04 nerve physiology.ppt
- 2. Introduction • The nervous system – neurons and supporting cells. • Neurons – basic structural and functional units of the nervous system. • The human CNS contains about 1011 neurons. • Neurons – excitable cells. Due to the presence of the ion channels. • Neurons – secretory cells.
- 3. The release of a neurotransmitter occurs only in the synapses. • Neurons are quiescent cells. Renew their constituents during their entire life. • Although neurons vary, they have two Cell body and Neuronal processes Neurons cont’d…
- 5. • Cell body – enlarged portion – contain the nucleus and its perikaryon. • Its shape is variable. Pyramidal soma for pyramidal cells; Ovoid soma for Purkinje cells; Granular soma for small multipolar cells; and Stellar soma for motoneurons. Neurons cont’d…
- 6. • Function of the soma – nutritional center of the neuron . The cell body and larger dendrites contain Nissl bodies. • The cell bodies within the CNS are frequently clustered – nuclei. • Cell bodies in the PNS usually occur in clusters – ganglia. • Neuronal processes – dendrites and axon. Based on morphological, ultrastructural, biochemical and functional. Neurons cont’d…
- 7. • Dendrites – thin, branched processes that extend from the cell body. Provide a receptive area. • The dendrites that emerge from the soma are simple perikaryal extensions – the primary dendrites. Between one and nine primary dendrites emerge from the soma. • The primary dendrites divide successively to give a dendritic tree with specific characteristics. • The dendrites are morphologically – Neurons cont’d…
- 8. Irregular outline, diameter which decreases along their branchings, acute angles between the branches, and ultrastructural characteristics. • The axon is a longer process that conducts impulses away from the cell body. • The axon is morphologically – Smooth appearance, a uniform diameter along its entire extent, by the right-angles with the main axon and by its ultrastructural characteristics. Neurons cont’d…
- 9. • Axons are narrow from their origin, and do not usually contain ribosomes or endoplasmic reticula. • The origin of the axon near the cell body – the axon hillock. • Axons vary in length. • The axon divides into presynaptic terminals. Each ending in a number of synaptic knobs which are also called terminal buttons or boutons. Neurons cont’d…
- 10. Neurons cont’d… o They contain granules or vesicles in which the synaptic transmitters secreted by the nerves are stored. cell body muscle tissue TYPICAL MOTOR NEURON Axon dendrite synapse cell body
- 11. Sensory Neuron Interneuron Motor Neuron Muscle Contracts Synapse Synapse Synapse • Functional classification – direction impulses conduction. 1. Sensory, or afferent, neurons. 2. Association neurons, or interneurons. 3. Motor, or efferent, neurons. Classification of Neurons
- 12. • Structural classification – number of processes that extend from the cell body. Classification of neurons cont’d…
- 13. Axonal Projection • Neurons with long axons – projection neurons (or principal neurons or Golgi type I cells). • Neuron that has all processes confined to one region of the brain – interneurons (or intrinsic neurons or Golgi type II cells). Some of these cells have very short axons. Others seem to lack a conventional axon altogether – anaxonal. Classification of neurons cont’d…
- 14. Dendritic Geometry • Pyramid-shaped spread of dendrites – pyramidal cells. • Radial-shaped spread of dendrites – stellate cells. • This classification often includes the presence or absence of dendritic spines. • All pyramidal cells appear to have spines, but stellate cells may have them (spiny) or not (aspiny). Classification of neurons cont’d…
- 16. • In the CNS, supporting cells – neuroglia/glial cells/. • Most of the supporting cells are derived from the ectoderm. • Supporting cells – aid the functions of neurons. About 10–50 times more abundant than neurons. Able to divide by mitosis. SUPPORTING CELLS
- 17. • Two types of supporting cells in the PNS – Schwann cells (neurolemmocytes) – form myelin sheaths around peripheral axons; and Satellite cells (ganglionic gliocytes) – support neuron cell bodies within the ganglia of the PNS. • Four types of supporting cells in the CNS – Oligodendrocytes – form myelin sheaths around axons of the CNS. Supporting cells cont’d…
- 18. Microglia –bone marrow origin. o They phagocytize foreign and remove toxic debris within the brain and o Secrete anti-inflammatory factors – functions that are essential for the health of neurons. o Overactive microglial contribute to neurodegenerative diseases. Supporting cells cont’d…
- 19. Supporting cells cont’d… Astrocytes – regulate the external environment of neurons in the CNS; and Ependymal cells – line the ventricles of the brain and the central canal of the spinal cord.
