Ross University School of Veterinary Medicine Neurophysiology PDF

The Nervous System Neurophysiology VP 2024 Clara Camargo, DVM Neurophysiology Study Guide 1. The Nervous System Explain: a) The structural and functional differences between the Central Nervous System and the Peripheral Nervous System b) The difference between a neuron and a neuroglial cell c) The c...

The Nervous System Neurophysiology VP 2024 Clara Camargo, DVM Neurophysiology Study Guide 1. The Nervous System Explain: a) The structural and functional differences between the Central Nervous System and the Peripheral Nervous System b) The difference between a neuron and a neuroglial cell c) The components of the neuron and their function 2. List the classifications of neurons as described functionally 3. Explain the different types of neurons and their location in the body 4. What is myelin and where does myelin originate? 5. Explain the correlation between an unmyelinated cell vs myelinated cell 6. Explain the function of the nodes of Ranvier and saltatory conduction 7. Explain resting membrane potential? What are the major factors maintaining RMP? 8. Explain action potential? 9. List the components to the Action Potential Graph and the Ions impacting each stage 10. Explain EPSP or IPSP and how does this influence membrane potential changes? 11. How are calcium ions involved with the synapse as the action potential travels down the axon? 12. List 6 major classes of Neurotransmitters The Nervous System 2 Categories of Cells in the Nervous System The Nervous System Neurons (Greek neuron=nerve)  MAJOR Functional Units of the Nervous System  Electrically excitable (action potential)  Specialized in information processing  Do not divide once they reach maturity  Injury leading to neuronal death will permanently change the structure & functions of the affected areas Neuroglia/ glial cells (Greek glia= glue)  The Helper Cells, The Support System  Involved in the nutrition & maintenance of the nerve cells  Astrocytes, Oligodendrocytes, Schwann cells NEURON The Nervous System DENDRITES – information-receiving area of the cell membrane, detect stimuli CELL BODY, SOMA or PERIKARYON – contains organelles AXON HILLOCK or TRIGGER ZONE – axon origin; originates Action Potential (AP) AXON – information-carrying extension of the cell membrane PRESYNAPTIC TERMINAL – end of axon; transmit information (neurotransmiters) MYELIN SHEATH – enhances speed of information transfer NODE OF RANVIER – gaps in the insulating myelin sheath AXON HILLOCK NEURONS - classified according to their function SENSORY or AFFERENT Send (INPUT) information from receptors towards the brain/spinal cord Somatic (skin or skeletal muscles) and visceral (internal organs) INTERNEURONS or ASSOCIATION NEURONS Found in the brain/spinal cord, connecting motor & sensory neurons MOTOR or EFFERENT Send information from the brain/spinal cord to muscle/glands (effectors, command) Somatic (voluntary) and autonomic (involuntary) The Nervous System NEURONS-classified according to their structure The Nervous System https://www.interactive-biology.com/3247/the-neuron-external-structure-and-classification/#peusdounipolar Bipolar Neuron  Have 2 processes that connect to the cell body 1 axon & 1 dendrite Found in specific areas of the nervous system (retina, inner ear and nose - olfactory epithelium)  INTERNEURONS The Nervous System Pseudounipolar Neurons  1 single stem axonal process that branches to form 2 processes Peripheral and central NS (sensory ganglia and cranial nerves) Do not have dendrites, axonal processes will receive and transmit information  SENSORY NEURONS Send information from receptors in sensory organs towards the brain/spinal cord The Nervous System Multipolar Neuron o 1 axon & many dendrites o Most common type o Found throughout the body MOTOR NEURONS & INTERNEURONS Send information from the brain/spinal cord to muscle/glands. The Nervous System THE NEURON Dendrites: receive signals from presynaptic terminals of other neurons Cell body: contains organelles such as: Nucleus Free ribosomes Rough Endoplasmic Reticulum (rER) Golgi apparatus Mitochondria  these are contained everywhere in neurons in large quantities as nerve cells require large amounts of ATP Axon hillock and Initial axon segment: Integrates different signals (often opposing each other) & Generates and shape the AP before it is propagated along the axon Axon: can be very long, is the conducting unit, adult axons often don’t contain ribosomes and depend on proteins from cell body. Presynaptic Terminals: signaling to adjacent cells The Nervous System The Nervous System NEURON AND SYNAPSE Neurons communicate via Synapses Greek Synapsis = connection Specialized contact areas with other neurons, muscle fibers or glands Synapses are formed by:  The presynaptic terminal of one cell  The receptive surface of the adjacent cell (post synaptic cell)  Synaptic Cleft ( space b/t the 2 cells)  Action potentials travel along the axon  Speed varies from 0.5 to 120 meters per second  Larger axons are myelinated  Smaller ones (< 1 μm in diameter) are not myelinated Electrical activity of neurons FYI https://media.hhmi.org/biointeractive/click/Neuron_Activity/01-vid.html Myelin Sheath The myelin sheath is a greatly modified plasma membrane  Wrapped around the axon in a spiral fashion  Originate from and are part of the: Schwann cells in PNS Oligodendrocytes in the CNS  Each myelin-generating cell furnishes myelin for only one segment of the axon  The periodic interruptions are the NODES OF RANVIER Critical to the functioning of myelin The Nervous System NEURON AND SYNAPSE The Nervous System The Myelin Sheath Facilitates  Conduction  “Electrical Insulation”  Saltatory Conduction of the impulse Latin Saltare = to “Jump” Action Potentials “jumps” from node to node Depolarization occurs more rapidly in myelinated axons MYELIN & CONDUCTION VELOCITY The Nervous System Myelinated fibers conduction is proportional to the diameter of the fiber  wider axon and longer internode → faster the conduction velocities (up to 