Tuesday, June 22, 2010

Nervous Tissue

The nervous systems works in conjunction with the integument system.  It helps the body keep controlled conditions within limits to maintain health and homeostasis.  The nervous system is responsible for all of our behaviors, memories and movements.  Neuroscience is the branch of medicine that deals with the normal functioning and disorders of the nervous system.
The nervous system has several functions.  The sensory function senses changes in internal and external environment through sensory receptors.  Sensory neurons, aka afferent neurons, serve this purpose.  Integrative function analyzes sensory information, stores some aspects and makes decisions regarding appropriate behaviors.  Association or inter-neurons serve this function.  The motor function responds to stimuli by actions.  Motor neurons or efferent neurons serve this function.  

Nerve tissue contains two types of cells: Neurons and Glial Cells.   The neurons are cells that transmit impulses.  Neurons are the functional unit of the nervous system. They produce action potential (which I'll describe better in a bit).  Action potential is the electrical excitability of a cell.  Neurons are made of the neuron, axons and dendrites.  Neurons have a single nucleus and organelles. dendrites bring information to the cell, while Axons send information away from the cell.  Glial cells support and nourish the neurons.  Within the nervous system, there are also blood vessels and connective tissues.
Neurons have the capacity to produce action potentials.  They are electrically excitable and then act upon that impulse.  Cell bodies have a single nucleus with a prominent nucleolus.  They also have nissl bodies that are chromatophillic substances.  There is an endoplasmic reticulum and lots of free ribosomes for protein synthesis.  There are more neurofilaments that give the cells shape and support, as well as microtubules that move material in the cell.
Dendrites conduct impulses to the cell body.  They are typically short and highly branched.  They have surfaces that are specialized for contact with other dendrites.  They also contain neurofibrils and Nissl bodies.
Axons move neurotransmitters which are made in the Golgi.  Fast axonal flow occurs in one direction and moves at 1-5mm per day.  Slow axonal flow moves organelles and materials along the surface of microtubles at 200-400mm per day.  Transports in both directions.

One way Neurons are classified by how many branches they have.  If they are multipolar, there are several dendrites and one axon and are the most common in the body.  Bipolar neurons have one main dendrite and one main axon.  They are found in the retina, inner ear and olfactory.  Unipolar neurons have only one process, an axon.
Another classification mechanism used to define neurons is their function. Sensory neurons (afferent neurons) transport sensory information from skin, muscles, joints, sense organs and viscera to the central nervous system.  Motor neurons (efferent) send motor nerve impulses to muscles and glands.  Interneurons (association) connect sensory neurons to motor neurons and are the most common in the body.

Neuroglial cells are more than 50% of the central nervous system and are 50 times more numerous.  They have many functions including holding the nervous tissue together, metabolizing glucose, producing myelin and phagocytosis.  They can divide.  If they divide too quickly, it can cause tumor formations. There are four types of neuroglial cells in the central nervous system:
astocytes: astrocytes are star-shaped cells.  They form a sheath by covering blood capillaries.  The control chemical movement around neurons.  They metabolize glucose and provide structural support.
oligodendrocytes: this is the most common type of glial cell.  each forms myelin around more than one axon in the central nervous system. They are analogous to Schwaan cells of the peripheral nervous system.  There is little regrowth of these cells after damage occurs.
Microglia and small cells found near blood vessels.  They have a phagocytic role and clear away dead cells.  They are derived from cells that also give rise to macrophages and monocytes.
Ependymal cells: form epithelial membrane lining cerebral cavities and central canal cord.  They produce cerebrospinal fluid along with capillaries in brain.

*the blood brain barrier prevents large molecules from passing through.  however, small molecules can.  But the problem is that it cannot distinguish between harmful and helpful chemicals. 
*gray and white matter. white matter is mylinated processes. gray matter is nerve cell bodies dendrites, axon terminals in bundles of unmyelinated axons and neuroflia.  IN the cord, gray matter is H-shaped inner core surrounded by white matter.  IN the brain, there is a thin outer shell of gray matter and in clusters called nuclei inside the central nervous system.  A nucleus is a mass of nerve cell bodies and dendrites inside the central nervous system.  A nucleus is a mass of nerve cell bodies and dendrites inside the central nervous system.

Neuroglia of the peripheral nervous system is completely surrounded by axons and cell bodies.  Schwann Cells encircle the peripheral nervous system and satellite cells are flat cells that surround cell bodies of neurons in the peripheral nervous system ganglia. 
Satellite cells are flat cells that surround the neuronal cell bodies in periheral ganglia.  They support the neurons in the PNS.
Each Schwaan cell encircles a PNS axon.  Each cell also produces part of the myelin sheath surround an axon in the PNS.  They help speed us neurtransmission.

