3.2 Nerve Impulses

Nerve impulses

• neurons send messages electrochemically
• chemicals that are ‘electrically charged’ = ions

Resting Membrane Potential

• when a neuron is not sending signal - at rest
• inside of a neuron is negative relative to outside (doesn’t mean it is negatively charged, just more negative than outside)
• K+ cross through membrane easily
• Cl- and Na+ have more difficult time crossing

negatively charged protein molecules inside neuron can’t cross membran

Movement of Potassium (K+)

• potassium ions move freely across the membrane
• they will move out of cell into extracellular fluid
• movement of positive ions = potassium leakage → makes it more negative inside cell
• electrical potential difference across the cell membrane builds to a high enough level and causes an electrical resistance
• charge imbalance = leak NOT flood because inside cell is negative - positive attrct to negative
• high concentration → low concentration
• inside (more K+) → outside (less K+)

Sodium potassium pumps

• both Na+ and K+ are able to leak through respective channels
• channels change in protein structure (denature) to pump sodium and potassium in and out
• over time, this will decrease potential difference (-70 mV will become closer to 0 mV) = no charged difference - but the neurons will need a charge difference to send messages - need to constantly maintain electrochemicals
• this is because diffusion will never stop - will go from high concentration → low concentration till it reaches equilibrium
• need a way of moving Na+ out of the cell and K+ into the cell - done by the pump
• $moves 3 Na+ out and 2 K+ in$
• requires energy as it moves against concentration gradient = ATP = adenosine triphosphate
• ATP transfers a phosphate group = ADP + P - need mitochondria for energy = cellular respiration
• Na+ constantly being pumped in and then out
• K+ constantly being pumped out then in
• more positives out than in = keeps difference, allows negative inside
• moving K+ and Na+ maintains chemical concentration gradient = ⬆️ polarity (more difference in charge) ⬇️ charge
• e.g. diabetes - neurons need glucose → energy to function - always moving or faint

Action Potential (AP)

How do nerve impulses start?

• neurons are stimulated by receptor cells (respond to a stimulus e.g. chemicals, pain, temperature)
  • contain special sodium channels that aren’t voltage-gated but gated by appropriate stimulus
• stimulus causes sodium channels to open
  • causes sodium ions to flow into channel because of negative inside and concentration gradient
  • causes depolarisation of membrane potential → affects voltage-gated sodium channel and starts an action potential
• e.g. chemical-gated in tongue - taste receptor cells open when a certain chemical in food binds to them
• mechanically-gated ion channels in hair cells of inner ear open when they are distorted by sound vibrations

What is action potential?

• resting membrane potential tells us about what happens when a neurons is at rest
• a nerve impulse = whole process, not just action potential - action potential is the letter being sent
• occurs when a neurons sends info down an axon

Depolarisation

• flipped polarity
• -55 mV causes more Na+ channels open for 0.5 ms - gate threshold
• causes Na+ to rush in → cell becomes more positive
• membrane potential will reach 30 mV by the time sodium is in the cell

Repolarisation

• depolarisation of membrane causes Na+ channels to close
• K+ channels open
• sodium potassium pump helps return to resting membrane potential
• K+ rush out → make inside more negative - attracted to
• restores original polarity = repolarisation

All-or-none response

• as soon as -55 mV is reached, rest of action potential is triggered - nothing can stop it
• frequency of impulse carries info → strong stimulus = high frequency
• intensity = frequency of action potential

Refractory period

• limit number of AP in a given time
• time after depolarisation where no new AP can start
  • impulses travel in 1 direction
  • time is needed to restore proteins of voltage sensitive ion channels to original resting conditions
  • Na+ channels cannot be opened - can’t be deporalised again
  • can last up to 10 milliseconds - limits frequency of impulses
• absolute = second stimulus doesn’t cause new AP
• relative = second AP can be produced ONLY if stimulus is greater than threshold

How do impulses travel down the neuron?

• different action potentials along neuron = propogate - triggers down the axon
• depolarises sections along the membrane

How fast are impulses?

