2011 Group 1 Project

From CellBiology
Neurons

Synaptic Junction

Synaptic junctions form the connection point between two neurons. Neurons are a specialized cell which form the network of nerves, spinal cord and brain. This nervous system controls the actions of the body while also transmitting information to different parts of it. The brain and spinal cord make up the central nervous system (CNS) while nerves and ganglia which span out from the spinal cord to limbs and organs make up the Peripheral Nervous System (PNS). Information including sensory stimuli, motor control and involuntary activity are all communicated throughout the body by the nervous system.

Electrical signals that carry information throughout the body are transmitted through the nervous system by the communication of neurons. The synaptic junction is where neurons transmit this signal from one to one another, either electrically or chemically. Without synaptic junctions, information and signals cannot be transmitted. These characteristics will be addressed later, as this page explores the nature the synaptic junction.

History

Charles Sherrington, John Eccles, and the Modern Study of Synaptic Transmission


1897- Term synapse introduced at turn of the century by Charles Sherrington to describe the zone of contact between neurons[1].

1900- Notion of a receptor introduced by German bacteriologist Paul Ehrlich, he wrote “Chemical Substances are only able to exercise an action on the tissue elements with which they are able to establish a chemical relationship”.

1906- English pharmacologist John Langley postulated sensitivity of skeletal muscle to curare and nicotine was caused by a “receptive molecule.”

1920s- Otto Lowei shows that ACh, a chemical compound, conveys signals from the vagus nerve to the heart.

1930s- This discovery led to a debate over how chemical signals could generate electrical activity at other synapses. One side argued that all synaptic transmission is electrical; the action potential in the pre synaptic neuron generates the potential that flows passively into the post synaptic cell. The other that transmission is chemical, initiating the current flow in the post synaptic neuron[2].

1939- Discovery that action potential in the presynaptic fibre does not lead to direct initiation of action potential in the post synaptic fibre.

1950s John Eccles and colleagues experiment on synaptic mechanisms if the spinal motor neuron, giving insight into synapses in the CNS mediated by ionotropic receptors.

1958- Furshpan and Potter first demonstrated electrical synaptic transmission to occur at the giant motor synapse of crayfish. First example of a real electrical synapse.

1963- Krnjevic and Phyllis release a paper titled “Iontophoretic studies of neurons in the cerebral cortex.” This is the first demonstration of glutamate and GABA as neurotransmitters in the CNS.

1970s- freeze-fracture techniques applied by Thomas Reese and John Hueser, discovering properties of transmitter release.

1976- First successful recordings of single ACh gated channels by Erwin Neher and Bert Sakmann showing that channels open and close in a step like manner, generating very small rectangular steps of ionic current.

1981- Watkins provides evidence for three different types of ionotropic glutamate receptors.

1999- Studies by Galarreta and Hestrin indicate electrical coupling is a fundamental feature of local inhibitory circuits and suggest that electrical synapses define functionally diverse networks of GABA-releasing interneurons[3]. .

2005- Gillispie, Kim and Kandler discover inhibitory synapses in the developing auditory system are glutametergic[4].

2006- Chávez, Singer and Diamond have a study published entitled "Fast neurotransmitter release triggered by Ca influx through AMPA-type glutamate receptors". The study reveals a previously unknown form of feedback inhibition within a neural circuit[5].

What is a Synaptic Junction

Neuron Cell Synapse junction
Em Synapse
By definition, synapse is a site between to nerve cells, and junction means; to join. Thus the term synaptic junction describes the region of contact between two neurons where impulses is transmitted from one to another. This communication normally occurs between pre-synaptic side ( axon terminal of the transmitting neuron ) and post-synaptic side (receiving neuron, muscle or gland). The nerve impulses are conveyed through the action of what is called neurotransmitters including; Acetylcholine (ACh), Norepinephrine (NE), Dopamine (DA), Gamma-aminobutyric acid (GABA). These neurotransmitters are stored and carried by membrane bound organelles called synaptic vesicles. The number of the vesicles varies and depends on whether the synapse is active or not. Moreover, the size is actually dependent upon the neurotransmitter type; fast-acting with 50nm diameter with clear centers, whereas, slow-acting with more than 100nm in diameter with dense core under the electron microscope. [6]

[7] [8]

The components of a synapse are demonstrated through the schematic sketch. These include;

1) Synaptic bulb axon terminal:

2) Synaptic Vesicle:

3) Neurotransmitter molecules:

4) Synaptic cleft:

5) Post-synaptic membrane:

6) Receptor site:

7) Neurotransmitter binding site:

Types of Synaptic Junctions

Cell to cell communication occurs through a variety of synapses. The most significant synapses are classified under either electrical or chemical synapses.

1)Electrical Synapses

Electrical Synapses

Electrical synapses; or what is known as bridging junctions are defined as electrical and mechanical conductive links that correspond to the narrow gap (gap junction) between the pre and post synaptic neurons of soma (neuron cell bodies). Protein channels are found to be connecting both pre and post synaptic cytoplasmic membranes. As a result, a low resistant electrical pathway is generated between the adjacent neurons, permitting a current flow passively from one neuron to the next. Neurons that are joined this way are referred to as electrically coupled. Moreover, electrical synapses conduct impulses very rapidly due to the short distance gap(3.5nm). Therefore, they are found in regions that depend on fast responses such as reflexes from the brain (jerky movements of the eye). Another key feature of these synapses is the synchronizing activity between the connected neurons. Finally, electrical synapses are usually bidirectional; allows transmission of ions in either direction. Nonetheless, some of these synapses can be unidirectional. [9] [10] [11]

2)Chemical Synapses

Chemical Synapse

On the contrary, Chemical synapses are known as specialized junctions by which chemical neurotransmitter are released and received. A single chemical synapse has two significant parts; axon terminal and receptive region. The key feature in the first part is membrane bound organelles, referred to as synaptic vesicles. These vesicles store and carry the chemical neurotransmitter molecules. Next, the molecules are secreted by exocytosis from the pre-synaptic part through the synaptic cleft , which is fluid filled region. The neurotransmitters bind to the receptors located on the pos-tsynaptic membrane. Consequently, post-synaptic channels are opened or closed, resulting in a change in the ions ability to flow in or out of the post-synaptic region. The flow of ions in chemical synapses is merely unidirectional. Furthermore, due to the distance difference between the pre and post-synaptic membranes (20-40nm), transmission of impulses is slower than electrical synapses. Finally, the transmission of signals across these synapses requires the secretion of neurotransmitters and binding to the receptors to generate a synaptic current and membrane potential. [12] [13]


It should be noted that there are other synapses. These include;

Axo-axonic synapse: connects between axon terminal and another.

Axodendritic synapse: connects between axon terminal and dendrites

Axosomatic synapse: connects between axon terminal and cell body

Dendrodendritic synapse: connects between dendrite and another dendrite

Summary of electrical and chemical synapses:

Types of synapses Distance between pre and postsynptic cell membrane intercellular continuity between pre and postsynaptic cells Key features agents of transmission Direction of transmission Location
Electrical 3.5nm Yes Gap junction channels Ion current mostly bidirectional Fast responses regions(brain), i.e. reflexes.
Chemical 20-40nm No Pre-synaptic vesicles and; synaptic cleft Chemical transmitter(neurotransmitter) Unidirectional. Muscles, glands, neurons.



Synaptic Integration and Modulation

To understand the synaptic junction’s role in neural modulation and integration, it is important to understand why these functions exist. As stated previously, the nervous system contains a network of many interlinking neurons, allowing communication between the brain and spinal cord and the rest of the body[14]. However, the human body does not need all nerve cells to be innervated at one time. It must coordinate the firing of all the nerves so that people can function normally. Included in this is the actions and processing performed subconsciously. For example, we do not focus on contracting our hamstrings then allowing them to be extended while we are walking. Instead, our nervous system allows these movements to occur subconsciously. This coordination occurs through integration and modulation. Modulation occurs at the synaptic junction, where pre synaptic inhibition allows for greater control[15].


Steps of Transmission in Neural Networks[16]
[17]


The interlinking pathways between neurons form converging and diverging organisations. Convergence describes many neurons synapsing onto a single post synaptic neuron, whereas divergence is when a neuron itself synapses onto multiple neurons. Almost all neurons receive convergent input, which allows for flexible control[18].

Convergence and Divergence

The Action Potential and Summation

In chemical synaptic junctions, the action potential arriving at the pre synaptic terminal is responsible for the release of neurotransmitters into the synaptic cleft, allowing excitation to continue to the post synaptic cell[19] .

Briefly, the action potential is a rapid depolarization that travels down the axon of a neuron. This is due to the actions of several channel types which are voltage dependent, allowing the depolaristion to propagate down an axon. It is an all-or-nothing event, where a certain threshold must be reached before the action potential is produced and fired from the axon hillock. The action potential is always the same size once activated, and also travels down collateral branches of the axons, reaching the pre synaptic terminal of many synaptic junctions[20]. This organising allows for greater control of signalling, as each neuron receives many inputs from numerous dendrites, called graded potentials. The summation of these graded potentials into the cell must add up to a set threshold potential, or greater, to induce an action potential. This table gives the differences between the graded potential and action potential:


Graded Potential Action Potential
Travel a short distance long distance, fast, regenerative
Mostly chemically gated ion channels Require voltage gated ion channels
Strength varies with stimulus Same strength-frequency increases

[21]


Graded potentials can vary in size because they depend on the amount of neurotransmitter released. This is achieved through the regulation of free calcium inside the cell[22]. Axoaxonic synapses control calcium influx into the pre-synaptic terminal of . If more neurotransmitters bind to receptors, the amplitude of the depolarization increases. Summation to threshold can occur in two ways, or a combination of these two ways;

Spatial summation: a graded potential can travel to a neuron from several synapses. Spatial summation is the addition of simulation graded potentials arriving from different synapses.

Temporal summation: a second graded potential from a single synapse can occur before a previous one is completed. This means that the second graded potential is added to the potential at which the membrane is in that point in time, not the resting membrane potential. If above threshold, this summation also leads to an action potential.

Pre-synaptic Modulation

Pre Synaptic and Post Synaptic Inhibition


There are two main types of modulation; pre-synaptic and post synaptic. Pos-tsynaptic modulation occurs through either IPSPs or EPSPs. The axon terminals synapsing onto either the cell body of the post synaptic cell (axosomatic synapse) or its dendrites (axodendratic synapse) are the structures involved in post synaptic inhibition. No modulation of the synapse itself occurs. Instead, these work by either adding to or reducing the combined total of the graded potentials onto the post synaptic cell. Our focus is on the synaptic junction, and since no modulation of the synapse itself, we will shift our attention to pre-synaptic modulation[23].

Pre-synaptic modulation occurs on the pre-synaptic terminal, directly affecting the release of neurotransmitters onto the target cell. This is known as volume modulation, as the number of neurotransmitters which bind to receptors affects the amplitude of the graded potential to be received by the post-synaptic cell body[24]. Axoaxonic synapses occur in both pre-synaptic facilitation and inhibition. For the modulatory mechanism to be facilitating, the modulating axon when it is excited causes an increase in the amount of neurotransmitter released from the pre synaptic terminal of the axon it synapses on to. The opposite happens for inhibitory modulation, where excitation of the modulatory neuron causes a decrease in neurotransmitter release from the pre-synaptic terminal[25].

