Nerve Impulses.

1. The cytoplasm inside the axon, known as the axoplasm, has a high concentration of potassium ions (K⁺) and a low concentration of sodium ions (Na⁺).
2. In contrast, the fluid outside the axon has a low concentration of K⁺ and a high concentration of Na⁺.
3. A potential difference (charge) exists across the cell surface membrane of all cells. This potential is typically negative inside the cell relative to the outside.
4. The potential difference across the membrane when the neuron is at rest is called the resting potential, which is approximately -70 mV. The negative sign indicates that the inside of the cell is more negative than the outside.

Key characteristics of the resting neurone

1. Selective Permeability:
   1. The axon membrane is relatively impermeable to sodium ions (Na⁺).
   2. It is freely permeable to potassium ions (K⁺).
2. Sodium-Potassium Pump:
   1. The axon membrane contains an active sodium-potassium pump that uses ATP to transport ions.
   2. It moves sodium ions (Na⁺) out of the axon and potassium ions (K⁺) into the axon.
   3. This pump reduces the concentration of sodium ions inside the axon. Since Na⁺ is pumped out and cannot diffuse back in, it maintains a low internal Na⁺ concentration.
   4. At the same time, K⁺ ions are pumped in but can diffuse out along their concentration gradient.
3. Polarization:
   1. As a result of these ion movements, the inside of the cell becomes slightly negatively charged relative to the outside.
   2. This state is known as polarization, and the potential difference of -70 mV across the membrane is termed the resting potential.

When a nerve impulse travels along an axon, the permeability of the axon membrane to sodium ions (Na⁺) changes. This change is triggered by a stimulus.

Sequence of events in an active neurone

1. Stimulation and Sodium Ion Entry:
   1. When the neuron is stimulated, specific sodium channels (or sodium gates) in the axon membrane open.
   2. These channels allow sodium ions (Na⁺), which are rich in both concentration and electrochemical potential, to rush into the axon.
   3. As a result, the potential difference across the membrane is reversed, making the inside of the cell positive compared to the outside.
2. Depolarization:
   1. This change in charge distribution is known as depolarization.
   2. Depolarization lasts for approximately 1 millisecond (ms).
   3. During this brief period, the potential difference reaches around +40 mV, a state known as the action potential.
3. Repolarization:
   1. After depolarization, the sodium channels close, preventing further entry of Na⁺.
   2. The sodium-potassium pump becomes active, rapidly pumping excess sodium ions (Na⁺) out of the axon.
   3. At the same time, the membrane temporarily becomes more permeable to potassium ions (K⁺).
   4. Potassium ions (K⁺) then move out of the axon along their electrochemical gradient, helping restore the negative charge inside.
4. Refractory Period:
   1. It takes around 3 milliseconds (ms) for the neuron to restore its resting potential (-70 mV) and become ready to transmit another impulse.
   2. This period is known as the refractory period.
   3. During this time, the section of the nerve fiber that just experienced the impulse cannot conduct another impulse, ensuring that the nerve impulse travels in one direction only.

The refractory period is the time during which an axon is unable to respond to a new stimulus. This period is part of the recovery phase following an action potential.

Types of refractory period

1. Absolute Refractory Period:
   • This is the initial phase of the refractory period.
   • During this time, the neuron cannot be restimulated, no matter how strong the stimulus.
   • This is because the sodium channels are completely blocked, and the resting potential has not yet been restored.
   • It lasts for approximately 1 millisecond (ms).
2. Relative Refractory Period:
   • Follows the absolute refractory period.
   • The neuron can be stimulated, but it requires a stronger stimulus than normal.
   • This is because the membrane is still recovering, and the threshold for triggering an action potential is temporarily higher.
   • It lasts for several milliseconds.

Importance of the refractory period

• Controls Impulse Frequency: Limits the number of impulses that can travel along a nerve fiber to about 500–1000 per second.
• Ensures One-Way Impulse Transmission: Prevents impulses from moving backward along the nerve fiber, maintaining a one-way flow of information.
• Supports Distinction Between Motor and Sensory Systems: Prevents internal confusion by ensuring that sensory and motor impulses remain separate.

