Histology: Nervous Tissue PDF

HISTOLOGY: NERVOUS TISSUE The nervous system contains two parts, namely, the central nervous system (CNS) which is surrounded by the cranium and vertebral column, namely the brain and spinal cord. The second part is the peripheral nervous system (PNS) which contain...

HISTOLOGY: NERVOUS TISSUE The nervous system contains two parts, namely, the central nervous system (CNS) which is surrounded by the cranium and vertebral column, namely the brain and spinal cord. The second part is the peripheral nervous system (PNS) which contains all other nervous tissue. The nervous system receives stimuli from both the internal and external environment, via the PNS. The information is analysed and integrated in the CNS so that an appropriate response can occur via the PNS in effectors. Functions: The nervous system (NS) have therefore two functions namely 1) It is a control system for homeostasis in the body and controls movement, co-ordination, bodily functions and emotions (i.e. controls “life”). 2) It is a physiological communication system. For control to be effective messages must be communicated; control system must receive and interpret messages, and then communicate appropriate responses. ORGANISATION OF THE NERVOUS SYSTEM Specialised parts of the PSS which are able to detect physical or chemical stimuli are called receptors. The sensory stimulus is conducted in the form of a nerve impulse via an afferent nerve of the PNS, to the CNS. Nerves which conduct information to the CNS are also called sensory nerves because the impulses that they conduct usually reach the conscious brain. There the impulse may follow different pathways, but eventually an efferent nerve is stimulated. The impulse leaves the CNS via the efferent nerve and makes contact with an effector e.g. a muscle which contracts when the impulse reaches it. Nerves which conduct impulses from the CNS to various organs are called efferent nerves. Because their impulses are conducted to muscles and glands, and result in a reaction (contraction of muscles or secretion by glands) they are also called motor nerves. This process is called the reflex arc. The somatic nervous system: The part of the nervous system which is concerned with sensation (perceptions) and voluntary reactions. The autonomic nervous system: Those parts which control the functions of organs such as the heart, glands, digestive tract, and all smooth muscle. The somatic and autonomic systems have peripheral and central components, and co-operate closely to ensure effective functioning of the body. SHOULD: The efferent part of the autonomic nervous system is subdivided into the sympathetic and parasympathetic divisions. Updated: 2015 CC Chase and A Alblas  2 Diagram of the reflex arc receptor (e.g. touch effector (e.g. muscle) receptor in skin) PNS PNS CNS afferent Brain & spinal efferent nerve cord nerve Reflex arc of large peripheral nerve Drawing from "Basic Histology" 8th edition, by Junqueira LC. et al, 1995, Prentice –Hall International Hist_Neuro CC Chase and A Alblas  3 CELLS IN THE NERVOUS SYSTEM The characteristic cells of the nervous system (NS) are 1) neurons and the 2) neuroglia (glial cells). 1. Neurons 1.1 General structure of a neuron The typical neuron has a cell body, an axon and a dendrite or dendrites. i. The cell body contain and maintain the cytoplasm and organelles (Nissl substance). The nucleus is in the cell body and is vesicular (has little heterochromatin) with a large nucleolus. ii. The dendrites become thinner towards their ends, and are usually very branched to enlarge the surface area for better contact with axons of other neurons. They contain all the organelles of the cell body, except for the Golgi complex. No myelin sheath. They conduct impulses, usually towards the cell body. iii. The axon usually has the same cross section throughout its length, and only branches at the end. It does contain organelles but not the Golgi complex and also not the granular endoplasmic reticulum nor ribosomes. The axon itself is often called a fibre. The axon can be covered by a myelin sheath. It also conducts impulses, usually away from the cell body. iv. The ends of the axon branches are enlarged and are called “boutons” (French for buttons), synapse or axial terminal. It establishes direct contact between two cells. Neurons are long-lived. Mature neurons do not undergo mitosis. Most of the neurons are formed before birth, with a small number being formed in the first year after birth. The baby then has its total number of neurons and no mitosis of mature neurons takes place after this. Neurons which die are not replaced. In these notes “neuron” means a mature neuron, i.e. one which is fully developed and cannot divide. There is however a limited