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Synaptic knob in the nervous system and the organelles involved

Explaining the functions and role they play in the Synaptic knob and why there are in large numbers.

Daniel Jones

on 20 September 2012

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Transcript of Synaptic knob in the nervous system and the organelles involved

The Nervous System and Synapses By Daniel Jones The nervous system is an organ system containing a network of specialized cells called neurons that can make up nervous tissue and coordinate the actions of an animal and transmit signals between different parts of its body.

Nervous tissue is composed of two main cell types: neurons and glial cells. Neurons transmit nerve messages. Glial cells are in direct contact with neurons and often surround them.

The neuron is the functional unit of the nervous system. The nervous system is packed with neurons as we have about 100 billion neurons in their brain alone! While variable in size and shape, all neurons have three parts.
1) Dendrites receive information from another cell and transmit the message to the cell body.
2) The cell body contains the nucleus, mitochondria and other organelles typical of eukaryotic cells.
3) The axon conducts messages away from the cell body.

There are also three types of neurons;
1) Sensory Neurons
2) Motor Neurons
3) Interneurons

In a reflex arc, you can see where and how the different types of neurones work. The Nervous system - In General How the impulse is transmitted across the
synaptic cleft in a Cholinergic Synapse Action Potential Functional synapses require mitochondria to supply ATP and regulate local Ca2+ for neurotransmission.

Mitochondria provides adenosine triphosphate (ATP) which in turn supplies the energy for synthesizing new neurotransmitter or transmitter substance.

Since the nervous system is always active throughout your life and sending nerves impulses every fraction of a second, relatively it needs alot of energy (ATP) to do this and therefore mitochondria are found in abundance there.. Mitochondria Synapse The space in between the cells is called the synaptic cleft and to cross it requires the actions of neurotransmitters which are stored in small synaptic vesicles clustered at the tip of the axon (Synaptic knob).

Synapses consist of:
- pre-synaptic ending (where neurotransmitters are made)
- post-synaptic ending (has neuroreceptors in the membrane)
- synaptic cleft

The Nerve impulse is carried by neurotransmitters across the synaptic cleft

You group synapses into 5 types:
1. Excitatory Ion Channel Synapses.

2. Inhibitory Ion Channel Synapses.

3. Non Channel Synapses.

4. Neuromuscular Junctions. (eg - Cholinrgic synapse)

5. Electrical Synapses. ...is the junction between a nerve cell and another cell. 1) A nerve impulse stimulates a muscle cell to contract.

2) This starts when the nerve impulse reaches the nerve terminal and depolarizes the plasma membrane of the terminal.

3) At the end of the pre-synaptic neurone there are voltage-gated calcium channels. When an action potential reaches the synapse these channels open, causing calcium ions to flow into the cell.

4) This causes the Ca2+ concentration outside the cell 1000 times greater then the free Ca2+ concentration inside. Ca2+ flows into the nerve terminal.

5) These calcium ions cause the synaptic vesicles to fuse with the cell membrane, releasing their contents (the neurotransmitter chemicals) by exocytosis into the synaptic cleft.

6) The neurotransmitters diffuse across the synaptic cleft.

7) The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane, causing the channels to open, so sodium ions flow in.

8) The depolarization opens voltage-gated Na+ channels allowing more Na+ to enter, which further depolarizes the membrane.

9) This makes an action potential that spreads over the entire plasma membrane.

10) The neurotransmitter is broken down by a specific enzyme in the synaptic cleft; for example the enzyme acetylcholinesterase breaks down the neurotransmitter acetylcholine. The breakdown products are absorbed by the pre-synaptic neurone by endocytosis and used to re-synthesise more neurotransmitter, using energy from the mitochondria. This stops the synapse being permanently on. Synaptic Knob Post-synaptic Membrane The post-synaptic membrane is at the receiving end of the Neurotransmitter and as such need Neurotransmitter receptors to continue sending on the nerve impulse.

The second function of the transmitter-bound receptors is that they alter the ionic permeability of the post-synaptic membrane because they're coupled, directly or indirectly, to ion channels in the post-synaptic membrane. Opening or closing these channels as a result of transmitter binding allows ionic currents to flow, thus changing the post-synaptic membrane potential. bibliography Internet research
- http://www.biologymad.com/nervoussystem/nervoussystemintro.htm
- http://www.biologymad.com/nervoussystem/synapses.htm
- http://www.emc.maricopa.edu/faculty/farabee/biobk/biobooknerv.html
- http://webspace.ship.edu/cgboer/genpsyneurotransmitters.html
- http://www.rcsb.org/pdb/101/motm.do?momID=54

Book research
-Molecular Biology of the Cell (fifth edition) by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter What's in the Presentation ? 1. General Background on the nervous system.
2. Synapses
3. Action Potential
4. Transferring a Nerve impulse over a Synapse
5. Mitochondria - Why are they there ?
6. Post-synaptic Membrane
7. Summary answer of the question.
8. Bibliography Summary Answer of the Question Question:
The Cytoplasm in the Synaptic Knob has a high proportion of certain organelles. These include smooth endoplasmic reticulum, mitochondria and vesicles. Each organelle has a specific role to play in the functioning of the cell. Describe the role of each of these organelles and explain why they are found in relatively large numbers in the synaptic knob? Answer:
Smooth ER - It serves as a transitional area for vesicles that transport ER products to various destinations.
- It can deactivate drugs/poisons to stop them disturbing the fragile nervous system.
- It is in abundance at the Synaptic Knob to help and ensure the transfer of neurotransmitters.

Mitochondria - Mitochondria are the cell's power producers. They convert energy into forms that are usable by the cell.
- They produce ATP by cell respiration that helps for synthesizing new neurotransmitter or transmitter substance.
- They are in Abundance because they are working all day, every day sending thousands of Nerve impulses across synapses every second.

Vesicles - They hold the neurotransmitters that relay the message on across the synapse.
- They fuse with the cell membrane and release the neurotransmitters by exocytosis.
- Later then, the Choline is reabsorbed by endocytosis.
- They are in abundance to provide enough neurotransmitters to accommodate the bodies every reaction. - Action potentials are generated by special types of voltage-gated ion channels embedded in a cell's plasma membrane.

- These channels are shut when the membrane potential is near the resting potential of the cell, but they rapidly begin to open if the membrane potential increases to a precisely defined threshold value.

- When the channels open, they allow an inward flow of sodium ions, which changes the electrochemical gradient, which in turn produces a further rise in the membrane potential. This then causes more channels to open, producing a greater electric current, and so on. The process proceeds explosively until all of the available ion channels are open, resulting in a large upswing in the membrane potential.

- The rapid influx of sodium ions causes the polarity of the plasma membrane to reverse, and the ion channels then rapidly inactivate. As the sodium channels close, sodium ions can no longer enter the neuron, and they are actively transported out of the plasma membrane.

- Potassium channels are then activated, and there is an outward current of potassium ions, returning the electrochemical gradient to the resting state.

- After an action potential has occurred, there is a transient negative shift, called the afterhyperpolarization or refractory period, due to additional potassium currents. This is the mechanism that prevents an action potential from traveling back the way it just came. How it is Generated...
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