150m/s) Unmyelinated fibers conduction is proportional to the square root of the diameter (0.5 to 10m/s) THE NEURON INFORMATION CONDUCTION Dendrites: receive signals from presynaptic terminals of other neurons Dendritic spines: small protrusions of the dendritic membrane, they greatly increase the receptive surface of the postsynaptic cell  Contain specialized receptors to recognize the chemical transmitters released from the presynaptic terminal The Nervous System RESTING MEMBRANE POTENTIAL The Nervous System The resting membrane potential is determined by the uneven distribution of ions (charged particles) between the inside & the outside of the cell and by the different permeability of the membrane to different types of ions. → Especially sodium (Na⁺) and potassium (K⁺) Although net concentration of + and - charges is similar in both intraand extracellular fluids: Excess positive charges accumulates just outside the cell membrane, & excess of negative charges immediately inside the cell membrane This makes the inside of cell (-) charged compared to outside of cell This electrical difference (voltage) across membrane: varies with cells, in mammalian neurons: ~ -70mV (average) RESTING MEMBRANE POTENTIAL Resting membrane potential is a result of 3 major factors: 1. The concentration of ions on the inside and outside of the cell. An ion species will move toward a dynamic equilibrium if it can flow across the membrane. This is called the equilibrium potential for that ion. Ions always flow towards it! the concentration difference across the membrane creates a chemical driving force (simple diffusion though leak channels – ions move towards concentration gradient) 2. Differential permeability of the membrane to diffusion of ions: the resting membrane is much more permeable to K + than to Na+ ions because it has more K+ leak channels than Na+ leak channels 3. Na+, K+ pump (ATPase): this energy-dependent pump in cell membranes pumps Na+ out of the cell and draws K+ into the cell against their concentration gradients Takes 3 Na+ ions out and 2 K+ ions into de cell The Nervous System MEMBRANE POTENTIAL CHANGES The Nervous System Resting membrane potential can be changed by synaptic signals All cells have an electrical potential (voltage) across their cell membrane neurons and muscle cells are unique, their membrane potential can be changed by a synaptic signals from another cell Neurotransmitters (released from presynaptic axon terminal) bind to receptors on postsynaptic membrane:  open or close ion selective channels and change the membrane potential of the postsynaptic cell  Can change it in 2 ways, creating postsynaptic potential: Make more negative (–) → INHIBITORY (IPSP) Make more positive (+) → EXCITATORY (EPSP)  This depends on which receptors are activated HOW MEMBRANE POTENTIAL CAN CHANGE? The Nervous System ESPS (excitatory postsynaptic potential) → makes inside of cells more positive than resting membrane potential → increases the chances for reaching the cellular threshold and trigger an AP Neurotransmitter → is quickly removed from synapse: change only lasts milliseconds as channels close again HOW MEMBRANE POTENTIAL CAN CHANGE? The Nervous System ISPS (inhibitory postsynaptic potential) → Makes inside of cell more negative than resting membrane potential → decreases the chance for triggering an AP What would happen if I use a drug that inhibits the postsynaptic potential of a sensory neuron? THE ACTION POTENTIAL The Nervous System Neurons and muscle cells electrical potential can be changed in response to: Synaptic signaling from other cells Transduction of environmental energy (sensory organs) When change in membrane potential (neuron/muscle cells) reaches a threshold value, this causes a dramatic change in the membrane potential:  an ACTION POTENTIAL (AP) Neuron resting potential https://youtu.be/YP_P6bYvEjE ACTION POTENTIALS: The Nervous System  Begin at the axon’s initial segment (hillock or trigger zone) and spread down the entire length of the axon  Result from the integration of the various EPSPs and IPSPs received by the cell Depend on the sequential opening and closing of voltage-gated ion channels:  Na+ gated channels  K+ gated channels VOLTAGE-GATED ION CHANNELS AND ACTION POTENTIALS 1. Stimulus: ESPS + ISPS 2. Depolarization: Na+ voltage-gated channels open: Na+ flows in 3. Repolarization: Na+ voltage-gated channels close AND K+ voltage-gated channels open: K+ flows out 4. Hyperpolarization: K+ flows out through leak channels as well as K+ voltage-gated channels 5. Return to resting potential as voltage-gated K+ channels gradually close PROPAGATION OF ACTION POTENTIALS APs propagate from their origin down the axon Positive charges passively spread to adjacent resting segments of the membrane and trigger an action potential there In this way, the AP spreads from the axon’s initial segment down to the presynaptic terminal at the axon’s far end ACTION POTENTIALS - CONDUCTION VELOCITY The Nervous System The speed of AP conduction varies → diameter of axon and degree of myelination  In smaller, unmyelinated axons: conduction velocity is slow (i.e. 0.5 m/s)  Wider axons and myelination can speed up velocity > 90 – 150 m/s In myelinated axons, the current can ‘jump’ from one node to another (saltatory conduction) → Flows very rapidly under the myelin THE SYNAPSE The Nervous System The action potential travels down the axon to the axon terminal  causes the opening of Ca2+ voltage-gated channels Ca2+ enters the cell → high levels of intracellular Ca2+ cause fusion of synaptic vesicles with the plasma membrane Neurotransmitters contained in those synaptic vesicles are released into the synaptic cleft by exocytosis  can bind to receptors of the postsynaptic cell  triggering cellular response The Nervous System Major classes of NEUROTRANSMITTERS Amino acids and derivatives: GABA (γ-aminobutyric acid), glutamate Amines: acetylcholine, serotonin Catecholamines: dopamine, norepinephrine, epinephrine Peptides: endorphins and endogenous opioids (leu-encephalin, met-encephalin, β-endorphin)