- 21. • All axons in the PNS are surrounded by neurilemma/sheath of Schwann/. The axons of the CNS lack a neurilemma. • One Schwann cell forms a myelin sheath around only one axon. Schwann cells roll around the axon, much like a roll of electrician’s tape is wrapped around a wire. However, the Schwann cell wrappings are made in the same spot. Myelin Sheath In PNS
- 22. The number of times Schwann cells wrap and number of layers in the myelin sheath is greater for thicker than for thinner axons. • The cytoplasm is forced into the outer region of the Schwann cell. • Each Schwann cell wraps only about a millimeter of axon, leaving gaps of exposed axon between the adjacent Schwann cells. These gaps – the nodes of Ranvier – transmission nerve impulses. Myelin sheath in PNS cont’d…
- 23. • Those axons smaller than 2 μm in diameter are usually unmyelinated whereas those that are larger are likely to be myelinated. But unmyelinated axons are also surrounded by a neurilemma. • Myelinated axons conduct impulses more rapidly than those that are unmyelinated. Myelin sheath in PNS cont’d…
- 24. Myelin sheath in PNS cont’d…
- 25. • The myelin sheaths around axons of the CNS – oligodendrocytes. Occurs mostly postnatally. • Each oligodendrocyte form myelin sheaths around several axons. • Areas of the CNS that contain a high concentration of axons – white matter. • The gray matter of the CNS – cell bodies, dendrites and axons that lack myelin sheaths. Myelin Sheath In CNS
- 26. Myelin sheath in CNS cont’d…
- 27. • Large stellate cells with numerous cytoplasmic processes. • They are the most abundant of the glial. • Astrocytes – end-feet surrounding the capillaries of the CNS. Also have extensions adjacent to the synapses. Astrocytes
- 28. • Functions of astrocytes:- Take up K+ from the ECF. o Important in maintaining the proper ionic environment for neurons. Take up some neurotransmitters released from the axon terminals. o For example, the neurotransmitter glutamate. Take up glucose from the blood. Astrocytes cont’d…
- 29. Needed for the formation of synapses in the CNS. o Few synapses form in the absence of astrocytes. Regulate neurogenesis in the adult brain. Induce the formation of the blood-brain barrier. Release transmitter chemicals. o Glutamine, ATP, adenosine, D-serine. Astrocytes cont’d…
- 31. ELECTRICAL SIGNALS IN NEURONS • Excitable cells – electrical signals Graded potentials Action potentials o In a muscle fiber – muscle action potential. o In a neuron – nerve action potential (nerve impulse).
- 32. • Open – specific ions to move down their electrochemical gradient. As ions move, they create a flow of electrical current. • Open and close due to the presence of “gates.” The gate is a part of the channel protein that can seal the channel pore shut or move aside to open the pore. • The electrical signals – rely on four types of ion channels. Ion Channels
- 33. 1. Leak channels – gates randomly alternate between open and closed. • Found – dendrites, cell bodies, and axons of all types of neurons. • The membrane’s permeability to K+ is much higher than its permeability to Na+. Plasma membranes – many more K+ leak channels than Na+ leak channels. The K+ leak channels are leakier than the Na+ leak channels. Ions channels cont’d…
- 35. 2. Ligand-gated channel – opens and closes – binding of a ligand. • Chemical ligands – NT, hormones, and particular ions. • Located – dendrites and cell bodies of neurons. Ions channels cont’d…
- 36. 3. Mechanically gated channel – opens or closes – mechanical stimulation. The force distorts the channel from its resting position. Ions channels cont’d…
- 37. 4. Voltage-gated channel – opens – change in membrane potential. Participate in the generation and conduction of action potentials. Ions channels cont’d…
- 39. • Build up of charge difference – only very close to the membrane. Separation of +ve and –ve form of potential energy – millivolts. • A cell that exhibits a membrane potential is said to be polarized. • In neurons, the RMP ranges from -40 to -90 mV. A typical value is - 70 mV. Resting Membrane Potential
- 40. • The resting membrane potential arises 1. Unequal distribution of ions 2. Inability of most anions to leave the cell. 3. Electrogenic nature of the Na+/K+ ATPases. RMP cont’d…
- 41. RMP cont’d…
- 42. Graded Potentials • Small deviation from the membrane potential. Membrane more polarized – hyperpolarizing graded potential. Membrane less polarized – depolarizing graded potential. • Occurs – mechanically gated or ligand-gated channels. Mainly in the dendrites and cell body.
