Neural signalling

nervous system

• central nervous system - brain and spinal cord

• peripheral nervous system - all of the nerves in the body

neurones

• have main, long fibre - axon

• axon is insulated by Schwann cells forming the myelin sheath

• have cell body containing nucleus and other cell structure

• cell bodies and axon terminals contain extensions - dendrites

• dendrites allow to connect to many other neurones and receive impulses

generating resting potential

• neurones transmit information through impulses which travel quickly from neurone to neurone

• impulses are a momentary reversal in the electrical p.d across neurone cell surface membrane

• in an axon not transmitting an impulse, the inside of axon has negative electrical potential, and outside has positive electrical potential

  • this membrane potential in resting neurone is called the resting potential

• 2 processes which establish and maintain resting potential

  • active transport of sodium and potassium ions

  • difference in rates of diffusion of sodium and potassium ions

active transport of sodium and potassium ions

• sodium-potassium pumps are present in cell surface membrane of neurone

• pumps use ATP to transport Na+ out of axon and K+ into axon

• 3 Na+ pumped out, 2 K+ pumped in - generates concentration gradient across membrane

difference in rates of diffusion of sodium and potassium ions

• due to conc gradient generated by sodium potassium pump, both K+ and Na+ will diffuse back across membrane through facilitated diffusion

• neurone membrane much less permeable to Na+ than K+, so K+ can diffuse out at a faster rate than Na+ can diffuse in

• this results in more positive ions on the outside than the inside

nerve impulses

• once resting potential reached, neurone is polarised

• to initiate nerve impulse in neurone, it must be depolarised

  • the reversal of electrical p.d across membrane

• depolarisation of membrane occurs when action potential is generated

  • rapid movement of Na+ and K+ across membrane of axon

• action potential: changes in voltage produced across axon membrane during nerve impulse

factors affecting speed of transmission

• myelination of neurone

  • myelinated neurones conducted impulsed much quicker than unmyelinated fibres

  • small gaps between Schwann cells - nodes of Ranvier, allow impulse to jump from one node of another, speeding up transmission along axon

• diameter of neurone

  • axon with wider diameter conducts impulse quicker than a narrow axon

  • wider axon offers less resistance

• squid have giant unmyelinated axons, despite being significantly wider, the speed of transmission is much faster in axon which is insulated by myelin sheath

myelination

• schwann cells wrapped around axon form the myelin sheath

• myelin sheath acts as electrical insulator

synapses

• 2 neurones never meet, instead a very small gap called synaptic cleft separates them

• 2 neurones and synaptic cleft form a synapse

• synapses - junctions between any cells in the nervous system

• ensure one-way transmission of impulses because neurotransmitter is released on one side and its receptors are on the other

release of excitatory neurotransmitters

• impulse arrives at the end of axon on presynaptic neurone, membrane of presynaptic neurone becomes depolarised, triggering release of Ca+ into presynaptic cell via Ca+ channels

• influx of Ca+ in presynaptic membrane causes vesicles containing neurotransmitter to moves forward to membrane where they fuse w/ it and release neurotransmitters into synaptic cleft by exocytosis

• neurotransmitters diffuse across synaptic cleft and bind w/ receptor molecules on postsynaptic membrane

generating excitatory postsynaptic potential, example: ACh

• ACh is released into synaptic cleft when ACh containing vesicles fuse w/ presynaptic membrane

• ACh binds to receptors on postsynaptic membrane, causing the receptor proteins to change conformation, allowing Na+ to flow through

• Na+ diffuse down their conc gradient across postsynaptic membrane causing membrane potential to become less negative

• if potential rises from -70 mV to -50 mV, an action potential is triggered in the postsynaptic neurone

• to prevent Na+ channels staying permanently open and to stop permanent depolarisation of postsynaptic membrane, ACh is broken down and recycled

depolarisation

• some ion channels of neurone are voltage gated, meaning they open/close in response to changes in electrical potential