Proposed Model for the voltage dependent calcium channels[26]

This is achieved through the regulation of free calcium inside the cell[27]. Axoaxonic synapses control calcium influx into the pre-synaptic terminal of its post synaptic cell (that is, the cell which is having its neurotransmitter release modified). By altering the amount of calcium that that flows into a terminal, small changes in the resting membrane potential can make an effective synapse inoperative or a weak synapse highly effective[28].

Three mechanisms have been recognised as causing pre-synaptic inhibition (experimenting with the neurons of invertebrates and mechano-receptor neurons of vertebrates):


• Activation of metabotropic receptors that leads to simultaneous closure of Ca2+ channels and opening voltage gated K+ channels, both of which decrease the influx of of Ca2+ and enhances re-polarization of the cell

• Activation of ionotrpoic GABA-gated Cl- channels, resulting in greater Cl- conductance, decreasing amplitude of action potential in pre-synaptic terminal. As a result, less depolarization occurs and fewerCa2+ channels are activated

• The third is less understood, but seems to work by decreasing sensitivity of one or more steps involved in the release process [29].


Pre-synaptic facilitation can be caused by an enhance influx of Ca2+. This can be achieved by closing K+ channels, thereby allowing Ca2+ influx to occur for longer. It is thought that facilitation generally occurs by depolarizing the pre-synaptic terminal, enhancing Ca+ influx in this way.

It is this ability to regulate Ca2+ in the pre-synaptic terminal that gives chemical synapses their flexible controlcell[30]. Many types of neurotransmitters exist and the ability to manipulate their release is fundamental in prioritizing and organizing communication in the nervous system.

Neurotransmitters

Neurotransmitters

[31] As previously mentioned synapse could either be “chemical” or electrical and the presence of neurotransmitters is the distinguishing feature of the chemical synapse. General Neurotransmitters are chemicals produced by the pre-synaptic neuron which is stored in synaptic vesicles and released into the synaptic cleft. The major role for neurotransmitter is to allow transmission of signals between neurons.These neurotransmitters would only bind to specific receptors on the post-synaptic neuron to produce any response. In the synaptic cleft the neurotransmitter could either diffuse and metabolize or undergoes reuptake back into the pre-synaptic neurons. However, special types of neurotransmitters such as Peptide neurotransmitters (histamine, cholecystokinin, neuropeptide Y, and somatostatin) are not produced from terminal buttons of axon but rather from the cell body. Moreover, in rare cases some neurotransmitters such as Nitric oxide and carbon monoxide transport retrograde (from axon back to the soma)which is opposite to the general neurotransmitter where the synaptic vesicles transport is through anterograde.

Neurotransmitters could be categorized into 2 major groups inhibitory and excitatory according to their reaction on the post-synaptic neuron membrane.

Inhibitory neurotransmitters-causes hyper-polarization that prevent action potential in post-synaptic neuron (calm the brain)

Excitatory neurotransmitters-causes depolarization which promotes action potential in post-synaptic neuron(stimulate the brain)


Types of neurotransmitters [32]
Types of neurotransmitters General Names Function Abnormalities
Biogenic amines [33] Dopamine,epinephrine,noreepinephrine ,serotonin and histamine • Involved in mood and behavior

• Blood pressure control

• Sexual behavior

• Arousal

• Motivation

• Fight or Flight response

• Parkinson’s disease

• Mood disorder

• Anxiety

• Schizophrenia

Neuropeptide [34] Neuropeptide Y Influences

• behavior

• learning

• memory

• appetite

• sleep cycle

• Anorexia

• Bulimia

• Obesity

Acetylcholine none • Involved in activation of skeletal muscle

• Cognitive function-especially memory

• Myasthenia gravis

• Lambert-Eaton

• Botulism

Excitatory amino neurotransmitters Glutamate,glycine and aspartic acid Major excitatory neurotransmitter in spinal cord • Hyperekplexia (myoclonic startle syndrome)

