Overview of the Nervous System Lecture 10 Overview of the Nervous System

Outline Organization of Nervous System Constituent Cells of Nervous System Electrical Signals in Neurons Source of Resting Membrane Potential Gated Ion Channels Qualitative Description of Action Potential Graded Potential Action Potential Refractory Period

Outline Organization of Nervous System Constituent Cells of Nervous System Electrical Signals in Neurons Source of Resting Membrane Potential Gated Ion Channels Qualitative Description of Action Potential Graded Potential Action Potential Refractory Period

Organization of the Nervous System Central nervous system Brain and spinal cord Peripheral nervous system Afferent neurons Sensory neurons Efferent neurons Send response to effector cells Somatic motor division Control skeletal muscle Autonomic division Controls smooth and cardiac muscle and exocrine/endocrine Two components: Sympathetic Parasympathetic Commonly exert antagonistic control over a single target

Fig 8.1 – Organization of the nervous system Silverthorn 2nd Ed

Outline Organization of Nervous System Constituent Cells of Nervous System Electrical Signals in Neurons Source of Resting Membrane Potential Gated Ion Channels Qualitative Description of Action Potential Graded Potential Action Potential Refractory Period

Cells of the Nervous System Neurons Basic signaling units of nervous system Consist of: Cell body Axons – carry outgoing information Dendrites – receive incoming signals Glial Cells Support cells Outnumber neurons by 10-50X Provide physical support for neural tissues Direct growth of neural tissue during repair and development Insulate axons creating myelin

Fig 8.2 – Model neurons Silverthorn 2nd Ed

Fig 8.6 – Formation of myelin Silverthorn 2nd Ed

Outline Organization of Nervous System Constituent Cells of Nervous System Electrical Signals in Neurons Source of Resting Membrane Potential Gated Ion Channels Qualitative Description of Action Potential Graded Potential Action Potential Refractory Period

Resting Membrane Potential Nernst Equation GHK Equation

Electrical Signals in Neurons If membrane permeability to ion changes: Membrane potential changes To substantially change Vm: Only small # of ions need to cross membrane Changes in Vm do not alter ion concentrations inside and outside cell

Depolarization At rest: Depolarization: Membrane potential mostly due to K+ Membrane almost impermeable to Na+ Depolarization: Cell becomes permeable to Na+ Na+ rushes in, membrane potential drops Moves towards +60mV of Na+

Gated Ion Channels How do cells change their membrane potential? Open or close existing channels in membrane Four major types of selective ion channels: Na+, K+, Ca+, Cl- Ions channel can be: Normally open Normally closed Mechanically gated – sense pressure Chemically gated – respond to neurotransmitters Voltage gated – important in initiation and conduction of electrical signals

Outline Organization of Nervous System Constituent Cells of Nervous System Electrical Signals in Neurons Source of Resting Membrane Potential Gated Ion Channels Qualitative Description of Action Potential Graded Potential Action Potential Refractory Period

Action Potentials When ion channels open: Ions move in or out depending on electro-chemical gradient Resulting influx changes membrane potential Two types of electrical signals: Graded potentials: Variable strength signals that travel short distances Action potentials: Large uniform depolarizations that travel rapidly over long distances without losing strength

Fig 8.7 – Graded potentials decrease in strength as they spread out from the point of origin Silverthorn 2nd Ed

Graded Potentials Amplitude directly proportional to strength of triggering event Begins on membrane at point where ions enter from ECF e.g. where neurotransmitter combines with receptors on dendrite to open Na+ channels Strength depends on how much charge enter cell Travels until: It dies out OR Reaches trigger zone IF - graded potential reaching trigger zone exceeds threshold, then AP IF NOT – dies out

Fig 8.8 – Subthreshold and suprathreshold graded potentials in a neuron Silverthorn 2nd Ed

Action Potentials All action potentials are identical in strength Do not diminish in strength as they travel

How Are APs Generated? Start at resting membrane potential Graded potential exceeding threshold reaches trigger zone Voltage gated Na+ channels open suddenly Sharp depolarization of cell  Cell reaches peak positive voltage Voltage gated K+ channels open slowly, Na+ channels close

How Are APs Generated? K+ moves out of cell  Reduces membrane potential K+ continues to leave, hyperpolarizes cell Voltage gated K+ channels close, some K+ enters through leak channels Cell returns to resting membrane potential

Fig 8.9 – the action potential Silverthorn 2nd Ed

Na+ Channel Dynamics Use a two-step process for opening and closing: Activation gate: Closed at resting membrane potential Opens when cell depolarizes, allowing Na+ to enter Inactivation gate: Open at resting membrane potential Closes when cell depolarizes, but has 0.5 ms delay Both reset when cell repolarizes

Fig 8.10 a – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

Fig 8.10 b – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

Fig 8.10 c – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

Fig 8.10 d – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

Fig 8.10 e – Model of the voltage-gated Na+ channel Silverthorn 2nd Ed

Fig 8. 11 – Ion movements during the action potential Fig 8.11 – Ion movements during the action potential Silverthorn 2nd Ed

Refractory Period Double gating of Na+ channel leads to refractory period Absolute refractory period Once an AP has begun, for about 1 ms, a 2nd AP can’t be generated Relative refractory period After Na+ channel gates have been reset, but before Vm has returned to normal, a STRONG graded potential can start a 2nd AP Graded potential opens Na+ channels, but Na+ entry is offset by continuing K+ loss through K+ channels that are still open

Fig 8.12 Refractory periods Silverthorn 2nd Ed

Features of APs Stimulus intensity is coded by AP frequency Conduction of APs: Travel from trigger zone to axon terminal Refractory period  APs travel in only one direction Speed of conduction: Depends on neuron diameter  diameter   speed Resistance of membrane to current leak Myelination increases speed of conduction

Fig 8.14 – Action potentials along an axon Silverthorn 2nd Ed

Fig 8.15 – Conduction of action potentials Silverthorn 2nd Ed

Myelination Nodes of Ranvier AP Propagation Saltatory Conduction Membrane resistance lowest at these points AP Propagation Starts at trigger zone AP flows to 1st Node of Ranvier Node has high density of voltage gated Na+ channels Na+ re-entry boosts strength of AP Saltatory Conduction “Leapfrogging” of APs

Fig 8.17 – Saltatory conduction Silverthorn 2nd Ed

Summary Organization of Nervous System Constituent Cells of Nervous System Electrical Signals in Neurons Source of Resting Membrane Potential Gated Ion Channels Qualitative Description of Action Potential Graded Potential Action Potential Refractory Period

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Due Dates Tuesday, October 5th HW5