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Today: • • • • • • Response and feedback loops: setpoints Speed and specificity of control systems Patterns of control pathways The organization of the nervous system Neurons Midterm test: – – – – Monday Oct. 7: 8:30 – 9:20 multiple choice questions Location: A- P in the Education Gym Q-Z in Erie Hall (lecture hall) Nervous system • Neurons • Glial cells • Electrical signals – Membrane potential – Ion movements – Gated channels – Graded potentials Cells of the nervous system: -neuron Axonal transport of membranous organelles Glial cells Electrical signals • Neurons are excitable cells, can propagate electrical signals. • As all other cells, neurons have a resting membrane potential As all other cells, neurons have a resting membrane potential Electrochemical gradients: Ca++ and Na++ concentrations are higher extracellular than intracellular. Intracellular K+ concentration is higher than extracellular K+ outward current is much stronger than Na+ inward current. Lots of K+ channels are open, few Na+ channels are open at rest. Factors that influence the membrane potential • Concentration gradient of ions across the membrane • Membrane permeability to ions (channels) • If these change, the membrane potential changes Ion movement across the cell membrane creates electrical signals • If Na channels open, Na+ enters the cell down its electrochemical gradient. Depolarization when Na+ channels open: electrical signal Ion movement causes hyperpolarizaton. Outward K+ movement. Inward Cl- movement Channel families • Selective ion channels: – Na channels – K+ channels – Ca++ channels – Cl- channels – monovalent cation selective channels (allow both Na+ and K+) How are channels opened or closed? • Leaky channels are open most of the time • Mechanically gated channels • Chemically gated channels (e.g. opened by neurotransmitters) • Voltage gated channels Graded potentials • Ions move across membrane when channels open Vm changes. Graded potentials • Amplitude depends on the strength of the stimulus Graded potentials • Spread across the membrane • Diminish away from origin Graded potentials Spread across the membrane Diminish away from origin Setpoints can vary • Acclimatization – adaptation to a given set of environmental conditions (first day of spring like weather) • Acclimation – adaptation to laboratory setting Endocrine and Nervous Reflexes • • • • • Specificity of target cells Nature of signal Speed Duration of action Stimulus intensity coding Specificity of target cells A neuron has a specific target cell. Many tissues may have receptor proteins for a hormone and may respond to the hormone. Nature of signal In neurons, electrical signals travel long distances and chemical signals (neurotransmitters) move to the target cell. Nature of signal The endocrine system uses chemical signals. Nature of signal: Neuroendocrine: Electrical signals travel long distances and chemical signals (neurohormones) move into blood vessels. Speed • Electrical signals conducted very quickly • Rapid signal transduction with many neurotransmitters • Hormones move by diffusion from endocrine cell to target (slower) • Some responses take quite a long time Duration of action • Nervous control is shorter than endocrine control • Nervous – usually neurotransmitter is rapidly removed Stimulus intensity coding • Intensity is coded by neuronal firing frequency amount of neurotransmitter • The amount hormone that is released codes the intensity. Knee jerk reflex Lactation, oxytocin Autonomic stimulation of insulin release Growth hormone Testosterone Low glucose and beta cells Organization of the nervous system • Sensory Receptors afferent pathway central nervous system (CNS, brain and spinal cord efferent neurons (peripheral nervous system PNS) target • PNS: – Somatic motor division – Autonomic division: – Sympathetic – Parasympathetic Cells of the nervous system: -neuron