Neurons as cells within the nervous system that carry electrical impulses
Two body systems are used for internal communication the endocrine system and the nervous system. The endocrine system consists of glands that release hormones. The nervous system consists of nurse cells called neurons. There are about 85 billion neurons in the human nervous system. Neurons help with internal communication by transmitting nerve impulses. A nerve impulse is an electrical signal.
Neurons have a cell body with a cytoplasm and a nucleus. They also have narrow outgrowths called nerve fibres along with nerve impulses travel.
-Dendrites are branched nerve fibres for example those used to transmit impulses between neurons in one part of the brain or spinal cord
-Axons are very elongated nerve fibres for example those that transmit impulses from the tips of the toes or the fingers to the spinal cord
Generation of the resting potential by pumping to establish and maintain concentration gradient of sodium and potassium ions
If microelectrodes are placed inside and outside any living cell of voltage across the membrane will be detected. This voltage is usually between 10 and 100 milli volts. It is known as the membrane potential. This potential is due to an imbalance between the net charge of cytoplasm and the fluid Outside. The cytoplasm of cells is generally electrically negative compared with the fluid outside. For this reason the membrane potential is expressed as a negative value.
When a neuron transmits an impulse its membrane potential suddenly changes. When it is not transmitting an impulse the potential across the membrane usually remains close to -70mV. This is called the resting potential. Three factors contribute to it:
1. Sodium potassium pumps in the membrane transfer sodium ions out of the neuron at the same time as they transfer potassium ions in. This is active transport and uses energy from ATP. The number of ions pumped are unequal when three sodium ions are pumped out. Only two potassium ions are pumped in. This creates a charge imbalance and concentration gradient for both ions.
- The pumped ions leak back across the membrane by diffusion. The diffusion rates are slow but the membrane is about 50 times more permeable to potassium than to sodium. Therefore, leakage of potassium ions is faster. This increase with the difference between the potassium and sodium concentration gradient in increasing the overall charge imbalance across the membrane.
- There are negatively charge proteins inside the nerve fibre which also contribute to the charge and balance.
Nerve impulses as action potentials that are propagated along nerve fibres
Electrodes can be used to monitor the membrane potential at one position along a nerve fibre. The potential can be displayed on a screen with time on the X axis and voltage on the Y. A horizontal line at about -70 mV represents the resting potential. A sudden spike represents an action potential. This is an all or nothing sequence of changes in membrane potential with two main phases:
- Depolarisation meaning a change in membrane potential from negative positive
- repolarisation , which is a change back from positive to negative
Both deflation and repolarization are due to the movement of positively charged ions across the membrane not to movement of electrons.
Action potentials are propagated along nerve fibres because the ion movements that depolarise one part of the fibre trigger depolarisation in the neighbouring part of the fibre. This is how neural signals pass along nerve fibres. A nerve impulse is an action potential that starts at one and a neuron and is propagated along the axon to the other end of a neuron.
In humans and other vertebrates, nerve impulses always move in one direction along neurons. This is because an impulse can only be initiated at one terminal of neuron and can only be passed on to other neurons at different cell types at the other terminal. Also, there is a refractory period after a depolarisation that prevents propagation of an action potential backwards along an axon.
Depolarisation
Depolarisation is due to the opening of sodium channels in the membrane allowing for sodium ions to diffuse into the neuron down the concentration gradient. The concentration of sodium ions outside is about 10 times as high as that inside the entry of sodium ions reverses the charge imbalance across the membrane so the inside is positive relative to the outside. This raises the membrane potential, typically from about -70 to a positive value of about 30.
Repolarization
Repolarization happens rapidly after depolarisation and is due to the closing of the sodium channels and opening of potassium channels in the membrane. Potassium ions diffuse out of the neuron down their concentration gradient and no more sodium ions diffuse in. As a result, the inside of the neuron becomes negative again relative to the outside. The potassium channels remain open until the membrane potential has fallen close to -70.
The diffusion of potassium repolarises in neuron. But it does not fully restore the resting potential as a concentration gradient of sodium and potassium ions have not yet been re-established. This takes a few milliseconds of actively pumping sodium ions out and potassium ions in before there can be another action potential.