• Seizures due to ingestion of domoic acid

• Rasmussen's encephalitis

Inhibitory Amino neurotransmitters [35] Gamma Aminobutyric acid (GABA) Major inhibitory neurotransmitter in brain • Stiff person syndrome

• Epilepsy

Gaseous neurotransmitters [36] Nitric oxide and Carbon monoxide • Mediate vasodilatation

• Aids gaseous exchange [37]

• Muscle relaxer

• Regulate gastrointestinal motility

• Ischemia

• gastrointestinal, cardiovascular and neurogical disorder

• erectile dysfunction

Diseases Associated with Synaptic Junctions Dysfunctions

Parkinson’s disease


Parkinson's disease is one of the most common movement disorders, mostly effecting people over 60. Historically a person suffering from Parkinson's generally has a hunched posture due to 'tremors' that slowly worsen and cause mobility degeneration. James Parkinson studied the disease over the late 1800's, noticing the debilitating effects over time. This particular disease effects the production of the neurotransmitter Dopamine in the substantia nigra for reasons still unclear. Skilled movement in distal muscles is hindered as the synapse function is distorted by an imbalance of Dopomine and Acetylcholine in the synaptic junction.

Epidemiology: Is the most common movement related disorder in the world (effects 1% of adults over 60). Global burden of Parkinson's disease, measured in disability adjusted life years per 100,000 inhabitants in 2004

Epidemiology of Parkinson's disease

Etiology: Dopomine is produced in the Substantia nigra in the brain (rostral midbrain, Superior Colliculus) by specialised cells. These cells are destroyed in a Parkinson’s patient by reasons which are still unclear, theories of the cause of Parkinson’s include:

o Genetic disorder

o Free radicals

o External toxins

o Ageing

Pathogenisis: Without the appropriate amounts of Dopomine being produced, the synaptic junctions begin to deterioate in function. In addition MAO-B is a chemical in the synapse that breaks down dopamine to maintain a balance with acetylcholine. Due to this imbalance the synapse cannot function properly, especially in fine or smooth movement of muscles. [38]

Morphology : Macroscopically there is distinct atrophy of the substantia nigra. Microscopically the neuropil shows a loss of pigmentation and lewy bodies are present. As seen in the histological section below, the brown lewy body is present In the substantia nigra[39]
Lewy body in substantia nigra


Clinical manifestations [40]: Symptoms of a typical patient will include pain, rigidity and lack of mobility, all of which deteriorate. There are three major sighns of Parkinson’s:

o Resting tremor: Common to occur in hands and worsens over time.

o Rigidity : During movement there is increased resistance on the joint.

o Bradykinesia: Inability to perform fine movement tasks like writing or tieing shoe laces.

o Other signs include a loss of balance and increasingly poor posture. A retropulsion test can be be carried out to test balance by pulling the patient backwards and noting if they correct themselves properly.

Treatment: This varies with every patient and also to what extent the disease has destroyed the dopamine producing cells in the substantia nigra.

1) In its early stages, exercise and physical therapy is recommended to maintain mobility and deal with tremors that result from Parkinson’s.

2) When this fails, medication can be sought. Non-dopomine drugs are used first including Amantidine which increases mobility and ability to exercise.

3) Ropinirole is a dopamine agnostic that helps stop tremors and stiffness and is used if the condition worsens.

4) The most effective treatment is the drug Levodopa which is a dopamine replacement and dramatically increases motor function. However this may have long-term effects from its use including drug-induce dyskinesia causing erratic involuntary movements. Motor movement complications were found in 1/3 of patients after only a 2 year use of the drug making this a last resort. [41]

As of yet there is no cure for Parkinson’s disease.

Prognosis[42] : This disease takes its course over a long period of time and isn’t the direct cause of death in its patient. Symptoms intensify over approximately a ten year period before a sufferer is either completely bed ridden, needed to be placed in perminant care or occurances of complications take place that can lead to death.

