Neural signalling C2.2

• What is a neuron?

  A neuron is a cell that processes and transmits information through electrical and chemical signals in the nervous system.

  Neurons are the basic structural and functional unit of the nervous system.
• What are sensory neurons, and what do they contain?

  Sensory neurons carry information towards the CNS. They consist of: Cell body - contains nucleus and other organelles 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. Most sensory neurons have a single axon that branches into two parts, one extending to the periphery and the other to the spinal chord.
• What are motor neurons, and what do they contain?

  Motor neurons carry information from the CNS to a muscle or gland. They consist of:

  Cell body - contains nucleus and other organelles 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

  Axon - A long, slender extension that carries the electrical signal. Most sensory neurons have a single axon that branches into two parts, one extending to the periphery and the other to the spinal chord.

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

  Neuromuscular junction- the point of contact between a motor neuron and a muscle fibre, where the nerve transmits an electrical impulse to the muscle, causing it to contract.
• What is membrane potential?

  The difference in charge between the inside and outside of the cell membrane is known as the membrane potential.

  The inside of cells is generally negatively charged, so the membrane potential is expressed as a negative value.
• What is a polarised membrane?

  A polarised membrane is a membrane that has a positive electrical charge on one side and a negative chare on the other side. Almost all cells exhibit some degree of polarity.

  If microelectrodes are placed inside and outside the cell, the amount of polarisation can be measured as the voltage difference between the inside and the outside of the cell membrane.
• What is resting membrane potential?

  When a neuron transmits an impulse, its membrane potential changes suddenly.

  When not sending an impulse, the neuron is said to be 'at rest'.

  The resting membrane potential of a neuron is about -70mV.
• What are some factors that contribute to the negative charge of 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.
• What are the steps of sodium-potassium pump action?

  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 action potential?

  The basic unit of neuron function is called an action potential.

  An action potential is the electrical signal that a neuron generates to transmit information along its cell membrane, from the dendrite to the synaptic terminal.

  This is characterised as a 'flip-flop' of charge. It is also known as a nerve impulse.
• Where does action potential travel?

  Dendrites receive information and send action potentials towards the cell body and axon of the neuron.

  The synaptic terminal is the region at the end of an axon. It is where the electrical signal from the action potential is converted into a chemical signal that is sent to a neighbouring cell.

  Action potential always moves in one direction, due to the refractory period after depolarisation.

  Action potentials are propagated along nerve fibres as the ion movements that depolarise one part of the fibre trigger depolarisation in a neighbouring part of the fibre.
• How does action potential occur?

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

  The action potential sequence is as follows:

  1- Resting potential

  2- Stimulus

  3- Threshold

  4- Depolarization

  5- Repolarization

  6- Refractory

  Both depolarisation and repolarisation are due to movement of positively charged ions (sodium and potassium), NOT the movement of electrons.
• What is depolarisation?


  Depolarisation is due to the opening of sodium channels in the membrane, allowing sodium ions to enter the neuron. As the concentration of sodium ions is far higher outside than inside the cell, this means that it diffuses inside the neuron. This makes the inside of a neuron positive in charge, relative to the outside, raising the resting potential from -70mV to +30mV.
• What is repolarisation?


  Repolarisation is the closing of sodium channels and the opening of potassium channels in the membrane. Potassium ions diffuse out of the neuron, and no more sodium ions diffuse in when this occurs. As a result, the inside of the neuron becomes negatively charged, in relation to the outside. The potassium channels remain open until the membrane potential return back to resting membrane potential (-70mV)
• What factors impact the speed of nerve impulses?

  The body size of the animal

  The axon diameter size

  Whether or not the axon is myelinated
• How does body size impact the speed of nerve impulses?

  Generally, an animals body size does not significantly affect the speed of its neuron impulses, meaning that nerve conduction velocity remains relatively constant among different sized animals.

  However, larger animals experience longer delays in nerve signal transmission simply because their neurons need to travel further distances due to their larger bodies. This results in slower reflexes in larger animals compared to smaller ones.
• How does axon diameter impact the speed of nerve impulses?

  The conduction velocity of an action potential (speed) is directly proportional to the diameter of the axon, meaning that larger diameter axons have a faster conduction velocity, allowing for quicker signal transmission.

  This is because a larger axon experiences less resistance to ion flow (a wider space allows ions to move freely without colliding with obstacles, leading to less resistance), which facilitates faster propagation of the action potential.

