S. D. V. Stephens

notes / Basic Neuroscience and Geometry of Neuroscience / chapter 1: Neurons and Signalling / From my notebook

N7 Organelles of the Soma

In progress: these notes stop partway.

Ribosomes and the rough ER

  • Ribosomes: structures in the cytoplasm that translate mRNA to manufacture proteins from raw materials in the form of amino acids.
  • Rough endoplasmic reticulum (rough ER): what many ribosomes are attached to. More abundant in neurons than in most other non-neural cells. These are also the Nissl bodies (the things that got dyed!).
  • Ribosomes that are not attached to the rough ER are called free ribosomes (as they float freely through the cytoplasm).
  • Polyribosomes: free ribosomes attached together by a thread; the thread is a single strand of mRNA. The ribosomes on any one strand are working together to create many copies of the same protein.

If the protein is to be inserted into the membrane of a cell or organelle, then the mRNA goes to the rough ER; otherwise it stays free-floating in the cytosol. The rough ER traps the proteins to send them to the Golgi apparatus.

Special membrane proteins (assembled in the neuron’s rough ER) are what provide our neurons with information-processing abilities.

Smooth ER and the Golgi apparatus

Smooth ER: heterogeneous (not all the same); it does a bunch of different things and can even be extensions of the rough ER.

Golgi apparatus: sorts proteins post-assembly for chemical processing and delivery (such as to the axons and dendrites).

Mitochondria

Mitochondrion/mitochondria: 1 µm long. Consists of folds of an inner membrane called cristae (singular crista). Between the cristae is the matrix. The mitos provide “cellular respiration,” a process where they take in pyruvic acid (from the sugars, proteins, and fats that you digest) and oxygen floating in the cytosol.

Inside the mito, the pyruvic acid begins the Krebs cycle, which then provides energy for reactions in the cristae (the electron-transport chain) that add phosphate to adenosine diphosphate (ADP), creating adenosine triphosphate (ATP). ATP is the energy source of the cell. The mito then “exhales” 17 ATP molecules for every molecule of pyruvic acid taken in. ATP is VERY important and powers most neural activity.

The paragraphs above are bracketed in the margin and tied to two pasted textbook figures. Figure 2.11 compares protein synthesis on a free ribosome, where the newly created protein ends up in the cytosol, with protein synthesis on the rough ER, where the newly synthesized protein is inserted into the ER membrane as it is assembled. Figure 2.12 shows the Golgi apparatus beside the rough ER, sorting newly synthesized proteins for delivery to the right places in the neuron; an arrow links the membrane-associated protein in Figure 2.11 to the rough ER and Golgi in Figure 2.12.

The cytoskeleton

Cytoskeleton: holds the structure of the cell; the cell membrane is kind of draped over the cytoskeleton.