Muscle and nerve are both excitable tissues specialized for specific functions. The types and physiology of muscle has been covered in detail elsewhere. Blood can be thought of as a specialized connective tissue with cells embedded in a fluid extracellular matrix containing soluble fibrillar proteins. Therefore, here we discuss the basic features of nervous tissue and blood.
Nervous tissue derives from the embryonic ectoderm under the influence of the notochord. The ectoderm is induced to form a thickened neural plate that then differentiates and the ends eventually fuse to form the neural tube from which all of the central nervous system derives. The central nervous system consists of the brain, cranial nerves and spinal cord. The peripheral nervous system derives from cells next to the neural groove called the neural crest.
Nerve tissue is distributed throughout the body in a complex integrated communications network. Nerve cells (neurons) communicate with other neurons in circuits ranging form very simple to very complex higher-order circuits. Neurons do the actual message transmission and integration while other nervous tissue cells called glial cells assist neurons by support, protection, defense and nutrition of the neurons.
There are about 10 times more glial cells than neurons in the brain. Glial cells create the microenvironment needed for neuronal function and sometimes they assist in neural processing and activity. Neurons are excitable cells. This means that when properly stimulated, an action potential can be initiated that may be propogated over the cell membrane to transmit information to distant cells. Neurons are independent functional units responsible for the reception, transmission and processing of stimuli.
In general, neurons consist of three parts; the cell body, where the nucleus and cellular oganelles are located; dendrites, which are processes extending from the cell body that receive stimuli from the environment or other neurons; and the axon, which is a long single process extending from the cell body for the transmission of nerve impulses to other cells. The axon usually branches at its distal end and each branch terminating on another cell has a bulbous end. The interaction of the end bulb with the adjacent cell forms a structure called a synapse. Synapses are specialized to receive a signal and convert it into an electrical potential.
Most neurons found in the human body are multipolar, meaning they have more than two cell processes with only one being an axon and the remaining processes being dendrites. Bipolar neurons of the retina or olfactory mucosa have one dendritic process and an axon coming off the cell body. Pseudounipolar neurons found in the spinal cord ganglia enable sensory impluses picked up by the dendrites to travel directly to the axon without passing through the cell body.
Neurons may also be classified according to function. Sensory neurons are involved in the reception and transmission of sensory stimuli. Motor neurons send inpulses to control muscles and glands. Other neurons, interneurons, act as go-betweens between neurons as part of functional networks.
Synapses are specialized functional cell junctions to propagate cellular signals. Most synapses are chemical synapses where vesicles in the presynaptic terminal contian a chemical messenger that is released to the synapic cleft when the presynaptic membrane is stimulated. The chemical messenger diffuses across the synaptic cleft to bind to receptors in the postsynaptic membrane. This induces a change in the polarization state of the postsynaptic membrane effecting cellular action. A special type of synapse is the neuromuscular junction.
More than 35 neurotransmitters are known and most are small molecules (nitric oxide, acetylcholine), catacholamines (norepinephrine, serotonin), or neuroactive peptides (endorphin, vasopressin). Once used, the neurotransmitters are removed quickly by enzymatic breakdown, diffusion or endocytosis by the presynaptic cell.
Some neurons are wrapped in an insulating material called myelin. This lipid rich material is formed by glial cells; Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system. The insulation enables faster nerve conduction by reducing the membrane surface area that must be depolarized. In myelinated neurons the nerve impulse jumps from one unmyelinated segment to another over the length of the axon. It is the myelin sheath and lack of neuron cell bodies within the tissue that makes some nervous tissue appear white as in the large peripheral nerves and white matter of the brain.
Other glial cells, called astrocytes, are involved in structural integrity, neuronal nutrition and maintaining the microenvironment of nervous tissue. Astrocytes, are in direct communication with one another via gap junctions and can affect the survival of neruons in their care by the regulation of the the local environment. Ependymal cells line spinal cord and the ventricles of the brain and secrete the cerebrospimal fluid. Other small glial cells, called microglia, are phagocytic cells that are involved with inflammation and repair in the adult central nervous system.
Blood consisits of fluid and cells that flow in one direction in a closed circulatory system. The formed elements are blood cells and platelets. The liquid in which they are suspended is called plasma. Plasma is an aqueous solution containing proteins, inorganic salts, amino acids, vitamins, hormones and other organic compounds. The main plasma proteins are albumin, globulins, and fibrinogen. Albumin is necessary in maintaining osmotic pressure and acts as a transport protein for various substances. The globulins are, for the most part, antibodies and fibrinogen is involved in the clotting process.
