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Lect. 9 nervous tissues
- 2. Nervous System: General Functions Respond to internal and external stimuli. Transmit nerve impulses to and away from CNS. Interpret nerve impulses at the cerebral cortex. Assimilate experiences in memory and learning. Initiate glandular secretion and muscular contraction. Program instinctual behavior.
- 3. Nervous System: Anatomical Division Central nervous system (CNS) Includes Brain and Spinal cord (SC) Peripheral nervous system (PNS) composed of nerve fibers and nerve ganglia out side the CNS Nerve fibers / nerves – cordlike extension of neurons from the brain and spinal cord Ganglia - small aggregates of nerve cells; nerve “relay” stations.
- 4. Nervous System Designed to deliver rapid and precise communication between different parts of the body by the action of specialized nerve cells called neurons. Neurons operate by generating electrical signals that move from one part of the cell to another part of the same cell or to neighboring cells.
- 6. Nervous Tissues Highly cellular – 80% (Neurons and supporting cells) Extracellular space - 20% (in the brain and spinal cord) CNS: Brain and spinal cord Do not have connective tissue component within. Only the meninges (covering) has CT component. Fresh CNS tissue have very soft, “jelly-like” consistency. Covered by meninges and protected by bony structure (skull and vertebral column)
- 7. Nervous Tissues NEURON: The structural and functional unit of the nervous system. highly specialized for irritability, conductivity, and synthesis of neuroactive substances such as neurotransmitters and neurohormones.
- 8. Neurons After a mechanical or chemical stimulus: these neurons react (irritability) to the stimulus and transmit (conductivity) the information via axons to other neurons in different regions of the nervous system. Strong stimuli create a wave of excitation, or nerve impulse (action potential), that is then propagated along the entire length of the axon (nerve fiber).
- 9. Nerve Cell Body / Soma 1) Nucleus – centrally located in the soma of most neurons; euchromatin and dark nucleolus (“owl-eyed nucleus”) 2) Cytoplasmic organelles and inclusions: Nissl bodies – dark clumps in the cytoplasm made of polysomes and rough ER Cytoskeletons: neurofilaments (regulates neuronal axon diameter); microtubules (in cytoplasm); microfilaments (in plasma membrane)
- 10. Nerve cell body
- 11. In general, the cell bodies of all neurons are located in the central nervous system Motor neurons, integrating neurons Except: the cell bodies of most primary sensory neurons and the terminal effector neurons of the autonomic nervous system (the cell bodies lie in aggregations called ganglia in peripheral sites).
- 12. Cytoplasmic processes Dendrites – a series of highly branched outgrowths of the cell body. major sites of information input into the neuron. with abundant mitochondria receive stimuli (signals) from adjacent sensory cells, or from other neuron and convert the stimuli into electrical impulse.
- 13. Dendrites: Processes that either become: specialised sensory receptors (as in primary sensory neurones) Sensory receptors • visual receptors in the eyes • auditory receptors in the ears • tactile receptors on the surface of the body form synapses with neighbouring neurons from which they receive stimuli (from an axons or other neurons).
- 14. * Break in the axon indicates the length of up to 1 meter..
- 15. Skin Sensory Receptors Merkel’s disks – free nerve endings (unencapsulated), unmyelinated nerve fibers (skin: stratum basale) Meissner’s corpuscle – sensitive to light touch (skin dermis) Pacinian corpuscle – deep pressure or coarse touch (skin dermis) End bulb of Krause – sensitive to cold (oropharynx lining & eye conjunctiva) Ruffinian corpuscle – sensitive to heat (soles of feet)
- 17. Cytoplasmic processes Axon – is a single long process that extends from the cell body and carries output to its target cells. also “nerve fibers”. conduct impulses away from the soma and sends it to effector/target organs or adjacent nervous tissue.
- 18. Parts of the axon: Axon hillock – specialized area of soma with high neurofilaments; initial segment. Axoplasm (axon cytoplasm) – contains sER, rER and long mitochondria Axolemma – plasma membrane of axon. Axon collaterals - branches that form from the main axon along its course. Axon terminals – ends of axon; with small swellings called terminal boutons. responsible for releasing neurotransmitters
- 21. The axons of most but not all neurons are covered by myelin, which consists of 20 to 200 layers of highly modified plasma membrane wrapped around the axon by a nearby supporting cell that produced it. Myelin – an insulating sheath of fatlike lipid that wraps around the axon.
- 22. Myelin sheath – formed by oligodendrocyte (CNS) or Schwann cells (PNS). Schwann cell covering only a segment of the axon. Between the Schwann cells there are short intervals at which the axon is not covered by a myelin sheath; these points are known as nodes of Ranvier.
- 23. Basic types of neurons: - three main patterns based on the number of dendrites and axons that originate from the cell body. Bipolar one dendrite and one axon. Multipolar numerous dendrites and one axon Pseudounipolar one process that branches into two.