Variation in the speed of nerve impulses
Nerve fibres are circular and cross-section, with a plasma membrane enclosing cytoplasm. In humans the diameter is typically about one micro meter, although some nerve fibres are wider. Nerve impulses are conducted along nerve fibres at a speed of about one meter per second.
Some animals have nerve fibres with larger diameter. An increase in diameter reduces resistance, so impulses are transmitted along wider fibres more quickly. For example a giant axon in squids have a diameter up to 500 micro meters and conduct impulses at 25 meters per second. These axons are used to coordinate a rapid jet proportion escape response when a squid is in danger. Animals do not have the space or resources for many giant axons so they can only use them to coordinate actions where speed is vital for example earthworms have just three giant axons that they use for an escape response for predator attack.
Myelination is another modification of nerve fibres that increases the speed of nerve impulses. This is a coding of nerve fibres that consists of a series of Schwan cells, with gaps between called nodes of Ranvier. In myelinated nerve fibres the nerve impulse can jump from one note of ran beer to the next, speeding up transmission along the nerve fibre as much too as 100 meters per second.
Synapses
A synapse is a junction between two cells and the nervous system. There are three main types of junction:
- Synapses between sensory receptor cells and neurons in sense organs.
- Synapses between neurons in both the brain and spinal cord
- Synapses between neurons and muscle fibres or gland cells. Muscles and glands are called effectors, because they effect a response to a stimulus.
Signals can only pass in One Direction across a synapse. The pre-synaptic neuron brings the signal to the synapse in the form of a nerve impulse or action potential. The postsynaptic neuron carries the signal away from the synapse again in the form of a nerve impulse. Chemicals called neurotransmitters carry signals across a narrow fluid filled gap between the synaptic and postsynaptic neurons this gap is only about 20 nm wide.
Release of neurotransmitters from a presynaptic membrane
Synaptic transmission occurs very rapidly as a result of these events:
- A nerve impulse is propagated along the presynaptic neuron until it reaches the end of the neuron and the presynaptic membrane
- Depolarisation of the presynaptic membrane causes calcium ions to diffuse through channels in the membrane into the neuron
- Influx of calcium ions causes vesicles containing neurotransmitters to move to the pre-synaptic membrane and fuse with it.
-The neurotransmitter is released into the synaptic gap by exocytosis
Excitatory postsynaptic potential
Release of neurotransmitters from a pre-synaptic neuron leads to a series of events that trigger an action potential in the post synaptic neuron.
- Neurotransmitter molecules diffuse across the synaptic gap. This happens extremely rapidly because the distance is so short 20 to 40 nm. The gap between the membranes is only 2 to 4 times the thickness of a typical phospholipid by layer.
- The neurotransmitter binds to the receptors in the postsynaptic membrane causing ion channels to open. Some receptors have an ion channel as a part of their structure while others cause an ion channel to open in a separate membrane protein.
- Ions diffuse down their concentration gradient into the postsynaptic neuron causing the membrane potential to change. In most cases, the potential rises becoming less negative and this is called excitatory postsynaptic potential.
- if the excitatory postsynaptic potential is strong enough, it triggers an action potential which propagates away from the synapse
- The neurotransmitter is rapidly broken down and removed from the synaptic gap
Acetylcholine
Many different neurotransmitters are used at synapses, with different effects. For example acetylcholine is used as a neurotransmitter in many synapses, including neuromuscular junctions. In the pre-synaptic neuron choline is combine with an acetal group produced by aerobic respiration. This produces acetylcholine, which is loaded into vesicles and then released into the synaptic gap during synaptic transmission.
When acetylcholine binds to its receptor in the postsynaptic membrane a channel opens in the receptor. Sodium ions diffuse through this channel and into the postsynaptic membrane causing an excitatory postsynaptic potential.
The acetylcholine only remains bound to the receptor for short time and only one action potential is initiated in the person in tic neuron. This is because the enzyme acetylcholinesterase is present in the synaptic gap and rapidly breaks acetylcholine down into choline and acetate. The choline is reabsorbed into the presynaptic neuron where it is converted back into acetylcholine by recombining with an acetyl group.
