The nervous system consists of CNS central nervous system (enclosed in the bones) and the peripheral nervous system SNP. The specialized cells of the SN neurons are (100 billion) and glia or glial cells (1000-2000 billion). Makes up 5-6% of body mass but consumes 20% of energy.
neurons (10-100 μm) are responsible for electrical activity and, with certain exceptions, not mitosis. The glia are
support or electrical isolation to the neuron, are 10 times smaller and divide to life. The child at birth is less than glia to decrease the size of the skull.
Neurons are divided into three constituent parts: the soma (cell metabolism, integration of signals), the dendrites (reception) and 'axon (signal transmission). Dendrites and axons are collectively called neurites .
Dendrites (20-2000 um length), the diameter tends to shrink away from the soma. Sometimes they are provided with specializations called dendritic spines that receive the nerve impulse.
The axon, in humans, has a variable length and may exceed one meter (motoneurons foot), the diameter is between 0.2 and 20 μm and tends to remain constant. Generally part of the axon from the soma is unique, but you can branch out in axons side. The axon starts from the soma in the region called axon mound or cone emergency or cone integration and end buttons in synaptic terminals or . In its length it is covered by the myelin sheath produced by glial cells. Often the axons of many neurons run parallel and form the nerve (SNP) and tract (SNC) to gain more strength.
The classification of neurons
There are different classifications and complementary.
or size.
or function:
- sensory neurons (afferent neurons, sensory systems),
- Interneuron (system integration)
- motor neurons (efferent neuron, effector systems).
The sensory system that receives the stimulus and the effector system that produces the answer varies for different species of organisms, but the system integration is species-specific. To form or
(dendrites)
- Unipolar (sensory neurons without dendrites, eg. Somatosensory cells)
- Biploari (sensory neurons, a dendrite, for example. Cells retinal)
- Multipole (most common form, eg. spinal motor neurons, hippocampal pyramidal cells, Purkinje cells of cerebellum)
or length of the axon:
- The second type of Golgi or pyramidal cells (axon length, projection neurons)
- type II Golgi or stellate cells (short-axon, local circuit neurons) or type of neurotransmitter
:
- cholinergic neurons (acitelcolina)
- dopaminergic neurons (dopamine)
- glutamergici neurons (glutamate)
- GABAergic neurons (GABA )
- etc ...
Glial cells
- fill the space separating one neuron to another and electrically insulate axons
-
nourish neurons - Maintain a stable composition of the extra-cellular space
- led the growth in the re-growth of neuronal cells
- repair tissues and defend by pathogens (replace the immune system)
Classification:
- Microglia
They embryological origin from the mesoderm (especially macrophages).
The main function is to repair damaged tissues and engulf the dead cells.
- Macroglia
They have the same embryological origin of the neurons.
-
Astrocytes are the cells more numerous in the CNS and have a star-shaped
nourish the neuron
The CNS is not in direct contact with the blood stream even if there are large vessels passing through it.
Astrocytes levying substances from the bloodstream and nourish the neuron. Astrocytes and endothelial cells of blood vessels to form the blood-brain barrier , in fact the brain is bathed in the blood but from cerebrospinal fluid or CSF .
• Avoid the entry of virus
• Maintains constant ion concentration
• Avoid contact between molecules in the circulatory system that affect neurons
working as phagocytosis
Astrocytes can move through the pseudopodia and move en masse to the damaged area
- Capture the neurotransmitters and potassium after the exchange synaptic
- Dab the extra-cellular concentration of neurons
Astrocytes are all connected together by joints Communicating (gap junctions or tight junctions) so that their network should send the substances, when they are requested.
- They produce growth factors
are chemical molecules that indicate the direction in which the axons grow during growth or to repair damaged tissues.
-
Oligodendrocytes produce myelin sheaths in the CNS
- Schwann cells
produce myelin sheaths in the PNS
The myelin sheath
The myelin is made from 80% to 20% lipids and proteins and works as an insulator for transmission. In the PNS, the Schwann cell wraps itself around a section of the axon. In the CNS oligodenrocyte form numerous tracts of myelin on different axons. The axon is not completely covered, but there are segments of 1-2 um discovered every stroke of 1mm myelin called nodes Ravier. The
myelination begins at the 5th month of pregnancy and continues after birth than the 2nd year that at birth the newborn's brain takes 400cc in volume 1300cc instead of the average although the number of neurons is the same. After adolescence, the number of neurons begins to decrease and there are gender differences in weight for age, which is greater in the male proportion to the rest of the body.
Schwann cells act as a guide when you cut the axon, but the oligodendrocytes in the CNS axon occupy space stopped. The multiple sclerosis
is an autoimmune disease of the CNS, with incidence in the northern hemisphere, due to attack myelin from some components of the immune system can cross the blood-brain barrier. The myelin sheath produces delays missing very important areas of the brain affected.
0 comments:
Post a Comment