- 24. Types of neurons (based on structure): Bi-polar neurons – a single dendrite and a single axon are associated with the cell body. are purely sensory neurons. Multipolar neurons - numerous branched dendrites arise from the cell body with a single axon on the opposite pole. all motor neurons and interneurons of the brain, cerebellum, and spinal cord. Pseudo-unipolar neurons* –exhibit only one process leaving the cell body. Most neurons in the adult organism - most other primary sensory neurons
- 25. Example of a neuron: Multipolar neuron http://education.vetmed.vt.edu/Curriculum/VM8054/Labs/Lab9/lab9.htm#
- 26. Functional classes of neurons
- 27. Functional classes of neurons Sensory neurons or Afferent neurons Transmit impulses to the CNS. Somatic sensory – carry impulses from the skin, bones, and muscles. Visceral sensory – carry impulses from the visceral organs. Motor or Efferent neurons Transmit impulses away from the CNS. Somatic motor – innervate skeletal muscles. Visceral motor (autonomic motor) – innervate cardiac, smooth muscles and glands. Associated neurons or Interneurons connect neurons within the CNS Conduct impulses from sensory to motor neurons
- 28. Characteristics of three classes of neurons I. Sensory or Afferent neurons Transmit information into the CNS from receptors at their peripheral endings Cell body and the long peripheral process of the axon are in the PNS; only the short central process of the axon enters the CNS Have no dendrites (do not receive inputs from other neurons)
- 29. II. Motor or Efferent neurons Transmit information out of the central nervous system to effector cells, particularly muscles, glands, or other neurons Cell body, dendrites, and a small segment of the axon are in the central nervous system; most of the axon is in the peripheral nervous system
- 30. III. Interneurons Function as integrators and signal changers Integrate groups of afferent and efferent neurons into reflex circuits Lie entirely within the central nervous system Account for 99 percent of all neurons
- 31. Neuroglial cells Supporting cells of CNS Small-sized and numerous than neuron; origin: neural crest cells Support, nourishes and protect neurons Types: astrocytes, oligodendrocytes, microglia, ependymal cells, Schwann cells - in the PNS
- 33. Astrocytes – largest neuroglial cell contain many processes forming pedicels (vascular feet) around a blood capillary forming part of the “blood-brain barrier” control the chemical environment around neurons clearing intercellular spaces of increased K ions* and released neurotransmitters, such as glutamate* support metabolic exchange between neurons and the capillaries of the CNS. contain reserves of glycogen, from which they release as glucose for energy metabolism of the CNS.
- 35. Oligodendrocytes have small, round, condensed nucleus that lives symbiotically with neurons; necessary for neuron survival; produces myelin in CNS present in CNS gray and white matter.
- 37. Microglia small, phagocytotic neuroglial cell that arised from monocytes in the bone marrow; they become APCs (antigen-presenting cell) in the CNS tissue when activated by trapped foreign bodies.
- 38. Microglia
- 39. Ependymal cells epithelial cells that line the neural tube and ventricles of the brain they have cilia that aids in moving the CSF (cerebrospinal fluid); cells of the choroid plexus (CSF production)
- 41. Schwann cells the only neuroglial cell-equivalent in the PNS; function is similar with oligodendrocytes that produce myelin the myelin sheath consists of several Schwann cell plasmalemma.
- 42. Myelinated nerve fibers – faster impulse conduction Unmyelinated fibers – slower conduction Schwann cells
- 45. Peripheral Nervous System Branches from the brain and spinal cord Cranial nerves and spinal nerves Ganglion, peripheral nerves Nerve fiber – individual axons enveloped by myelin sheath Nerves – bundles of nerve fibers; appear whitish due to (+) myelin; CT wrappings Ganglia – encapsulated aggregations of soma or cell bodies
- 46. Myelin sheaths not continuous, solid sheets along the axon nodes of Ranvier - punctuations / gaps / intervals significantly accelerate the conduction of nerve impulses (action potentials) along the axons. Saltatory conduction - impulse propagation in myelinated axons where the nerve impulse or action potential jumps from node to node, resulting in a more efficient and faster conduction of the impulse.
- 48. Ganglia Discrete aggregations of neuron cell bodies located outside the CNS. The spinal ganglia lie on the posterior nerve roots of the spinal cord as they pass through the intervertebral foramina; They contain the cell bodies of primary sensory neurones which are of the pseudo-unipolar form.
- 49. Connective tissue wrappings of Peripheral Nerve Epineurium – covers the whole nerve bundle that forms external coat of nerves formed by dense CT(fascia). Perineurium – surrounds each nerve bundle (fascicle); flattened cells are joined by tight junctions (z. occludens) prohibiting passage of macromolecules. Endoneurium – forms a thin layer of reticular fibers produced by Schwann cells that surrounds individual nerve fibers (neurofiber).
- 52. Central Nervous System Brain and Spinal Cord Matter: Gray Matter and White Matter Gray Matter (GM) – neuronal cell bodies (nucleus), more neuroglial cells and many unmyelinated fibers White Matter – mostly myelinated fibers and some neuroglial cells Brain GM – cortex; WM – medulla Spinal cord WM – cortex; GM – medulla (H-section) Nucleus - encapsulated aggregations of soma or cell bodies
- 53. Spinal Cord
- 55. Dura mater – connected to the endosteum of the skull Arachnoid mater – sublayer between endosteum and brain, contains network (cobweb) of blood vessels. Pia mater – covers the brain parenchyma Meninges
- 58. Da Lamarcke’s theory of use and disuse can be associated with microcephaly. Such condition may happen if you don’t exercise your brain!
- 59. Merci beaucoup! (thanks a lot!)
Editor's Notes
- * initially bipolar during embryonic dev’t. and the two processes fuse during later dev’t and form one process.
- blood-brain barrier, which restricts the movement of molecules from the blood into the interstitium of the CNS. -In general, the blood–cerebrospinal fluid and blood-brain barriers are highly permeable to water, carbon dioxide, oxygen, and most lipid-soluble substances such as alcohol and anesthetics; slightly permeable to electrolytes such as sodium, chloride, and potassium; and almost totally impermeable to plasma proteins and most non–lipid-soluble large organic molecules. Therefore, the blood-cerebrospinal fluid and blood-brain barriers often make it impossible to achieve effective concentrations of therapeutic drugs, such as protein antibodies and non–lipid-soluble drugs, in the cerebrospinal fluid or parenchyma of the brain. * If these metabolic chemicals are not quickly removed from these sites, they can interfere with neuronal functions.