  For example, squids have giant axons of diameters of 500 micrometres. This allows them to have very fast responses, which is a key evolutionary adaptation for the squid in its marine environment.
• How does myelination impact the speed of nerve impulses?

  The conduction speed of neural impulses depends on whether the axon is myelinated or unmyelinated.

  Myelination increases the speed of neuron impulses.

  Schwann cells grow around the neuron axon, creating layers of myelin. Myelin is a fatty substance that insulates nerve axons.

  Gaps between the Schwann cells are called Nodes of Ranvier.

  In myelinated neuron axons, the impulse can jump from one Node of Ranvier to the next, speeding the transmission to as quick as 100 meters per second.
• What is the correlation coefficient?

  The correlation coefficient is a measure of how strongly two variables are linearly related.

  +1 is a perfect positive correlation (as one goes up, the other goes up).

  -1 is a perfect negative correlation (as one goes up, the other goes down).

  0 means no linear relationship.
• What is the coefficient of determination?

  The coefficient of determination is the square of the correlation coefficient (r²).

  It tells you what proportion of the variation in one variable is explained by the other in a linear model.
• What is a presynaptic neuron, synaptic gap and postsynaptic cell?

  A neuron connects to another cell at a junction that is known as a synapse.

  The presynaptic neuron is the cell that sends chemical signals to the postsynaptic cell at the synapses. It releases neurotransmitters into the synaptic gap.

  The synaptic gap (or the synaptic cleft) is the space between the axon terminal of the presynaptic neuron and the postsynaptic cell.

  The postsynaptic cell receives chemical signals from the presynaptic cell at the synapse.
• What are the three main 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 information between cells at the synapse. Although they work locally between individual cells, neurotransmitters have a significant impact on an entire organism, impacting sleep, homeostasis, memory, cognition and more.

  Neurotransmitters are released from a presynaptic neuron, and bind to receptors in the postsynaptic cell membrane. They diffuse across the synaptic gap rapidly (due to a short distance), and trigger a series of events in the postsynaptic cell.
• What are amino acid neurotransmitters?

  Amino acid neurotransmitters are the main inhibitory and excitatory messengers in the nervous system. One example is glutamic acid, which is an excitatory neurotransmitter.
• What are amine neurotransmitters?

  Amines are small molecules that are synthesised through modification of amino acids, and have a wide range of functions as neurotransmitters. One example is dopamine, which can be both excitatory or inhibitory.
• What are neuropeptides?


  Neuropeptides are neurotransmitters made of small chains of amino acids that are synthesised and released by neurons. They are structurally diverse and play important roles in many physiological and behavioural events. One example are endorphins, which are inhibitory neurotransmitters.
• What are ester neurotransmitters?

  An ester is the general name for a chemical formed when an alcohol bonds to an acid. One example of an ester neurotransmitter is acetylcholine, which is acetic acid bound with choline, and is an excitatory neurotransmitter.
• What are gas neurotransmitters?

  There are only three gas molecules that have known inhibitory neurotransmitter function. They are toxic at high concentrations, and are nitric oxide, carbon monoxide and hydrogen sulphide. In small quantities, they are essential. Carbon monoxide aid vasodilation, for example.
• What happens when a neurotransmitter is released from a presynaptic cell?

  1) A nerve impulse arrives at the axon terminal of the presynaptic cell.

  2) Depolarisation pf the presynaptic cell membrane causes voltage gated calcium channels in the axon terminal to open.

  3) Calcium moves into the cell via facilitated diffusion

  4) The increase in calcium concentration causes vesicles containing neurotransmitters to move towards the end of the axon terminal

  5) The vesicles fuse with the presynaptic cell membrane

  6) There, they release the neurotransmitter into the synaptic gap via exocytosis

  7) The neurotransmitter diffuses across the synaptic gap and binds to transmembrane neurotransmitter receptors

  8) This triggers a response in the postsynaptic cell.
• 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, which depolarises and triggers an action potential in the postsynaptic cell.
• 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 acetylcholine?


  Acetylcholine is one of the most common neurotransmitters in invertebrates and vertebrates. It forms in the presynaptic neuron when acetic acid binds with choline, vis the enzyme choline acetyltransferase. It has many functions, including triggering muscle contraction at neuromuscular junctions.
• What is the process of acetylcholine release?


  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 moves 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

  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