Depolarisation and reparation during action potentials detailed
Opening of the sodium and potassium channels that cause depolarisation and repolarization is triggered by changes in the transmembrane voltage. This is called voltage gating. If the resting potential of -70 increases to -50 sodium channels in the membrane start to open. This allows sodium ions to diffuse into the axon, further reducing the membrane potential and causing more sodium channels to open. This is an example of positive feedback and causes the very rapid change in membrane potential from -50 to positive 30.
The voltage that causes sodium channels to open is called the threshold potential. Depolarisation will not occur unless the threshold potential is reached, instead the sodium potassium pump will re-establish the resting potential of -70. A nerve impulse is all or nothing because the threshold potential must be reached.
Sodium channels remain open for a very short time one to 2 ms before they close again. They’re opening allows a pulse of sodium ions diffuse out. The resulting depolarisation causes voltage gated potassium channels to open. These channels also remain open for one to 2 ms before closing. Even in this short time, enough potassium ions diffuse out of the axon to repolarise the axon. The membrane potential returns to -70, it may briefly become more negative than this before the sodium potassium pump re-establishes concentration gradient.
Local currents
The propagation of an action potential along an axon is due to movements of sodium ions. Depolarisation of part of the axon is due to diffusion of sodium ions into the axon through sodium channels. This reduces the concentration of sodium ions outside the axon and increases it inside. The depolarised part of the axon therefore has different sodium ion concentrations to the neighbouring part of the axon that has not yet depolarised. As a result, sodium ions diffuse between these regions both inside and outside the axon.
Inside the axon there is a higher sodium ion concentration in the depolarise part of the axon. As a result sodium ions diffuse along the axon to the neighbouring part that is still polar. Outside the axon, the concentration gradient is in the opposite direction of sodium ions diffuse from the polarise part back to the part that was just depolarise. These movements are called local currents.
Local currents reduce the concentration gradient in the part of the neuron that has not yet depolarised. This makes the membrane potential rise from the resting potential of -70 to about -50. Sodium channels in the cell membrane our voltage gated and open when a membrane potential of -50 is reached. This is the threshold potential. Opening of the sodium channels causes deflation plus local currents cause a wave of depolarisation and then repolarization to be propagated along the axon at a rate between one and 100 or more meters per second
Oscilloscopes
Membrane potentials in neurons can be measured by placing electrodes on each side of the membrane. The potentials can be displayed using an oscilloscope. The display is similar to a graph with time on the X axis and the membrane potential on the Y axis. If there is a resting potential a horizontal line appears at the oscilloscope screen at the level of 70. If an action potential occurs the narrow spike is seen the rising and falling phases of the spike showed the depolarisation and repolarization.
Saltatory conduction
Some nerve fibres are myelinated. Myelin is multiple layers of phospholipid membrane that are deposited around the nerve fibre, as one sells grow around it. Each time they grow around the nerve fibre a double layer of phospholipid bilayer is deposited. There may be 20 or more layers when the one cell stops growing.
The myelin sheath prevents ion movements, so action potentials only occur at nodes of Ranvier. Sodium potassium pumps and both sodium and potassium channels are clustered at Notes, with very few where the axon is coded in myelin local currents allow the nerve impulses to jump from one note of Ranveer to the next. This is called saltatory conduction and give speeds of transmission of the nerve impulse a boost of as high as 100 meters per second.
Exogenous chemicals
An exogenous chemical is one that enters the body of an organism from an outside source. These chemicals can enter through the skin the lungs or the gut, or they can be injected. Some exogenous chemicals affect synaptic transmission, either by blocking it or by promoting it.
Neonicotinoids as exogenous chemicals
Neonicotinoids are synthetic compounds similar to nicotine. They bind to the acetylcholine receptor in cholinergic synapses in the central nervous system of insects. Acetylcholinesterase does not break down Neonicotinoids , so the binding is irreversible. Because the receptors are blocked, acetylene is unable to bind and synaptic transmission is prevented. This leads to paralysis and death of insects. Neonicotinoids are therefore very effective insecticides.
One of the advantages of Neonicotinoids is that they are not highly toxic to humans and other mammals. This is because insects have a much greater proportion of cholinergic synapses in their central nervous system, compared with mammals. Neonicotinoids also bind much less strongly to acetylcholine receptors in mammals compared to insects.
