C2.2 NEURAL SIGNALLING

using the given diagram, label the different parts on a neuron

outline neurons as cells within the nervous system and outline what they do

• nervous system transmits signals by neurons which are specialised cells with a function to transmit electrical impulses

• cytoplasm and a nucleus form the cell body of a neuron, with elongated nerve fibres of varying length projecting from it

  • dendrites are short branched nerve fibers while an axon is a long single nerve fiber

• transmits an electrical impulse along dendrites and the axon

  • releases a chemical signal to other neurons or effector cells (eg muscles)

  • chemicals released are called neurotransmitters

  • space between two neurons are called a synapse

what is membrane potential

• difference in electrical charge between the inside and outside of its cell membrane

  • neurons must have a membrane resting potential to be able to send an electrical impulse

• can be resting or active

explain the generation of the resting potential by pumping to establish and maintain concentration gradients of sodium and potassium ions

• resting potential: difference in charge across neurons when the neuron is not transmitting an impulse (-70mV inside)

  • membrane is polarised due to unequal distribution of charges (more + on membrane exterior, move - on membrane interior)

• neurons conduct signals by pumping Na+/K+ across the plasma membrane

  • controlled by a sodium-potassium pump which is a transmembrane protein or antiport

  • this pump pumps 3Na+ out of the cell and admits 2K+ into the cell, creating an electrochemical gradient whereby the cell interior is relatively negative

    • pump requires ATP as it is an active transporter

• because there is an uneven exchange, where the pump moves three sodium ions out of the cell for every two potassium ions it brings in, this creates a negative charge in the cell's interior

explain the process of nerve impulses as action potentials that are propagated along nerve fibers

• action potential: change in charge across membrane when neuron is firing (+30mV inside)

  • a neuron sends a signal along its dendrites and axon to form a wave of depolarisation

    • depolarisation is caused by movement of positively charged sodium ions

  • action potential = depolarisation + repolarisation

• depolarisation:

  • the positive charge propogating from the previous neuron stimulates sodium channel to open, causing Na+ to move into the membrane interior, creating a positive potential relative to the interior membrane

    • this depolarises the area and raises the voltage on the inside from -70mV to +40mV

• repolarisation:

  • immediately after the interior membrane is positively charged (once the na channels close, the k channels open), K+ flow out from the potassium channels to re-establish the negative membrane potential, which is a process called repolarisation returning the voltage back to -70/-80mV

note: depolarisation spreads across neuron until it reaches the axon terminals, where it triggers the release of neurotransmitters into the synaptic cleft

explain the variation in the speed of nerve impulses

• axons are circular in transverse section with a plasma membrane enclosing cytoplasm

• speed of nerve impulses are dependent on 2 factors

  • size of axon diameter:

    • axons with wide diameters have less longitudinal resistance resulting in faster conduction speeds

  • myelinated/unmyelinated fibres:

    • myelin sheaths act as an insulator and amplifies the affect of ion concentration gradients

    • myelin sheaths can reduce the generation of action potential to fewer points which increases the current flow

explain the process of synaptic transmission

• synapse: physical gap that separates neurons from other neurons or receptor/effector cells

  • transmit information by converting electrical signals into chemical signals using neurotransmitters

• neurotransmitters: chemical messengers released from neurons functioning to transmit signals across synaptic cleft

1. when an action potential reaches the axon terminal, voltage-gated Ca²⁺ channels are opened

2. Ca²⁺ diffuse into the cell and promote the fusion of vesicles filled with neurotransmitters with cell membrane

3. these vesicles with neurotransmitters are released from the axon terminal by exocytosis and diffuse across the synaptic cleft

4. neurotransmitters bind to receptors on the post-synaptic membrane and open Na+ ion channels

5. the opening of the ion channel generates an electrical impulse, creating a new action potential in the post-synaptic neuron

