| What is the nervous tissue? (histologic and anatomic level) | At the histological level, nervous tissue is formed of 2 types of cells: nerve cells (or neurons) and glial cells. These cells, along with their intercellular medium and a rich blood capillaries network, form the nervous system.
Considering the anatomical, physiological or functional levels, we distinguish:
- The central nervous system (CNS) consisting of the brain and spinal cord; it is the body’s central control system, and is protected by the skull and the vertebral column.
- The peripheral nervous system (PNS) consisting of the nerve fibers emerging from the brain (cranial nerves) and the spinal cord (spinal nerves) and reaching all the organism; these nerves link the CNS to receptors, muscles and glands |
| Talk about the roles of nervous tissue | The nervous system is the major regulatory system and communication network of the body. It is specialized in reception, treatment, storage and transfer of information received either from the external environment or from the organism itself. It accomplishes therefore 3 basic functions: the sensory function, by detecting any internal or external body modification, the integrative function, by interpreting the detected changes, and finally the motor function by reacting to the interpretation through addressing to the effector organs (muscles and glands) the orders necessary for the individual life and the species survival. |
| What is the CNS composed of? | The central nervous system (CNS) is composed of nerve cells (and their extensions), glial cells and blood capillaries |
| Talk about nerve cells (neurons) | Neurons are the structural and functional units of the nervous system. They are highly specialized cells that do not divide. They are responsible for genesis, treatment and propagation of information. They conduct nerve impulses and allow rapid and specific communication of distant regions of the body through three essential properties: excitability (response to stimulation), conductivity (propagation of this remote response) and communicability (transmission of a message to another cell). |
| Talk in general about the morphology of neurons. | The neuron has a particular morphology: it is constituted by a cell body (perikaryon or soma) from which two types of processes emerge: the dendrites and the axon. A plasma membrane (or axolemma) covers the cell body and its extensions; it is the anatomical support of the propagation of nerve impulses. |
| Talk in general about cell body of neurons. | The cell body of the vast majority of neurons is located in the central nervous system (CNS). It is of varied shape and size. It can be rounded or oval, sometimes triangular or pyramidal. It contains the usual organelles as well as other elements |
| What is the cell body of neurons consisting from? | --a nucleus (most neurons possess in the middle of their cell body a single large spherical nucleus containing a large dense rounded nucleolus, visible in optical microscopy),
--a prominent Golgi apparatus
--many mitochondria
--well-organized cytoskeleton that maintains the shape of the neuron and allows specific functions such as axonal transport
--a smooth endoplasmic reticulum (SER)
--Nissl bodies: in optical microscopy it is a very basophilic material in the form of fine granulations. In electron microscopy these granulations correspond to RER cisternae, between which there are numerous free ribosomes reflecting the intense activity of protein synthesis. The neuronal cytoskeleton is composed of actin microfilaments, microtubules (MT) and neurofilaments (NF). Neurofilaments are specifically axonal. |
| Talk about dendrites. | they are non-myelinated expansions of the cell body,
differentiated for the reception of the nervous message (afferent). Most often they are short and very ramified. Their number, their length and the mode of spatial distribution of their ramifications vary from one neuron to another. They contain the same cytoplasmic organelles but no neurofilaments or Golgi apparatus. Their surface is irregular and presents lateral protrusions called dendritic spines |
| Talk about the axon | cylindrax: unlike dendrites, the axon is a unique cytoplasmic
process of the neuron having a constant diameter. Usually it is thinner, longer, more rectilinear than the dendrites of the same neuron. The axon originates in a cone-shaped region of the cell body called implantation cone (axon hillock) where there are some ribosomes but no Nissl bodies and no Golgi. Its initial segment is the site of the action potential of nerve impulses and contains microtubules and neurofilaments. Its main segment (that could be more or less long) contains neurofilaments, microtubules and mitochondria. On its way the axon can
emit lateral (collateral) branches and it ends with fine branches, called axonal endings, at the end of which are the synaptic buttons. |
| Talk about types of neurons (sensory ones) | Depending on the number, polarity and general arrangement of the extensions in relation to the cell body, there are 4 types of neurons:
- Unipolar neurons: they have only one process, the axon which starts from the rounded cell body. This is the typical type in invertebrates. In humans they are present at the embryonic stage of neuroblasts
-Pseudo-unipolar neurons: they have a single main process which, near the cell body, is divided into an afferent prolongation (dendrite) and an efferent prolongation (axon). Ex: T-neurons of the spinal ganglia. |
| Talk about types of motor neurons. | - Bipolar neurons: two main diametrically opposite processes of equal size extend from the oval or elongated cell body, and can branch, one dendritic, the other axonal. These are interneurons. Ex: cells of the retina.
