1. Physiology of the Neuron 1_Powerpoint-2.pdf

PHYSIOLOGY OF THE NEURON 1 Hans Strijdom Division of Medical Physiology 1 The Nervous System (NS) is a: CONTROL SYSTEM Homeostasis Controls movement, co-ordination, bodily functions, emotions (i.e. controls “LIFE”) PHYSIOLOGI...

PHYSIOLOGY OF THE NEURON 1 Hans Strijdom Division of Medical Physiology 1 The Nervous System (NS) is a: CONTROL SYSTEM Homeostasis Controls movement, co-ordination, bodily functions, emotions (i.e. controls “LIFE”) PHYSIOLOGICAL COMMUNICATION SYSTEM For control to be effective, messages must be communicated; control system must receive and interpret messages, and then communicate appropriate responses 2 Messages, messengers and transport: For the NS to control and communicate: Messages must be created and delivered Messengers must be available to deliver messages Transport systems must be available to move messengers and messages from A to B 3  Application of Structure & Function Sensory Receptors in skin, eyes, ears, GI tract, bladder etc Sensory Neurons The Transport System that carries the Messengers and Brain & Spinal Cord Messages is made up of billions of NEURONS Efferent Neurons: Somatic motor neurons or Autonomic neurons Glands, muscles 4 Anatomy BRAIN CENTRAL NERVOUS SYSTEM (CNS) SPINAL CORD PERIPHERAL NERVOUS SYSTEM (PNS) PNS / PSS CNS / SSS PNS / PSS 5 The cells of the NS The NS is made up of mainly two cell types: NEURONS GLIAL CELLS ** Self Study: Read up on what glial cells are and what their purpose is (Silverthorn 6th Edition p. 244-248) 6 Glial Cells of the NS SELF STUDY: SEE SILVERTHORN 6TH EDITION P. 244-248 FOR MORE INFO ON GLIAL CELLS 7 NB: Please read through the research article by Guo et al. on the potential of glial cells to be re‐programmed into functional neurons following brain injury or in a rat model of Alzheimer’s Disease.  The Neuron: functional unit of the NS ONE SINGLE NEURON CAN BE UP TO 1 METER IN LENGTH! CELL BODY (SOMA) AXON NUCLEUS AXON HILLOCK MYELIN SHEATH AXON TERMINALS DENDRITES DIRECTION OF COMMUNICATION / SIGNAL TRANSFER: MESSAGE RECEIVED BY DENDRITES AND CELL BODY, TRANSFERRED VIA CELL BODY AND AXON AND RELEASED TO THE NEXT NEURON OR TO THE FINAL EFFECTOR ORGAN VIA THE AXON TERMINALS 9  The Neuron: functional unit of the NS CELL BODY (SOMA) AXON TERMINALS DENDRITES Sensory Neurons look slightly different… 10 Neuronal axons are bundled together Radial Nerve NEURONAL AXONS OF THE AFFERENT AND EFFERENT PATHWAYS ARE OFTEN CONTAINED IN BUNDLES OF HUNDREDS OF AXONS (AXONS ALSO CALLED “NERVE FIBRES”) FROM DIFFERENT NEURONS. ONE SUCH BUNDLE IS CALLED A “NERVE”. NERVES CAN THEREFORE POTENTIALLY RELAY SIGNALS THAT CARRY COMPLETELY DIFFERENT MESSAGES (“MIXED NERVES”). 11  Neurons: The Axo-Dendritic Synapse DIRECTION OF COMMUNICATION / SIGNAL TRANSFER PRESYNAPTIC NEURON POSTSYNAPTIC NEURON (FIRST ORDER NEURON) (SECOND ORDER NEURON) THE REGION OF ANATOMICAL AND FUNCTIONAL CONNECTION (JUNCTION) BETWEEN TWO NEURONS IS CALLED AN AXO-DENDRITIC SYNAPSE: THE AXON TERMINAL OF THE PRESYNAPTIC NEURON CONNECTS WITH THE DENDRITIC OR CELL BODY MEMBRANE OF THE POSTSYNAPTIC NEURON. THE SMALL GAP FOUND IN THE SYNAPSE IS CALLED THE SYNAPTIC CLEFT. CRUCIAL TRANSFER OF SIGNALS FROM ONE NEURON TO THE NEXT TAKES PLACE AT THE SYNAPSES. A