Neuronal communication

• What are sensory receptors?

  specialised cells that detect and respond to a stimulus in the external and internal environment of an organism and create action potentials.
• What is an action potential?

  a brief electrical impulse that travels along the membrane of a neurone, allowing for the transmission of signals between neurones.
• Pacinian corpuscles are pressure sensors that detect changes in pressure on the skin.

  > when pressure on skin changes it deforms the rings of connective tissue which push against nerve ending, converting movement to energy.

  > An increase in pressure causes Na+ channels to open as they are pressure sensitive.

  
• Sensory receptors respond to specific types of stimuli and act as energy transducers. For example:


  > change in light intensity is detected by roads and cones in retina converting light energy into electrical.

  > change in pressure on skin detected by Pacinian corpuscles in skin converting movement energy into electrical.

  > chemicals in food are detected by chemical receptors in taste buds on the tongue which sends electrical impulses.
• Functions of neurones

  > electrical impulse needs to be transmitted to other parts of the body which is carried as an AP.
• Types of neurones

  > sensory neurones: carry an AP from sensory receptor to the CNS

  > motor neurones: carry an AP from the CNS to an effector like a muscle or gland

  > relay neurones: connect sensory and motor neurones in the CNS
• Structure of neurones

  > very long so they can transmit an AP over long distances

  > cell surface plasma membrane has many gated ion channels that control entry/exit of Na, K, Ca2+.

  > Na/K pumps use ATP to actively transport Na ions out of cell and k ions in.

  > neurones maintain a Pd across their surface plasma membrane

  > cell body contains the nucleus, many mitochondria and ribosomes

  > numerous dendrites connect to other neurones, carrying impulses towards cell body

  > an axon carries impulses away from cell body
• Why do neurones have numerous dendrites, particularly motor neurones

  highly branched dendrites provide a large surface area for the axon terminals of other neurones.
• Why do neurones have many mitochondria and ribosomes in their cell body?

  > Mitochondria are central for ATP production for Na/K pumps and development of dendrites and axons.

  > Ribosomes synthesise and produce high amounts of protein neurotransmitters.
• Differences in neurone structures


  >Sensory neurones have a long dendron carrying AP from sensory receptor to cell body near CNS. They also have a short axon carrying AP into the CNS.

  >Motor neurones have cell body in CNS and a long axon which carries AP to the effector.

  >Relay neurone has a short axon and many short dendrites.

  > Sensory and motor neurones are insulated by myelin sheath made of schwann cells.

  A) Nodes of Ranvier

  B) Schwann cells

  C) Axon

  D) Axon

  E) Dendrite

  F) Cell body

  G) Myelin sheath

  H) Axon

  I) Dendron

  J) Receptor cell

  K)

  
• Myelinated neurones - Sensory and Motor

  >Neurones are insulated by myelin sheath made up by schwann cells and cytoplasm which is wrapped thick and tight around the neurone, preventing movement of ions across the neurone membranes.

  >1-3mm intervals along neurone are gaps in myelin sheath called nodes of Ranvier. Here only movement of ions can occur so APs jump from each mode making conduction more rapid, enabling a rapid response.
• Non-myelinated neurones

  Several neurones may be wrapped loosely in Schwann cells.

  On these neurones, AP moves slower and are shorter so carry AP over shorter distances.

  Often used in breathing and digestion.
• How resting potential is established

  The Neurone is not transmitting an AP, but the Na and K pumps are still exchanging 3 Na+ for every 2 K ions in the plasma membrane.

  The gated Na+ channels are kept closed and some K channels remain open. There is a higher concentration of Na+ outside and a higher concentration of K+ inside however, as the membrane is more permeable to K ions, some diffuse out of cell. The cytoplasm also contains anions which overall maintains the cell's interior at a negative potential compared to the outside.

  The membrane is polarised and the pd across it is -60mv.
• Generating an action potential Pt.1

  >membrane starts in resting state

  >generator effect - when gated Na channels are opened by action of a synapse so some Na ions enter cell, causing it to depolarise.

