Nervous Tissue and Nervous System. Zhong Jinjie

Transcription

1 Nervous Tissue and Nervous System Zhong Jinjie

2 Nervous System Central nervous system Brain Spinal cord Peripheral nervous system Ganglia Nerves Nerve endings

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4 In the histological slide Deep color grey matter (cortex), more nuclei, less nerve fibers Light color white matter (medulla), less nuclei, more nerve fibers

5 grey matter Nerve tissue is distributed throughout the body as an integrated communications network. white matter Dark blue dots: nuclei of nerve cells

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7 Nerve fiber : formed by neurons and neuroglial cells together. Nerve ganglia : small aggregates of nerve cells.

8 Nerve Tissue Nerve tissue consists of two cell types: nerve cells, or neurons, which usually show numerous long processes, and several types of glial cells (neuroglia cells), which have short processes, support and protect neurons, and participate in neural activity, neural nutrition, and the defense processes of the central nervous system.

9 Neurons consist of three parts : 1. Dendrites, which are multiple elongated processes specialized in receiving stimuli from the environment, sensory epithelial cells, or other neurons; 2. Cell body, which is the center for the whole nerve cell and is also receptive to stimuli; 3. Axon, which is a single process specialized in generating or conducting nerve impulses to other cells (nerve, muscle, and gland cells).

10 Neuron - Neuroglia cell - Nerve fiber Synapse - Another neuron

11 Nerve : formed by many nerve fibers.

12 Classification of neurons I Number of processes: pseudounipolar/bipolar/multipolar neurons Ⅱ Length of axons: Golgi type Ⅰ / Ⅱ neurons Ⅲ Functions: sensory (afferent) neurons / motor (efferent) neurons/ interneurons Ⅳ Chemical substances: cholinergic / adrenergic / peptidergic neurons

13 Neuron

14 Cell membrane is excitable one. Axon and dendrite have different functions.

15 dendrite synapse glial Nissl body synapse microfilament glial dendrite Nissl body synapse microfilament axon hillock microtubule Ultrastructure of a neuron

16 Chromophil Substance (Nissl bodies) LM: strongly basophilic, throughout the cytoplasm of the cell body and dendrites, but absent from the axon hillocks and axons). EM: RER(rough endoplasmic reticulum), free ribosomes. Functions: synthesis proteins

17 Neurofibrils consisting of aggregates of microtubules and microfilaments. abundant in cell bodies and axons and dendrites. silver impregnation : black function: supporting neurons as cell skeleton transportation of substances

18 Comparison Between Axon and Dendrite processes numbers shape hillock spines functions dendrites 1~numerous branched, like trees axons 1 longer, thinner, not branched no Yes, numerous reception of information from other neurons yes no conducting impulses away from cell body to other neurons, or effectors structures

19 Chemical Synapse A synapse is a specialised area of contact where the transmission of nerve impulses occurs from one neuron to another in one direction.

20 Structure of Chemical snapses Presynaptic element: axonal terminals/terminal buttons, mitochondria, synaptic vesicles,presynaptic membrane(neurotransmitters) Synaptic cleft:10-30nm wide Postsynaptic element: postsynaptic membrane(receptor molecules)

21 Neuroglia cells Glial cells are 10 times more abundant in the brain than neurons; they surround neurons that occupy the interneuronal spaces. Nerve tissue has only a very small amount of extracellular matrix, and glial cells furnish a microenvironment suitable for neuronal activity. a-d In central nervous system e In peripheral system

22 Origin and Principal Functions of Neuroglial Cells Glial Cell Type Origin Location Main Functions Oligodendrocyte Neural tube Central nervous system Myelin production, electric insulation Schwann cell Neural tube Peripheral nerves Myelin production, electric insulation Astrocyte Neural tube Central nervous system Structural support, repair processes Blood brain barrier, metabolic exchanges Ependymal cell Neural tube Central nervous system Lining cavities of central nervous system Microglia Bone marrow Central nervous system Macrophagic activity

23 Neuroglia cells in central nervous system astrocyte oligodendrocyte Spinal cord ependymal cell capillary myelin sheath in internode unmyelinated axon microglia cell neurons myelinated axon myelin nodes Grey matter White matter

24 astrocytes oligodenrocytes microglia

25 Blood-brain barrier A. the continuous endothelium of capillaries. B. a continuous basal membrane around the endothelium. C. the vascular foot processes of astrocytes.

26 Formation and organization of a nerve in PNS

27 Formation and organization of myelinated fibers Myelin is formed by Schwann cells in the peripheral nervous system (PNS) and oligodendrocytes in the central nervous system (CNS). Each Schwann cell forms a single myelin sheath around an axon. In contrast, each oligodendrocyte forms multiple sheaths (up to 30 or more) around different axon.

