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LO: Nervous System Histology 1. Identify and describe the major cellular components of the central and peripheral nervous systems Central nervous system The CNS consists of the brain (encephalon), which is enclosed in the skull, and the spinal cord, which is contained within the vertebral canal. Nervous tissue of the CNS does not contain connective tissue other than that in the meninges and in the walls of large blood vessels. Collagenous fibres or fibrocytes/blasts are consequently not observed. The two major classes of cells that make up the nervous tissue are nerve cells, neurones, and supporting cells, glia. The cerebrum, cerebellum and spinal cord consist of white and grey matter: - White matter: does not contain neuronal cells bodies, but consists of myelinated axons + myelin producing oligodendrocytes. - Grey matter: contains neuronal cell bodies, dendrites and initial unmyelinated portion of axons and glial cells. Synapses occur in this region. The Meninges include; - Dura mater: outermost layer composed of dense CT continuous with periosteum of the skull. Dura mater separated from periosteum of vertebrae by epidural space – contains thin walled veins, loose CT and adipose tissue. Dura mater separated from arachnoid mater by thin subdural space. - Arachnoid mater: has two components (i) a layer in contact with dura mater & (ii) a system of trabeculae connecting the layer with the pia mater. Cavities between the trabeculae are the subarachnoid space (communicates with ventricles of the brain) which is filled with CSF and completely separated from subdural space. It functions to protect CNS from trauma. Composed of CT devoid of BV’s. - Pia mater: loose CT containing BV’s. Close to nerve tissue but not indirect contact as it is separated by a thin layer of neuroglial processes which keeps the CSF away. Blood brain barrier: functional layer that prevents the passages of substances (antibiotics, chemical and bacterial toxins) from blood to nerve tissue. Results from reduced permeability of blood capillaries in nervous tissue due to occluded cell junctions, unfenestrated cytoplasm of endothelial cells and very few pinocytotic vesicles Choroid plexus: consists of invaginated folds of pia mater rich in dilated fenestrated capillaries that penetrate the interior of the brain ventricles. Found in the roofs of the 3rd + 4th ventricles and partly in walls of the lateral ventricles. Produces CSF that acts as a protective device against mechanical shocks. CSF: it is clear, has a low density and protein content. Few desquamated cells and lymphocytes are present. Choroid plexus ventricles subarachnoid space venous circulation. Peripheral nervous system The PNS comprises all nervous tissue outside the brain and spinal cord. It consists of groups of neurones (ganglion cells), called ganglia, feltworks of nerve fibres, called plexuses, and bundles of parallel nerve fibres that form the nerves and nerve roots. Nerve fibres, which originate from neurones within the CNS and pass out of the CNS in cranial and spinal nerves, are called efferent or motor fibres. Nerve fibres which originate from nerve cells outside the CNS but enter the CNS by way of the cranial or spinal nerves are called afferent or sensory nerve fibres. The principal neurotransmitters in the PNS are acetylcholine and noradrenalin. 2. Describe the microscopic structure of peripheral nerves, ganglia and synapses Peripheral Nerve A peripheral nerve is a bundle of nerve fibres held together by connective tissue. Fibre types o Type A fibres (myelinated) are 4 - 20 µm in diameter and conduct impulses at high velocities (15 - 120 m per second). E.g. motor fibers, which innervate skeletal muscles, and sensory fibres. o Type B fibres (myelinated) are 1 - 4 µm in diameter and conduct impulses with a velocity of 3 - 14 m per second. E.g. preganglionic autonomic fibres. o Type C fibres (unmyelinated) are 0.2 - 1 µm thick and conduct impulses at velocities ranging from 0.2 to 2 m per second. E.g. autonomic and sensory fibres. Connective tissue components o Endoneurium: the loose connective tissue surrounding each individual nerve fibre Consists of collagen fibres secreted by Schwann cells. Fibrils run parallel and around nerve fibres binding them together into a fascicle Fibrocytes, macrophages and mast cells are present in the endoneurium o Perineurium: the specialised connective tissue surrounding each nerve fascicle Serves as a metabolically active diffusion barrier, “blood nerve barrier”, which maintains the ionic milieu of the ensheathed nerve fibres Cells in this layer are squamous, have an external basal lamina on both surfaces, are contractile (contain actin filaments), and have tight junctions. o Epineurium: the dense irregular connective tissue that surrounds and binds nerve fascicules into a common bundle. Outermost tissue of peripheral nerves, may contain adipose tissue in larger nerves. BV supplying the nerves travel in the epineurium and then the branches penetrate into the nerve and travel within the Perineurium. Tissue is poorly vascularised, metabolic exchange relies on diffusion from the BV through perineurial sheath. Cell bodies of peripheral nerves may be located within the CNS or outside the CNS in the peripheral ganglia. Ganglia contain cluster of neuronal cell bodies and the nerve fibres leading to and from them o Cell bodies in dorsal root ganglia + cranial nerve ganglia belong to sensory neurons (somatic and visceral afferents). o Cells bodies in paravertebral, prevertebral and terminal ganglia belong to postsynaptic “motor” neurons (visceral efferents) of the ANS. o Motor neuron cells bodies of the PNS lies in the CNS Cell bodies of motor neurons that innervate sk mm (somatic efferent) are located in the brain, brain stem and spinal cord. The axons leave the CNS and travel in peripheral nerves to the sk mm they innervate. A single neuron conveys impulses from the CNS to the effector organ. o In the ANS a chain of two neurons connect the CNS to sm mm, cardiac mm, and glands (visceral efferents). The cell bodies of presynaptic neurons of the ANS are located in the CNS. Their axons leave the CNS and travel in the peripheral nerves to synapse with the postsynaptic neurons in the peripheral ganglia. o Sensory neuron cell bodies are located in the ganglion outside of, but close to the CNS. In the sensory system, (both somatic and visceral afferent components), a single neuron connects to the receptor, through a sensory ganglion, to the spinal cord or brainstem. Sensory ganglion re located along the dorsal roots of spinal nerves (DRG) and in association with the sensory components of CN V, VII, VIII, IX, and X. Ganglia Ganglia are ovoid structures containing neuronal cell bodies and glial cells supported by CT. Sensory ganglia: receive afferent impulses going to CNS. There are two types o Cranial ganglia: associated with cranial nerves o Spinal ganglia: associated with dorsal root of spinal nerves that comprise of large neuronal cell bodies with prominent fine Nissl bodies surrounded by abundant small glial cells called satellite cells. o A CT framework and capsule support ganglion cells. The neurons of ganglia are pseudounipolar and relay information from the ganglion’s nerve endings to the grey matter of the spinal cord via synapses with local neurons. Autonomic ganglia: appear as bulbous dilatations in autonomic nerves. Some are located within viscera especially their walls i.e. intramural ganglia. They are devoid of CT capsule and their cells are supported by a stroma. They usually have multipolar neurons, and neuronal perikaryon’s with fine Nissal bodies. A layer of satellite cells frequently envelops autonomic ganglia. Synapses Synapses are specialised junction between neurons that facilitate transmission of impulses from one (presynaptic) neuron to another (postsynaptic) neuron. Synapses also occur between axons and effector (target) cells such as muscles and glands. Synapses between neurons can be classified morphologically as: o Axodendritic: occurring between axons and dendrites o Axosomatic: occurring between axons and the cell body o Axoaxonic: occurring between axons and axons. Chemical synapses: in which conduction of impulses is achieved by the release of chemical substances (neurotransmitters) from the presynaptic neuron. Neurotransmitters then diffuse across the narrow intracellular space that separates a presynaptic neuron from a postsynaptic neuron or target cells