nerve impulse) alone a neurone - an action potential is only triggered if the generator potential reaches the threshold level

6. If the stimulus is too weak, the generator potential won't reach the threshold, so there's no action potential - 'all or nothing' response

Pacinian corpuscles are mechanoreceptors - they detect mechanical stimuli eg. pressure and vibrations

They contain the end of a sensory neurone (sensory nerve ending) which is wrapped in lots of layers of lamellae (connective tissue)

1. When a Pacinian corpuscle is stimulated, the lamellae are deformed and press on the sensory nerve ending

2. This causes deformation of stretch-mediated channels in the sensory neurone's cell membrane

3. The sodium ion channels open and sodium ions diffuse into the cell, creating a generator potential

4. If the generator potential reaches the threshold, it triggers an action potential

1. In a neurone's resting state, the outside of the membrane is positively charged compared to the inside, because there are more positive ions outside the cell than inside

2. The membrane is polarised - there's a difference in charge. The voltage across the membrane when at rest is about -70 mV - the resting potential

3. The resting potential is created and maintained by sodium-possium pumps and potassium ion channels in a neurone's membrane

4. The sodium-potassium pumps move sodium ions out of the neurone, but they can't diffuse back in as the membrane is not permeable to sodium ions. This creates a sodium ion electrochemical (concentration) gradient because there are more positive sodium ions outside the cell than inside

5. The sodium-potassium pumps also move potassium ions in to the neurone, but the membrane is permeable to sodium ions so they diffuse back out through potassium ion channels

6. This makes the outside of the cell positively charged compared to the inside

1. Stimulus causes sodium ion channels to open. The membrane becomes more permeable to sodium ions, so the sodium ions will diffuse into the neurone down the sodium ion electrochemical gradient. This makes the inside of the neurone less negative

2. If the threshold potential is reached, the membrane becomes depolarised. Voltage gated sodium ion channels will open, so more sodium ions diffuse into the neurone - postive feedback

3. Repolarisation - at a potential difference of around +30 mV the sodium ion channels close and the voltage gated potassium ion channels open. The membrane is more permeable to potassium so the potassium ions diffuse out of the neurone down the potassium ion concentration gradient. This starts to get the membrane back to its resting potential - negative feedback

4. Potassium ion channels are slow to close so there is a slight 'overshoot' where too many potassium ions diffuse out of the neurone - hyperpolarisation. The potential difference becomes more negative than the resting potential

5. The sodium-potassium pumps return the membrane to its resting potential

1. When an action potential happends, some of the sodium ions that enter the neurone diffuse sideways

2. This causes sodium ion channels in the next region of the neurone to open and sodium ions diffuse sideways into that part

3. This causes a wave of depolarisation to travel along the neurone

4. The wave moves away from the parts of the membrane in the refractory perion because these parts can't fire an action potential

Refractory period - the concentrations of sodium ions and potassium ions inside and outside of the cell are wrong, so the concentrations must be restored by the sodium potassium pumps. While the concentrations are being restored, the cell membrane cannot be stimulated to reach another action potential. This allows the cell to recover after an action potential and ensures the action potentials are only transmitted in one direction

Once the threshold potential is reached, an action potential will always fire with the same change in voltage, no matter how big the stimulus is

If the threshold potential isnt reached, an action potential wont fire - 'all or nothing'

A bigger stimulus does not cause a bigger action potential, it causes action potentials to fire more frequently - if the brain receives a higher frequency of action potentials, it interprets this as a big stimulus and responds accordingly

Myelinated neurones have a myelin sheath, made of Scwann cells, which acts as an eletrical insulator

Between the Schwann cells are tiny patches of bare membrane - nodes of Ranvier. Sodium ion channels are concentrated at the nodes

In a myelinated neurone, depolarisation only happens at the nodes of Ranvier 

The neurone's cytoplasm conducts enough electrical charge to depolarise the next node, so the impulse 'jumps' from node to node - saltatory conduction, which is very fast

In a non-myelinated neurone, the impulse travels as a wave along the whole length of the axon membrane - slower than saltatory conduction

1. An action potential arrives at the synpatic knob of the presynaptic neurone, which stimulates voltage-gated calcium ion channels in the presynaptic neurone to open

3. Calcium ions diffuse into the synaptic knob

4. The influx of calcium ions causes the synaptic vesicles to move to the synaptic membrane, and they fuse with the membrane

5. The vesicles release the neurotransmitter into the synaptic cleft by exocytosis

6. The neurotransmitter diffuses into the synaptic cleft and binds to specific receptors on the post synaptic membrane

7. This causes sodium ion channels on the postsynaptic neurone to open, and the influc of sodium ions causes depolarisation. An action potential on the post synaptic membrane is reached

8. The neurotransmitter is removed from the synaptic cleft by acetylcholinesterase so the response doesn't keep happening

Allow information to be dispersed or amplified

• When one neurone connects to many neurones information can be dispersed to different parts of the body - synaptic divergence
• When many neurones connect to one neurone information can be amplified - synaptic convergence

Allow summation

• One action potential produces one EPSP  - EPSPs are combined (summed) at the presynaptic membrane to cause the membrane to cause sufficient depolarisation to reach the threshold
• Temporal summation - from several action potentials in the same presynaptic neurone
• Spatial summation - from action potentials arriving from different presynaptic neurones
• IPSPs can reduce the effects of summation and prevent an action potential in the postsynaptic neurone

Ensure impulses are transmitted one way

• Receptors are only on the postsynaptic membranes, so impulses can only travel in one direction