number of stem cells in certain places in the brain, which can develop into new neurons. (For example, in the hippocampus.) A large neuron is a very active cell. It contains many organelles and has a high turnover of organelles, cell membrane, transmitter substances, enzymes. The granular endoplasmic reticulum and free ribosomes are especially abundant. Together they are called the Nissl substance. Hist_Neuro CC Chase and A Alblas  4 The neuron (longitudinal section) 1.2 Classification of neurons A. Classification of neurons: Types of neuron There is considerable variation between different neurons in their size and shape. Three types are common in the body. They are named according to the number of their processes: multipolar, bipolar en pseudo-unipolar. The pseudo-unipolar neuron’s whole long process is called an axon these days. Only the branches at one end can be called dendrites. This is according to the latest tendency to call the neuron cell process an axon if it can conduct an action potential. synapse Hist_Neuro CC Chase and A Alblas  5 B. Classification of neurons: Function of neuron Neurons can also be named according to their function: motor, sensory and interneurons. i. The motor neurons carry messages from the central nervous system to effectors e.g. muscle cells and are usually multipolar neurons. ii. The sensory (receptor) neurons carry messages from receptors to the central nervous system and are usually pseudo-unipolar or bipolar. iii. The interneurons are in the central nervous system (brain) and spinal cord and are involved in integration or processing of information. They are mostly multipolar neurons. 1.3 Myelination of neurons Myelin is a fatty material, which surrounds some axons. Function: The myelin sheath serves as an electrical insulator. Conduction is therefore much faster in these neurons than in unmyelinated axons with the same cross section. In the peripheral nervous system (PNS), the Schwann cell is responsible for myelination. In the central nervous system (CNS) it is the oligodendrocyte. Many Schwann cells / oligodendrocytes are needed for each axon. The true number depends on the length of the axon. Each Schwann cell is only involved with one axon, but one oligodendrocyte supplies several axons with myelin. Between successive Schwann cells / oligodendrocytes there is a short distance of the axon which is not covered by myelin. It is covered by Schwann cell cytoplasm in the PNS. Forming of myelin: The Schwann cell / oligodendrocyte folds around the axon and forms a mesaxon (double layer of cell membrane) which turns around the axon, repeatedly. The cytoplasm between the membranes is pressed out so that the membranes can bind to each other to form a continuous layer of myelin around the axon. Myelin therefore consists of layers of cell membrane. Cell membranes contain a large amount of phospholipid, and also protein, which is therefore also true of myelin. Hist_Neuro CC Chase and A Alblas  6 The process of myelination axon Schwann cell nucleus mesaxon Schwann cell cytoplasm "Basic Histology" 8th Ed, by Junqueira LC. et al, 1995, Mesaxon elongated myelin mesaxon layers form myelin Myelinated axons (cross section) Cytoplasm associated with myelin. In certain parts of the Schwann cell e.g. on the outside around the nucleus and on the inside next to the axon and in thin connecting strips between the two, there is still cytoplasm present between the membrane layers of the myelin. Therefore membrane components which are made by the Schwann cell for maintenance of its cell membrane can still reach the innermost membrane. Drawing from Wheater's Functional Histology by Burkitt HG et al, 3rd edition, 1996, Churchill Livingstone. Hist_Neuro CC Chase and A Alblas  7 Unmyelinated axons Unmyelinated axons are also enclosed by Schwann cell cytoplasm. The axons are never "bare". A number of axons are serviced by one Schwann cell. The Schwann cell cytoplasm folds around the axon. A mesaxon forms but it does not turn around the axon. There is still a number of Schwann cells needed to cover the whole length of the axon. Cross section of unmyelinated axons. 