ØA. Axoplasmlc(Axonal) Transport: Movement of metabolic products through the axoplasm Can be fast (up to 400 mm/d) or slow (eg, 1 mm/d), and it involves neurotubules and neurofilaments. Anterograde or Orthograde axoplasmic transport moves newly synthesized products and synaptic vesicles toward the axon's terminal arborization and can be fast or slow. Retrograde axoplasmic transport, the return of worn materials to the perikaryon for degradation or reutilization, is usually relatively fast.
ØB. Signal Generation and Transmission: The basic function of nerve tissue is to generate and transmit signals, in the form of nerve impulses or action potentials, from one part of the body to another. The arrangement of neurons in chains and circuits allows integration of simple on-off Signals into complex information. The microscopic structure of nerve tissue (axon diameter, presence or absence of myelin, etc) exploits physicochemical phenomena to regulate the rate and sequence of signal transmission.
Ø1. Resting membrane potential.
Ø2. Firing and propagation of action potentials.
Ø3. Refractory period.
Ø4. Direction of signal transmission.
Ø5. Saltatory conduction.
Ø6. Blocking signal transmission,
HISTOPHYSIOLOGY OF NERVE TISSUE
SYNAPSES (CHEMICAL)
Synapses are specialized junctions by which a stimulus is transmitted from a neuron to its target cell. Artificially stimulated axons can propagate a wave of depolarization in either direction, but the signal can travel in only one direction across a synapse, which functions as a unidirectional signal valve. Synapses are named according to the structures they connect, eg, axodendritic, axosomatic, axoaxonic, and dendrodendritic synapses. The 3 major structural components of each synapse are the pre and postsynaptic membranes and the synaptic cleft that separates them.
A. Presynaptic Membrane: This is the part of the terminal bouton membrane closest to the target cell. It consists of an electron-dense thickening into which insert many short intermediate filaments, as in a hemidesmosome. On stimulation, neurosecretory vesicles in the bouton fuse with the presynaptic membrane and exocytose their neurotransmitters into the synaptic cleft. Neurosecretory vesicles are present only in the presynaptic component of the junction. The vesicle membrane added to the presynaptic membrane is recycled by endocytosis of the mem brane lateral to the synaptic cleft. Intact vesicles do not cross the synaptic cleft.
B. Synaptic Cleft (Synaptic Gap): This is a fluid-filled space, generally 20 nm wide, between the pre- and postsynaptic membranes. It is shielded from the rest of the extracellular space by supporting cell processes and basal lamina material that binds the pre- and postsynaptic mem branes together. Some clefts are traversed by dense filaments that link the membranes and perhaps guide neurotransmitters across the gap.
C. Postsynaptic Membrane: This is a thickening of the plasma membrane of the next neuron or target cell leg, muscle). It resembles the presynaptic membrane but also contains receptors for neurotransmitters. When enough receptors are occupied, hydrophilic channels open, resulting in depolarization of the postsynaptic membrane. Neurotransmitter leg, acetylcholine) remaining in the cleft after stimulation of the postsynaptic neuron (or other target cell) is degraded by enzyme leg, acetylcholinesterase) in the cleft. Degradation products are endocytosed by coated pits in the membrane of the bouton, lateral to the presynaptic thickening. Removal of excess transmitter allows the postsynaptic membrane to reestablish its resting spotential and prevents continuous firing of the postsynaptic neuron in response to a single stimulus.
Supporting Cells of the PNS:
1. Schwann cells are the supporting cells of the peripheral nerves. One Schwann cell may envelop segments of several unmyelinated axons or provide a segment of a single myelinated axon with its myelin sheath. Each myelinated axon segment is surrounded by multiple layers of a Schwann cell process with most of its cytoplasm squeezed out; the remaining multilayered Schwann cell plasma membrane, called myelin, consists mainly of phospholipid. The gaps between the myelin sheath Se8ments are the nodes of Ranvier. Ovoid or flattened Schwann cell nuclei lie peripheral to the axon they support. They are usually more euchromatic than the nuclei of the fibrocytes scattered among the axons.
2. Satellite cells are specialized Schwann cells in craniospinal and autonomic ganglia, where they form a one-cell-thick covering over the cell bodies of the neurons (ganglion cells). Their nuclei are spheric with mottled chromatin. In sections, the nuclei typically appear as a "string of pearls" surrounding the much larger ganglion cell bodies.
SUPPORTING CELLS
A. Supporting Cells of the CNS: There are about 10 neuroglial cells per neuron in the CNS. Glial cells are generally smaller than neurons. Their processes, although abundant and exten sive, are indistinguishable without special stains. Identification is usually based on nuclear morphology. The major supporting cells in the CNS are the macroglia, including astrocytes and oligodendrocytes, the microglia, and the ependymal cells.
