Nerve,structure and function

MBBS Lecture

1. Nerve,structure and function

2. Cells of the Nervous System Two main types of cells Neurons, and glia

3. Neurons Are excitable cells Conduct the impulses that make possible all nervous system functions They are the WIRING of the nervous system’s information circuits The human brain is estimated to contain about 100 billion, or about 10% of the total number of nervous system cells in the brain

4. STRUCTURE OF NEURON Nerve cell with all it’s processes is neuron

5. The Make-up of Neurons All neurons consist of: Cell body Also termed SOMA OR PERIKARYON Dendrites Process Are thread like and often known as nerve fibers Axon Process Are also thread like and often known as nerve fibers

6. Cell body of a Neuron Largest part of the nerve Contains a nucleus, cytoplasm, and various organelles such as mitochondria and a Golgi apparatus The cytoplasm also extends into the processes and a plasma membrane encloses the entire neuron The rough ER (endoplasmic reticulum) has ribosome's that can be stained to form easily apparent structures call NISSL BODIES , which provide protein molecules needed for the transmission of nerve signals and are useful in maintaining and regenerating nerve fibers

7. Dendrites of a Neuron Usually branch extensively from the cell body (like a tree) (which the name comes form the Greek word for tree) The distal ends of dendrites of sensory neuron may be termed receptors because the receive the stimuli that initiate nerve signals They receive stimuli and conduct electrical signals toward the cell body and/or axon of the neuron.

8. Axon of the Neuron Is a single process Usually extends form a tapered portion of the cell body at a location termed AXON HILLOCK Axons conduct impulses away from the cell body Even though neurons have only one axon, that axon often has one or more side branches termed axon collaterals The distal tips of axons form branches called telodendria , which each terminate in a synaptic knob , which contain mitochondria and numerous vesicles

9. Size of Axons Some are a meter long and some are only a few millimeter Their diameter also varies. The larger the diameter the faster the impulse An axon can be myelinated or not However only axons can have myelin sheath, dendrites do not.

10. Parts:- I. Axon - i ) generally long ii) arises from axon hillock iii) axis cylinder has axoplasm , neurofibrils & mitochondria iv) axons end in terminal buttons v) carry impulses away from cell body II. Dendrite:- i ) multiple & short ii) contain nissl granules iii) carry impulses towards soma

11. III. Cell body:- Neurocyton or Soma i) Nucleus- pale, large, spherical, central ii) Neuroplasm- has neurofibrils, nissl granules, mitochondria, golgi apparatus, neurosecretory material

12. Classifying Neurons Two ways: Structural functional

13. Structural Classification of Neurons Can be classified according to the number of extensions form the cell body, there are three types Multipolar Bipolar unipolar

14. CLASSIFICATION OF NEURONS (a) Golgi bottle type I (b) Golgi bottle type II II. Anatomic classification- a) Unipolar b) Pseudounipolar c) Bipolar d) Multipolar e) Apolar

15. III. Physio-anatomic classification- a) afferent i) somatic ii) visceral b) efferent i) somatic ii) visceral IV. Depending on myelination a) myelinated b) unmyelinated

16. Multipolar Neurons one axon Several dendrites Found in the brain and spinal cord

17. Bipolar Neurons one axon Branched dendrites Are the least numerous kind of neuron Found in the: retina, inner ear, and olfactory pathway.

18. Unipolar Neurons Also termed pseudounipolar neurons They only have a single process extending form the cell body Are always sensory neurons , conducting information toward the CNS This single process branches to form a central process toward the CNS and a peripheral process (away from the CNS), which together for an axon It conducts impulses away form the dendrites found at the distal end of the peripheral process

19. Functional Classification of Neurons Can be classified according to the direction in which they conduct impulses, there are three types: Afferent Efferent Interneuron's

20. Afferent Neurons Are SENSORY Transmit nerve impulses to the spinal cord or brain

21. Efferent Neurons MOTOR Transmit nerve impulses away form the brain or spinal cord to or toward muscles or glands

22. Interneuron's Conduct impulses from afferent neurons to or toward motor neurons Located entirely within the CNS (brain and spinal cord)

23. Glia Cells

24. Glia or Glial Cells Do not usually conduct information themselves but support the function of neurons in various ways

