Neurons and Neuroglia
- Nissl stain: Marks cell nuclei and surrounding matter; Nissl bodies. Separates neurons from neuroglia and enables the study of cytoarchitecture, architecture of cells. It does not show neurites.
- Golgi Stain: Silver chromate, darkens neurons with their neurites.
The central body is the soma and the neurites are axons and dendrites. There is generally a single axon that subdivides later on, but many dendrites rising from the same soma. The branches of the axon are always at right angles and are called Axon Collaterals. Axons can be up to a meter in length. Axons are diameter-uniform but dendrites are tapered.
Cajal then drew all the neurons. Golgi believed they were linked continuously, Cajal believed they were only linked by proximity and contact. The former would be a counterexample to ordinary cell theory and the latter was the idea that neurons were nothing more than cells, that is, the Neuron Doctrine. Proof of the Neuron Doctrine had to wait until the development of electron microscopy.
Sub-Cellular characteristics of the neuron
- Soma: Spherical, 20 microns, full of potassium-rich salt solution (Cytosol), isolated by the neuronal membrane, containing the organelles.
- Nucleus: The usual.
- Rough ER: Studded with ribosomes so the proteins they produce will be sent to the cellular membrane or an organelle.
- Free-floating Ribosomes: Synthesize proteins that reside in the cytosol. Sometimes there are multiple free ribosomes attached by a thread, these are polyribosomes. Proteins meant for the cytosol float towards the free ribosomes.
- Smooth ER and Golgi Apparatus: ER without the ribosomes. Some smooth ER is continous with rough ER, and is apparently where protein folding happens. Sometimes it just regulates calcium etc. concentrations. The stack of disks in the soma that lies farthest from the nucleus is the Golgi apparatus, which does post-translational chemical processing of proteins, that is, sorting the proteins for delivery to different parts of the neuron. This is where cytosol proteins are routed to the axons or the dendrites.
- Mitochondria: You know the drill. A micron long. Produces ATP. 17 ATP molecules, in fact, for every molecule of pyruvic acid that is taken in.
- Neronal Membrane: Five nanometers thick, studden with proteins (Durr hurr). "The function of neurons cannot be understood without understanding the structure and function of the membrane and its associated proteins", says the book, because ion channels and shit.
- Cytoskeleton: Microtubules, neurofilaments and microfilaments. I won't bother.
Axons: Starting from a tapered cone (Axon hillock) from the soma, the axon proper expands away. It has no ER and few ribosomes, and the protein composition of its membrane is fundamentally different from that of the soma's membrane. There is thus no protein synthesis and proteins for the axon must be mounted on a kinesis transport system that travels down microfilaments. Length varies from a millimeter to over a meter. Branches are at a right angle and called axon collaterals. Some axon collaterals return to dendrites of the originating cell, and are called recurrent collaterals.
The diameter is around a micron.
The signal that travels down the axon is called the nerve impulse.
Axon terminal: Where the axon or the collateral ends, in contact with dendrites of other dendrites, cell bodies or cells other than neurons, and transmits information to them, this point is the synapse. The set of all terminations in an axon tree is the terminal arbor. Synapses that occur somewhere other than in the termination form boutons en passant, or boutons in passing.
In any case, when a neuron contacts another cell (Neuron or neuroglia) it innervates the cell, or provides innervation.
Differences in the axonal cytoplasm:
- Microtubules don't reach terminals
- The terminal has small bubbles of membrane called membrane vesicles
- The inside of the membrane is dense in proteins
- Numerous mitochondria due to high energy usage.
Synapse: Has two sides, post and pre synaptic, showing the direction of information transfer. The pre side is the axon terminal and post is either a cell body or a dendrite, with the gap between being the cleft where neurotransmitters float around during a synaptic transmission.
The incoming charge suffers an electrical-to-chemical conversion with the release of the neurotransmitters, then back to electrical, allowing the neurons to exchange information.