1. Astrocytes are the largest glial cells. Their nuclei, also the largest, are irregular, spheric, and pale-staining with a prominent nucleolus. Their branching cytoplasmic processes often have, at their tips, expanded pediclcs, or vascular end-feet. These surround capillaries of the pia mater and are important components of the blood-brain barrier . Proteplasmic astro cytes (messy cells) are more common in gray matter. They have ample granular cytoplasm and short, thick, highly branched processes. Fibrens astrecytes are more common in white matter. Silver stains show their cytoplasm to be full of fibrous material. Their long, thin processes are less branched than those of protoplasmic astrocytes. 2. Oligedendroglia or oligodendrocytes, the most numerous glial cells, are found in both gray and white matter. Their spheric nuclei fall between those of astrocytes and microglia in terms of size and staining intensity. Like the Schwann cells of the PNS, oligodendrocytes form myelin and occur in long rows as required to myelinate entire axons. Unlike a Schwann cell, each may have several cell processes and may provide myelin for segments of several axons. Unmyelinated axons of the CNS are not sheathed. 3. Microglia, the smallest and rarest of the glia, are found in both gray and white matter. Their nuclei are small and elongate (often bean-shaped), and their chromatin is so condensed that they often appear black in H&E-stained sections. Their processes are shorter than those of astrocytes and are covered with thorny branches. Microglial cells may derive from mes enchyme, or they may be glioblasts (immature oligodendrocytes) of neuroepithelial origin. Some microglia may be components of the mononuclear phagocyte system and have phago cytic capabilities. When neural injury is unaccompanied by vascular injury, phagocytic cells in the lesioned area appear to derive from macroglia. 4. Ependymal cells derive from ciliated neuroepithelial cells of the internal lining of the neural tube. In adults, they retain their epithelial nature and some cilia, and they line the remnants of the neural tube (ventricles and aqueducts of the brain and the central canal of the spinal cord). The lining resembles a simple columnar epithelium, but epcndymal cells have basal cell processes that extend deep into the gray matter. The ependymal lining is con tinuous with the cuboidal epithelium of the choroid plexus.
Neurons
ØA. Cell Body: The cell body (soma, perikaryon) is the synthetic and trophic center of the neuron. It can receive signals from axons of other neurons through synaptic contacts on its plasma membrane and relay them to its axon. The abundant free and RER-associated polyribosomes appear as clumps of basophilic material collectively called Nissl bodies.
ØB. Dendrites: These extensions of the soma are specialized to increase the surface available for incoming signals.
ØC. Axon: Each neuron has one axon, a complex cell process that carries impulses away from the soma. An axon is divisible into several regions. The axon billock, the part of the soma leading into the axon, differs from the rest of the perikaryon in that it lacks Nissl bodies.
ØD. Classification of Neurons
properties of nervous system - 2
H. Blood-Brain Barrier: Nerve tissue of the CNS receives oxygen and nutrients from capillaries in the pia mater. These capillaries are relatively impermeable because (1) their endothelial cells lack fenestrations and are joined at their borders by tight junctions, and (2) they are partly surrounded by the cytoplasmic processes of neuroglia called astrocytes. These features contribute to a structural and functional barrier that protects CNS neurons from many extraneous influences and prevents certain antibiotics and chemotherapeutic agents from reaching the CNS.
Properties of nervous system
E. Embryonic Development of Nerve Tissue: All neurons and supporting cells derive from embryonic ectoderm.
F. Aging and Repair: Mature neurons are incapable of mitosis and are often used as examples of terminally differentiated cells. Neurons of the elderly may contain abundant lipofuscin pigment. The inability of neurons to divide makes repair of injured nerve tissue more difficult than for most other tissues. Neuron cell bodies lost through injury or surgery cannot be replaced, but if an axon is severed or crushed and the cell body remains intact, regeneration of the injured axon is possible. Supporting cells, unlike neurons, can divide if stimulated by injury.
G. Meninges: The brain and spinal cord are separated from the bony compartments that house them (skull and vertebral canal) by 3 connective tissue layers termed the meninges. The outer layer, or dura mater, is dense connective tissue bound tightly to the periosteum of the surround ing bone. The middle layer, or arachnoid, has 2 components: (1) a layer of loose connective tissue in contact with the dura mater, and (2) many connective tissue trabeculae (strands) that attach the arachnoid to the underlying pia mater. The spaces between the arachnoid trabeculae contain cerebrospinal fluid. Projections of the arachnoid into sinuses in the dura are called arachnoid villi. The innermost layer, or pia mater, is a thin, richly vascularized layer of loose connective tissue that is firmly attached to the surface of the brain or spinal cord but separated from the neurons by neuroglial cells processes. Ramified, cuboidal epithelium-covered projections of the pia matter into the ventricles of the brain are collectively termed the choroid plexus; they produce the cerebrospinal fluid by selective ultrafiltration of the blood plasma.
