Neurons, Synapses and Signaling Chapter 48

Warm Up Exercise What types of cells can receive a nerve signal?

Nervous Organization Neurons- nerve cells. Brain- organized into clusters of neurons, called ganglia. Central Nervous System- includes the brain and the spinal cord. Peripheral Nervous System- all of the neurons extending from the brain and the spinal cord. Communication between neurons comes from long distance electrical and short distance chemical signals. Neurons transmit sensory information, control heart rate, hand and eye movement, record memories, generate dreams, etc. Nerves- bundles of neurons in the body. Senses may be from the five external senses, or internal conditions- such as blood pressure, CO2 levels, muscle tension, etc.

Types of Neurons Sensory Neurons- transmit information from the senses to processing centers in the brain or ganglia. Interneurons- neurons in the brain that analyze and interpret sensory input. Motor Neurons- transmit signals for muscle and gland activity.

Neuron Structure and Function Cell Body- contain the organelles and nucleus. Dendrites- branched extensions that receive signals from other neurons. Axon- extension from the cell body that transmits signals to other cells. Synapse- junction between neurons. Neurotransmitters- chemical messengers that pass information between neurons. Glial Cells- supporting cells that insulate the axons of neurons and regulate fluid surrounding neurons. Presynaptic Cell (the transmitting neuron) and Postsynaptic Cell (neuron, muscle, or gland) Neurons can be very simple, or complex with highly branched axons that can transmit information to many target cells. Neurons with highly branched dendrites can receive signals though large number of synapses- as many as 100,000 in the case of some interneurons. Glial cells outnumber neurons in the brain 10-50 fold in mammals.

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Ion Pumps and Resting Potential Membrane Potential- the difference in voltage between the inside and outside of the cell membrane. Resting Potential- the membrane potential of a resting neuron is -60 to -80 mV. Formed by a high concentration of K+ ions inside the cell, and high Na+ ions outside the cell. Resting = not transmitting a signal This difference in charge is a source of potential energy needed to transmit a signal and must be maintained (cannot allow for simple diffusion to allow the ions to flow freely across). Rapid changes in membrane potential allow you to see, read, sense, run, etc. WHY A BIG CHARGE??? NET MOVEMENT OF IONS GENERATES MEMBRANE POTENTIAL

Sodium-Potassium Pumps Sodium-Potassium Pumps- maintain resting potential in the cell membrane. Transport 3 Na+ ions out for every 2 K+ ions in. In addition to the Sodium- Potassium pump, ions diffuse across the concentration gradient. Many K+ channels are open, allowing for a large amount of K+ to move out of the cell, few Na+ channels are open allowing little flow inside, leading to a negative membrane potential inside. Ion channels are selectively permeable- allowing only 1 ion to pass. At rest, many potassium channels are open, which allow a large outflow of K+, but very few sodium channels are open, leading to a negative charge inside the cell. This provides the membrane potential. Net outflow of K+ continues until chemical and electrical forces are in balance.

Warm Up Exercise Explain how a nervous response is transmitted through a series of neurons? How does the sodium-potassium pump maintain a membrane gradient?

Ion Gated Channels Ion Gated Channels- ion channels that open or close in response to stimuli. The opening and closing of ion gated channels alters the membrane potential. Hyperpolarization- an increase in the magnitude of the membrane potential. Depolarization- a reduction in the magnitude of the membrane potential. Changes in the membrane potential occur because neurons contain ion gated channels. How would hyperpolarization and depolarization change the charge on the inside and the outside of the membrane. *Think about the potassium channels in the Na/K pump- if more of them open, K+ floods out of the cell, shifting the membrane potential to be more negative. In a resting neuron, hyperpolarization results from an stimulus that increases the outflow of positive ions or inflow of negative ions. Depolarization is usually a result of sodium gated channels. If they open, sodium moves intot he membrane, lowering the potential.

Graded and Action Potentials Graded Potential- a shift in the membrane potential; is a response to hyperpolarization or depolarization Action Potential- a massive change in membrane voltage, caused by depolarization. Action potentials can be regenerated to spread along the axon at a constant magnitude. Action potentials have a constant magnitude and can regenerate in adjacent regions, spreading along the axon- making them well suited for transmitting signals over long distances. The magnitude of the graded potential varies with the strength of the stimulus, with a larger stimulus causing a greater change in the membrane potential. Graded potentials induce a small electrical current that leaks out of the neuron as it flows along the membrane, thus decaying with the distance from their source. These are not nerve signals, but have a major effect on the generation of nerve signals.