6. the neurotransmitters are released back into the synapse where they are broken down by enzymes and transported back into the presynaptic neuron to be reused

outline 3 functions of a neurotransmitter

• neurons: trigger propagation of action potential in the next neuron

• muscle: trigger contraction or relaxation of muscle tissue

• gland: trigger release of chemicals

use acetylcholine as an example for neurotransmitters

• acetylcholine: commonly released at neuromuscular junctions and binds to receptors on muscle fibers to trigger voluntary muscle contractions

  • acetylcholine is stored in vesicles within the axon terminal until they are released via exocytosis in response to impulse

  • this activates the post-synaptic cell by the binding of acetylcholine to the muscle receptors

what are effector cells and describe the relationship between nerves and its function

• effector cells are cells in muscles, glands and organs that respond to stimulus by nerves

  • skeletal muscle cells contract when neurons signal them

  • adrenal glands release epinephrine (adreneline) when stimulated

  • the heart beats faster when signalled by the brain

explain gated ion channels in neurons using nicotinic acetylcholine receptors as an example

• gated ion channels are responsible for receiving synaptic transmissions and propagating the action potential along the nauron

  • these proton channels are gated meaning they can be opened and closed, there are 2 types

    1. neurotransmitter-gated ion channels which act as receptors and receive synaptic transmissions

    2. voltage-gated ion channels that propagate action potentials

• initially the ion channel is closed when no neurotransmitter is bound to the binding site

• when then neurotransmitter nicotinic acetylcholine binds to these receptors, they open the gated ion channels

• this allows sodium ions to diffuse through, creating an action potential in the post-synaptic neuron or muscle cell

explain depolarisation and repolarisation during action potentials

• depolarisation:

  • neurotransmitter-gated channels open in the dendrites, allowing Na+ to diffuse into the cell

    • this causes depolarisation near the neurotransmitter-gated channels

  • if the depolarisation levels are greater than the threshold potential of -55mV, the nearby voltage-gated Na+ channels are stimulated to open

    • note: when depolarisation reaches peak at +30mV, the channels close

  • the depolarisation propagates along the membrane by causing the threshold potential to be reached, opening the nearby voltage gated ion Na+ channels

• repolarisation:

  • after depolarisation and the influx of Na+ ions, the charge across the membrane causes the voltage gated potassium channels to open and voltage gated sodium channels to close(which is when the voltage is at around +30mV)

    • this resores the resting potential, making the exterior of the membrane positive and the interior of the membrane negative

• NOTE: there is a refractory period which prevents a second action potential while the Na+/K+ pumps restore the resting potential distribution of ions outside and inside the cell, respectively

explain local ion currents

• once voltage gated sodium ion channels open as the threshold potential has been exceeded (past -55mV), it allows for the influx of sodium ions, further depolarising that region of the neuron, starting the action potential

  • once sodium ions are in the axon, they can move freely, allowing for the generation of local ion currents

  • local sodium ions in the interior of the neuron move from an area of high to low concentration through simple diffusion in he direction of the axon terminal

    • this increases the positive charge in the region ahead of the action potential

  • exterior sodium ions move from an area of high to low concentration towards the action potential, in the direction of the dendrites

    • this decreases the positive charge in the region ahead of the action potential which disrupts the membrane resting potential

• decrease in positive charge in the region ahead of the action potential → disrupts the resting membrane potential in the cell → allowing it to reach the threshold potential → opening voltage-gated sodium channels → rapid depolarisation → creating a chain that propagates the action potential forward

explain how saltatory conduction in myelinated fibres are used to achieve faster impulses

• saltatory conduction: the phenomenon whereby an action potential jumps from one node of ranvier to the next node as an impulse as it progresses along a myelinated axon

• in myelinated axons, action potentials jump from node to node which contain clustered transport proteins which only transfer ions to cause resting and action potentials

  • the influx of Na+ at one node pushes ions inside the axon towards the next node, triggering the action potential and causing depolarisation at the next node

  • action potentials move from node to node rather than continuously along as no transport occurs within the myelinates areas

• this causes much faster speeds than in unmyelinated axons (100x more faster)

what does the term exogenous molecule mean

• molecules from the environment that can have an effect on the body are called exogenous

