P. Ch 48 – Nervous System pt 1

Peripheral nervous system (PNS) Central nervous system (CNS) Overview Sensor Effector Sensory input Motor output Integration Peripheral nervous system (PNS) Central nervous system (CNS)

Types of neurons Sensors detect external stimuli and internal conditions and transmit information along sensory neurons Sensory information is sent to the brain or ganglia, where interneurons integrate the information Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity

Parts of a neuron Dendrites Stimulus Axon hillock Nucleus Cell body Presynaptic cell Signal direction Axon Synapse Neurotransmitter Synaptic terminals Postsynaptic cell Parts of a neuron

Evolutionary Adaptation of Axon Structure The speed of an action potential increases with the axon’s diameter In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS

Axon Myelin sheath Schwann cell Nodes of Ranvier Node of Ranvier Layers of myelin Nucleus of Schwann cell

Nerve Signals Membrane potential - the electrical charge difference across a membrane Due to different concentrations of ions in & out of cell Anions more concentrated inside, Cations more concentrated outside Resting potential – the membrane potential of an unstimulated neuron About -70 mV (more negative inside)

Changes in membrane potential act as signals Concentration of Na+ highest outside, concentration of K+ highest inside cell. Sodium –potassium pumps use ATP to maintain concentration gradients Energy is potential chemical energy

A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell The resulting buildup of negative charge within the neuron is the major source of membrane potential

What determines the membrane potential?

Key Na K Sodium- potassium pump Potassium channel Sodium channel OUTSIDE OF CELL

Maintenance of Resting Potential - Note gradients for sodium and potassium - To maintain resting potential, you need to keep concentration gradients constant - ions have a charge, so need facilitated diffusion through membrane (is this with or against the gradient?) - there are selective ion channels that allow “leakage”

Maintenance of Resting Potential To keep sodium & potassium in right gradients, sodium-potassium pump uses ATP to maintain gradients The sodium-potassium pump pumps 2K+ in and 3Na+ out each time.

What is the result of changes in membrane potential? Graded potentials – based on magnitude of stimulus Action potentials – all or nothing depolarization

Types of ion channels: Ungated ion channels – always open Gated ion channels – open or close in response to stimuli Ligand gated ion channels (chemically gated) ion channels–in response to binding of chemical messenger (i.e. neurotransmitter) Voltage gated ion channels – in response to change in membrane potential Stretch gated ion channels – in response to mechanical deformation of plasma membrane

Graded potentials The magnitude of the change (in polarization) varies with the strength of the stimulus These are not the nerve signals that travel along axons, but they do have an effect on the generation of nerve signals

Hyperpolarization Stimulus Threshold Resting potential Hyperpolarizations 50 50 Membrane potential (mV) When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative This is hyperpolarization, an increase in magnitude of the membrane potential

Depolarization Stimulus Threshold Resting potential Depolarizations 50 50 100 1 2 3 4 5 Membrane potential (mV) Opening other types of ion channels triggers a depolarization, a reduction in the magnitude of the membrane potential For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell

Graded potentials occur up to a particular voltage, the threshold voltage If depolarization reaches the threshold voltage, then an action potential is triggered.

Action Potentials Signals conducted by axons, transmitted over long distances Occur as the result of gated ion channels that open or close in response to stimuli A specific dynamic change in the charge across the membrane of a cell, one that occurs either totally or not at all (“all or nothing”

Action Potential Steps: 1) resting state 2) threshold 3) depolarization phase 4) repolarization phase 5) undershoot Threshold Resting potential 50 50 100 1 2 3 4 5 Membrane potential (mV) 6 Action potential

Resting state Most voltage gated sodium(Na+) and potassium gated (K+) ion channels are closed, membrane potential is at -70 mV

Threshold An electric stimulus causes the voltage-gated Na+ channels open first and Na+ flows into the cell The flow of Na+ causes more voltage – gated Na+ channels to open, so more depolarization (what kind of feedback is this) If this depolarization reaches threshold potential, then more Na+ gates open

Depolarization phase Membrane potential rises rapidly as sodium ions rush into cell Also known as “rising” phase

Repolarization phase The Na+ voltage gates close shortly after opening, so Na+ inflow stops The K+ voltage gates open, so K+ rushes out There is a loss of positive charge, so cell returns to resting state “Falling” phase

Undershoot The membranes permeability to K+ is higher than at rest, so membrane potential dips down lower than resting potential K+ gates close, so membrane potential returns to normal

Refractory period After an action potential occurs, there is a period of time when another action potential cannot be triggered During undershoot – sodium channel inactivation gates are closed – haven’t had enough time to reopen yet

Membrane potential (mV) 50 OUTSIDE OF CELL INSIDE OF CELL Inactivation loop Sodium channel Potassium channel Action potential Threshold Resting potential Time Membrane potential (mV) 50 100 50 Na K Key 2 1 3 4 5 Resting state Undershoot Depolarization Rising phase of the action potential Falling phase of the action potential

How do action potentials “travel” along a neuron? Where action potential is generated (usually axon hillock), the electrical current depolarizes the neighboring region of membrane Action potentials travel in one direction – towards synaptic terminals

Action potentials are self-propagating across the axon. Plasma membrane Action potential 1 Cytosol Na Action potentials are self-propagating across the axon.

Axon Plasma membrane Action potential 1 Cytosol Action potential 2 Na K 2 Na K

Axon Plasma membrane Action potential 1 Cytosol Action potential 2 Na Action potential K 2 Na K Action potential K 3 Na

Why doesn’t it travel backwards? The refractory period is due to inactivated Na+ channels, so the the depolarization can only occur in the forward direction.

Speed of action potentials Speed is proportional to diameter of axon, the larger the diameter, the faster the speed Several cm/sec – thin axons 100 m/sec in giant axons of invertebrates such as squid and lobsters

http://www.youtube.com/watch?v=omXS1bjYLMI Nerves with giant axons Ganglia Brain Arm Nerve Eye Mantle Nerves with giant axons http://www.youtube.com/watch?v=omXS1bjYLMI

Speeding up Action potential in vertebrates Myelination (insulating layers of membranes) around axon Myelin is deposited by Schwann cells or oligodendrocytes. Cell body Schwann cell Depolarized region (node of Ranvier) Axon

Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction