This document was created by Alex Yartsev (
[email protected]); if I have used your data or images and forgot to reference you, please email me.
Cells of the Central Nervous System The GLIAL cells
“glia” means glue in greek. - 2 major types: MICROGLIA and MACROGLIA - Microglial cells are scaveneger cells that resemble tissue macrophages; they remove debris - MACROGLIA come in 4 flavours: Oligodendrocytes form the myelin sheaths in the central nervous system Schwann cells form the myelin sheaths of the peripheral nervous system Each cell forms a segment of myelin about 1mm long The cell waraps itself around the axon in concentric layers Fibrous astrocytes contain many intermediate filaments and are found in white matter Protoplasmic astrocytes have a granular cytoplasm and are found in grey matter both types of astrocytes send foot processes to the capillaries in the brain, forming the blood-brain barrier by setting up tight junctions. They also send processes to envelop the surface of neurons, and especially their synapses - WEIRDLY, astrocytes have a membrane potential like neurons, but they do not generate propagating action potentials. - They maintain an appropriate concentration of ions in the ECF of the brain; especially important is the fact that they take up K+ ions - They also absorb a lot of the secreted glutamate and GABA
The NEURONS
Initial segment of the axon
-
Presynaptic terminals
Terminal buttons
Nodes of Ranvier Axon Hillock
Dendrites
Soma Dendritic spines
-
- -
The soma is the metabolic centre of the neuron, and contains the nucleus The dendrites are extensively arborised (branching) projections which in the cerebrum and cerebellum have small projections called “dendritic spines” The long fibrous axon originates from a protruding area of the cell body called the axon hillock The first denuded portion of the axon is called the “initial segment” The axon divides into several presynaptic terminals, and eah presynaptic terminal divides into a number of little synaptic knobs or buttons. The buttons contain the vesicles wherein the neurotransmitter is stored
Depending on how many processes emerge from them, the neurons can be classified as unipolar, bipolar, or multipolar unipolar neurons have one process; that one process has both receiving and transmitting terminals Axon
Dendrites
bipolar neurons have a DENDITE which carries information to the cell, and an axon which carries information away; an example is the bipolar cells of the retina
Axon Axon to spinal cord
Axon to skin
pseudo-unipolar cells have a single axon split into two, one of which goes to the skin or muscle, and the other to the spinal cord; ad example is a ganglion cell of the dorsal root of spinal cord
This document was created by Alex Yartsev (
[email protected]); if I have used your data or images and forgot to reference you, please email me.
multipolar cells have one axon and many dendrites; for example, the motor neurons of the motor cortex, hippocampal pyramidal cells, and purkinje cells which have an insanely huge number of dendrites
Dendrite
Axon
Huge branching mass of dendrites Axon
Motor neuron -
Apical dendrite
Axon
Cerebellar Purkinje neuron
Basal dendrite
Pyrmaidal hippocampal neuron
Which is the axon and which is the dendrite? -
Its easy to say which is which in the motor neurons and the interneurons neurons In others, its generally assumed that there are 4 major zones of functional importance: o RECEPTOR REGION or dendritic zone; Multiple local action potentials from several synapses are integrated into one
o ACTION POTENTIAL GENERATION REGION which in spinal motor neurons is the initial segment of the axon, and in sensory neurons is the initial node of Ranvier o An AXONAL PROCESS which propagates the generated potential to the nerve endings o THE NERVE ENDINGS which release the neurotransmitter to the receptor region of the next neuron in the system
-
The cell body is usually a the dendritic end, but can be anywhere- in cutaneous neurons it is attached to the side of the axon, or it could be inside the axon like in auditory neurons
MYELINATION -
The Schwann cells wrap themselves around an axon up to 100 times to myelinate them Unmyelinated axons still have Schwann cells around them, but not wrapped as tightly
AXONAL TRANSPORT -
-
Like all kinds of secretory cells, neurons synthesise something and then release it via vescles. The difference is that the neurons have to release this secretion far away from the cell body AXOPLASMIC FLOW is the movement of the substances along the axon to the nerve endings This means not just neurotransmitters but all kinds of maintenance material o Thus, an axon that gets cut degenerates distally to the cut; this is WALLERIAN DEGENERATION ORTHOGRADE TRANSPORT TOWARDS THE NERVE ENDINGS occurs along the axon: dragging substances along the microtubules with the use of dynein and kinesin o FAST orthograde transport happens at 400 mm/day o SLOW orthograde transport occurs at 5-10mm/day RETROGRADE TRANSPORT goes back from the nerve endings towards the
References: Ganong's Review of Medical Physiology, Chapter 4
