Neuron Ultrastructure Flashcards
Which two main cell types forms nervous tissue?
1) Neurons - functional units of the nervous system
2) Glial cells - supporting cells
What are the 4 different types of neurons?
- Multipolar: have numerous dendrites extending from the cell body.
a) The vast majority of neurons are multipolar ex. motor neuron, pyramidal neuron (cortex neuron), Purkinje cell from the cerebellar cortex - Bipolar - have only two processes extending from the cell body, one axon + one dendrite
a. In humans, bipolar neurons are only found in the eye, nose and the ear. - Unipolar - have only one process arising from the cell body.
a. In vertebrates, unipolar neurons are only found in the sensory ganglia of spinal and cranial nerves - these are said to be pseudounipolar - Anaxonic - only dendrites no axons - just balance electronic movement
What is grey matter composed of?
Nerve cell bodies embedded in neuropil (a dense network of interwoven nerve fibres and their branches and synapses, together with glia)
What are the 3 main staining methods?
1) Myelin stains: stains myelin sheath
* clearly delineates areas of grey matter from white matter –> useful for studying general topography (mapping) and location of fibre tracts
2) Nissl stains: cationic stain for nucleic acids, including RER
* Useful for identification of nuclei (group of neuron cell bodies in the CNS) + laminar patterns of cells
3) Golgi stains: stains cell membranes
* Selectively stains about 1% of neurons entirely. Useful technique for studying the dendritic morphology of neurons
* Does not stain the Golgi apparatus
What are the 2 newer staining methods?
1) Immunohistochemical staining - uses antibodies to stain for specific enzymes or proteins e.g NeuN (a neuronal nuclear protein)
2) Fluorescent proteins - inserted into the genome of transgenic animal e.g Brainbow staining
What is white matter composed of?
Myelinated (lipid) axons with associated cells
Outline the cell membrane of a neuron.
Cell membrane - a semipermeable, bilipid layer with proteis embedded in it.
a. Receptors: formed by membrane proteins that can either be metabotropic (linked to 2nd messengers) or ionotropic (linked to ion channels)
Outline key features of ion channels.
Ion channels: ligand gated or voltage sensitive
- Membrane proteins form ion channels –> allow selective passage of electrolytes into and out of the cell
- Ion concentration differ within and outside the cell –> resting membrane potential (increased intracellular K+, increased extracellular Na+)
- When neuron is stimulated, ion channels open up –> influx of Na+ and efflux of K+ –> graded potential change
- If strong enough, graded potential change may generated an action potential in the axon hillock –> spreads along the axon as a nerve impulse.
Outline the nucleus of the neuron.
a. Centrally located
b. Contains: prominent nucleolus, which can be revealed with a Nissl stain and other cationic stain
Outline the cytoplasm of the nucleus.
The cytoplasm is a gelatenous liquid containing:
a. Prominent RER –> site of protein synthesis
b. Golgi apparatus - for packaging protein
c. Many mitochondria - for energy production
d. Cytoskeleton
i. Microtubules/neurotubules - hollow, important for axoplasmic transport
ii. Neurofilaments (10nm in diameter)
iii. Microfilaments (5nm in diameter)
Neurofilaments and microfilaments are important for:
a. Maintaining the shape of neurons
b. Anchoring membrane molecules in place
c. Movement of axon and dendrites in growing neurons
Outline the dendrite component of neurons.
Dendrites - extensions of the cytoplasm that usually branch close to the soma.
a. Up to 90% of cytoplasmic volume is located in the dendrites - contains organelles similar to the soma.
b. Dendrites are specialised to receive signals from other neurons, at junctions called synapses.
c. Dendrites may have surface projections called dendritic spines, which increase surface area to allow for more synapses.
i. They are dynamic, not fixed –> thought to underlie mechanisms of learning and memory
Outline axons as part of neurons.
Axons - single long processes, which do not branch close to the soma.
a. Functions to transmit an electrical impulse away from the cell body, towards another neuron.
b. Near its termination, the axon branches into telondria (terminal branches) which end as small swellings (axon terminals) that form synapses.
c. The axon arises from a Nissl-free area of the soma called the axon hillock. It has an initial segment, just beyond the axon hillock, which is devoid of myelin and is the trigger zone for action potentials.
d. Most axons are surrounded by a myelin sheath.
What is the myelin sheath and what are its features?
Myelin sheath - insulating lipid envelope of myelin that surrounds/insulates the core of an axon and facilitates the transmission of nerve impulses.
a. Schwann cells + oligodendrocytes wrap themselves around axons to form a tight lipid rich myelin sheath
i. In the PNS, the myelin sheath is formed by Schwann cells - one Schwann cell forms internode for one axon
ii. In the CNS, the myelin sheath is formed by oligodendrocytes - one oligodendrocyte forms internodes for multiple axons.
b. This sheath is segmented (each segment is an internode), separated from the others by a Node of Ranvier containing voltage gated ions channels
c. The myelin sheath increases conduction velocity from 0.1m/s to 80m/s by forcing current to jump from node to node.
d. Not all axons are myelinated - they are surrounded only by a single layer of membrane from the glial cell and lack Nodes of Ranvier, making the conduction much slower.
What are the two types of synapses?
Chemical or electrical.
i) Chemical synapse has 2 compartments:
1) Presynaptic - contains neurotransmitter vesicles
2) Post-synaptic - contains neurotransmitters receptors
ii) Electrical synapse (gap junctions)
What is the process of a chemical synapse?
i. There is a narrow space between the neurons called a synaptic cleft
ii. Neurotransmitters are released from synaptic vesicles (exocytosed) into the synaptic cleft, where they bind to receptors on the post-synaptic membrane.
iii. This results in changes to the membrane potential of the post-synaptic neuron. Depending on the type of transmitter and the type of receptor, the effect on the postsynaptic neuron may be to depolarise (excite) or hyperpolarise (inhibit)