Treatments mentioned earlier have become more effective over time and make life dealing with Parkinson’s a lot easier. Over time there is no stopping the continual deteriation, immunity and other systems are effected. The synaptic junctions where Parkinson’s clinically manifests, eventually become unbalanced all over the body which cause complications which cause death.


Myasthenia Gravis


Myasthenia Gravis is another disease which causes dysfunction within a synapse. It has a major effect on the neuromuscular junction, causing muscle weakness. Parkinson’s disease showed how the effects of a disruption in the neurotransmitters can affect a synaptic junction, this time the receptors on the post-synaptic membrane are the cause.
Neuromuscular junction


Epidemiology:[43] It is common disorder of the neuromuscular junction, effecting over 60 000 people in the USA alone, expected to rise with an aging population. Studies have shown two groups that have a high correlation with Myasthenia gravis: 1. Age 10-40 in women.

2. Age 50-75 in males


Etiology: This is still unclear.


Pathogenesis : Myasthenia Gravis is highly mediated by production of Antibodies to the Acetylcholine receptor (AChR) by the thymus (75% of patients have abnormalities in the thymus). These antibodies are responsible for dysfunction of the synaptic junctions within skeletal muscle fibers in three ways that lead to muscle weakness:

a. These antibodies are directed at the AChR’s located on the sodium/potassium ion channels in the post-synaptic membranes of the neuromuscular junctions. This disrupts depolarization of the muscle if the number of antibodies is sufficient to block the opening of this channel. This is the main cause of muscle weakness in cases of Myasthenia Gravis.

b. Antibodies increase internalization, which causes an increased rate of degradation in AChR’s. The replacement of receptors cannot keep up with the increased internalization leading to a low number of AChR’s in the postsynaptic membrane.

c. The bodies own immune response to antibodies binding to AChR’s causes a complement cascade of inflammatory molecules. In the process of inflammation, the problem won’t be resolved and attempts to repair it only leads to the destruction of the receptors over time. [44]


Morphology: Histological sections show very little change in skeletal muscle anatomy. All that is shown is slight myofiber II atrophy from weakness and disuse of the muscle. Within the connective tissue, lymphocytes are prominent. Within the synaptic junction affected, the number of AChR’s can decrease by 2/3 in chronic cases. This number falls over the period of the disease.


Clinical manifestations: As mentioned the major symptom is muscle weakness, which worsens with increased amounts of contractions, for example caused by exercise. Main affected area is the cranium (mastication and head rigidity) mainly the muscles of the eyelid causing ptosis (drooping). [45]


Diagnosis: Done through blood tests looking for elevated levels of AChR antibodies seen in Myasthenia Gravis patients. The Edrophonium test to see improvements in

response to drugs that block the antibodies (Tensilon). Electromyography can be used to examine an effected synaptic junction in skeletal muscle to measure action potentials with electrical shocks. With diagnosis of Myasthenia Gravis, it is rarely carried out quickly due to the symptoms being related to many other neurological disorders. A severity criteria is also available depending on the level of neurological deteriation:
Edrophonium test for Myasthenia Gravis


Osserman’s scale [46]

(Myasthenia Gravis=MG)

0. No MG symptoms.

1. MG with purely ocular muscle weakness.

2. MG patients with mild generalized weakness, usually with ocular muscle weakness but without bulbar involvement. Respiratory muscles are not involved.

3. MG with mild generalized weakness including bulbar involvement with dysarthria, dysphagia, and poor mastication.

4. MG patients with moderate generalized weakness, usually with moderate bulbar and respiratory muscle weakness.

5. MG with severe generalized, bulbar, and respiratory muscle weakness, or death due to MG-related complications.

[47]


Treatment: To treat the symptoms of Myasthenia gravis, immunosuppressive corticosteroids are administered to relive the large proportion of antibodies. In worse cases plasmapheresis is carried out with good results. [48]. Thymectomy is the removal of the thymus which is responsible for the over-production of AChR antibodies and can relieve the symptoms or show total remission.


