Neural signalling, C2.2

• What are neurons?


  Neurons are cells that conduct electrical impulses along their fibres to communicate with other neurons, muscles, or glands.

  Neurons are the basic structural and functional unit of the nervous system.
• What is the direction of a nerve impulse?

  Dendrites → Cell body → Axon → Axon terminals
• What do sensory neurons do?

  Sensory neurons carry information towards the CNS.
• What do motor neurons do?

  Motor neurons carry information from the CNS to a muscle or gland. They contain neuromuscular junctions.
• What do neurons contain?

  Cell body - contains nucleus and other organelles. Processes the information before sending signals down the axon.

  
  

  Dendrites- Finger like projections that act as the primary site for receiving sensory information from the environment. These may have specialised receptors to detect specific stimuli.

  
  

  Schwann cells- Glial cells that wrap around the axon and form the myelin sheath

  
  

  Myelin sheath- Layers of lipid that act as an insulator, allowing for faster signal transmission along the axon through a process called saltatory conduction

  
  

  Axon terminal - The part of the neuron that releases neurotransmitters into the synapse to transmit signals to other neurons or cells

  
  

  Axon - A long, slender extension that carries the electrical signal away from the cell body.
• What is resting potential?

  The resting potential is the electrical charge difference across the plasma membrane of a neuron when it is not transmitting a signal.

  It's typically around -70 mV, meaning the inside of the neuron is more negative than the outside.
• What are the factors that contribute to the negative charge of the resting potential?

  The sodium-potassium pump actively transports three sodium ions out of the cell, for every two potassium ions that it brings in, leaving the inside of the cell relatively negative compared to the outside.

  
  

  Potassium ions can leak out of the cell membrane, leaving the cell and contributing to the negative charge of the resting membrane potential.

  
  

  Negatively charged proteins within the neuron contribute to the negative charge of the resting potential.
• How does sodium-potassium pump action work?

  1- Three sodium ions from inside the cell bind to the pump

  2- ATP is hydrolysed, and a phosphate group is attactched to the pump.

  3- The pump undergoes a conformational change using ATP to do so, allowing the three sodium ions to be released.

  4- Two potassium ions from outside the cell bind to the pump

  5- The phosphate group detaches and the pump reverts back to its original conformation

  6- The two potassium ions are released into the cell.
• What is an action potential?

  An action potential is a temporary reversal of the electrical charge across a neuron's membrane. It is also known as a 'flip-flop' of charge. It travels along the axon from the dendrites to the axon terminal, as a nerve impulse- propagated along the axon.
• What is the action potential sequence?

  - It is an all-or-nothing sequence of changes due to the movement of positively charged sodium and potassium ions across the membrane.

  
  

  - If sufficient sodium ions diffuse into the membrane and the neuron reaches the threshold potential (-55mV), then more will diffuse into the cell, causing it to have a positive charge, raising its membrane potential to +40mV. This process is known as depolarisation. If not, then the neuron will return back to resting potential.

  
  

  - Depolarisation in one part of the nerve cell triggers the next section to depolarise. Therefore, the action potential is propagated across the nerve cell as a nerve impulse.

  
  

  - After depolarisation, the potassium gated facilitated diffusion channel opens briefly. This causes potassium ions to diffuse out of the nerve cell, making it less positive. Sodium ions are also pumped out of the cell via the sodium potassium pump. This causes the the nerve cell to return to its resting potential of -70mV, and is known as repolarisation.

  
  

  - There is also a refractory period afterwards, which ensures that the action potential occurs in one direction.
• What are nerve impulses?

  Nerve impulses are action potentials propagated along the axons of neurons.
• How is the speed of nerve impulses affected by body size?

  Larger bodies have slower nerve impulses because they need to travel across further distances.
• How is the speed of nerve impulses affected by axon diameter?

  The speed of a nerve impulse is directly proportional to the diameter of the axon. Larger diameters have a faster speed of nerve impulses, as there is less resistance of ion flow.
• How is the speed of nerve impulses affected by myelination?

  Myelination increases the speed of nerve impulses.

