Microcirculation •       Capillaries are the sites for exchange of materials between blood and tissue cells. •    ...


           Microcirculation




•      Capillaries are the sites for exchange of materials between blood and tissue cells.
•      The walls of the capillaries are extremely thin, constructed of single-layer of highly permeable endothelial cells. Therefore water, cell nutrients, and cell excreta can interchange quickly and easily between the tissues and the circulating blood.
•      The peripheral circulation of the whole body has about 10 billion capillaries with a total surface area estimated to be 500 to 700 square meters.

Structure of the Capillary Wall


•      The wall is composed of a single layer of endothelial cells and is surrounded by a very thin basement membrane on the outside of the capillary. The total thickness of the capillary wall is only about 0.5 micrometers. The internal diameter of the capillary is 4 to 9 micrometers.
Structure of Microcirculation and Capillary System
•      Each nutrient artery entering an organ branches six to eight times before the arteries become small enough to be called arterioles, which generally have internal diameter of only 10 to 15 micrometers. Then the arterioles branch two to five times, reaching diameter of 5 to 9 micrometers at their ends where they supply blood to the capillaries.
•      The arterioles are highly muscular, and their diameter can change manyfold.The metarterioles (the terminal arterioles) do not have a continuous muscular coat, but smooth muscle fibers encircle the vessel at intermittent points.
•      At the point where each capillary originates from a metarteriole, smooth muscle fibers usually encircle the capillary. This is called the precapillary sphincter. This sphincter can open and close the entrance to the capillary.
•      The venules are larger than the arterioles and have a much weaker muscular coat.
•       The pressure in the venules is much less than that in the arterioles. The venules can contract despite the weak
      muscle.


Pores in the Capillary Membrane

•      An intercellular cleft thin-slit like curving channel that lies between adjacent endothelial cells. Each cleft is interrupted by short ridges of protein attachments that hold the endothelial cells together, but between these ridges fluid can percolate freely through the cleft. The cleft normally has a uniform spacing with a width of about 6 to 7 nanometers.
•      There are many minute vesicles called caveolae in the endothelial cells. These are formed from protein caveolins.
•      They are believed to play role in endocytosis.

Vasomotion

•      Intermittent contraction of the metarterioles and precapillary sphincters.
          Because of this blood does not flow                  continuously through the capillaries.

Regulation of Vasomotion

•      The most important factor found to affect the degree of opening and closing of the metarterioles and precapillary sphincters is the concentration of oxygen in the tissues.When the rate of oxygen usage by the tissue is great so that tissue oxygen concentration decreases below normal the intermittent periods of capillary blood flow occur more often, and the duration of each period of flow lasts longer, thereby allowing the capillary blood to carry increased quantities of oxygen (as well as other nutrients) to the tissues.
•      Exchange of water, nutrients and other substances between the blood and Interstitial fluid occurs by diffusion through the capillary membrane.



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HEART The Cardiac Muscle •      Branched •      Centrally located nucleus •      Muscle cells of the heart-...


HEART








The Cardiac Muscle


•     Branched
•     Centrally located nucleus
•     Muscle cells of the heart-more commonly called myocytes or myofibrils.
•     Outside membrane is called sarcolemma.
•     Cardiac muscles have the same arrangement of actin and myosin, and the same bands, zones and Z discs as skeletal muscles forming sarcomeres.
•     They do have less sarcoplasmic reticulum than skeletal muscles and require Calcium from extra cellular fluid for contraction as T-tubules are not well organized.

The Pericardium & the Pericardial Sac:
The heart is enclosed in the double-walled, membranous pericardial sac (peri means “around”). The  sac consists of two layers—a tough, fibrous covering and a secretory lining.
The outer fibrous covering of the sac attaches to the connective tissue partition that separates the lungs. This  attachment anchors the heart so that it remains properly positioned within the chest.
The sac’s secretory lining secretes a thin pericardial fluid, which provides lubrication to prevent friction between the pericardial layers as they glide over each other with every beat of the heart.
Pericarditis, an inflammation of the pericardial sac that results in a painful friction rub between the two pericardial
layers, occurs occasionally because of viral or bacterial infection.