2. Neuroglia There are different supporting cells in the nervous system: two cell types in the PNS and four cell types in the CNS: 2.1 Types of neuroglia in die PNS i. SATTELITE CELLS are neuroglia with the same function as the astrocytes of the CNS. They form the „connective tissue“ cells of the PNS and serve a supporting and protective function by forming the packaging material between the neurons. Furthermore, they are also responsible for the supplying nutrients to the surrounding neurons. ii. SCHWANN CELLS are responsible for myelination of the nerve cell processes in the PNS. 2.2 Types of neuroglia in die CNS i. ASTROCYTES are neuroglia which has many cytoplasmic processes, which are relatively short in comparison with those of many neurons. They are the “connective tissue” cells of the CNS, i.e. they are the packing material between the neurons. (Except for the blood vessels and in the meninges, there are neither fibroblasts nor collagen fibres in the CNS). They also help to form the blood-brain barrier (by means of astrocyte end-feet), and have other functions too. ii. OLIGODENDROCYTES have only a few cytoplasmic processes, and they are responsible for myelination of the nerve cell processes in the CNS. ("oligo" = few) iii. MICROGLIA have many cytoplasmic processes. They are phagocytic i.e. they play the role of the macrophage. iv. EPENDYMAL cells look like cuboidal epithelial cells and function as epithelial cells. They line the central canal of the spinal cord and the brain ventricles. They have microvilli, isolated cilia and long basal processes, but no basement membrane. They also form part of the choroid plexus, which is responsible for the forming of the cerebrospinal fluid (CSV). Hist_Neuro CC Chase and A Alblas  8 Types of neuroglia in the CNS MICROSCOPIC STRUCTURE OF THE PNS A large peripheral nerve consists of groups of axons arranged in fascicles. Connective tissue sheath surrounding a nerve 1. The whole nerve is surrounded by a layer of loose connective tissue viz the epineurium. There is also epineurium between the fascicles. 2. Special perineural cells form several layers around each fascicle/group of axons, the perineurium. 3. There is also a thin layer of loose connective tissue around the axon and Schwann cell, which is called the endoneurium. Small peripheral nerves consist of only one fascicle. They therefore have a perineurium but no epineurium. Hist_Neuro CC Chase and A Alblas  9 Drawing of a cross section of a large peripheral nerve. epineurium perineurium blood vessels fassikel endoneurium akson miëlien enlarged Axon (white) Myelin (black) Ganglia Ganglia are groups of neuron cell bodies, which are present outside the central nervous system (CNS). (Part of the peripheral nervous system; PNS.) Neuron cell bodies are NOT present in a peripheral nerve; they are only in the ganglia. A section through a sensory ganglion in the dorsal root of the spinal cord. cell bodies of neurons capsule (dense irregular connective tissue) (ganglion cells) loose connective tissue cytoplasm nucleus of Schwann cell nucleus axon of pseudo-unipolar neuron runs through ganglion nucleolus myelin satellite cells around the ganglion cell N.B. All the neurons have myelinated axons & satellite cells, but only one neuron is drawn in detail. Its axon is incomplete, however. Hist_Neuro CC Chase and A Alblas  10 Synapses: where communication between neurons takes place. Synapses are present in the peripheral and central nervous systems, and are abundant in both. The synapse is the structure where communication takes place between neurons. Here the nerve impulse is propagated from one neuron to another. The axon of one neuron and the dendrite of the other are usually involved in a synapse, or an axon of one neuron and the cell body of another. There are also other less common types. Drawing of a synapse/neuromuscular junctions axon of neuron mitochondrium bouton or axon synaptic vesicle which terminal contains transmitter substance synaptic cleft receptors for transmitter other neuron’s cell substances membrane - - - neuron cell body - - - Classification of synapses i. Axo-dentritic: between axons and dendrites (in CNS) ii. Axosomatic: between axons and neuron cell bodies (in CNS) iii. Dendrodentritic: between 2 dendrites iv. Axo-axonic: between 2 axons Degeneration and repair of neurons If an axon in the peripheral nervous system is severed, the axon distal to the cut degenerates, and is phagocytosed by macrophages. (“Distal” here means the part which is furthest from the cell body.) The Schwann cells do not degenerate however. They proliferate (by mitosis) and form a tube along which the remnant of the axon grows. When the axon makes contact with the neuron or muscle cell or other effector which it serves, a synapse / neuromuscular junction will be formed and the neuron can function once again. (NB: This process is not always successful!) Usually it is not a single axon which is severed, but a whole nerve. The same process then occurs in all the axons. If a cell body of a neuron is damaged, the neuron dies and no recovery is possible. Because neurons do not undergo mitosis, there will be no formation of new neurons to compensate for the loss. If axons in the central nervous system (CNS) are severed, there is no repair, and also no repair when their cell bodies are damaged, of course. Hist_Neuro CC Chase and A Alblas  11 Precise joining of a severed peripheral