25. Major Types of Glia Common term is NEUROGLIA Discovered by Italian cell biologist Camillo Golgi (whom also named the Golgi Apparatus in cells) He accidentally dropped a piece of brain tissue in a bath of silver nitrate. When he finally found it, golgi could see a vast network of various kinds of darkly stained cells surrounding the neurons—proof that glia existed Glia literally means “ GLUE ” Studies of glia is now one of the hottest areas in neurobiology Some say that there is well over 900 billion glia in the nervous system (9 times the amount of stars in the Milky Way)

26. More About Glia Most important thing about glia is that unlike neurons they retain their capacity for cell division throughout adulthood However, even though this gives glia the ability to replace themselves, it also makes them susceptible to abnormalities of cell division, such as cancer Most benign and malignant tumors in the nervous system begin in glial cells

27. So What is the Role of Glia Cells? They serve as various roles in supporting the function of neurons! There are five major types of glia Astrocytes; Microglia; Ependymal cells; Oligodendrocytes; and Schwann cells

28. Astrocytes Star shaped (derived their name from the Greek astron, “star” Found only in the CNS Are the largest and most numerous Their long delicate “points” extend through the brain tissue, attaching to both neurons and the tiny blood capillaries Also termed “stars of the nervous system”

29. What do astrocytes do? The “feed” the neurons by picking up glucose from the blood, converting it to lactic acid, and passing it along to the neurons that they are connected with Since astrocytes form tight sheaths around the brain’s blood capillaries they help form the BLOOD-BRAIN BARRIER (BBB)

30. BBB Is a double barrier made up of astrocyte “feet” and the endothelial cells that make up the walls of the capillaries Small molecules (oxygen; carbon dioxide; water; and alcohol) diffuse rapidly through the barrier to reach brain neurons and other glia Large molecules penetrate it slowly or not al all (box 12-1)

31. Recent news about astrocytes Astrocytes may not only influence the growth of neurons and how the neurons connect to form circuits, but may also transmit information along “astrocyte pathways” themselves

32. Microglia Small Usually stationary cells found in the CNS In inflamed or degenerating brain tissue, microglia enlarge and move about and carry on phagocytosis Therefore microglia are functionally and developmentally unrelated to other nervous system cell

33. Ependymal Cells Resemble epithelial cells, forming thin sheets that line fluid-filled cavities in the brain and spinal cord Some take part in producing the fluid that fills these spaces Others have cilia that help keep the fluid circulating within the cavities

34. Oligodendrocytes Smaller than astrocytes and have fewer processes Literally means “ cell with few branches ” ( oligo few; dendro branch; cyte cell) Some lie clustered around nerve cell bodies Some are arranged in rows between nerve fibers in the brain and spinal cord Function is to help hold nerve fibers together and also serve another more important function—they produce the fatty myelin sheath around nerve fibers in the CNS (Box 12-2)

35. Schwann Cells Only found in the PNS !!!!!!!!!!!!!!! Serve as the functional equivalent of the oligodendrocytes, supporting nerve fibers and sometimes forming a myelin sheath around them Many Schwann cells can wrap themselves around a single nerve fiber

36. How do Schwann cells form myelin sheaths? The myelin sheath is formed by layers of Schwann cell membrane containing the whit, fatty substance “ MYELIN ” Microscopic gaps in the sheath, between adjacent Schwann cells are termed NODES OF RANVIER The myelin sheath and its tiny gaps are important in the proper conduction of impulses along nerve fibers in the PNS

37. MYELINATION Nerve cells in grey matter are naked. As they enter white matter they acquire myelin sheath. As the nerve leaves CNS it acquires neurolemma (sheath of schwann) Myelin sheath- a protein-lipid complex Envelops the axon except at its ending & at nodes of ranvier

38. Myelinogenesis - Inside CNS myelin is produced by oligodendroglia & outside CNS by schwann cells Schwann cell wraps around axon up to 100times. This is compacted by apposition of protein zero. Nodes of ranvier are periodic 1 μ m constrictions which are 1mm apart where there is no myelination

39. Neurilemma As each Schwann cell wraps around nerve fibers, its nucleus and cytoplasm are squeezed to the perimeter to form the NEURILEMMA (sheath of Schwann) The neurilemma is essential to the regeneration of injured nerve fibers Nerve fibers with many Schwann cells forming a thick myelin sheath are called MYELINATED FIBERS OR WHITE FIBERS When several nerve fibers are held by a single Schwann cell that does not wrap around them to form a thick myelin sheath, the fibers are termed UNMYELINATED FIBERS OR GRAY FIBERS

40. Satellite cell Is a type of Schwann cell They surround the cell body of a neuron They support neuronal cell bodies in regions called GANGLIA in the PNS Ganglion Block Shots: are how doctors numb an entire limb or portion of the body such as epidurals.

