Unlike most human cells (which are rounded as in appendix 1) the motor neurones are not (see appendix 2), they have a very specific structure. They resemble wires, and it is these wire-like structures whose function it is to carry the signals (nerve impulses) from the central nervous system to effector organs - for example, the muscles and glands within the body that respond to stimulus.
These cells are stretched out into long thin fibres that can be over a metre in length (unlike the bacteria cell which are amongst the smallest living organisms), and the nerve impulses travel along these fibres in one direction. These singular fibres are known as axons and are covered with a myelin sheath for insulation. This myelin sheath is made up of fatty cells and as fat is a good insulator the electrical impulses are able to jump across the gaps in the myelin sheath at a phenomenal rate, sometimes up to 120 metres a second. These gaps are called the nodes of Ranvier and they are approximately 1 micrometer wide. The function of these nodes is saltatory conduction (jumping across the gaps in the sheath). The bacillus cell also has a protective surface the cell wall, made up from a substance called peptidolycan, consisting of sugars and amino acids. This substance gives the cell a rigid layer and this provides the cell with strength. Often the baccilus is further protected by a sticky outer layer known as a slime capsule. Beneath the cell wall of both cells is the plasma membrane which is filled with a watery fluid known as cytoplasm and it is this membrane that is responsible for the flow of molecules into and out of the cell membrane. (Wikipedia 2007, Boyle & Senior 2002)
In order to increase the surface area of the neurone available for incoming information, the cell has short fibres that branch out from the cell body. They receive this information from other neurons and transmit it away from the brain, to the muscles and glands and are referred to as dendrites. The salmonella cell is surrounded by fibres similar in appearance to the dendrites known as flagella, however, the function of the flagella is to allow the cell to move through the intestines of its host, the name given to this function is motile.
The bacteria cell can also increase its surface area although internally, by creating mesosomes - folds which protrude inside the cell membrane.
The point at which two neurons connect, or the connection between a neuron and muscles or glands occurs is the synapse. Some cells in the nervous system can have as many as 200,000 of these connections and it is at this connection that ‘electrochemical communication’ occurs and the electrical and chemical signals are sent and received. The bacteria cell also has the ability to attach to other cells but not for the purpose of electrochemical communication but for ‘conjugation’ – a primitive form of reproduction, which it does with the use of an external appendage known as pili.
The cell body or soma of the neurone is a hive of metabolic activity and contains the nucleus, a double membrane bound organelle, (usually rounded in the animal cell) with nuclear pores. The nucleus has the very specific function of storing the cells information - Deoxyribose Nucleic Acid (DNA). This DNA is contained within another organelle known as the nucleolus, and it is this DNA information that is copied onto another molecule – Ribose Nucleic Acid (RNA), which is then sent through the nucleus pores to the ribosomes. Instead of a membrane bound nucleus, the bacillus DNA has no physical boundary – it has a nucleoid region which contains the substance known as ‘naked DNA ‘. This naked DNA is formed into a ring-like structure with no beginning and no end. (Boyle & Senior 2002, cellsalive.com 2006)
The ribosomes are tiny factories responsible for the production of protein within both cells, and they use the information gathered from the RNA to make it. In the motor neurone they attach themselves to the nissl granules which are membrane bound organelles known as rough endoplasmic reticulum (ER), the ‘rough’ referring to ribosomes on their surface giving a rough appearance. The ribosomes within the bacterium are not attached to any other structure and float freely in the cytoplasm. Although both cells contain ribosomes as they both need protein, even there, there are differences. The motor neurone has larger and more complex ribosomes made up of five types of RNA and eighty types of proteins, whereas the bacterium has only three types of RNA and fifty types of protein.
Unlike the lonely ribosomes in the prokaryote, there are other ‘membrane bound organelles’ present in the cytoplasm of the eukaryote, each with its own purpose. Aside from the nucleus, the mitochondria are the ‘power houses’ of animal cells whose function it is to provide cells with the energy they needs to divide, move and secrete – this is achieved by turning glucose into ATP (Adenosine 5’-triphosphate - the cells primary energy source). A motor neurone cell has many mitochondria simply because it needs huge amounts of energy.
There are powerful enzymes contained in organelles known as Lysosomes that ‘clean up’ by digesting obsolete organelles and cells.
The golgi complex is also present within the cell body and this is a series of flattened sacks, stacked like pennies - their function is to receive the vesciles of protein from the rough ER. Vesciles bud onto the golgi body whilst it modifies and finishes off the protein molecules. The completed vescicles bud off for exocytosis - which is the process of releasing the protein where the vesicle fuses and becomes part of the cell membrane.
Aside the obvious visual differences between the motor neurone and the salmonella cells, there are many differences in their internal structures. The motor neurone cell has a membrane bound nucleus, the bacteria does not. It also lacks any of the membrane bound organelles that are present in the motor neurone. In fact it is far easier to recognise those few features these two cells do share.
They both contain a type of genetic blueprint known as DNA and RNA, needed for both species to function and reproduce. They possess the specialised machinery to manufacture this, that said, there are even differences here. The motor neurones DNA is linear or has closed ends and contains proteins named ‘histones’, which are organised into chromosomes. The salmonella bacteria has naked DNA that forms a chain and does not contain these histones, therefore, has no end and no beginning, it is just a loop of DNA. (class notes 2007, Boyle & Senior 2002)
Both cells In conclusion, whilst these two organisms are the building blocks of life, they share minimal features. Both have a cell membrane, contain DNA and contain ribosomes within cytoplasm. Here the similarities end, the motor neurone cell is far more complex in structure and function than the basic salmonella cell. Its contents include many membrane bound organelles including the nucleus, synapse, dendrites and the axon with its own specialised features. The salmonella has flagella to facilitate movement, naked DNA and the protection of a capsule. They each have aspects of their own make-up that are unique, but basically they are one and the same, however, one has, over many millions of years evolved from the other. The bacterium and its simplistic form has clearly been a key to its longevity.
References
Boyle, Mike and Senior, Kathryn. (2002). Biology. 2nd Edition. London, HarperCollinsPublishers Limited.
Milner, Bryan and Martin, Jean. (1997). Biology. Cambridge, Cambridge University Press.
Beckett, Brian and Gallagher, RoseMarie. (2001). Modular Science, Biology. Oxford, Oxford University Press.
Wikipedia contributors [online] The Free Encyclopedia. (URL ?) (Accessed October 2007).
Cellsalive.com [online] Cell Models (URL ) Copyright 2006 Quill Graphics (Accessed October 2007).
Humanillnesses.com [online] images (URL . Copyright 2007 – Advameg Inc. (Accessed October 2007).