  >when there are enough generator potentials to reach the threshold potential, they cause voltage gated channels (opened by changes in membranes Pd) to open at -50mV, driven by positive feedback as small depolarisation causes a change which increases APs further.

  >when these open, it allows a large influx of Na+ into cell, allowing depolarisation to reach +40mV on the inside of the cell.
• Generating an action potential Pt.2

  >once the inside of the cell reaches a depolarisation of +40mV, the neurone will transmit the AP.

  > after this, Repolarisation occurs as Na+ channels close and K+ channels open and diffuse out of cell, which brings the pd back to negative inside the cell compared to outside.

  > the pd then overshoots making the cell hyperpolarised.

  > then the original pd is restored and cell is back to resting state.
• Generating an action potential (short)

  1> membrane is in resting state

  2> generator effect occurs

  3> generator effects reach threshold potential and voltage gated channels are opened due to pd change in membrane

  4> depolarisation reached +40mV as a result of positive feedback of APs

  5> repolarisation occurs and the pd returns to normal

  6> hyperpolarisation occurs and the pd is overshot

  7> original pd is restored and cell is returned to resting state.
• Generating Action Potentials:

  A) Threshold

  B) Depolarisation

  C) Repolarisation

  D) Hyperpolarisation

  E) Refractory period

  
• What is meant by the All-or-nothing?

  when a neurone transmits an AP, they are all the same intensity and they all produce a depolarisation of +40mV. (meeting the threshold value)
• Refractory period 


  > occurs shortly after Hyperpolarisation and before resting period.

  > here, it is impossible for cell to stimulate another action potential.

  > concentration of Na/K ions need to be restored by their pumps.

  > this period allows cell to recover after an AP and ensures they are transmitted in one direction.
• How local currents are formed (how APs move in neurones)

  1> Na+ channel open as a result of an AP allowing Na+ ions to diffuse into the neurone from region of higher concentration outside.

  2> causes a localised increase in concentration of Na+ inside neurone.

  3> Na+ ions diffuse along the axon/dendron away from the region of higher concentration.

  4> this movement is a local current and causes a slight depolarisation further along neurone, opening voltage-gated Na+ channels, enabling Na+ to flow in. This influx then causes a full depolarisation (AP) further along neurone.
• Saltatory conduction in myelinated neurones

  >Myelin sheath causes saltatory conduction as Na and K+ ions cannot diffuse through fatty layers of Schwann cells. These ionic movements can only occur at the Nodes of Ranvier.

  >The local currents are elongated and Na+ ions diffuse along neurone to each node of Ranvier. When the AP 'jumps' this is saltatory conduction.

  > Saltatory conduction conducts APs more rapidly as it doesnt have to wait for ions to be exchanged across membrane.
• The Significance of the frequency of impulse transmission

  > A higher frequency of APs means a more intense stimulus

  > A stimulus with higher intensity opens more Na+ channels in sensory receptor, producing more generator potentials, resulting in more frequent APs in sensory neurone and more frequent APs entering the CNS.
• What is meant by depolarisation?

  the process by which the membrane potential of a neuron becomes less negative, leading to an action potential
• What is meant by an action potential?

  Electrical signal that travels down the membrane of a neurone.
• Structure of a Cholinergic Synapse

  >A synapse is a junction between 2 or more neurones and between these is a gap called the synaptic cleft. APs travel along neurone through a series of ionic movements across its membrane. However, the AP cannot move across the gap between the two neurones.

  >SO: AP in pre-synaptic neurone causes release of a neurotransmitter, Acetylcholine, which diffuses across the synaptic cleft generating a new AP in the Post-synaptic neurone. Synapses that use Ach as a neurotransmitter are called cholinergic synapses.
• Structure of the Pre-synaptic bulb

  The pre-synaptic bulb is a swelling at the end of the pre-synaptic neurone. It has many specialised features:

  > many mitochondria as the bulb has active processes needing ATP.