41. V. Erlanger-Gasser’s Classification:- Type Function diameter conduction ( μ m) velocity (m/s) A α proprioception , somatic 12-20 70-120 motor A β touch, pressure 5-12 30-70 A γ motor to muscle spindle 3-6 15-30 A δ pain, temperature, 2-5 12-30 touch B preganglionic autonomic <3 3-15 C i ) Dorsal root- pain, touch, 0.4-1.2 0.5-2 ii) postganglionic 0.3-1.3 0.7-2.3 sympathetic

42. VI. Numerical classification Number origin fiber type Ia Muscle spindle, A α annulospiral ending Ib Golgi tendon organ A α II Muscle spindle, flower-spray A β ending, touch, pressure III Pain, temperature, touch A δ IV Pain dorsal root ‘C’ fibers

43. PROPERTIES OF NERVE EXCITABILITY- it’s the ability of a cell to produce action potential in response to a stimulus. action potential- it’s a self-propagating change in potential across a cell membrane.

44. LOCAL RESPONSE

45. ELECTROTONIC POTENTIAL ACTION POTENTIAL Produced due to application of subthreshold stimulus Produced due application of threshold stimulus It is a local response Propagative type of response It is a graded response All or nothing response It has no latent period It has a latent period It has no refractory period It has a refractory period Not affected by hypoxia, anaesthesia Not produced during hypoxia, anaesthesia

46. Stimulus- it’s a change in environment which brings about a change in potential across a membrane in an excitable tissue Types of stimuli- Electrical Chemical Thermal Mechanical Electromagnetic it can also be classified into subliminal, minimal (threshold), sub-maximal and maximal, depending on the strength of stimulus.

47. STRENGTH-DURATION CURVE TIME UTILISATION TIME STRENGTH RHEOBASE 2 X RHEOBASE CHRONAXIE

48. RHEOBASE - minimum current required to produce action potential. UTILIZATION TIME- time taken for response when rheobase current is applied. CHRONAXIE - time taken for response when twice rheobase current is applied. It is a measure of excitability of tissues.

49. Factors affecting excitability Temperature Mechanical pressure Blood supply Chemicals- CO 2 & narcotics pH- increased excitability in alkaline and reduced excitability in acidic media. Ions- Na + , Mg ++ and K + are neuro-excitatory and Ca ++ is neurosedative

50. II. CONDUCTIVITY  Action potential is self-propagative  Conduction may orthodromic or antedromic  In axon, conduction is towards terminal buttons physiologically.  In myelinated nerves, conduction is saltatory type.

51. Factors affecting conductivity Temperature Mechanical pressure Blood supply Chemicals pH Ions Size of the nerve Myelination

52. III. ALL OR NONE RESPONSE The action potential doesn’t occur in a nerve if the stimulus is sub-threshold. If the stimulus is threshold and above, the action potential produced will be of same amplitude, regardless of intensity of stimulus. * The frequency of action potential increases with the increasing intensity of stimulus.

53. IV.REFRACTORY PERIOD Absolute refractory period- it is the period during an action potential, during which a second stimulus can’t produce a second response. Relative refractory period- it is the period during an action potential, during which a stimulus of higher intensity can produce a second response

54. V.ACCOMODATION When a stimulus is applied very slowly, no matter however strong it might be, it fails to produce an action potential. Cause: a slowly applied stimulus causes slower opening of Na + channels with concomitant opening of K + channels. The influx Na + of is balanced by efflux of K + .

55. COMPOUND ACTION POTENTIAL Multi-peaked action potential recorded from a mixed nerve bundle is called a compound action potential.