• travel 0.1-100 m/s along axons
• allows for fast responses to stimuli
• speed is affected by:

1. temperature
2. axon diameter - like straw and milkshake - bigger it is, easier it flows

1. whether or not it has myelin sheath

Myelinated Neurons

• axons of many neurons are encased in fatty myelin sheath (Schwann cells)
• where the sheath of one Schwann celll meets the next, the axon is unprotected
• voltage-gated sodium channels of myelinated neurons are confined to spots (nodes of Ranvier)
• in rush of Na+ at one node creates just enough depolarisation to reach threshold of next
• AP jumps from one node to next = saltatory propogation = faster

Summary

• nerve impulse conduction is bumping of positive charge down the axon
• AP initiated at one end of axon can only propagate in 1 direction
• AP doesn’t turn back because membrane just behind is in its refractory period (voltage gated Na+ channels are inactivated)

Summarised steps of AP

1. A resting neuron has a positive charge on the outside of the membrane and a negative charge on the inside (resting membrane potential -70 mV)
2. High concentration of Na+ on the outside and high concentration of K+ on the inside
3. greater concentration of negatively charged ions - due to organic anaions than K+ = negative on the inside
4. stimulus causes voltage gated sodium channels to open and sodium ions rush in intracellular fluid
5. -55 mV is threshold for voltage gated sodium ions
6. inward movement of Na+ ions reverses the charge = cell becomes depolarised, polarisation has flipped - inside is positive, outside is negative - membrane potential will reach +30 mV
7. After inside of membrane becomes flooded with sodium, gated potassium channels open and allow potassium ions to move to the outside
8. as soon as potassium ions are released, sodium ion channels closed (membrane is repolarised)
9. hyperpolarisation occurs = too many potassium ions cause the inside of the cell to be more negative than -70 mV
   10. sodium potassium pump restores concentration of sodium and potassium when membrane is at resting state

Synapse

• junction between branches of adjacent neurons
• neurons don’t join at synapse = small gap (for axon and skeletal muscle = neuromuscular jinction)
• occurs between a branch at the end of an axon and dendrite or the cell body of another neuron
• AP cannot cross synaptic cleft
• impulse is carried by chemicals called neurotransmitters

Neurotransmitters

• made by cell sending impulse (pre-synaptic neuron) and stored in synaptic vesicles at the end of the axon
• cell receiving impulse (post-synaptic neuron) has chemical gated ion channels = neuroreceptors

  ### Synapses explained

• at the end of the pre-synaptic neuron - voltage gated calcium channels
• when AP reaches synapse, channels open
• calcium ions flow into the cell

• cause synaptic vesicle to fuse with cell membrane
• neurotransmitters diffuse across synaptic cleft

• neurotransmitter binds to neuroreceptors in the post-synaptic membrane
• channels open, Na+ flow in
• causes depolarisation
• AP initiated in post-synaptic neuron

Transmission across a synapse (in steps)

1. AP arrives at pre-synaptic axon terminal
2. Local depolarisation caused voltage gated calcium ion channels to open
3. Calcium ions from extracellular fluid diffuses through presynaptic membrane of axon terminal (AT) and enters cytoplasm of AT
4. Calcium causes neurotransmitter vesicles to migrate to pre-synaptic membrane of AT
5. Neurotransmitters leave vesicles and enter synaptic cleft through exocytosis
6. Neurotransmitter diffuses across synapse to post-synaptic membrane of dendrite of adjacent neuron
7. Na+ floods in, causing depolarisation in postsynaptic dendrite
8. AP is generated

Function

• prevent impulses travelling in wrong direction
  • impulse can pass along an axon in either direction, but can only cross a synapse in one direction because the vesicles are only found in knobs and end plates
• vast number of synaptic connections allow flexiblity
  • equivalent to switchboard in an elaborate telephone exchange, enabling messages to be diverted from 1 line to another

What happens to neurotransmitters? + Examples

• broken down by specific enzyme in synaptic cleft
• breakdown products are absorbed by pre-synaptic neuron
• used to re-synthesis more neurotransmitter
• Acetylcholine (ACh)
  • released by motor neurons onto skeletal muscle cells
  • released by neurons in parasympathetic nervous system
• Noradrenaline
  • released by neurons in sympathetic nervous system