  • they can affect synaptic transmission

• they are taken in, consumed (can be through drugs, medication, etc)

explain the effects of exogenous chemicals on synaptic transmission using neonicotinoids as an example of a pesticide that blocks synaptic transmission

• neonicotinoids are pesticides used in agriculture

  • neonicotinoids bind to the nicotinic acetylcholine receptors (usual neurotransmitter that binds is acetylcholine)

    • the binding causes the Na+ channel in the receptor to open

  • neonicotinoids are not broken down by enzymes so the binding is irreversible

    • this causes Na+ channels to remain open, resulting in a continuous stimulating of action potential

  • the continuous stimulus can lead to overstimulation which can cause paralysis, convulsions and eventually death

explain the effects of exogenous chemicals on synaptic transmission using cocaine as an example of a drug that blocks reuptake of the neurotransmitter

• dopamine is a neurotransmitter which is associated with feelings of reward, pleasure, motivation and being productive

• cocaine is a drug that binds to neurotransmitter proteins

  • this blocks their reuptake function which are membrane proteins that pump dopamine back into the presynaptic neuron

• as cocaine blocks these transporters, there is a high concentration of dopamine in the synaptic cleft, continuously stimulating the postsynaptic neurons

• note: the continuous feeling of euphoria can lead to addiction and brain damage

  • can eventually lead the brain to adapt to unnatural reward pathway and becomes less sensitive to natural reinforces which increase the likelihood cocaine intake

  • tolerance may develop, higher doses no longer affective

what are excitatory neurotransmitters

• neurotransmitters that sends a signal from the presynaptic neuron to the postsynaptic neuron or cell

what are inhibitory neurotransmitters

• neurotransmitters that inhibit the generation of action potentials in the postsynaptic neuron

  • as a result, these neurons require more excitatory stimulus to cause an action potential

using GABA as an example of inhibitory neurotransmitters, explain how it causes hyperpolarisation

chlorine ion influx:

• a type of inhibitory neurotransmitter known as GABA binds to the receptors on the postsynaptic neuron which triggers chlorine ion channels to open, allowing negative chlorine ions to diffuse into the cell

• these influx of negative ions inside decreases the charge inside the cell making it harder to cause in action potential (inhibitory)

  • as a result, neurons require more excitatory simulus to cause an action potential

what is summation

• it is the process by which multiple signals or inputs are combined in a postsynaptic neuron to determine whether an action potential will be generated

  • like the summing up of excitatory and inhibitory signals

explain summation of the effects of excitatory and inhibitory neurotransmitters in a postsynaptic neuron

• multiple presynaptic neurons interact with one postsynaptic neuron

  • they can send a combination of inhibitory and excitatory effects on the post synaptic neuron

  • the cumulative effect of both excitatory and inhibitory neurontransmitters in the dendrites and call body causes an “all-or-nothing” consequence for depolarising the axon

    • it either fires or doesnt fire

  • the postsynaptic neuron needs to reach the threshold potential (-55mV) to cause an action potential in the axon

explain the perception of pain by neurons with free nerve endings in the skin

• free nerve endings are found in the skin and in internal organs to sense pain

  • the signal flows in a unidirectional manner from sensory organs to the central nervous system

    • these nerve endings have channels for positively charged ions

• the channels open in response to a stimuli such as high temperature, acid, or certain chemicals (excess of something)

  • eg) capsaisin, a spicy compound found in chilli peppers

• entry of positively charged ions cause the threshold potential to be reached

• nerve impulses then pass through the neurons to the brain where pain in perceived

• the brain sends an impulse along motor neurons to affect a response behaviour

explain consciousness as a property that emerges from the interaction of individual neurons in the brain

• consciousness is a property that emerges from the interactions of individual neurons in the brain

  • simultaneous awareness of many things

• different states of consciousness arises between the interdependence and interactions between different neurons

• examples of diff kinds of consciousness

  • reduced consciousness: sleep

  • unconsciousness: anesthesia