neurological histology

Neurological histology:

Neurons make up the tissue of the nervous system. They are highly specialized for impulse conduction to control muscle activity, think, and regulate glands

The central nervous system is divided into the afferent (sensory) and efferent (motor) systems. The efferent system is then broken into the somatic nervous system, which controls voluntary muscular contractions, and the autonomic nervous system which is involuntary. The ANS is divided into the sympathetic and parasympathetic nervous systems

Neuroglia serve as special supporting cells in the nervous system, also called glial cells (glia=glue). Glial cells are generally smaller than neurons, but outnumber them by five to ten times. Many of the glial cells form a supporting network by twining around nerve cells are lining certain structures in the brain and spinal cord. A few glial cells also have specialized functions. Some produce a phospholipid covering, called the myelin sheath, around the axons of neurons. These myelin sheaths speed the impulse conduction rate and insulate fibers. Certain glial cells are phagocystic. Of clinical interest, neuroglia are a common source of tumor (gliomas), which account for some forty-five percent of intercranial tumors. There are four types of glial cells:

astrocytes-- as the name suggests, these cells are star-shaped, with many processes. Protoplasmic astrocytes are found in gray matter in the central nervous system. Fibrous astrocytes are located in white matter in the central nervous system. They form a supporting network in the brain and spinal cord, as well as attack neurons to their blood vessels
oligodendrocytes-- resemble astrocytes, but with fewer and shorter processes. The lend support to semirigid connective tissue rows between neurons in the brain and spinal cord and produce myelin sheathing for neurons of the central nervous system
microglia-- small cells with only a few processes, derived from monocytes. Mostly, these cells are stationary, but may migrate to the site of an injury. Microglia are also called brain macrophages. Microglia are phagocystic
ependyma (ependymocytes)-- these are epithelial cells arranged in a simple layer, ranging from squamous to columnar. Many ependyma are ciliated. This epithelium forms a continuous lining for the ventricles of the brain and central canal of the spinal cord, and probably assist in circulation of cerebrospinal fluid

Neurons are the nerve cells responsible for the conduction of impulses. They are made up of three distinct parts:

cell body, soma, or perikaryon-- contains a well defined nucleus and nucleolus surrounded by a granular cytoplasm. The cytoplasm contains all of the typical cellular organelles. Many neurons also have inclusions such as lipofuscin, yellowish in color, and may be a by-product of lysosomal activities. This lipofuscin grows in amount as we age. The chromatophilic substance, or Nissl bodies, is an orderly arrangement of rough endoplasmic reticulum. The proteins from these Nissl bodies are used to restore peripheral nerve fibers. Neurofibrils are long, slender fibers composed of microtubules. Neurofibrils assume the task of supporting the cell and transporting nutrients. Mature neurons do not have mitotic potential
dendrites-- carries impulses towards the cell body. They are the shorter of the two extensions of the cell body. Neurons usually have several main dendrites. Dendrites typically contain chromatophilic substance and typical organelles
axon, axis cylinder-- a single long, thin process of the neuron originating from a small, conical elevation-- the axon hillock. The axoplasm houses no chromatophilic substance, and thus cannot synthesize proteins. The axon is surrounded by a plasma membrane known as the axolemma. Axons can range in length from a millimeter to a meter. Branches of the axon are called axon collaterals. The axon and it's collaterals terminate in axon terminals, or telodendria. The axon terminals expand into synaptic bulbs. These bulbs contain membrane-enclosed sac referred to as synaptic vesicles

Neurons have two main types of intracellular systems for transporting synthesized material from the cell. The slower of the two is axoplasmic flow. It moves axoplasm in only one direction; from the cell body, down the axon for regeneration, restoration and replenishing the axon. The faster is called axonal transport. It conveys materials on both directions, possibly along tracks formed by microtubules and filaments. Axonal transport moves various organelle secretions that form the neurolemma, synaptic endbulbs and contents and membrane of the synaptic vesicles