An action potential is a rapid, temporary change in the electrical charge across the axon membrane, caused by the movement of ions.

How action potential occurs

1. Sodium Channels Open: In response to a stimulus, sodium channels in the axon membrane open, allowing sodium ions (Na⁺) to enter rapidly.
2. Depolarization: The influx of Na⁺ causes the inside of the membrane to become positive, creating a +40 mV charge (action potential).
3. Restoring Resting Potential:
   • The sodium channels close.
   • The sodium-potassium pump actively removes excess Na⁺.
   • Potassium ions (K⁺) move out along an electrochemical gradient.
4. This process returns the membrane to its negative resting potential (-70 mV).

The threshold of a neuron is the minimum stimulus required to open enough sodium channels for an action potential to occur.

Key points about threshold

1. If the stimulus is strong enough to reach the threshold, an action potential will occur.
2. If the stimulus is below the threshold, no action potential is generated.
3. This is known as an "all-or-nothing response."
4. The size of the action potential is always the same, regardless of the strength of the stimulus, as long as the threshold is reached.

An action potential does not remain confined to a single point on the axon. Instead, it travels along the entire length of the nerve fiber.

How impulses are propagated

1. When an action potential is generated at one point, it causes local ion currents.
2. These local currents cause depolarization of adjacent areas of the membrane.
3. As a result, the action potential is continually generated along the axon in one direction.
4. The refractory period prevents the impulse from moving backward.

In myelinated vertebrate nerves, the mechanism of propagation is strongly more complex. Ions can only pass freely into and out of the axon at the nodes of ranvier which are about 1mm apart.

This means that action potential can only occur at the nodes of ranvier and so they appear to jump from one node to the next as the diagram shows. The effects of this is to speed up transmission as the concentration movements associated with the action potential occurs much less frequently taking less time. This condition is known as saltatory condition from the Latin verb which means to jump.

The nerves are basic units of the nervous system adopted for the rapid passage of electrical impulses to inter communicates. Receptors must pass their information into the sensory nerves, which in turn must relay the information to the central nervous must be communicated to the effectors organ so that action can be taken.

Whenever two nerve cells meet, they are linked by synapse as shown in the figure below.

Every cell in the central nervous system is covered with synaptic knob from other cells several hundred some cases.

Neurons never actually touch their target cells so a synapse is a gap between two nerve cells with the nerve message must be somehow crossing. The electrical nature of the nerve impulse as detected long before to its could be accurately recorded and measured similarly it was suspected that transmission at the synapses was not electrical but chemical long before the electrons microscope and other technique could demonstrate this clearly.

Once the structure at the synapse had been seen using the electron microscope, the synapse gap would be measured. This settled the argument.

The arrival of an impulse at the synaptic knob increases the permeability at the synaptic membrane to calcium ions.

Calcium ions therefore move into the synaptic knob along concentration gradients the effect of these calcium ions is to cause the synaptic vesicles containing transmitters' substance to move to the pre-synaptic membrane. Each vesicle fuses with the membrane and release the transmitters.

Some of the vesicles fuse with the membrane and release the transmitters. Some of the vesicles fuse with the membrane and release the transmitter substance into the synaptic cleft.

The transmitters diffuse across the gap and become attached to the specific protein receptor sites on the post synaptic membrane.

As a result, ions channels are opened and there is usually a local depolarization and influxmof sodium ions, causing an excitatory post synaptic potential (EPSP) to be set up. If there are sufficient of the potential the positive charge in the postsynaptic cell build up to the threshold level and an action potential in set up this then travels on a long the post synaptic neuron.

Once the transmitter has its effects it is destroyed by enzymes. This is very important because unless the transmitter is removed from the synaptic cleft subsequent impulses would have as effect, as the receptors on the post synaptic membrane would the entire bound.

The most common transmitter substance found at the majority of synaptic is acetylcholine (Ach). It is synthesized in the synaptic knob using ATP produced in many mitochondria present.

