General Anatomy (Nerve Tissue) PDF

NERVOUS TISSUES A.L. IMAM Introduction Nervous system (NS) is a complex system which coordinate and control all the activities of the entire body system. The NS has two major divisions: Central nervous system (CNS); made up of brain and spinal cord Peripheral nervous system (PNS); cr...

NERVOUS TISSUES A.L. IMAM Introduction Nervous system (NS) is a complex system which coordinate and control all the activities of the entire body system. The NS has two major divisions: Central nervous system (CNS); made up of brain and spinal cord Peripheral nervous system (PNS); cranial, spinal, and peripheral nerves. The Nervous system consist of specialised tissue called Nerve tissue. The Nerve tissue has two (2) forms; (i) Neurons or Nerve cells (ii) Glial cells NEURONS Neuron forms the structural and functional unit of nervous system. Neurons respond to environmental changes (stimuli) It rapidly change electrical potential in response to stimuli i.e forms the excitable cells of the nervous system Neurons react quickly to stimuli by causing reversal of ionic gradient (membrane depolarization). The depolarization starts from the point where the stimulus begins and travel across the neuron’s entire plasma membrane. This propagation, called the action potential or the nerve impulse. Structure of the Neuron Neurons consist of three main parts: i. Cell body, or perikaryon, ii. Dendrites, and iii. Axon NERVE CELL BODY Also called Perikaryon is the main part of the neuron, from which the cytoplasmic processes arise. The cell body contains a large nucleus, Nissl bodies, neurofibrils, mitochondria and Golgi apparatus.  NERVE CELL BODY Each neuron has a centrally placed nucleus, present in the nerve cell body. Nucleus has one or two prominent nucleoli. Neurons lack centrosome, hence, the nerve cell cannot multiply like other cells.  Neurofilaments They are strands of proteins that appears as the neurofibrils. They are present in the cytoplasm of a neuron, and extend into dendrites and axons More observable in the distal part of the axon  Neurotubules Consist of proteins and measure 20–30 nm. Are of variable length as they extend into the different parts of the neuron; They are more abundant in dendrites than in axons Assist in the transport of materials through the cytoplasm and processes of the neuron.  Nissl bodies Nissl granules are small basophilic granules found in cytoplasm of neurons. Attached ribosomes and present only in the somata and dendrites of neurons. They are more prominent in highly active neurons, and in motor than sensory neurons Are responsible for the synthesis of proteins in neurons.  Mitochondria Mitochondria appear as filament in neurons. They occupy the somata and cytoplasmic processes. Are much more prominent and abundant at the synapses and end- plates They are also clustered at the tips of regenerating dendrites Form the powerhouse of the nerve cell, where ATP is produced Lipofuscin Is a yellow pigment found in the cell body of a neuron; it represents the product of degenerative activity (wear and tear) of neurons. Increases in quantity with advancing age. Centrioles Present in nerve centrosome Are not involved in the formation of mitotic spindle in neurons, as most neurons do not undergo cell division in adults May be involved in the generation and maintenance of neurotubules  Dendrites Dendrites are short, branched process associated with the neuron's cell body. They possess ribosomes, granular endoplasmic reticulum (Nissl bodies), mitochondria, microfilaments and microtubules. Dendrite transmits impulses towards the nerve cell body  Axon Axon is the longer process of nerve cell. Arise from the parts of the somata referred to as the axon hillocks. They lack ribosomes and Nissl substance, but contain other organlles Axon transmits impulses away from the nerve cell body and transmit to other cells (nerve, muscle, and gland cells). Classifications of Neuron Neurons can be classified according to; 1. Number of processes; 2. Functions of the neuron; and 3. Sizes of the neuron.  Classifications of Neuron (Number of processes) Based on the nmber of process, neurons can be; 1. Multipolar neurons: Which have one axon and two or more dendrites Function either as sensory or motor neurons Found majorly in the central nervous system, found in the sympathetic and parasympathetic ganglia.  