Nerve Impulses and Reflex Arcs November 28, 2016

Neuron Functions What are the functions of the nervous system? Receive sensory input, interpret & make a decision, and effect a response Individual neurons have two major functional properties: irritability and conductivity. Irritability = ability to respond to a stimulus and convert it to a nerve impulse Conductivity = ability to transmit the impulse to other neurons, muscles, or glands. How do irritability and conductivity relate to the functions of the nervous system?

Irritability Irritability = ability to respond to a stimulus and convert it to a nerve impulse What type of stimulus? Receptor neurons respond to specific stimuli (e.g. touch, temperature, pressure, tension, sight, hearing, chemical) All other neurons (interneurons, motor neurons) respond to neurotransmitters released by other neurons What is a nerve impulse? A quick switch in voltage potential (charge difference) across the membrane that travels all the way along the axon of the neuron Occurs due to flow of ions across the membrane All – or - nothing Now to understand the process of a nerve impulse / action potential in much more detail …

Resting Potential Neurons at rest have an electrochemical gradient across the cell membrane, known as the resting potential. The resting potential is approximately -70mV. Extracellular fluid Cell membrane cytoplasm

Resting potential The resting potential is maintained by: Sodium-potassium pump (3 Na+ pumped out for every 2 K+ pumped in) Greater membrane permeability of K+ (K+ can diffuse back out to some degree) Organic anions (- ions) within cell This ‘rest state’ takes energyto create Extracellular fluid Na+ Cell membrane K+ Organic anion cytoplasm

Action Potential An action potential involves the rapid depolarization and repolarization of the membrane.

Action potential When a stimulus is applied to a nerve, some Na+ gates open, allowing Na+ to diffuse in. Extracellular fluid Na+ Na gate Cell membrane K+ K gate Na / K pump Organic anion cytoplasm

Action potential Once a threshold is reached, all Na+ gates open, causing depolarization of the membrane. When the membrane is depolarized, the inside of the membrane is more positively charged than the outside. Extracellular fluid Na+ Na gate Cell membrane K+ K gate Na / K pump Organic anion cytoplasm

Action potential Membrane repolarization occurs when Na+ gates close and K+ gates open, allowing net diffusion of K+ outside. Repolarization returns the membrane to resting potential (more negatively charged inside) Extracellular fluid Na+ Na gate Cell membrane K+ K gate Na / K pump Organic anion cytoplasm

Action potential The K+ gates close and the resting potential is maintained by the Na+ / K+ pump Watch me! Extracellular fluid Na+ Na gate Cell membrane K+ K gate Na / K pump Organic anion cytoplasm

Action Potential – Turn & Talk Scholar with more siblings … At rest, what ions are most abundant outside the cell, and which are most abundant inside the cell? Which side of the membrane is more negative at rest? Scholar with less siblings … Describe how the movement of ions causes Depolarization Repolarization

Action Potential – Turn & Talk Scholar with more siblings … At rest, what ions are most abundant outside the cell, and which are most abundant inside the cell? Which side of the membrane is more negative at rest? At rest, Na+ ions are mostly outside the membrane, while K+ and anions are mostly inside. The inside of the cell membrane is more negatively charged. Scholar with less siblings … Describe how the movement of ions causes Depolarization – Na+ ions rush into the cell Repolarization – K+ ions rush out of the cell

Nerve Impulse Propagation The nerve impulse moves along the axon. The change in voltage of one area triggers the depolarization of the next area. Repolarization follows immediately.

Nerve Impulses In myelinated neurons the impulse “jumps” from node to node, rather than traveling the whole length of the axon – makes the impulse transmission much more efficient. Watch me! Saltatory conduction is 10-60X faster than smooth conduction

Irritability vs. Conductivity Generation and propagation of nerve impulse along one neuron= irritability Conductivity is the ability of one neuron to signal another. This occurs in an entirely different fashion at the synapse, or gap, between neurons.

Synapses The fluid-filled space between neurons is called a synapse Chemicals called neurotransmitters carry the nerve impulse across the synapse.

Synapses The nerve impulse reaches the axon terminal. Ca+ gates open, allowing Ca+ into the axon. The Ca+ causes vesicles containing neurotransmitters to empty into the synapse

Synapses The neurotransmitters diffuse across the synaptic cleft and bind with receptors of the next neuron. Na+ channels open in the dendrites of the post-synaptic neuron Post-synaptic neuron depolarizes 7. Remaining neurotransmitter is broken down.

Synapses Pre-synaptic neuron Action potential  calcium ions release  neurotransmitter release Post-synaptic neuron Neutrotransmitter uptake  sodium gates open  action potential Note: information travels as electrical signal within neurons and as chemical signal between them 2015 study on Alzheimer’s suggests that the deterioration seen with Alzheimer’s may occur partly because the beta tangles disrupt a protein that physically hold the synapses of neurons in the brain close together. Watch me!

Reflex Arcs Watch me! Reflexes are rapid, predictable, involuntary responses to stimuli. May be somatic or autonomic Contain 5 elements

Closure What were our objectives today, and what did we learn about them? What was our learner profile trait and how did we use it? How does what we did today relate to our unit question? HOMEWORK: Quiz on Thursday / Friday!

Exit Ticket Count off by three. Identify the steps that take place during an action potential. Make a diagram of the cell membrane demonstrating the changes that occur during each step. Draw a diagram of the voltage changes that occur during an action potential. Label the diagram with the changes that occur within the cell during each step. Identify the steps that take place during synaptic transmission. Make a diagram of the events that within the pre- and post-synaptic neurons during each step.