Prognosis: With treatment the outlook is very good for patients with <5% mortality rate. If death occurs it may be caused by complications due to muscle weakness in the respiratory system leading to cardiac failure. however with treatment patients have a high chance of living a normal life.


Current and Future Research and Developments

Mood Disorder
Synaptic junction could be involved in various diseases as mentioned above in the table hence most of the current researches are focused more on the treatment and prevention of those diseases. In order to produce therapeutic treatments firstly an in-dept research must be carried out to understand the mechanism of the disease itself. Although Alzheimer disease had been introduced first in 1906 however till date the mechanism is not fully understood.However there [49] had been various current researches on how a synapse could lead to mood disorders such as Parkinson , schizophrenia and more recent Alzheimer disease. Alzheimer disease was claimed to be influenced by various disorders in the receptors. These receptors include : glutamate receptor, AcetylCholine, Dopamine receptor and serotonin receptors . All of the receptors mentioned are involved in cognitive learning and memory functions. Hence disruption to any or all of these channels would lead to diseases like Alzheimer. Moreover, the current research on the area also proposed the possibility for synaptic cell adhesion proteins to be involved with these disorders. Synaptic cell adhesion proteins are molecules that bind the pre-synaptic neuron and the post synaptic neuron together to form a “synapse” . Abnormalities in these proteins (Neurexin and neuroligin) will create
Cancer cells
dysfunctional synapse enabling the neurons to carry out its normal function . It was concluded that Alzheimer's Dementia occurred mostly due to the loss of neurons (leading to reduction in neurotransmitters)as opposed to Schizophrenia [50]where the disorder was mainly due to "neuronal dysfunction".


[51]As for the therapeutic research there had been a huge controversy on how N-cadherin could be the next therapeutic drug for cancer in future. The spreading of cancer is known to be related to cell to cell adhesion especially during metastasis. N-cadherin serve the purpose of adhering nerve cells together (in synaptic junction)which could retrieve the cancer cells. By contrast, [52] it was also claimed that N-cadherin could essentially promote "motility and invasion" of cancer cells hence further research is still required.

Gallery


Glossary

  • Axon- nerve fibre conducting away from the neuron
  • EPSP- excitatory post-synaptic potential. A depolarizing graded potential. Brings a cell’s membrane closer to threshold
  • Collateral Axon Branch- a branch of an axon running parallel with the parent axon
  • IPSP- inhibitory post-synaptic potential. Hyperpolarizing graded potential. Takes the cell membrane away from threshold
  • Metastasis- Transference of a disease from one part of the body or organ to another via blood vessels.
  • Postsynaptic neuron- neuron after the synapse
  • Presynaptic neuron - neuron before the synapse (usually the one that secrete neurotransmitters)
  • Soma- Cell body of a neuron
  • Spatial summation- the combining of graded potentials from different neurons onto a cell
  • Synapse- A junction where impulses is transferred from one neuron to the next
  • Temporal summation- the combining of graded potentials from the save neuron but at different times. The time between the two potentials is small enough that the second potential adds on top of the previous
  • Terminal buttons- is an area located at the end of the axon responsible for secreting neurotransmitters
  • voltage dependent channels- channels in the membrane of cells which are activated or inactivated depending on the potential across the membrane
  • Volume modulation- Modulation of the amount of neurotransmitter released from a pre synaptic terminal
  • Synaptic Vesicle- membrane bound organelles within the presynaptic ending that contains a neurotransmitter substance which is released on depolarization.
  • Synaptic cleft- the gap or small space between the axon terminal and effector cell.
  • Postsynaptic membrane- the portion of effector cell’s membrane.
  • Neurotransmitter binding site- where neurotransmitter molecules bind to the receptor site located on the postsynaptic membrane

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2011 Projects: Synaptic Junctions | Gap Junctions | Tight Junctions | Desmosomes | Adherens Junctions | Neuromuscular Junction