Principles of Electrical Signaling in the Nervous System, Slides of Biology

Download Principles of Electrical Signaling in the Nervous System and more Slides Biology in PDF only on Docsity! Neural Signaling * Chapter 41 * [aS | | ) Introduction • Animal movements are triggered by electrical signals conducted by nerve cells – neurons • Complex processes based on seemingly simple events: – flow of ions across plasma membranes docsity.com The Anatomy of a Neuron • Most neurons: – have a cell body – highly branched short projections called dendrites – one or more long projections called axons – dendrite: – receives electrical signals from the axons of adjacent cells – cell body (soma): – includes the nucleus – integrates the incoming signals and generates an outgoing signal – axon: – sends the signal to the dendrites of other neurons docsity.com How Does Information Flow in a Neuron? Information flow through neurons Nucleus Dendrites Collect electrical signals Cell body Integrates incoming signals and generates outgoing signal to axon Axon Passes electrical signals to dendrites of another cell or to an effector cell Neurons form networks for information flow docsity.com An Introduction to Membrane Potentials • electrical potential: – difference of electrical charge between any two points – voltage • membrane potential: – a separation of charge across the membrane – ions carry a charge – cytoplasm and extracellular fluid contain unequal distributions of ions • Membrane potentials are a form of electrical potential and are measured in millivolts (mV). • A flow of charged ions is called an electric current. docsity.com How Is the Resting Potential Maintained? • As K+ moves out of the cell via K+ channels – the inside of the cell becomes more negatively charged compared to the outside • the membrane reaches a voltage at which there is equilibrium between the concentration gradient that moves K+ out and the electrical gradient that moves K+ in – called the equilibrium potential for K+ • Na+/K+-ATPase imports K+ ions and exports Na+ ions • As a result: – inside of the neuron is negatively charged with respect to the extracellular environment – the neuron has a negative resting potential docsity.com What Is an Action Potential? • action potential: – a rapid, temporary change in a membrane potential • It has three phases: depolarization, repolarization, and hyperpolarization. • The initial event is a rapid depolarization of the membrane: – For an action potential to begin, the membrane potential must shift from its resting potential of –70 mV to about –55 mV – If threshold potential (-55 mV) is reached… – (Na+) channels in the axon membrane open and ions rush into the axon…following their electrochemical gradients – this current flow causes further depolarization docsity.com What Is an Action Potential? • When the membrane potential reaches about +40 mV… – abrupt change is triggered by the closing of certain ion (Na+) channels and the opening of other ion channels (K+) in the membrane – the membrane experiences a rapid repolarization as ions flow out of the axon. • The repolarization event causes the membrane to become more negative than the resting potential – called hyperpolarization (-80 mV) • All three phases of an action potential occur in about a millisecond. docsity.com How Do Voltage-Gated Channels Work? • Voltage-gated channels: – ion channels that open and close in response to changes in membrane voltage • When the plasma membrane in the region of an axon is depolarized – voltage-gated Na+ channels open – sodium ions rush into the cell – generates an action potential • Shortly after the action potential is under way… – the Na+ channels close – voltage-gated K+ channels open docsity.com How Do Voltage-Gated Channels Work? • As K+ exits the cell… – the membrane potential drops back toward resting levels • Although Na+ flows in and K+ flows out… – the two currents do not cancel each other out – due to the movement of K+ is delayed – the Na+ channels close as the membrane potential approaches +54 mV – at which time the voltage-gated K+ channels open • Thus, action potentials result from the staggered activity of voltage-gated Na+ and K+ channels. docsity.com How Is the Action Potential Propagated? • When Na+ enters a cell during an action potential: – positive charges in the cell are repulsed and negative charges are attracted – charge spreads away from the sodium channels… – causes sections of the membrane near the site of the action potential to depolarize – nearby voltage-gated Na+ channels open in response… – an action potential results • Action potentials do not propagate back up the axon because Na+ channels are refractory – once they have opened and closed… – less likely to open again for a short period of time docsity.com How Is the Action Potential Propagated? • myelination: – prevents charge in the form of ions from leaking out as it spreads down an axon. • the influx of charge that results from an action potential spreads unimpeded until it hits an unmyelinated section of the axon – called a node of Ranvier • The node has a dense concentration of voltage-gated channels: – so action potentials can occur – electrical signals jump down the myelinated axon much faster than they could down an unmyelinated one docsity.com Action Potentials Propagate Quickly in Myelinated Axons Action potentials jump down axon. Nodes of Ranvier Action potential jumps from node to node Schwann cells (glia) wrap around axon, forming myelin sheath WHY ACTION POTENTIALS JUMP DOWN MYELINATED AXONS Schwann cell Node of Ranvier Axon Schwann cell membrane wrapped around axon 1. As charge spreads down an axon, myelination (via Schwann cells) prevents ions from leaking out across the plasma membrane. 2. Charge spreads unimpeded until it reaches an unmyelinated section of the axon, called the node of Ranvier, which is packed with Na+ channels. 3. In this way, electrical signals continue to jump down the axon much faster than they can move down an unmyelinated cell. docsity.com How Is the Action Potential Propagated? • If myelin degenerates, the transmission of electrical signals slows considerably • Multiple sclerosis (MS): – develops as damage to myelin increases and electrical signaling is impaired – causes the muscles to weaken and coordination to lessen docsity.com Neurons Meet and Transfer Information at Synapses Presynaptic neuron Postsynaptic neuron ACTION POTENTIAL TRIGGERS RELEASE OF NEUROTRANSMITTER Na+ and K+ channels Presynaptic membrane (axon) Postsynaptic membrane (dendrite or cell body) Synaptic cleft Action potentials 1. Action potential arrives; triggers entry of Ca2+. 2. In response to Ca2+, synaptic vesicles fuse with presynaptic membrane, then release neurotransmitter. 3. Ion channels open when neurotransmitter binds; ion flows cause change in postsynaptic cell potential. 4. Ion channels will close as neurotransmitter is broken down or taken back up by presynaptic cell (not shown). docsity.com What Do Neurotransmitters Do? • To qualify as a neurotransmitter, a molecule must satisfy three criteria: – (1) be present at the synapse and released in response to an action potential – (2) bind to a receptor on a postsynaptic cell – (3) be taken up or degraded • Many neurotransmitters function as ligands: – molecules that bind to a specific site on a receptor molecule docsity.com Postsynaptic Potentials and Summation • Synapses can be one of two general types: – lead to depolarization, or… – hyperpolarization of the membrane • excitatory postsynaptic potentials (EPSPs): – cause the membrane to depolarize • inhibitory postsynaptic potentials (IPSPs): – cause the membrane to be hyperpolarized docsity.com What Does the Peripheral Nervous System Do? • The functions of the PNS form a hierarchy • The PNS is divided: – afferent division: – which transmits sensory information to the CNS – efferent division: – which carries commands from the CNS to the body docsity.com What Does the Peripheral Nervous System Do? • The efferent division is divided: – somatic system: controls the skeletal muscles – responds to external stimuli and results in movement – autonomic system: controls internal processes – responds to internal stimuli and controls the activity of internal organs and glands – The autonomic system is further divided: – parasympathetic nervous system: promotes relaxation or digestion – sympathetic nervous system: prepares organs for stressful situations docsity.com The Functions of the PNS Form a Hierarchy Central nervous system (CNS) Information processing Peripheral nervous system (PNS) Sensory information travels in afferent division Most information travels in efferent division, which includes… Somatic nervous system Autonomic nervous system Parasympathetic division Sympathetic division docsity.com