Nerves using Acetylcholine has done its job it is very rapidly, hydrolyzed by the enzymes cholinesterase.

This ensures that it no longer affects the post synaptic membrane, and it also releases the components to be recycled they pass back into the synaptic knob and are resynthesized into acetylcholine Some vertebrates' nerves particularly those of the sympathetic nervous system produce noradrenaline in their synaptic vesicles and are known as adreneigic nerves.

Nerves have to communicate not only with each other but with receptors and effectors as well. Motor nerves need to communicate with muscles. Where a motor nerve and muscles fibre meet a special kind of synapse is formed known as a neuromuscular junction.

The membrane of the muscle fibre is very folded in this region and forms a structure known as an endplate to which the end of the motor nerve joins. Electron microscope shows us that the structure of the neuro muscular junction is remarkably similar to that of any other synapse as the figure below shows. The end of the motor neurons is full of mitochondria and synaptic vesicles which contain acetylcholine. It appears that when an impulse arrives at the end of the motor neurone acetylcholine is discharged into the synaptic cleft.

As a result of its effect on the postsynaptic membrane an end potential is set up which can be recorded, If sufficient end plate potentials are set up on action potential is fired off on the muscle fibre spreading through the tubules and leading to a contraction of the muscle.

Neurons interact in a variety of complex ways. Sometimes single nerve fibre will carry an action potential to a synapse with another cell and transmission. But in many cases the situation is much more complex than this. Often a single synaptic knob does not release enough transmitter substance to set up an action potential in the post synaptic fibre however, if two or more synaptic knobs are stimulated and release transmitters at sometimes onto the same post synaptic membrane the effects add together and a post synaptic action potential results. This is known as spatial summation; as illustrated below. In other cases, a single knob does not release enough transmitter substance to stimulate the post synaptic nerve fibre, but if a second impulse is received from the same knob in quick succession an action potential results. This effect is known as temporal summation (i.e. adding over time). It involves facilitation in other words, the first impulse does not trigger off a response but it has an effect which make easier (facilitate) the passage of the next impulse. The arrival of the impulse at one synaptic knob triggers an action potential in the post synaptic fibre.

Spatial summation

Action potential needs to arrive at several synapses at once to release the amount of neurotransmitter required to trigger as action potential in the postsynaptic fibre.

Temporal summation

One action potential arises and although it does not release sufficient transmitter substance itself to set up another potential it makes it easier for the next impulses which arises to do so.

On first applying perfume or after shave we tend to be very aware of the smell ourselves. After a short time we lose that awareness and it is other people who notice how pleasant we smell. If we apply our scent another day, we can smell it again. This reaction is the same as that of a sea anemone which when poked with a pointer, will withdraw its tentacles. If the sea anemone is pocked repeatedly the response is lost. If left alone for a while the sea anemone reacts to the results of process known as accommodation.

If a nerve is repeatedly, it eventually loses ability to respond. Each time on impulse arrives at a synapse, vesicles, full of transmitter discharge their contents into the synaptic cleft. The transmitter can only be synthesized at a certain rate if the synapse is used too often all of the vesicle are discharged into the synaptic cleft and the rate of synthesis simply cannot keep up. At this point the nerves can no longer respond to the stimulus, they are said to have accommodated or fatigued. A short rest restores the response as new vesicles and transmitter molecules are made. Some synapses nerve fatigue they have no extremely rapid synthesis is rate whilst others accommodate very quickly.

The nerve fibre and synapses which have been considering in isolation make up enormously complicated systems. Bundles of nerve fibres from nerves capable of carrying vast number of messages in different directions, together all the available information and control all the actions of the body.

Nervous and synapses in the central nervous system collect information and sends out instructions, synapses susceptible to both fatigue and drugs, allow for great flexibility, intercommunication between cells, facilitation and inhibition. They also play a vital role incompletely understood in the brain, closely linked with both learning and memory.