Classifications of Neuron (Number of Processes) 2. Bipolar Neurons: Possesses two cytoplasmic processes with one dendrite and one axon Is sensory in function They are found in the retina, olfactory mucosa, and the (inner ear) cochlear and vestibular ganglia  Classifications of Neuron (Number of Processes) 3. Unipolar Neurons: Have a single cytoplasmic process attached to their cell bodies The single process bifurcates close to the perikaryon Are functionally sensory neuron Are more numerous in the foetus They are found in the spinal ganglia (the spinal nerves) and in most cranial ganglia  Classifications of Neuron (Number of Processes) 4. Anaxonic neurons: They possess many dendrites but no true axon, Do not produce action potentials, but regulate electrical changes of adjacent neurons. example Amacrine cell of the retina and golgi cells in the cerebellum Classifications Based on Function of the Neuron Sensory Neurons: They are afferent neurons which receive stimuli from the receptors throughout the body Sensory neurons may be unipolar (pseudounipolar) e.g.the spinal ganglia, or bipolar, as found in the retina and olfactory epithelium Classifications Based on Function of the Neuron Motor Neurons: These are efferent, that send impulses from CNS to effector organs such as muscle fibers and glands. They are multipolar neurons found in the Cental Nervous system Somatic motor nerves are under voluntary control and typically innervate skeletal muscle; Autonomic motor nerves control the involuntary activities of glands, cardiac muscle, and most smooth muscle Classifications Based on the size of the Neuron Golgi type 1: Possess characteristically large somata and long axons Examples are:Purkinje cells of the cerebellum, pyramidal cells of the motor cortex and alpha motor neurons of the spinal cord Golgi type 1I: Possess characteristically small somata and short axons Are mostly inhibitory in function E.g: amacrine cells of the retina  Glial Cells (Neuroglia) Are the non-excitable cells of the central nervous system (CNS) Support neuronal survival and activities They are more abundant than the neurons. examples of glial cells are: Astrocytes Oligodendrocytes Microcytes (Microglia) Ependymal cells  Astrocyte Most abundant glia; highly branched glial cells; Star-shaped appearance with a rounded nucleus. Contains abundant mitochondria, lysosomes, Golgi apparatus, microfilaments and glycogen Exist in two forms fibrous astrocytes, which are numerous in the white matter, and p ro t o p l a s m i c a s t ro c y t e s , w h i c h a r e numerous in the grey matter Give support to neurons and help to transfer nutrients to them from capillaries Maintain the blood brain barrier  Oligodendrocytes Possess smaller and rounded cell bodies. Possess the ability to proliferate produce the myelin sheaths around axons that provide the electrical insulation for neurons in the CNS May also be involved in the metabolic activities of nerve cells  Microglia Are the smallest glial cells in the CNS Less numerous than oligodendrocytes or astrocytes Microglia migrate through the neuropil, scanning the tissue for damaged cells and invading microorganism Are rich in lysosomes Are normally quiescent in the absence of injury or inflammation of the CNS but active following injury  Ependymal Cells Ependymal cells are columnar or cuboidal cells. They line the ventricles of the brain and central canal of the spinal cord Ependymal cells have cilia and long microvilli, They are essential for the production and modification of cerebrospinal fluid (CSF) Facilitate the movement of cerebrospinal fluid (CSF), and involved in its absorption.  Schwann Cells Are specialized neuroglial cells associated with the peripheral nervous system Schwann cells have trophic interactions with axons and importantly allow for their myelination Schwann cell forms myelin around a segment of one axon, Blood-Brain Barrier(BBB) The blood-brain barrier (BBB) is a functional barrier that allows much tighter control passage of substances moving from blood into the CNS tissue. The primary structural component of the BBB is the capillary endothelium, with the cells joined by a well-developed occluding junctions and with little or no transcytosis activity. Another structural component is the limiting layer of perivascular astrocytic feet that completely envelops the basal lamina of the capillaries. These regulates passage of molecules and ions from blood to brain  The BBB protects neurons and glia from bacterial toxins, infectious agents, and other exogenous substances, It also helps to maintain the stable composition and constant balance of ions in the interstitial fluid that is required for normal neuronal function. A = Astrocytes: C = Capillaries Clinical Correlate Chromatolysis: degeneration of the Nissl substance of the cell body Multiple sclerosis: a disease condition that is due to damage myelin sheaths surrounding the axon Astrocytosis or gliosis: proliferation of the astrocytes Demyelination: Severe damage to oligodendrocytes