Cocaine as an exogenous chemical
Cocaine acts at synapses that use dopamine as a neurotransmitter. It binds to dopamine uptake transporters which are membrane proteins that pumped dopamine back into the pre-synaptic neuron. Because cocaine blocks these transporters dopamine builds up in the sign up the gap and the postsynaptic neuron is continuously excited. Cocaine is therefore an excitatory or stimulant psychoactive drug that gives feelings of euphoria that are not related to any particular reward activity.
Inhibitory neurotransmitters
Not all neurotransmitter stimulate action potentials in the postsynaptic neuron when some neurotransmitters respond to the postsynaptic membrane the membrane potential becomes more negative. This hyperpolarisation makes it more difficult for the postsynaptic neuron to reach the threshold potential. Nerve impulses are inhibited, rather than stimulated.
GABA also called aminobutyric acid is an inhibitory neurotransmitter. When it binds to its receptor a chloride channels, causing hyper portal of the postsynaptic neuron at the entry of chloride ions. In contrast acetylcholine is an excitatory neurotransmitter because it causes entry of positively charged ions to the postsynaptic neuron reducing polarisation.
Transient changes to the membrane potential caused by neurotransmitters such as GABA and acetylcholine are known as inhibitory and excitatory postsynaptic potentials.
Effects of inhibitory and excitatory neurotransmitters in a postsynaptic neuron
More than one pre-synaptic neuron can form a synapse with the same postsynaptic neuron especially in the brain where there may be hundreds or even thousands of presynaptic neurons. Usually a single release of excitatory neurotransmitter from one presynaptic neuron is insufficient to trigger an action potential, because one excitatory postsynaptic potential does not reach the threshold potential. Either one presynaptic neuron must repeatedly release neurotransmitters or several adjacent pre-synaptic neurons must release neurotransmitters more or less simultaneously. When multiple releases of excitatory neurotransmitters combined to cause an action potential this is called summation.
Summation can also combine the effect of inhibitory and excitatory neurotransmitters. Whether or not an action potential is initiated in the postsynaptic neuron depends on the balance between the effect of these two types of neurotransmitter. Inhibitory neurotransmitters counter the effects of excitatory neurotransmitters so the threshold potential is not reached. The threshold potential will only be reached if there are many more excitatory neurotransmitters than inhibitory neurotransmitters.
The synapses integrate signals from any different sources and this is the basis of decision-making processes in the central nervous system.
Perception of pain
Pain receptors in the skin and other parts of the body to text stimuli such as the chemical substances in a boosting excessive heat or the puncturing of skin by hypodermic needle. These receptors are the endings of sensory neurons that convey impulses to the central nervous system. The nerve endings associated with pain receptors have channels for positively charged ions. These channels open and response to stimuli such as high temperature, acid or certain chemicals. Entry of positively charged ions causes the threshold potential to be reached and nerve impulses then pass through the sensory neuron to the spinal column. Inter neurons in the spinal cord relay the impulse to the cerebral cortex.
When impulses reached sensory areas of the cerebral cortex we experienced the sensation of pain. Signals are transmitted to the prefrontal cortex allowing us to become fully aware of the pain and evaluate situation. This will often result in a signal from the brain to the effectors of Behaviour, reducing the exposure to the stimulus.
Consciousness as a property
If we are conscious of something we are aware of it, we do not have to be actively thinking about something to be aware of it, so we can be simultaneously aware of many things. This state of complex awareness is known as consciousness. There is an agreement that it exists, but philosophers and scientists have not yet agreed on how to define it.
Sleep is a state of reduced or partial consciousness. General anaesthetics use it during surgery to make us unconscious. However, scientist do not fully understand how these drugs work so they do not reveal much about the physiological basis of consciousness. The most we can say with certainty is that consciousness emerges from the interaction of individual neurons in the brain and it is an example of an emergent property.
An emergent property is caused by the interactions between the element of a system. It is not a property of one component rather it is a property of the system as a whole when we recognise that a system is more than the sum of its parts we are acknowledging the existence of emergent properties. Two biological examples are the catalytic activity of enzymes and flight in birds.
