Neurons, Synapses, & Signaling Campbell and Reece Chapter 48

Neurons nerve cells that transmit information within the body communication between neurons consists of: ◦ long distance electrical signals ◦ short distance chemical signals

Neurons use pulses of electrical current to receive transmit regulate the flow of information over long distances w/in the body

Neuron Organization

Nervous System

Types of Neurons Sensory Neurons ◦ transmit information (senses) from body  brain ◦ are afferent ◦ specialized dendrites that initiate action potential when stimulated

Types of Neurons 2. Motor Neurons transmit signals to muscle fibers & glands are efferent

Types of Neurons 3. Interneurons majority of neurons in brain ◦ form local circuits connecting neurons

Synapse junction between axon terminal & next cell (another neuron, muscle fiber, gland cell) neurotransmitters are chemical messengers most synapses that pass action potential to receiving cell

Synapse presynaptic cell: cell releasing neurotransmitter & passing on action potential postsynaptic cell: receiving neurotransmitter synaptic cleft: physical space between the 2; neurotransmitter released into this space & diffuses across it  attaching to receptors on postsynaptic cell

Synapse

Glial Cells cells that support neurons Greek: glue aka neuroglia nourish neurons insulate axons regulate ECF surrounding neurons

Ion Pumps ions unequally distributed across plasma membrane inside of cell slightly (-) compared to outside cell source of potential nrg called the membrane potential resting potential : the membrane potential of rest = -60 to –80 mV

Resting Potential

Formation of Resting Potential Na+/K+ pump generates & maintains the ionic gradients of membrane potential 1 turn of pump ◦ 1 ATP ◦ 3 Na+ out ◦ 2 K+ in

Membrane Potential

Ion Channels pores that span the membrane allowing ions to diffuse across (in or out) membranes are selectively permeable and variations in how easily any particular ion can cross a membrane depends on the # of channels & how often they are open

Types of Ion Channels

Action Potentials neurons have gated ion channels that open or close in response to stimuli ◦ open/close changes permeability for that ion neurons have K+ channels ◦ when open K+ diffuses out of cell ◦ changes resting potential from: -60 mV to -90 mV

K+ Ion Channels in Neurons

Resting & Action Potentials t/chp44/4401s.swf t/chp44/4401s.swf tanding-Action-Potential-and-Nerve-Impulses tanding-Action-Potential-and-Nerve-Impulses

Hyperpolarization when K+ channels open & resting potential decreases to -90 mV inside of cell becoming more (-) than normal resting potential called: hyperpolarization

Depolarization when Na+ ion channels open Na+ diffuse into cell making inside less (-) compared to outside cell membrane potential shifts toward (+) mv this reduction in magnitude of membrane potential called depolarization

Graded Potentials any shift in membrane potential magnitude of shift varies with strength of stimulus induce a small electrical current that flows along the membrane leaking out of the cell so only lasts short distance from source

Action Potential electrical signal that propagates along the membrane of a neuron as a nongraded (all or nothing) depolarization have a constant magnitude & can regenerate in adjacent regions of the membrane travel long distances

Voltage-Gated Ion Channels ion channels that open/close based on membrane potential passing a particular level Na+ channels in neurons are voltage gated: open when depolarization occurs  Na+ diffuses into cell  becomes more depolarized  more Na+ channels open (+ feedback)

imations/actionpotential_short.swf imations/actionpotential_short.swf hill.com/sites/ /student_vie w0/chapter14/animation__the_nerve_im pulse.html hill.com/sites/ /student_vie w0/chapter14/animation__the_nerve_im pulse.html Interactive site to try at home:

Threshold Action potentials occur when a depolarization increases the membrane voltage to a particular value (the threshold) for mammals the threshold is a membrane potential ~ -55mV once started the action potential has a magnitude independent of the strength of triggering stimulus

+ feedback loop of depolarization & channel opening triggers an action potential whenever the membrane potential reached the threshold membrane depolarization opens both Na+ & K+ channels but Na+ opens faster initiating the action potential Na+ channels become inactivated as action potential proceeds (gates close) & remain so until after membrane returns to resting potential

Refractory Period (-) membrane potential restored by inactivation of Na+ channels, which increases K+ outflow This is followed by a refractory period: ◦ no matter how strong the stimulus to initiate next action potential is cannot initiate one during refractory period

Conduction of Action Potentials

Myelin Sheaths glial cells oligodendrocytes (CNS) and Schwann cells (PNS) form layers of electrical insulation along length of axons

Saltatory Conduction

Neurotransmitters >100 neurotransmitters belonging to 5 groups: 1. Acetylcholine 2. Amino Acids 3. Biogenic Amines 4. Neuropeptides 5. Gases