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Biology 219 – Human Physiology Clemens Neurons and Neural Signals Text: Ch 8 I. Neurons and the Nervous System Functions of the nervous system: sensation communication integration control neurons - functional cells of the nervous system; - excitable cells - generate electrical signals (changes in membrane potential) - communicate information in the form of electrical and chemical signals DRAW NEURON HERE Structure of Neurons cell body contains the nucleus and most organelles dendrites branch from the cell body, receive signals from other cells through synapses axon extends from the cell body, conducts action potentials axon hillock - region where axon joins the cell body initial segment adjacent to the axon hillock is the trigger zone for AP axon terminals - contain vesicles with neurotransmitter, form synapses with other cells Nervous System Organization 1. Central Nervous System (CNS) - Brain and Spinal Cord. 2. Peripheral Nervous System (PNS) - nerves, ganglia and sensory receptors a. Afferent Division - input sensory information to the CNS b. Efferent Division - output motor signals from CNS to effector organs Functional types of neurons 1. sensory (afferent) neurons - input to CNS from sensory receptors; dendrites located at receptors, axons in nerves, cell bodies in ganglia outside the CNS 2. motor (efferent) neurons - output from CNS to effectors (muscles, glands) cell bodies and dendrites in the CNS, axons in nerves 3. interneurons - located entirely within the CNS; most abundant and diverse types II. Neural Signals A. Graded Potentials small, localized changes in membrane potential formed at the cell body and dendrites can be depolarization (↑) or hyperpolarization (↓) spread passively and weaken with distance size depends on stimulus strength +60 Vm 0 (mV) GRADED POTENTIALS -90 Time (ms) Page 2 B. Action Potentials (= nerve impulses) large change in membrane potential actively conducted along the axon rapid depolarization followed by repolarization “all or none” - constant size, does not depend on stimulus strength +60 Vm 0 (mV) ACTION POTENTIAL Phases of the action potential -90 Time (ms) 1. Depolarization (rising) phase - triggered by an initial depolarizing stimulus, must be above threshold to form an AP - voltage-gated Na+ channels open activation gate opens in response to initial depolarization → rapid Na+ inflow → depolarization → more activation gates open (positive feedback) 2. Repolarization (falling) phase - voltage-gated Na+ channels close inactivation gate closes when depolarization reaches a peak (~ +30 mV) - voltage-gated K+ channels open → rapid K+ outflow → repolarization 3. Hyperpolarization (undershoot) phase - voltage-gated K+ channels remain open, high K+ permeability results in hyperpolarization - resting states of channels and resting potential restored at the end of undershoot phase Properties of action potentials 1. threshold - stimulus must be greater than a certain strength to evoke an AP 2. "all or none" - once threshold is reached, size of the AP is constant regardless of stimulus 3. regenerative - AP is regenerated and does not decrease in strength along the axon 4. refractory period - short delay following an AP before another AP can be formed absolute refractory period (ARP) - another AP can not be formed relative refractory period (RRP) – a stronger stimulus is required to get another AP Importance of refractory period: - Refractory period prevents AP from traveling backward along the axon - During the RRP, a stronger stimulus can result in increased frequency of APs Stimulus intensity is coded by the frequency of APs. _I___I____I_I_I_I____IIIIIIIIIIII_ Conduction of action potentials a. unmyelinated axons - AP depolarization spreads a short distance down the axon (local current flow) - depolarization stimulates formation of AP farther down the axon - axons are “leaky” to Na+ and K+; need to regenerate AP often along the axon → slow conduction speed - increasing axon diameter increases conduction speed b. myelinated axons - myelin sheath formed by Schwann cells in the PNS, oligodendrocytes in CNS - insulates the axon, reduces leakage of Na+ and K+ - nodes of Ranvier - gaps in myelin sheath are sites of AP regeneration AP “jumps” from node to node (saltatory conduction) → very fast conduction speed (> 100 m/s) Ion Channels and the Action Potential 0. Resting Potential + K Leak Channel Voltage-gated Na+ Channel ECF ICF Channel: activation gate K+ OPEN 1. Depolarization Phase of A.P. inactivation gate CLOSED Channel: CLOSED Na+ activation gate ECF ICF Voltage-gated K+ Channel K+ OPEN OPENS CLOSED 2. Repolarization Phase of A.P. Na+ ECF ICF Channel: K+ OPEN inactivation gate K+ CLOSES OPENS 3. Hyperpolarization Phase of A.P. ECF ICF Channel: K+ OPEN gates reset CLOSED K+ OPEN, slowly closes