- 43. • Vary in amplitude – depending on the strength of the stimulus. How many ligand-gated or mechanically gated channels have opened (or closed) and how long each remains open. • Flow of current is localized – decremental conduction. • Useful for short-distance communication only. • Summation –add graded potentials together. GP cont’d…
- 44. • GP have different names. When a GP occurs in the dendrites or cell body in response to a NT – postsynaptic potential. Occur in sensory receptors and sensory neurons – receptor potentials and generator potentials of respectively. GP cont’d…
- 45. • Rapidly occurring events. • The threshold in a particular neuron usually is constant. • The generation of an AP depends on whether a particular stimulus is able to bring the MP to threshold. Occur in response to threshold or suprathreshold stimulus. • Action potential – the all-or-none principle. Generation of Action Potentials
- 46. Stages of the Action Potential
- 47. Voltage-gated sodium channel cont’d…
- 48. Voltage-gated potassium channel cont’d…
- 49. • The period of time – cannot generate another AP in response to a normal threshold stimulus. • ARP – very strong stimulus cannot initiate a second AP. Coincides with period of Na+ channel activation and inactivation. Inactivated Na+ channels cannot reopen. • Graded potentials do not exhibit a refractory period. Refractory Period
- 50. • Under normal body conditions, the maximum frequency of nerve impulses in different axons ranges between 10 and 1000 per second. Large-diameter axons – ARP – 0.4 msec. Small-diameter axons – 4 msec. • RRP – second AP can be initiated, but only by a larger-than-normal stimulus. Coincides with period when voltage-gated K+ channels are still open. RP cont’d…
- 51. • Unmyelinated axon every patch of membrane – Na+ and K+ channels. AP – produced along the entire length of the axon. • The type of conduction – continuous conduction. • AP must be produced at every fraction of a micrometer – conduction rate is relatively slow. Conduction rate is somewhat faster if unmyelinated axon is thicker. Conduction in an Unmyelinated Axon CONDUCTION OF NERVE IMPULSES
- 52. Conduction in unmyelinated cont’d…
- 53. • The myelin sheath provides insulation for the axon, preventing movements of Na+ and K+ through the membrane. The myelin has interruptions – the nodes of Ranvier. • The nodes of Ranvier – separated by very short distance (1 to 2 mm). Na+ channels are highly concentrated at the nodes (estimated at 10,000 per square micrometer) and almost absent in the regions of axon membrane between the nodes. Conduction in a Myelinated Axon
- 54. • AP occur only at the nodes of Ranvier and seem to “leap” from node to node – saltatory conduction. • Myelinated axons conduct the AP faster than unmyelinated axons. • Conduction rates vary from 1.0 m/sec to faster than 100 m/sec. • The speed of AP conduction is increased – Increased diameter of the axon and Myelination. Conduction in myelinated cont’d…
- 55. Conduction in myelinated cont’d…
- 56. • The functional connection between a neuron and a second cell. • In the CNS, this other cell is a neuron. Axodendritic, axosomatic, and axoaxonic synapses. • In the PNS, the other cell – neuron, muscle or gland. • Myoneural, or neuromuscular junctions. • Neuroglandular junctions A SYNAPSE
- 57. • Two major types – the chemical synapse and the electrical synapse. • In the CNS, almost all the synapses are chemical synapses. The first neuron secretes neurotransmitter. o ACh, NE, E, histamine, GABA, glycine, serotonin, and glutamate. • Electrical synapses – direct open fluid channels that conduct electricity from one cell to the next. Types of Synaptic Transmission
- 58. Consist of gap junctions – allow free movement of ions from the interior of one cell to the interior of the next. Only a few examples of gap junctions – in the CNS. Way of gap junctions – smooth muscle fiber and cardiac muscle cell. A synapse cont’d…
- 59. • Chemical synapses transmit the signals in one direction. It is quite different from conduction through electrical synapses, which often transmit signals in either direction. • Presynaptic terminals – 10,000 to 200,000 minute synaptic knobs. Motor neurons – about 80 to 95% of them on the dendrites and only 5 to 20% on the soma. • These are the ends of nerve fibrils that originate from many other neurons. “One-way” Conduction at Chemical Synapses
- 60. • Neurons in other parts of the cord and brain differ from the anterior motor neuron in – The size of the cell body; The length, size, and number of dendrites; The length and size of the axon; and The number of presynaptic terminals. A synapse cont’d…
- 61. • The presynaptic terminal is separated from the postsynaptic neuronal soma by a synaptic cleft. Width usually of 200 to 300 angstroms. • The presynaptic terminal contain terminal knobs, boutons, end-feet. • The terminal has two internal structures – transmitter vesicles and mitochondria. PRESYNAPTIC TERMINALS.
- 62. • The transmitter vesicles contain the transmitter substance. • The mitochondria provide ATP – supplies the energy for synthesizing new transmitter substance. A synapse cont’d…
- 64. Neurotransmitters Synapse (gap) The message is transferred when RECEPTORS receive neurotrans- mitters. (pink spheres) A synapse cont’d…