Neural Signaling PDF

Neural Signaling Ezgi G. Berkay, MD, PhD Neural Signaling (1) Reception of information by a sensory receptor (2) Transmission by an afferent neuron to the central nervous system (CNS) (3) Integration by interneurons in the central nervous system (CNS) (4) Transmission by an efferent neuron to other...

Neural Signaling Ezgi G. Berkay, MD, PhD Neural Signaling (1) Reception of information by a sensory receptor (2) Transmission by an afferent neuron to the central nervous system (CNS) (3) Integration by interneurons in the central nervous system (CNS) (4) Transmission by an efferent neuron to other neurons or effector (5) Action by effectors the muscles and glands 3 External stimulus (e.g., vibration, movement, light, odor) Internal stimulus (e.g., change in blood pH or blood pressure) RECEPTION Detection by internal sense organs Detection by external sense organs TRANSMISSION Sensory (afferent) neurons transmit information Central Nervous System (brain and spinal cord) INTEGRATION Interneurons sort and interpret information TRANSMISSION Motor (efferent) neurons transmit impulses ACTION BY EFFECTORS (muscles and glands) e.g., animal runs away e.g., espiration rate increases; blood pressure rises Stepped Art Fig. 40-1, p. 846 Neural signaling involves reception, transmission, integration, and action by effectors 6 Neurons Specialized to receive stimuli; transmit electrical and chemical signals    Cell body: contains nucleus and organelles Dendrites: Many branched dendrites extend from cell body of neuron specialized to receive stimuli and send signals to the cell body Axons: A single long axon extends from neuron cell body forms branches (axon collaterals), transmits signals into terminal branches which end in synaptic terminals -- Myelin sheath: surrounds many axons insulates -- Schwann cells: form the myelin sheath in the PNS -- In the CNS sheath is formed by other glial cells -- Nodes of Ranvier: gaps in sheath between successive Schwann cells Nerves and Ganglia Nerve  several hundred axons wrapped in connective tissue Ganglion  mass of neuron cell bodies in the PNS Glial Cells Support and nourish neurons Are important in neural communication 1. 2. 3. 4. 5. Astrocytes Oligodendrocytes Schwann cells Microglia Ependymal Cells Astrocytes physically support neurons regulate extracellular fluid in CNS (by taking up excess potassium ions) communicate with one another (and with neurons) induce and stabilize synapses Oligodendrocytes form myelin sheaths around axons in CNS Schwann cells form sheaths around axons in PNS Microglia Phagocytic cells Ependymal Cells line cavities in the CNS contribute to formation of cerebrospinal fluid serve as neural stem cells !!!Neurons are specialized to receive stimuli and transmit signals; glial cells are supporting cells that protect and nourish neurons and that can modify neural signals!!! Electrical signals transmit information along axons Plasma membrane of resting neuron (not transmitting an impulse) is polarized Inner surface of plasma membrane is negatively charged relative to extracellular fluid Resting potential Potential difference of about -70 mV across the membrane Magnitude of resting potential (1) differences in ion concentrations (Na+, K+) inside cell relative to extracellular fluid (2) selective permeability of plasma membrane to these ions Ions  Pass through specific passive ion channels - K+ leak out faster than Na+ leak in - Cl- accumulate at inner surface of plasma membrane  Large anions (proteins) - cannot cross plasma membrane - contribute negative charges Sodium–Potassium Pumps - Maintain gradients that determine resting potential transport 3 Na+ out for each 2 K+ in - Require ATP The resting potential of a neuron is maintained by differences in concentrations of specific ions inside the cell relative to the extracellular fluid and by selective permeability of the plasma membrane to these ions Membrane potential Membrane is depolarized  if stimulus causes membrane potential to become less negative Membrane is hyperpolarized  if membrane potential becomes more negative than resting potential Graded potential: A local response Varies in magnitude depending on strength of applied stimulus Fades out within a few millimeters of point of origin 19 Action Potential Action potential is a wave of depolarization that moves down the axon Generated when voltage across the membrane declines to a critical point (threshold level) Voltage-activated ion channels open Na+ ions flow into the neuron Voltage-Activated Ion Channels Voltage-Activated Ion Channels During an Action Potential Action potential An all-or-none response No variation in strength of a single impulse either membrane potential exceeds threshold level, or it does not Once begun, an action potential is selfpropagating Repolarization As an action potential moves down an axon, repolarization occurs behind it Refractory periods During depolarization, the axon enters an absolute refractory period when it can’t transmit another action potential When enough gates controlling Na+ channels have been reset, the neuron enters a relative refractory period when the threshold is higher Depolarization of the neuron plasma membrane to threshold level generates an action potential, an electrical signal that travels as a wave of depolarization along the axon Continuous Conduction Involves entire axon plasma membrane Takes place in unmyelinated neurons Saltatory Conduction Depolarization skips along axon from one node of Ranvier to the next More rapid than continuous conduction Takes place in myelinated neurons Nodes of Ranvier  sites where axon is not covered by myelin Na+ channels are concentrated Synapses Junctions between two neurons or between a neuron and effector Most synapses are chemical some are electrical synapses A presynaptic neuron releases neurotransmitter (chemical messenger) from its synaptic vesicles Neurotransmitters Acetylcholine triggers contraction of skeletal muscle Biogenic amines norepinephrine, serotonin, dopamine important in regulating mood dopamine is also important in motor function Some amino acids glutamate (excitatory neurotransmitter in brain) GABA (widespread inhibitory neurotransmitter) Neuropeptides (opioids) endorphins (e.g., beta-endorphin) enkephalins Nitric oxide (NO) gaseous neurotransmitter transmits signals from postsynaptic neuron to presynaptic neuron (opposite direction from other neurotransmitters) Synaptic Transmission Calcium ions cause synaptic vesicles to fuse with presynaptic membrane; releases neurotransmitter into synaptic cleft Neurotransmitter diffuses across the synaptic cleft; combines with specific receptors on a postsynaptic neuron Neurotransmitter Receptors Many are proteins that form ligand-gated ion channels Others work through  Neurons signal other cells by releasing neurotransmitters at synapses  Binding of Neurotransmitter to a Receptor - Binding causes either excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP) - Depending on the type of receptor EPSPs  bring neuron closer to firing IPSPs  move neuron farther away from its firing level Neural Integration Process of summing (integrating) incoming signals Summation  process of adding and subtracting incoming signals Each EPSP or IPSP is a graded potential vary in magnitude depending on strength of stimulus applied Summation of several EPSPs brings neuron to critical firing level Temporal summation  Occurs when repeated stimuli cause new EPSPs to develop before previous EPSPs have decayed Spatial summation  Occurs when several closely spaced synaptic terminals release neurotransmitter simultaneously stimulating postsynaptic neuron at several different places !!! During integration, incoming neural signals are summed; temporal and spatial summation can bring a neuron to threshold level !!! Neural circuits Complex neural circuits are possible because of associations such as convergence and divergence Convergence  A single neuron is affected by converging signals from two or more presynaptic neurons Allows CNS to integrate incoming information from various sources Divergence  A single presynaptic neuron stimulates many postsynaptic neurons allowing widespread effect Reverberating Circuits Important in  rhythmic breathing mental alertness short-term memory Depend on positive feedback new impulses generated again and again until synapses fatigue THANKS! CREDITS: This presentation template was created by Slidesgo, including icons by Flaticon, infographics & images by Freepik