nerve under a surgical microscope is always necessary, so that the correct axons will grow along the correct Schwann cell tube, for functional recovery. During the recovery of such a nerve, the axons grow at about 1mm per day. If the distance which they must grow is rather large, then the process can take months, and there is more chance of it being unsuccessful. DAMAGE & REPAIR OF AXON IN PERIPHERAL NERVE cell body Nissl substance myelin sheath Schwann cell nucleus NORMAL NEURON INJURY axon disintegrates AXON GROWS Schwann cells multiply and form a tube. Axon grows again, down the tube. REGENERATION Drawing from “Principles of Anatomy & Physiology” by GJ Tortora & SR Grabowsky, Harper Collins, 1996. Hist_Neuro CC Chase and A Alblas  12 MICROSCOPIC STRUCTURE OF THE CNS The grey matter of both the cerebrum and cerebellum is situated in an outer or peripheral cortex, while the white matter is centrally situated. In the spinal cord, the grey matter is central. The grey matter consists of cell bodies of neurons, as well as the proximal parts of their cell processes (axons and dendrites). There is no true connective tissue between the neurons, but there are neuroglia which function as connective tissue. The white matter consists of the processes of neurons (especially axons), but not their cell bodies. The white appearance is as a result of the myelin sheaths. Neuroglia are also present in white matter. Cells in the brain: neurons and neuroglia. Example of the CNS: Spinal cord (cross section) Drawing from "Basic Histology" 8th edition, by Junqueira LC. et al, 1995, Prentice –Hall International. Grey matter in the spinal cord: Grey matter inside: H shape. Central canal lined with ependymal cells. Anterior horns/columns: large cell bodies of multipolar motor neurons to voluntary muscles. Enlargement of grey matter: This drawing is simply to give an idea of the complexity of the brain structure, you do not have to be able to draw it! Can you see the capillaries? They are totally covered with astrocyte end-feet. Drawing from: "Basic Histology" 8th edition, by Junqueira LC. et al, 1995, Prentice –Hall International. Hist_Neuro CC Chase and A Alblas  13 White matter in the spinal cord: White matter outside. Contains parallel bundles of axons, mostly myelinated, the tracts of the white matter. Choroid plexus in third ventricle. connective tissue ( i h id) modified ependymal cells capillary (type 2) CSF in ventricle ordinary d l ll Cerebellum: cross section of a folium (fold). meninges dendrites of Purkinje cell Purkinje cells granular GREY MATTER cell climbing fibre mossy fibre axon To deep nuclei WHITE MATTER The function of the cerebellum is coordination of movements. For this purpose, it receives information from the cortex as well as the apparatus of movement, so that movements can be monitored, and smooth, even execution of movements can be ensured. Hist_Neuro CC Chase and A Alblas  14 The cortex (outside) consists of grey matter with white matter on the inside. The cortex is arranged in 3 layers: molecular layer (outermost), Purkinje cell layer, granular layer (innermost layer). 1. The molecular layer contains small neurons which are found only in this layer. Function: inhibition of Purkinje cells. [CAN: stellate neurons and basket cells (neurons). Both form inhibitory sinapses on Purkinje cells.] 2. Purkinje cell bodies form the middle layer. Axons run to deep cerebellar nuclei, or to lateral vestibular nucleus. The Purkinje cells receive multiple stimulatory and inhibitory messages, and integrate them. Then they send a message out – the only efferent tract out of the cerebellar cortex. 3. Granular layer: contains granular cells which are small neurons. Their axons run to the molecular layer, where they form long parallel fibres. The latter make sinapses with many Purkinje cells, and are stimulatory. [CAN: Golgi cells (neurons) are also present in the granular layer, and form inhibitory synapses on the granular cells.] NERVOUS TISSUE: SUMMARY Organisation: CNS, PNS. Somatic NS, Autonomic NS. Reflex arc: afferent neuron, interneuron(s) in CNS, efferent neuron. Neurons: PNS &CNS, Neuroglia: only CNS Neurons have: cell body and nucleus cytoplasmic processes: dendrites & axon myelinated, unmyelinated PNS: Schwann cell, CNS: oligodendrocyte Sinapse: Structure where contact between 2 neurons takes place Peripheral nerve = many axons (no dendrites, no cell bodies) endoneurium, perineurium, epineurium Ganglia: groups of neuron cell bodies in PNS Regeneration of neurons when axon injured. (Only PNS) CNS = grey matter (with cell bodies of neurons), white matter (with myelin) Spinal cord: grey matter inside, white matter outside. Hist_Neuro CC Chase and A Alblas  15 Important nerves in the body Hist_Neuro CC Chase and A Alblas