- Multipolar neurons: they are the most frequent. Several dendrites and an axon radiate from the usually bulky cell body. |
| Talk about the types of multipolar neurons | According to the length and the shape of the axon of the multipolar neurons, one distinguishes:
- type I Golgi neurons (projection neurons), with a long axon more or less rectilinear and most often myelinated,
- and type II Golgi neurons (association neurons), with a short branched and often amyelinated axon. |
| Talk about neuronal stem cells | For twenty years the notion of non-renewal of adult nervous tissue has been questioned. It is known that a small contingent of stem cells persists within the mature CNS in specific anatomical niches. Neural stem cells participate in the permanent renewal of certain categories of neurons and, where appropriate, the regeneration of nervous tissue. |
| Talk about glial cells | Glial cells of the CNS (central neuroglia) are more numerous than neurons. They occupy the spaces left free by the neurons and their extensions and they intervene between the neurons and the blood capillaries.
They have a supporting role because they make the nervous tissue more compact, and they provide multiple functions: defense of the nervous system, synthesis of myelin, regulation of neuronal activity, nutrition ... Their morphological and functional characteristics are very different from those of neurons.
In the CNS, glial cells are of four types: astrocytes, oligodendrocytes, microglia and ependymocytes. |
| Talk in general about astrocytes | These are the most numerous glial cells, they are large star-shaped cells, with a cell body and numerous cytoplasmic processes that vary from one cell to another (number, shape, aspect, ramifications). The processes in contact with blood vessels are called vascular feet.
The cytoplasm contains the nucleus as well as the usual organelles of the cell, but it is essentially characterized by an abundance of specific gliofilaments (GF).
Astrocytes form a relatively impermeable barrier at the periphery and on the surface of the CNS, close to the innermost meninge. There they are covered by a basal lamina and they are called marginal astrocytic covering (or limiting glial lamina). |
| What are the roles of astrocytes? | Astrocytes play a role of structural support within the CNS parenchyma. They also play a nutritive and metabolic role. |
| Talk about the types of astrocytes | Two types of astrocytes are described:
- Fibrous astrocytes, especially present in the white matter and having many fibrils and fine extensions. By controlling the immediate ionic environment of neurons, these astrocytes participate with endothelial cells, to the formation of the blood-brain barrier.
- Protoplasmic astrocytes mainly present in the gray matter. They have few fibrils but thick extensions. They surround neurons and the synaptic cleft and capture back neurotransmitters. Their nucleus is round and light colored. Their cytoplasm contains glycogen granules. |
| Talk about oligodendrocytes. | Their cell body is characterized by a regular contour and a non-starred shape. Extensions are fewer and finer than those of astrocytes.
In gray matter, the role of oligodendrocytes is essentially nutritive, whereas in white matter, they synthesize myelin; an oligodendrocyte, due to its cytoplasmic prolongations, can myelinate several axons but on a portion of their path |
| Talk about microglia | Microglial cells are characterized by a small elongated cell body with one or more fine, short and branched extensions. They are mesenchymal in origin and represent the defense system of the CNS. When the nervous tissue is damaged, they get activated and turn into phagocytic cells that can get rid of the degeneration products of damaged neurons. |
| Talk about ependymal cells | these are epithelial cells lining the cerebral ventricles and the ependymal canal. They form a simple ciliated cubic or prismatic epithelium separating the cerebrospinal fluid from the underlying nervous tissue. Their apical membrane has the appearance of a brush and presents, between the cilia, short microvillosities. They secrete the cerebrospinal fluid and play an important role in the exchanges between the cerebrospinal fluid and the cerebral parenchyma. |
| Talk about gray matter | in the brain, the gray matter is located in the outer region of the cerebral hemispheres (cerebral cortex) and in the very central part (central gray nuclei). It is also found on the surface of the cerebellum (cerebellar cortex), in the center of the brainstem (central nuclei) and in the inner part of the spinal cord.
The gray matter is characterized by its richness in blood capillaries, and by the presence of the cell bodies of neurons, their dendrites and the initial part of their axon as well as the terminal part of the axons derived from neighboring or distant cell bodies (some axons remain naked in the gray matter). It is therefore in the gray matter that the interneuronal connections (synapses) are established. Astrocytes are more numerous than oligodendrocytes in the gray matter. |
| Talk about white matter | the white matter occupies all the regions of the CNS parenchyma left unoccupied by the gray matter.