Heart walls are composed of 3 distinct layers:


1.  ENDOCARDIUM (inner): thin layer of endothelium that lines the entire circulatory system (endo means “within”)
2.  MYOCARDIUM (middle): composed of cardiac muscle that forms the bulk of heart wall (myo means “muscle”)
3.  EPICARDIUM (outer): thin external membrane covering the heart (epi means “on”)

Arrangement of the heart muscles
The myocardium consists of interlacing bundles of cardiac muscle fibers arranged spirally around the circumference of the heart.
What is the advantage of the spiral arrangement?
When the cardiac muscle contracts & shortens, a wringing effect is produced, efficiently pushing blood upwards towards the exit of the major arteries of the heart.

Intercalated Discs

Although the cardiac muscles interdigitate & branch, there is no anatomical continuity b/w the individual muscle fibers.
•     Cardiac muscle are branched, have a single nucleus and are interconnected to each other, end to end by specialized structures called as INTERCALATED DISCS. The intercalated discs are further composed of:
1.  Gap Junctions
2.  Desmosomes

HEART AS A DUAL PUMP:
Even thought the heart is a single organ, the left and the right side of the heart is anatomically and functionally separate.
This is done with the help of the interventricular spetum.
It ensures that the blood from the left and right side of the heart does not mix.
Although the left and the right sides are separated, the heart contracts in a co-ordinated fashion: the atria contract together and the ventricles contract together….
·      O2 poor blood returns from the body thru the Superior & Inferior Vena Cava
·      Enters the Right atrium
·      Right ventricle
·      Pulmonary artery
·      Lungs
·      Blood is oxygenated
·      Pulmonary Veins
·      Left Atrium
·      Left Ventricle
·      Aorta
·      Circulated to the body


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Classification of neurons: Neurons are classified on the basis of: STRUCTURE: •       Unipolar •       Bipolar •       ...


Classification of neurons:




Neurons are classified on the basis of:

STRUCTURE:

•      Unipolar
•      Bipolar
•      Multipolar



     FUNCTION:
•      Motor
•      Sensory
•      Interneurons

Classification of nerves     
–   Unipolar
–   Bi-polar
–   Multi-polar

Myelination
•      Myelination is the presence of myelin around the neuron. Myelin is not part of the structure of the neuron but consists of a thick layer mostly made up of lipids, present at regular intervals along the length of the axon.
•      Such fibers are called myelinated fibers.
•      The water-soluble ions carrying the current across the membrane cannot permeate this coat, it act as an insulator, just like the white coating of the electric wires and prevents the leakage of ions from the neuron through its membrane.

How does the process of myelination occur? 
Myelination is carried out by myelin-forming cells that wrap themselves around the axons in jelly-roll fashion. These myelin-forming cells are Schwann cells in the PNS (peripheral nervous system) and the Oligodendrocytes in the CNS (brain & the spinal cord)
Myelination
•      Outside CNS
1.          Schwann cells
2.          Neurons CAN regenerate
3.          Neuron can recover after injury

•      Inside CNS
1. Oligodendrocytes
  2. Neurons CANNOT regenerate
3.          Neurons DIE after injury
Outside the CNS: myelinated fibers
•      Myelination is not part of the neuron but is done by the schwann cells.
•      As the diagram shows, the nerve cell invaginates the schwann cell…
•      The schwann cell wraps around the axon in concentric spirals.
•      Collectively, the various layers form the myelin sheath (a patch of myelin might be made of upto 300 layers of wrapped lipid bilayers)

Nodes of Ranvier



•      In myelinated nerve fiber, the myelin sheath is not a continuous sheath, but is deficient at regular intervals.
•      Between the myelinated regions, at the NODES OF RANVIER, the axonal membrane is bare and exposed to the ECF.
•      Current can flow across the membrane only at these bare spaces to produce action potentials.
•      Voltage-gated Na+ channels are concentrated at these regions.