Nerves give rapid communication; they also give the ability to people at least for long and involved nervous activity to take place in the brain before a particular action to is undertaken. But for simpler organisms most nervous activity and behavior involves reflex action which have a minimum of input from the central system. Even human beings are ruled by reflexes to a remarkable extent.

Definition; synapse is a region where the branches of an axon are in contact with the dendrites of another neurone.

Types of synapses

Chemical synapses

These are synapses that neurons communicate with each other by means of neurotransmitters.

The neurons are not in direct contact with each other, they join at a synapse which have synaptic cleft, a small gap of about 20nm.

The neurotransmitter is released from one membrane is the synapse the pre-synaptic membrane then it diffuses across the synaptic cleft and binds into receptors on the post synaptic membrane. The post synaptic cell may be in an effectors organ such as muscle or gland.

Types of chemical synapses

The chemical synapses are of two kinds; excitatory synapses and inhibitory synapse.

Excitatory synapse

These are synapses where the pre synaptic neurone releases neurotransmitter that makes the post synaptic membrane more excitable and more likely to generate nerve impulses.

The diagram above shows various events takes place in a chemical synapse which uses acetylcholine as a neurotransmitter.

Acetylcholine is synthesized within the pre synaptic knob and stored in special organelles called synaptic vesicles (some neurotransmitters neither decrease nor increase the tendency of a postsynaptic cell to fire an action potential instead they work like hormones; e.g. noradrenaline released by axons of the sympathetic nervous system has an effect to on cells similar to that of the hormone adrenaline. When an action potential reaches the presynaptic membrane, it depolarizes the membrane, that is; it makes the membrane less negative than at rest. This depolarization triggers the opening of calcium ions channels in the pre synaptic membrane.

The calcium ions diffuse into the presynaptic knob; causing the pre synaptic vesicles containing acetylcholine to migrate and fuse with the presynaptic membrane.

The acetylcholine is released into the synaptic cleft and diffuses across the synapse. The synapse then it binds to specific protein receptor molecules on the post synaptic membrane a process known as receptor activities.

Receptor activation causes sodium ion channels to open making the membrane more permeable and produce a graded potential, if enough acetylcholine is released the graded potential may become large enough an action central.

If an excitatory synapse receives a continuous stream of action potential at high frequency eventually transmission across synapse stops.

This is because the neurotransmitter cannot be resynthesised fast enough and it runs out. The synapse becomes fatigued (i.e. adapted).

Acetylcholine is then broken down by acetylcholinesterace to its constituents groups acetic acid (or ethanoic acid) and choline groups; which diffuses back into the pre synaptic membrane and used to resynthesize the neurotransmitter under the influence of ATP from the mitochondria concentrated here and refilled in vesicles for future transmission.

Inhibitory synapse

These release neurotransmitter that makes the postsynaptic membrane less excitable and less likely to transmit an impulse.

E.g. In mammals, they occur in nerve pathways which central rapid eye movements, they are also common in the heart.

Diagram above: An electrical synapse in which the nerve impulse is transmitted through the protein pores that line the cytoplasm of the two cells

1. Transmission of impulses from one neuron to another.
2. To ensure the undirectional flow of impulses by the following mechanisms.
   • The neurotransmitters are only released in the pre-synaptic neuron.
   • The receptor molecule for the neurotransmitters are only located on the post synaptic membrane.
   • Enzymes for degrading neurotransmitters are found in the post synaptic knob.
   • The mitochondria and energy production to resynthesize the ACL are found at the pre- synaptic knob.
3. They act as a junction i.e. they allow spatial summation. This means that the impulses passing along the different neurone between them release a neurotransmitter substance sufficient to generate an action potential where as individually they would not i.e. facilitation.
4. Filter out low level stimulus i.e. They block the passage of stimuli that are enabled to release a sufficient neurotransmitter for propagating a new impulse in the post synaptic neurone.
5. To allow accommodation to intense stimulus i.e. is case where the rate of release of neurotransmitter substances exceeds the rate of its formation the synapse become fatigued. No further neurotransmitter substance is released and no further impulse is transmitted