It is characterized by the predominant presence of myelinated axons grouped in a parallel way in bundles or cords. Between myelinated axons are astrocytes and especially oligodendrocytes (formation and maintenance of myelin) and blood capillaries. No synapses are observed in the white matter which role is to drive and transmit nerve impulses. |
| Talk about meninges. | CT with epithelial cells that cover the CNS |
| Talk in general about PNS | Histologically, the peripheral nervous system (PNS) is composed of peripheral nerves (27 pairs of spinal nerves, and 12 pairs of cranial nerves), nerve ganglia, and nerve endings. It includes neurons, nerve fibers, glial cells, blood capillaries and connective tissue. |
| Talk about peripheral nerves and nerve fibers | ■ A peripheral nerve is a bundle of nerve fibers in the PNS.
■ Nerve fiber: The term nerve fiber refers to a peripheral axon or group of axons in addition to all surrounding Schwann cells. A nerve fiber can be motor or sensitive, myelinated or amyelinated. |
| Talk about schwann cells | these are the glial cells of the PNS. Their plasma membrane is coated by a continuous basal lamina. Their cytoplasm contains the usual organelles of the cell.
Schwann cells divide as the axon lengthens. In this way, all axons of the PNS are surrounded by Schwann cells. Schwann cells produce myelin in the PNS; they also have a nutritive role for the axon. They have a macrophagic role and are essential for the regeneration of axons. |
| Talk about myelinated nerve fibers | they are mono-axonal, that is to say constituted by a single myelinated axon, associated with a sequence of Schwann cells. Each Schwann cell extends over a segment of the nerve fiber called internode, which is the space between 2 neighboring Ranvier constrictions (or nodes). The constrictions of Ranvier thus represent the separation between a Schwann cell and its myelin on the one hand, and the neighboring Schwann cell and its myelin on the other hand.
In optic microscopy, the myelin sheath has the shape of a tube interrupted, at regular intervals, by the nodes of Ranvier. A single Schwann cell provides formation and covers with myelin each internode. |
| Talk about amyelinated nerve fibers | Remak fibers: most often an amyelinated peripheral nerve fiber is polyaxonic, consisting of 10 to 15 myelin-free axons associated with the same Schwann cell. Each axon lies within an invagination of the plasma membrane of the Schwann cell and is connected to this plasma membrane by a mesaxon. The entire fiber is finally covered with a basal lamina that separates the Schwann cells from the surrounding connective tissue. |
| Talk about the organization of nerve fibers | A peripheral nerve is a set of nerve fibers (myelinated and amyelinated, sensory/ afferent and motor/ efferent) grouped in fascicles or bundles and surrounded by connective tissue.
The dense connective tissue that surrounds the entire nerve is referred to as the epineurium. It brings the different bundles together. It contains many blood and lymphatic vessels.
Inside the nerve, each bundle of fibers is surrounded by a sheath of dense connective tissue thinner than the epineurium: this is the perineurium.
Fine connective septae derive from the perineurium and pass between the nerve fibers, forming the endoneurium, a loose connective tissue with many blood capillaries, that covers each nerve fiber. |
| Talk about nervous ganglia | A nerve ganglion contains several cell bodies of peripheral neurons, nerve fibers (arising from them, terminating in or passing through), capsular (supporting) cells, and blood capillaries. These elements are grouped in a stroma of connective tissue in continuity with a fibrous envelope that surrounds the ganglion. There are two types of ganglia |
| Talk about sensory ganglia | they are localized at the level of the posterior roots of the spinal nerves (called then spinal ganglia), and on the trunks (paths) of the cranial nerves (called then cranial ganglia). They contain the cell bodies of the pseudo-unipolar sensory peripheral neurons. No synapse is present there. |
| Talk about vegetative ganglia | autonomic ganglia: they are associated with the autonomic nervous system. They are located on the sympathetic chain and its ramifications as well as in the walls of the organs innervated by the autonomic nervous system. They contain the cell bodies of multipolar vegetative peripheral neurons. Many synapses are observed there. |
| Talk about nerve endings | - Afferent sensory nerve endings are receptors capable of transforming a mechanical, chemical, thermal or electrical stimulation into an afferent message.
- Efferent motor nerve endings: such as the neuromuscular junction or motor plate. |
| Talk in general about myelin | Myelin is synthesized by two types of specialized glial cells (myelinating or myelinogenic cells): oligodendrocytes in the CNS and Schwann cells in the PNS; it is arranged around large-caliber axons of the central and peripheral nervous systems.
Myelin is a good electrical insulator and allows the rapid spreading of the action potential along the nerve fiber; in the fresh state it appears white and refractive |
| Talk about chemical composition of myelin | Myelin is a lipoproteic substance, but very rich in lipids. However, the chemical composition of central myelin is different from that of peripheral myelin. Central myelin contains 70% lipids and 30% proteins. In peripheral myelin, lipids are even more: 80%. |