Fibers OUTSIDE the CNS
•      Myelinated (WHITE MATTER)
–   Only single nerve fiber invaginates single cell
–   Concentric layers of schwann cells wrapped around the fiber
–   No cytoplasm as all squeezed out- process called myelination
–   Outermost layer called Neurilemma or sheath of schwann
–   White appearance (white matter)
•      Unmyelinated (GREY MATTER)
–   Small diameter fibers
–   The nerve fiber only invaginates
–   No concentric layers or wrapping
–   A single schwann cell is invaginated by multiple nerve fibers
–   Nerve fibers surrounded by Schwann cell cytoplasm
–   Gray appearance (gray matter)

Nerve fibers lying WITHIN the CNS
•      Myelinated fibers
–   Myelin sheath produced by Oligodendrocytes
–   Myelinates upto 6 nerve fibers at a time.
–   Do not aid in regeneration
•       Unmyelinated fibers
–   Not supported by Oligodendrocytes
–   Indirectly supported by mass of surrounding tissues.
–   Do not aid in regeneration.
SALTATORY CONDUCTION
In a myelinated nerve fiber, the nerve impulse “jumps” from node to node skipping over the myelinated sections of the axons. This process is called Saltatory conduction.
Basis: Saltatory conduction propagates nerve impulse more rapidly because the nerve impulse has to be generated only at the nodes of ranvier and not repeatedly. Thus, it is faster.
In unmyelinated fibers, the nerve impulse is like a grasshopper walking while in a myelinated fiber, the nerve impulse is like grasshopper jumping.

Layers of nerve fibers
•      Endoneurium: finely reticular tissue lying just next to neurilemma.
Surrounds individual fibers separating them from each other.
Forms the endoneurial tube.
•      Perineurium: Several nerves surrounded by layer of connective tissue.
•      Epineurium: Nerve trunk itself surrounded by a loose layer of elastic tissue and CT.




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NEURON/ NERVE   PARTS OF NERVOUS SYSTEM: Brain Spinal cord Peripheral nerves NERVOUS SYSTEM The system that c...


NEURON/ NERVE




 PARTS OF NERVOUS SYSTEM:

  • Brain
  • Spinal cord
  • Peripheral nerves

NERVOUS SYSTEM

The system that control all of the activities of the body
It is made of:
  • The brain       
  • The spinal cord
  • The nerve
  • The senses

NERVOUS SYSTEM
1.          CENTRAL NERVOUS SYSTEM
·    Brain
·    Spinal cord
         
2.          Peripheral nervous system
·    Sensory output
·    Motor output

NEURON
It is a basic structural and functional unit of the nervous system.
It is a highly differentiated and specialized excitable tissue.
•      The Human NS contains 100 billion neurons.
•      The nerves allow you to react to a stimulus.
Functions
•      Reception of the stimulus
•      Generation of the nerve Impulse
•      Transmission of the nerve Impulse
Structure of the A Typical Neuron
•      A typical neuron thus has the following parts:
1.                      Soma or Nerve Cell body
2.                      Axon with the axon terminals
3.                      Dendrites
Nerve Cell Body
•      Nissl bodies
–   Are rough endoplasmic reticulum with ribosomes
–   Stained with basic dyes
–   Composed of RNA & polysomes.
–   Tigroid substance (due to striped appearance)
–   Not present in the axon
–   Synthesis of proteins
–   Dissolve & disappear if cell injured (nerve cut, injured, fatigued, poisoned)
•      Neurofibrils
–   Formed by clumping of neurotubules & neurofilaments
–   Delicate threads running from cytoplasm of the nerve cell body into the axon and the dendrite
–   Functions:
–   Neuronal microtubules transport substances from the cell body to the distal cell processes.
–   Neurofibrils give support and shape to the neuron.
Axon
•      Also called axis cylinder or nerve fiber.
•      Longest process
•      A single axon arises  from a cone-shaped area of the neuronal cell body called the axon hillock
•      Axon hillock & first 100 µm of axon (no myelin sheath) is called Initial segment.
•      Trigger zone: is the name given to the axon hillock & the initial segment. It is an area that shows high excitability and a nerve impulse is generated here.
AXON IS MADE UP OF:
•      Jelly-like semi fluid substance called Axoplasm
•      Plasma membrane called Axolemma
•      Mitochondria and ER
•      No Nissl granules so Does NOT synthesize proteins.
AXON ENDS IN:
Terminal Buttons (Synaptic knob or Bouton Terminaux)
–   Axon break up into no. of terminal branches called Telodendria or Terminal filaments
–   At their end is a small swelling called Terminal knob.
–   These knobs contain granules or vesicles with neurotransmitter substance
Dendrites
•      Short, tree-like, highly branched tapering processes of the nerve cell
•      Receive and then carry impulses to the cell body
•      Small knob-like projections called dendritic spines
•      Have all the components of the cell body 





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