  Myelinated axons consist of Schwann cells that create layers of myelin around the neuron. There are gaps between the Schwann cells, known as Nodes Ranvier. In Myelinated axons, the impulse can jump from one Node of Ranvier to the next. This speeds up the nerve impulse, and is also known as salutatory conduction.
• What are synapses?


  Synapses are tiny gaps (Junctions) between two neurons, a neuron and a sensory cell, or a neuron and an effector cell.
• Why do signals only pass in one direction across a synapse?

  A signal can only pass in one direction across a synapse, from a presynaptic neuron to a postsynaptic cell. This is because vesicles with neurotransmitters are only present in the presynaptic neuron, and receptors for the neurotransmitters are only present on the postsynaptic membrane.

  The synaptic cleft is the gap between neurons at a synapse.
• What are the three types of synapses?

  - Synapse between a sensory cell (e.g. photoreceptor in the eye) and a sensory neuron

  
  

  - Synapse between neurons where one neuron transmits a signal to another neuron

  
  

  - Synapse between a motor neuron and a muscle or gland (effector). This is also known as a neuromuscular junction.
• What are neurotransmitters?


  Neurotransmitters are chemical messengers that carry signals between neurons and target cells in the brain and body.
• What are some different types of neurotransmitters?

  - Amino acid neurotransmitters - main inhibitory/excitatory neurotransmitters

  - Amine neurotransmitters- synthesised through modification of amino acids

  - Neuropeptides - small chains of amino acids

  - Ester neurotransmitters- alcohol bonded with acids (e.g. acetylcholine)

  - Gas neurotransmitters- gas molecules that are toxic at high concentrations (nitric oxide, carbon monoxide, hydrogen sulphide)
• How are neurotransmitters released from a presynaptic cell?

  1- Depolarization of the presynaptic membrane occurs as the action potential arrives.

  2- Voltage-gated calcium channels open in the membrane.

  3- Calcium ions (Ca²⁺) rush into the presynaptic neuron from the synaptic cleft.

  4- Calcium acts as an internal signal, triggering synaptic vesicles.

  5- Vesicles move toward and fuse with the presynaptic membrane.

  6- Neurotransmitters are released into the synaptic cleft by exocytosis.
• What is the role of calcium in neurotransmission?

  Calcium ions are essential signalling molecules inside the neuron. They cause vesicle fusion with the membrane so neurotransmitters can be released. Without calcium uptake, neurotransmission would not occur.
• What are neurotransmitter receptors?

  Each neurotransmitter has a specific transmembrane receptor to which it can bind.

  Often, the neurotransmitter receptor is a ligand-gated channel. Binding of the neurotransmitter to the receptor leads to the opening of an ion channel within the postsynaptic cell membrane.

  When this ion channel opens, ions can move through facilitated diffusion into the postsynaptic cell, causing its membrane potential to change.
• What are excitatory neurotransmitters?

  Excitatory neurotransmitters have an excitatory effect. They trigger the influx of positive ions, also known as an excitatory post-synaptic potential (EPSP) which is a temporary depolarisation of the postsynaptic membrane caused by the flow of positive ions.
• What are inhibitory neurotransmitters?

  Inhibitory neurotransmitters have an inhibitory effect. They trigger the influx of negative ions, which hyperpolarises and prevents an action potential in the postsynaptic cell.
• What is the process of acetylcholine neurotransmission?

  1- Acetylcholine is synthesised from choline (from the diet) and acetic acid (produced in aerobic respiration)

  2- It is loaded into vesicles and released from the presynaptic cell vis exocytosis (due to increase of calcium in the presynaptic cell)

  3- It diffuses across the synaptic gap

  4- It binds to the acetylcholine receptor in the postsynaptic cell membrane

  5- This receptor is a ligand-gated channel, so when it binds to it, the sodium channel opens and sodium ions move into the cell through facilitated diffusion.

  6- If the threshold potential (-55mV) is reached, then an action potential will be triggered and propagated as an impulse in the postsynaptic cell. This is the EPSP.

  7- An enzyme (acetylcholinesterase) in the presynaptic gap rapidly breaks down the acetylcholine into choline and acetate. If this does not occur, then it will continue to bind to the receptors which would result in multiple action potentials and a continuous response.

  8- The choline is recycled back into the presynaptic cell and the acetate is excreted as waste