Axoplasmic Transport: Since there are no ribosomes in the axon, proteins are manufactured in the soma and shipped to the entire axon: Material is enclosed in vesicles that walk down the microtubules one ATP-powered kinesin proteins, this is called anterograde transport, and there is also retrograde transport for moving material down intpo the soma. The system is the same only that the vesicles are moved by a protein called dynein instead of kynesin. Anterograde and retrograde transport can be exploited for tracing connections (For example, by using radioisotopes in aminoacids to trace the motion of proteins as they are transported). There are too kinds of anterograde transport, fast and slow, which move at 1000m/day and (1 to 10m/day), resp.
Dendrites: The term comes from the Greek for 'tree'. The whole: Dendritic tree. Each branch: Dendritic branch. Morphology is used to classify neurons, sometimes. They are the postsynaptic size of a synapse. They have receptor molecules for the neurotransmitters. In some mneurons, dendrites are covered in spines that receive specific types of input. They are believed to isolate chemical reactions. Their structure is sensitive to the type and amount of synaptic activity.
Cytoplasm in a dendrite is the same as that of axons, full of mitochondria and cytoskeletal elements. Polyribosome complexes can be found in dendrites, under the spines. Protein synthesis can be regulated by synaptic activity.
Classification of Neurons
By number of neurites: A neuron with a single neurite is unipolar, two neurites, bipolar, more: Multipolar. Obviously, most neurons are multipolar.
By morphology of dendrites: Stellate cells are star-shaped or explosive, pyramidal cells are the straight ones (Both in the cerebral cortex). Spiny neurons have spines, aspinous neurons don't. In the cerebral cortex, all pyramidal cells are spiny.
By connections: Primary sensory neurons are those that neurites in the sensory surfaces. Motor neurons control muscles. Interneurons are all that route between them.
By axon length: Cells with trans-brain-section axons are Golgi Type I, or projection neurons. Short axons are Golgi Type II neurons, or local circuit. Pyramidal cells for example are usually type I.
By neurotransmitter: Motor neurons that command voluntary movements all use acetylcholine in their synapses, so the neurons are cholinergic, meaning they use that neurotransmitter. Neurotransmitter systems are set of neurons that use a particular neurotransmitter.
You know, supporting neurons and shit.
Astrocytes: Most numerous glia. Neuron/astrocyte gap = 20nm. Regulate chemical environment. Restrict spread of neurotransmitters and adsorb them.
Myelinating Glia: Oligodendroglial cells or Schwann cells are a subset of this. Provide membrane to insulate axons: Myelin, which forms the myelin sheath. Periodical interruptions are the nodes of Ranvier. Myelin speeds up imnpulses. Oligodendroglial cells myelinate several axons, Schwann cells only one.
Others: Ependymal cells line the fluid ventricles in the brain, direct cell migration during growth. Microglia: Phagocytes (Cleaning, immune system). Also the cells in the vascular system.
Review questions for this section:
- State the neuron doctrine in one sentence and who stated it: Neurons are the fundamental unit of 'consciousness' - Cajal
- Which parts of a neuron are shown by Nissl and Golgi stain resp.? Nissl stain shows the soma, Golgi stain shows the soma and neurites.
- What are the physical characteristics that distinguish dendrites from axons? Axons are long, branch at 90º, start through the hillock. Dendrites are short, and sprout directly from the soma.
- Which one of the following is unique to neurons? Nucleus, mitochondria, rough ER, synaptic vesicle, Golgi apparatus? Synaptic vesicle.
- Name the steps of the process in which DNA directs synthesis of a membrane-associated protein: DNA -> mRNA -> RNA -> Ribosome on the rough ER
- Colchicine is a drug that depolymerizes microtubules. What effect would this drug have on anterograde transport? What would happen in the axon terminal? It would derp
- Classify the cortical pyramidal cell based on a) number of neurites b) presence or absence of spines c) connections d) axon length: a) Bipolar, b) Present, c) Interneurons, d) Golgi Type II
- What is myelin, what does it do? Which cells provide it in the CNS?: An insulating molecule? It insulates hur bur: Myelinating neuroglia (Oligodendroglial cells and Schwann cells)