Neurons Synapses Signaling Campbell and Reece Chapter 48

Neurons, Synapses, & Signaling Campbell and Reece Chapter 48

�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 Neurons

Neuron Organization

Nervous System

�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 Types of Neurons

�junction between axon terminal & next cell (another neuron, muscle fiber, gland cell) �neurotransmitters are chemical messengers released @ 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

Synapse

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

�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 neuron @ rest = �-60 to – 80 m. V Ion Pumps

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 Formation of Resting Potential

Membrane Potential

�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 Ion Channels

Types of Ion Channels

�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 m. V to -90 m. V Action Potentials

K+ Ion Channels in Neurons

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

�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 Depolarization

�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 Graded Potentials

�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 Action Potential

�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) Voltage-Gated Ion Channels

�http: //highered. mcgraw- hill. com/sites/0072495855/student_view 0/chapter 1 4/animation__the_nerve_impulse. html �Interactive site to try at home: http: //outreach. mcb. harvard. edu/animations/ actionpotential_short. swf

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

�+ 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

�(-) 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 Refractory Period