The axons of many neurons are wrapped in a segmented myelin sheath. The cells of this sheath are neurolemmocytes, or Schwann cells. They secrete the white phospholipid covering that speeds the rate of impulse conduction. To make the myelin sheath, the neurolemmocyte wrapped itself about the axon, and spirals about it. The spiraling squeezed most all of the cytoplasm and organelles, including the nucleus, to the outside, stopping when there is about twenty to thirty layers of it's phospholipid membrane embrace the axon. The part of the cell membrane which encloses the cytoplasm and organelles of the Schwann cell is called the neurolemma. Myelin is responsible for 'white' matter in the brain, nerves and spinal cord. The neurolemma is only found in the peripheral nervous system. Gaps in the myelin sheath are neurofibral nodes, or nodes of Ravier. Unmyelated axons are also sometimes wrapped in neurolemmocytes, but without the multiple layers. The central nervous system has fewer neurofibral nodes, and uses oligodendrocytes to aid in myelation. The unfinished process of myelation in an infant accounts for it's slower reaction time

Neurons can be categorized by form and function:

multipolar neurons-- have several dendrites and one axon. Most of the neurons in the brain and spinal cord are of this type
bipolar neurons-- have one dendrite and one axon. Found in the retina of the eye, inner ear, and olfactory area
unipolar neurons-- have on process, which splits into two branches. The central branch acts as the axon, the peripheral, a dendrite. Originate as bipolars in the fetus. Unipolar neurons are found in the posterior (sensory) root ganglia of spinal nerves
sensory, afferent neurons-- usually unipolar

exteroceptors-- provide information about the outside world. Located near the surface of the body

free, naked nerve endings-- touch receptor

tactile, Merkel's discs-- touch receptors, deep epidermal

corpuscles of touch, Meissner's corpuscles-- touch receptors, deep dermal

type II cutaneous mechanoreceptors, end organs of Ruffini-- receptors for heavy, continuous touch. Located usually more deep to the other touch receptors

lamellated, Pacinian corpuscles-- pressure receptors

nociceptors-- pain receptors. Actually the branching ends of certain sensory neurons

visceroceptors-- provide information about the internal environment
proprioceptors-- provide information about position and movement

muscle spindles-- delicate skeletal muscle receptors

tendon organs, Golgi tendon organs-- receptors at the junction between a tendon and muscle

joint kinesthetic receptors-- similar in structure to type II mechanoreceptors

motor, efferent neurons-- convey impulses from the brain to to effectors (glands, organ, muscles)
association, connecting, or interneuron neurons-- carry information from sensory neurons to the brain and spinal cord. Examples of association neurons are stellate cells, cells of Martinotti, horizontal cells of Cajal, and pyramidal cells. All are found in the cerebral cortex. Granule cells and Purkinje cells are association cells in the cerebellum. Up to ninety percent of neurons in the body are association neurons

The sodium-potassium pump keeps a -70 millivolt resting potential of the plasma membrane. The concentration of sodium outside the cell is much greater then that on the inside. The opposite is true of potassium on the inside of the cell. The outside of the membrane keeps a positive charge. On the inside are potassium, calcium, and negatively charged, nondiffusable proteins to keep a negative charge. The ability of the cells to respond to stimulus is it's excitability. If a stimulus of adequate strength, a threshold stimulus, is applied, the membrane become permeable to sodium ions. The potential moved from -70 towards 0, then to a positive value. This loss of polarization is called depolarization, and begins at -69 millivolts. The high point of the depolarization is at about 30 millivolts. This action moves like a wave down the neuron. Once the events of depolarization have taken place, it is said that an action potential, or nerve impulse, has been initiated. By the time the depolarization has moved to another point, the previous has begun repolarization. The sodium-potassium pump restores the resting potential. The period directly after depolarization in which the cell cannot generate another action potential is called the refractory period, which may last up to 0.4 milliseconds. Threshold stimulus can be either one strong stimulus, several smaller ones from different dendrites, or several smaller in direct succession from one dendrite. When the depolarization reached the end of the axon, the influx of sodium sets the calcium ions on the synaptic vesicles in the presynaptic end bulb. They then release neurotransmitters across the synaptic cleft to the postsynaptic neuron. The acetylcholine is disassembled by acetylcholinesterase and sent back to the presynaptic neuron after the depolarization has moved on

In myelinated neurons, the impulse jumps from one neurofibral node to the next, greatly increasing the rate of impulse conduction. The speed of conduction depends on temperature, the diameter of the nerve fiber, and the presence of myelin. Cold is applied to painful areas because it slows the painful nerve impulses

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