  > large amount of smooth endoplasmic reticulum which packages neurotransmitters into vesicles.

  > large numbers of vesicles containing Ach - the neurotransmitter which diffuses across the cleft.

  > number of voltage-gated Ca2+ channels on cell surface membrane. The Ca2+ allows vesicles to move within synapses through motor proteins and cytoskeleton.
• Structure of the Post-synaptic membrane

  > Contains specialised Na2+ channels that respond to neurotransmitter.

  > Also contains channels that consist of 5 polypeptide molecules of which two have special receptor site specific to Ach, which is complementary to the molecules shape.

  > When Ach is present in synaptic cleft, it binds to the 2 receptor sites, causing Na2+ channels to open.
• Transmission of AP across the Synaptic CleftPt.1

  1> AP arrives at the Synaptic bulb and Voltage-gated Ca2+ channels open

  2> Ca2+ diffuse into synaptic bulb causing synaptic vesicles to move to, and fuse, with the pre-synaptic membrane

  3> Acetylcholine is released by exocytosis and Ach molecules diffuse across cleft

  4> Ach molecules bind to receptor sites on Na+ channels in post-synaptic membrane. The Na+ channels then open.

  5> Na+ diffuse across pre-synaptic membrane into post-synaptic neurone.
• Transmission of AP across the Synaptic Cleft Pt.2

  6> Once Na+ ions diffuse into post-synaptic neurone, a generator potential / EPSP (excitatory post-synaptic potential) is created. If sufficient potentials combine, then the pd across the post-synaptic membrane reaches threshold potential.

  7> A new AP is created in post-synaptic neurone, passing it down.
• Role of Acetylcholinesterase in the synaptic cleft 


  > Acetylcholinesterase is an enzyme found in the synaptic cleft.

  > It hydrolyses acetylcholine -> acetic acid + choline which stops transmission of signals so synapse does not continue to produce APs in post-synaptic neurone.

  > acetic acid and choline are recycled and re-enter synaptic bulb by diffusion. They are then recombined to make acetylcholine using ATP from respiration in mitochondria, which is stored in synaptic vesicles for future use.
• Nerve junctions involve 7 neurones whether from different places joining onto one or one sending signals out to 7 neurones that split to different effectors.

  When one AP passes down an axon to a synapse, it causes a few vesicles to move and fuse with the pre-synaptic membrane. A small number of Ach molecules diffusing across cleft, produces a small depolarisation, which is known as an EPSP which is not sufficient to cause an AP in the post synaptic neurone alone.

  SO: 7 EPSPs combine to increase membrane depolarisation until it reaches the threshold. This is known as summation.
• Two types of Summation

  >Temporal: when 7 APs in same pre-synaptic neurone combine together to increase membrane depolarisation until it reaches threshold and AP can be transported to different parts of nervous system.

  >Spatial: when APs arriving from 7 different pre-synaptic neurones combine together to increase membrane depolarisation until it reaches threshold and AP in post-synaptic neurone generates a particular response.
• Control of communication in synapses

  some pre-synaptic neurones can produce inhibitory post-synaptic potentials (IPSPs). These reduce the effect of summation and prevent an AP in the post-synaptic neurone.
• Control of communication in synapses Pt.2

  >combination of 7 EPSPs could be prevented from producing an AP by one IPSP.

  >synapses ensure APs are transmitted in the right direction and can stop impulses to different cells. They can signal out unwanted low-level signals. 7 vesicles of acetylcholine must be released to create an AP in the post-synaptic neurone.

  >If the low-level stimulus is persistent, it will create 7 successive APs in the pre-synaptic neurone. The release of many vesicles of Ach over a short period will enable post-synaptic EPSPs to combine together to produce an AP.
• How can mammals become habituated to a stimuli?

  After repeated stimulation, a synapse may run out of vesicles containing the neurotransmitter and the synapse becomes fatigued. Then we have become habituated to it.