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29 Saltatory conduction (from the Latin saltare, to hop or leap) is the propagation of action potentials along myelinated axons from one node of Ranvier to the next node, increasing the conduction velocity of action potentials. The uninsulated nodes of Ranvier are the only places along the axon where ions are exchanged across the axon membrane, regenerating the action potential between regions of the axon that are insulated by myelin, unlike electrical conduction in a simple circuit.

30 Blood vessel Nerve Connective tissue

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32 Nodes of Ranvier Each Schwann cell myelinates about 100 µm of an axon. Gaps between neighboring Schwann cells are called nodes of Ranvier. Action potentials jump from node to node while traversing an axon.

33 Myelinated Unmyelinated

34 Nerve Endings Sensory nerve endings Motor nerve endings

35 Sensory nerve endings in the finger Encapsulated nerve endings consist of branched axon enclosed in a discrete connective tissue capsule. Free nerve endings are the branched terminations of the axons. Myelinated nerves lose their myelin sheath, and end in a number of branches that penetrate the area being innervated.

36 Meissner s corpuscles 1. Are found in thick skin of the palms and soles, and in the skin of the nipples and genitalia. 2. Are sensitive to discriminatory touch. 3. Are critically located in the dermal papillae where the overlying epidermis is thinnest 4. Are oval structures 5. Have branched, unmyelinated nerve terminals within their core 6. Have transversely arranged, flat cells around the nerve terminals, considered to be modified Schwann cells of Neural crest origin. 7. Are enclosed within an external connective tissue sheath epidermis Meissner s corpuscles

37 Pacinian corpuscles 1. Are found in the deep tissues, particularly in the deep layers of the dermis, subcutaneous tissue, around joints, in the parietal pleura and peritoneum 2. Are sensitive to deep touch, pressure and vibration 3. Are large structures measuring 1-2 mm in diameter 4. Have branched, unmyelinated nerve terminals in the core of the corpuscle 5. Have concentric lamellae of flattened cells arranged longitudinally along the corpuscle 6. Are enclosed within an external connective tissue sheath

38 Golgi tendon organs 1. Are situated at musculo-tendinous junctions. 2. Are enclosed within a capsule 3. Have a core of intracapsular tendon bundles 4. Have elaborately branched nerve endings twisted around the tendon core 5. Have skeletal muscle fibres inserted into them 6. Signal the strength of muscle contraction 7. Are involved in reflexes that inhibit the homonymous (agonist) muscle and stimulate antagonistic muscles Tendon Dendrite Axon

39 Neuromuscular spindles Neuromusclar spindles are abundant in antigravity muscles, muscles of the neck and intrinsic muscles of the hand. They consist of a group of 6 to 12 intrafusal muscle fibres, surrounded by connective tissue continuous with the perimysium. The intrafusal muscle fibres are modified skeletal muscle fibres that have the characteristic striations but do not have their nuclei situated below the sarcolemma.

40 Nerve endings on smooth muscle, cardiac muscle and secretory cells 1. These are the nerve endings of postganglionic autonomic nerves. 2. They are always free nerve endings and do not have motor end plates or other specialised endings. 3. They may be cholinergic or adrenergic. 4. Gastrointestinal smooth muscle and cardiac muscle contract independently of motor nerve stimulation. The autonomic endings on these two types of muscle may affect the rate of contraction of the muscles.

41 Brain - Cerebrum

42 gray matter Gray matter contains neuronal cell bodies, dendrites, and the initial unmyelinated portions of axons and glial cells. white matter Dark blue dots: nuclei of nerve cells Gray matter (Cortex) : 6 layers

43 Cerebral cortex: 6 layers

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45 Neurons in gray matter

46 The cerebellar cortex has three layers: an outer molecular layer, a central layer of large Purkinje cells, and an inner granule layer. The Purkinje cells have a conspicuous cell body and their dendrites are highly developed, assuming the aspect of a fan.

47 The Purkinje cells

48 Nerve fiber In cross sections of the spinal cord, white matter is peripheral and gray matter is central, assuming the shape of an H. In the horizontal bar of this H is an opening, the central canal, which is a remnant of the lumen of the embryonic neural tube. It is lined with ependymal cells. Glial cell Neuron

49 Poliomyelitis Poliomyelitis, often called polio or infantile paralysis, is an infectious disease caused by the poliovirus. In about 0.5% of cases there is muscle weakness resulting in an inability to move. Polio vaccine