Red blood cells are anucleate and filled with the oxygen carrying protein hemoglobin. Human erythrocytes are biconcave discs 7.5 microns in diameter. The biconcave shape provides a large surface-to-volume ratio for oxygen delivery and better flexibility in narrow capillaries. There are normally 3.9-5.5 million per microliter in women and 4.1-6 million per microliter in men. A decrease in the number of red blood cells is called anemia.
Within the cell membrane of erythrocytes is the respiratory pigment hemoglobin and enzymes to break down glucose. Erythrocytes loose their mitochondria, nucleus, ribosomes and many cytoplasmic enzymes during maturation to become specialized for carrying oxygen. Maturation in bone marrow takes 24-48 hours and the average red blood cell survives in circulation for 120 days. Old erythrocytes are removed from circulation by phagocytic cells of the spleen and bone marrow.
White blood cells are found in circulation but may leave the circulatory system and migrate to the tissues to perform various functions. White blood cells are involved in cellular and humoral defense of the organism. They are capable of motility and often leave the circulatory system by slipping through the vessel endothelium by a process called diapedesis. The number of leukocytes varies according to the age, sex, and physiologic conditions with normal adults having about 6,000-10,000 leukocytes per microliter of blood. Leukocytes are classified into granulocytes and agranulocytes based on the presence or absence of visible granules within the cellular cytoplasm. Granulocytes include neutrophils, eosinophils, and basophils. Agranulocytes are the monocytes and lymphocytes.
These cells make up the bulk of ciculating leukocytes (60-70%). Neutrophils are short-lived cells (6-7 hours in blood and 1-4 days in tissue) that are12-15 microns in diameter with a multilobed nucleus. Immature neutrophils have a nonsegmented horseshoe shaped nucleus. Older neutrophils have more than five lobes and are called hypersegmented. The cytoplasm contains small granules that are primary lysosomes containing numerous enzymes. There are few mitochondria in the cytoplasm and netrophils mostly survive by the anaerobic use of glycogen for energy. These phagocytic cells surround and engulf bacteria and constitute a defense against invasion.
These granulocytes are much less numerous than neutrophils and make up 2-4% of leukocytes in normal blood. Eosinophils have a diameter of 12-15 microns and contains a characterisitic bilobed nucleus. The main identifying feature of eosinophils is the presence of many large and elongated granules that are stained red by eosin. An increase in the absolute number of eosinophils in circulation is associated with allergic reactions and parasitic infections. These cells also produce substances that modulate inflammation.
These cells contain numerous granules that contain heparin and histamine and may mediate hypersensitivity reactions. These granules stain blue with the typical hemotoxylin/eosin stain used in blood smears. Basophils are often difficult to find in blood smears because they are so few in circulating blood (less than 1% of blood leukocytes).
Lymphocytes are spherical cells with a diameter of 6-18 microns with a spherical nucleus and very little cytoplasm. Some large lymphocytes are thought to be memory cells that differentiate into effector T cells or B lymphocytes having specific antigen receptors on the cell surface. T cells participate in cellular immunity while B cells are involved in humoral immunity as they differentiate into plasma cells that make specific antibodies. Lymphocytes vary in life span with some living only a few days while others ciculate for many years. These cells are the only white blood cells that return to the blood stream after migrating to the tissues.
These agranulocytes vary in diameter from 12-20 microns and have an oval, horseshoe or kidney shaped nucleus with more cytoplasm than lymphocytes. The nucleus does not stain as darkly as that of lymphocytes in blood smears. Monocytes represent 3-8% of circulating white blood cells. After leaving the ciculatory system, monocytes differentiate into macrophages in connective tissues.
These cell fragments originate from large cells in the bone marrow called megakaryocytes, have a lifespan of about 10 days and are nonnucleated. Platelets promote blood clotting and repair gaps in the walls of blood vessels. Upon injury to the vessel endothelium the platelets aggregate and form a platelet plug. They then release substances that induce further platelet aggregation and blood clotting. After the vessel is repaired by the formation of new tissue the clot is removed by plasma and platelet derived enzymes. Normal platelet count ranges from 200,000 to 400,000 per microliter of blood.