NEURON CAN RECEIVE SIGNALS FROM HUNDREDS NB: SYNAPSES ALSO OCCUR BETWEEN AN EFFERENT OR THOUSANDS OF AXON NEURON AND THE FINAL EFFECTOR CELL, TISSUE OR TERMINALS... ORGAN (E.G. MUSCLE OR GLAND). IS CALLED: NEURON- TARGET SYNAPSE 12 The Neuronal Network of the Nervous System can be compared to an Electrical Circuit or Power Grid  Neurons: The Myelin Sheath SCHWANN CELLS ARE IMPORTANT GLIAL CELLS OF THE PNS IN THE CNS, MYELIN IS PRODUCED BY OLIGODENDROCYTES. MYELINATED NEURONS IN THE BRAIN ARE COLLECTIVELY CALLED “WHITE MATTER” MYELIN PROVIDES SUPPORT TO, AND INSULATES THE AXONS (“ELECTRICALLY INSULATED”), WHICH EFFECTIVELY SPEEDS UP SIGNAL TRANSMISSION DOWN THE AXONS. MYELIN 14 CONSISTS OF 80% LIPID AND 20% PROTEIN Diseases affecting the myelin sheath: DEMYELINATING CONDITIONS (e.g. Multiple Sclerosis): **See Article by Love S uploaded on SunLearn** Genetic (gene mutations) DYSMYELINATING CONDITIONS (“leukodystrophy”) Neurons: Transporting messages (signals) 1. ELECTRICAL IMPULSES CREATED BY SUDDEN CHANGES IN THE ELECTRICAL CHARGE ON THE INSIDE AND OUTSIDE OF THE CELL MEMBRANES (MEMBRANE POTENTIAL DIFFERENCE) OF THE NEURONAL CELL BODIES AND AXONS 2. THE ARRIVAL OF THE ELECTRICAL IMPULSES IN THE AXON TERMINAL TRIGGERS THE RELEASE OF HORMONE-LIKE CHEMICAL MESSENGERS, CALLED NEUROTRANSMITTERS, INTO THE SYNAPSE WHICH THEN BIND TO A RECEPTOR ON THE CELL MEMBRANE OF THE POSTSYNAPTIC NEURON OR EFFECTOR CELL... 3. A THIRD AND LESS COMMON FORM OF COMMUNICATION BETWEEN NEURONS IS VIA SO-CALLED “ELECTRICAL SYNAPSES”. HERE, ELECTRICAL SIGNALS ARE PASSED DIRECTLY FROM THE CYTOPLASM OF 16 ONE NEURON TO THE NEXT VIA GAP JUNCTIONS. Knowledge required: Ion Channels BBH lectures Ion Channels allow ions to flow into and out of cell via FACILITATED DIFFUSION… this movement is driven by the concentration gradient of the particular ion. Na+ channels, K+ channels, Ca2+ channels, etc. “LIGAND GATED” 17 Knowledge required: Nernst Potential AN EQUILIBRIUM IS REACHED WHEN POTASSIUM’S CHEMICAL AND ELECTRICAL GRADIENTS ARE EQUAL. AT EK (EQUILIBRIUM FOR POTASSIUM) THE DRIVING FORCE FOR THE K+ EFFLUX (CHEMICAL FORCE = CONCENTRATION GRADIENT) VIA K+ CHANNELS EQUALS THE INWARD FORCE (ELECTRICAL FORCE). THE NET FLOW IS NOW ZERO. THE POTENTIAL DIFFERENCE AT WHICH K+ IS AT ITS EK, IS KNOWN AS ITS NERNST POTENTIAL, WHICH IN THE CASE OF K+ IS –90MV. FOR MOST CELLS, THE PERMEABILITY FOR K+ IS ABOUT 40 TIMES MORE THAN FOR NA+. FOR THIS REASON, THE RESTING MEMBRANE POTENTIAL OF MOST CELLS IS CLOSER TO THE NERNST POTENTIAL OF K+ (‐90MV) THAN TO THAT OF NA+ (+60MV). IF A CELL WAS PERMEABLE TO K+ ONLY, THE CELL’S RESTING MEMBRANE POTENTIAL WOULD HAVE BEEN –90MV. BUT BECAUSE CELLS ARE NOT TOTALLY IMPERMEABLE TO NA+, THE SMALL AMOUNT THAT DOES FLOW THROUGH THE LEAKING CHANNELS RESULTS IN A SLIGHTLY MORE POSITIVE RESTING MEMBRANE POTENTIAL OF ‐70 MV. 18 Knowledge required: Membrane Receptors BBH lectures 4 MEMBRANE RECEPTOR CLASSES: LIGAND‐GATED ION CHANNEL RECEPTORS G‐PROTEIN COUPLED RECEPTORS RECEPTOR‐ENZYME RECEPTORS INTEGRIN RECEPTORS (NOT SHOWN) PLEASE REVISE MEMBRANE PHYSIOLOGY NOTES OF LIFE FORMS AND FUNCTIONS… KNOWLEDGE OF MEMBRANE RECEPTORS IMPORTANT, AS THE CHEMICAL SIGNALS OF NEURONAL COMMUNICATION INVOLVE THE BINDING OF LIGANDS TO NEURONAL MEMBRANE RECEPTORS… 19