• voltage gated Na+ channels open if the membrane potential rises from -70 mV to -50mV - action potential is generated

• Na+ moves into axon down conc gradient, raising membrane potential further

• if enough Na+ enters axon and membrane potential is raised enough, more Na+ channels open, and more Na+ flows in

• this causes membrane potential to rise from -50 mV to around 30/40 mV

action potential threshold

• action potential only initiated if the threshold potential is reached

• nerve impulse is “all or nothing”, because unless threshold potential is reached, there is no action potential

• if the voltage reaches around -50 mV, the threshold potential is reached and the voltage gated Na+ channels can open

how action potential is propagated

• once generated, action potential is propagated along length of axon

• this movement happens because an action potential in 1 part of the axon triggers an action potential in the next part, this occurs due to local currents

  • local currents are movements of Na+ by diffusion between one part of axon that has depolarised, and adjacent part that is still polarised

  • this movement makes the potential in the still polarised region less negative

  • if potential in membrane rises from -70 mV to threshold potential, voltage gated Na+ channels start to open triggering action potential

repolarisation

• positive membrane potential developed during depolarisation causes voltage-gated K+ channels to open

• K+ diffuses out of axon down conc gradient

• movement of K+ ions causes inside of axon to become negatively charged again - repolarisation

• membrane potential returns to -70 mV and may briefly overshoot by becoming more negative than this, before Na+/K+ pump restablishes conc gradients - this is hyperpolarisation

interpreting oscilloscope traces

• resting potential at -70 mV

• action potential causes a spike rising up to +30/40 mV

• instead of repolarisation causing the membrane potential to return immediately to normal resting potential of -70 mV, the trace shows a short period of hyperpolarisation

  • when membrane potential becomes more negative than resting potential

saltatory conduction

• the presence of myelin increases the speed at which action potentials travel along the neurone

  • in section of axon surrounded by myelin sheath, depolarisation cannot occur as the myelin sheath stops the diffusion of Na+ and K+

  • small, uninsulated sections of the axon - nodes of Ranvier, contain clusters of ion pumps and channels allowing the action potential to occur

  • action potentials jump from one node to another - saltatory conduction

neonicotinoids - exogenous chemical

• synthetic compounds similar to nicotine commonly found in pesticides

• block synaptic transmission at cholinergic synapses in insects by binding to ACh receptors

• ACh is unable to bind, stopping impulses from being transmitted, leading to death in insects

cocaine - exogenous chemical

• blocks reuptake of neurotransmitters into presynaptic neurone

• cocaine primarly affects reuptake of dopamine as it binds to dopamine transporter protein, preventing it from moving back into presynaptic neurone

• dopamine builds up in synapses, leading to feelings of pleasure

• also blocks neurotransmitters serotonin and norepinephrine

inhibitory postsynaptic potentials

• neurotransmitters that prevent generation of action potential in postsynaptic neurone

• neurotransmitter opens a gated K+ channel in membrane so K+ is able to diffuse out

  • postsynaptic neurone becomes hyperpolarised

  • threshold isn’t reached, so action potential can’t be triggered

summation of neurotransmitter effects in postsynaptic neurone

• initiation of an action potential is an “all-or-nothing” event

• a single release of excitatory neurotransmitter from one neurone is often insufficient to trigger action potential, bc it doesn’t cause threshold potential to be reached

• to reach threshold, neurone must release neurotransmitter multiple times in short time, or several neurones must release neurotransmitter simultaneously - this is summation

perception of pain

• pain receptors have free nerve endings which are unencapsulated

• nerve endings have channels for positively charged ions which open in response to stimulus e.g temperature, acid or chemicals e.g capsaicin in chilli peppers

• entry of positively charged ions causes threshold potential to be reached and action potential to be generated

• impulse passes through neurones to the brain

consciousness

• emergent properties: result of interactions between the elements of a system

• interactions between neurones lead to consciousness