Regeneration and repair:
Plasticity is maintained throughout life.  New dendrites sprout and new proteins are made.  There are also changes in the synaptic contacts with other neurons.  There is limited ability for repair.  The central nervous system cannot repair itself but the peripheral nervous system can repair some damaged dendrites or axons.  They can only be repaired if the neuron body remains intact, the Schaan cells remain active and form their tubes and scar tissue does not form too rapidly.  When an axon is damaged, there are changes called chromatolysin in the cell body.  24-48 hours after the injury, Nissl bodies break up into fine granular masses in the cell body.  This causes the swelling of cell bodies and peaks 10-20 days later.  By the 3rd-5th day, degeneration of the distal portion of axons and myelin sheath occurs.  Afterward, marcophages phagocytize the remains.  Retrograde degineration of the proximal portion of the fiber extends only to the first neurofibral stage.  Within several months, regeneration follows choratolysis.  The synthesis of RNA and protein accelerate, favoring the rebuilding of the axon, but it can take several months.  Neurolemma is on each side of the injury repairs the tube.  Nerve growth factors are released bu the glia and axonal buds grow down the tube to reconnect.
Neurogenesis is the formation of new neurons from the stem cells occurs in the hippocampus. This is a critical area for learning.  There is a lack of neurogenesis in other regions of the brain and spinal cord.  Factors that prevent neurgenesis in the central nervous system include inhibition by neuroglial cells creates and absence of growth stimulating factors, a lack of neurolemma and rapid formation of scar tissue by astrocytes.

An action potential is a nerve impulse that is an electrochemical change that travels along the length of a neuron fiber.  Transmission of signals between neurons is dependent on neurotransmitter molecules.
The resting potential (RMP) is when a cell has more positive ions on the outside than the inside.  There are more sodium molecules outside than inside and more potassium molecules inside than outside.  The axon is not conducting an impulse.  Resting membrane potential exists because of the concentration of ions that are different between the inside and outside of a cell.  The extracellular fluid is rich in sodium and chloride.  The intracellular fluid is full of potassium, organic phosphate (to change ADP to ATP) and amino acids.  Membrane permeability differs for sodium and potassium with a 50-100 times greater permeability for potassium.  There is an inward flow of sodium that cannot keep up with the outward flow of potassium.  A sodium/potassium pump removes sodium as fast as it leaks in.
The action potential is a rapid change in the membrane carried down the axon.  Sodium gates will open and let sodium into the cell creating a positive charge (depolarization). The cell says oh shit and opens potassium channels to give the cell a negative charge again (repolarization).  Depolarization and repolarization  occur like a wave going down an axon.  The sodium/potassium pump will then kick into gear and bring the levels back to normal.
If and when a neuron responds, it will respond completely.  A nerve impulse is conducted whenever a stimulus of threshold  intensity or above is applied to an axon.  The strength of the impulse will remain the same for the entire length of an axon. 
Local anesthetics are used to block pain and or other sensations.  They prevent the opening of sodium channels so nerve impulses cannot pass the obstructed region. 
An action potential spreads or propagates over the surface of an axon membrane.  As the sodium flows into the cell during depolarization, the voltage of neighboring areas is effected and their sodium channels also open.  This is called self-propagation.  The traveling action is called the nerve impulse. 
A synapse is the functional junction between neurons or between a neuron and an effector (which could be a muscle or a gland).  Depending on the neurotransmitter and the receptor, response by the post-synaptic neuron can be excitation or inhibition. Integration is the summing of the signals received by a postsynaptic neuron. 

Small molecule neurotransmitters include acetylcholine, amino acids, drugs such as marijuana, biogenic amines, neuropeptides and alcohol. Acetylcholine is released by many peripheral neurons and some central nervous system neurons.  They are excitatory on neuromuscular junctions.  They are inactivated by acetylcholinesterase.  Amino acids include glutamate which is release by nearly all excitatory neurons in the brain.  They are inactivated by glutamate specific transporters.  GABA is inhibitory neurotransmitter for 1/3 of all brain synapses.  (Valium is a GABA agonist-enhancing its inhibitory effect.) Biogenic amines are modified amino acids (from tyrosine).  Norephinephrine regulates mood, dreaming, and awakening from deep sleep.  Dopamine regulates skeletal muscle tone, the reward center in the brain and ecstasy.  Seretonin is similar to epinepherine and controls the mood, temperature regulation and induction of sleep.  These are removed from synapse and recycled or destroyed by enzymes.  Marijuana is a GABA antagonist and prevents the release of GABA.  GABA usually inhibits the second neuron from firing, but without it, the 2nd neuron fires like crazy.  It is also a dopamine reuptake inhibitor.  It allows dopamine to hang out in the cleft between neurons longer so it can bind to more receptors.  This stimulates the reward center and produces happy feelings and analgesia. 

Neuropeptides are synthesized by neurons in the brain or spinal cord.  THey are modulators of neurotransmitters. Enkephalins block pain.  Beta endorphins are a natural pain reliever. Substance P enables transmission to brain and cord.

Removal of neurotransmitters occurs through diffusion (down the concentration gradient), enzymatic degradation (acetylcholinesterase) and reuptake by neurotransmitters.  Both excitatory and inhibitory neurotransmitters are present in the central and peripheral nervous systems.  The same neurotransmitter might do the opposite effect in different locations.

Drugs that effect neurotransmitters:
synthesis drugs can stimulate or inhibit
releasae drugs can block or enhance
removal drugs can be stimulated or blocked
receptor site can be blocked or activated
agonist drugs enhance a transmitter's effects while antagonists prevent the neurotransmitter from working

Alcohol is a central nervous system depressant.  It inhibits NMDA.  It is an agonist for GABA, seretonin and dopanine and opiods.  Alcohol enters the first neuron and makes it release even more neurotransmitters.  All the GABA is dumped out and inhibits motor skills and makes you sleepy! It also dumps out more and more serotonin, which causes an increase in sexual desire, food cravings and aggression.  Dopamine is also released and tells us that alcohol is a good thing.  Endorphins are also released which prevent pain!

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