Graded Potentials and Action Potentials kk The huge change in shape represents the large change in membrane potential from the ions moving through the voltage gated sodium and potassium channels. Membrane depolarization opens both types of channels, but they respond independently and sequentially.

Voltage Gated Ion Channels Voltage-Gated Ion Channels- open and close when the membrane potential passes a particular level. Action potentials occur when depolarization increases the membrane potential to a certain level, called the threshold. Action potentials arise because some of the ion channels are voltage gated. Voltage gated ion channels are a demonstration of positive feedback. If depolarization opens the channels, the flow of Na into the neuron causes further depolarization, causing more channels to open, leading to more current. This leads to a rapid change in membrane potential that defines an action potential. For mammals, this threshold is about -55mV,. Once it reaches this level, the action potential has a magnitude independent of the strength of the trigger stimulus. They exhibit an all or none response.

kk Sodium channels open first initiating the action potential. As the action potential proceeds, the sodium channels become inactivated, blocking flow through the opening until after the membrane returns to resting potential and the channels close. Potassium channels open more slowly than sodium channels but remain open and functional until the end of the action potential. At Resting Potential- sodium potassium pump (not shown) and simple diffusion (ungated channels) maintain resting potential. Sodium and Potassium channels are closed. Stimulus leads to the opening of some sodium channels- Na flows in causing depolarization (less difference between inside and outside). If depolarization reaches threshold, an action potential is triggered. Depolarization causes the opening of most sodium channels, potassium channels remain closed. Sodium influx makes inside very positive with respect to outside. Sodium channels become inactivated, blocking Na inflow. Most potassium channels open, permitting K+ outflow, restoring the negative membrane potential. All sodium channels closed, but some potassium remain open. As potassium channels begin to close, the membrane returns to its resting state. This is called undershoot.

Warm Up Exercise Explain what happens in hyperpolarization and depolarization? Which ions move in which direction? Describe what happens in an action potential.

Action Potentials Refractory Period- “downtime” when a second action potential cannot be initiated. Occurs because of the inactivation of the sodium channels- during the falling phase and early part of the undershoot. Refractory period also ensures that all signals in an axon travel in one direction, from body to terminals. For most neurons the time from the onset of an action potential to the end of the refractory period is only 1-2 milliseconds. Because these action potentials are so brief, the neuron can produce hundreds per second. The frequency of an action potential is also correlated to its strength. For example, louder sounds have more frequent action potentials in the neurons connecting the ear to the brain. The difference in the time interval between action potentials is the only variable in transmission of information by an axon. Defects in ion gated channels lead to seizures- excessive synchronized firing of groups of nerve cells. At the site where an action potential is initiated Na inflow creates an electrical current that depolarizes the neighboring region of the axon membrane, causing the action potential to be reinitiated there. REPOLARIZATION

Action Potentials Saltatory Conduction- how the action potentials jumps from node to node along the axon. The wide axon diameter allows for extremely fast conduction of action potentials. Large axons (in invertebrates) can transmit about 30 meters per second. In vertebrates, the axons are more narrow, but the myelin sheaths help the action potential to travel faster. Also, more nerves can be packed into a smaller space (almost 2,000 vertebrate nerves for 1 invertebrate nerve) The voltage gated sodium channels are only found within the nodes of ranvier. Because of the myelin sheath, the axon is insulated, which allows the depolarization current to travel to the next node where the action potential is regenerated. Thus a time consuming process of opening and closing ion channels happens at fewer places along the axon.

Communication With Other Cells Electrical Synapses- contain gap junctions which allow electrical currents to flow from one neuron to the next. Chemical Synapses- release a chemical neurotransmitter between cells. In most cases, action potentials are not transmitted from neurons to other cells. Information is transmitted in another form, at the synapse. Electrical- responsible for very rapid responses Chemical synapses- the majority.

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Chemical Synapse Presynaptic neuron synthesizes neurotransmitter and packages in synaptic vesicles. The arrival of action potential at axon/synaptic terminal depolarizes plasma membrane, opening voltage-gated channels, which allow Ca2+ to diffuse into the synaptic terminal, which forces vesicles to fuse with membrane causing the release of neurotransmitter into the synaptic cleft. Neurotransmitters diffuse across the cleft and binds to and activates a specific membrane receptor (called a ligand-gated ion channel).

Ligand-Gated Ion Channels Ligand-Gated Ion Channel- located in postsynaptic cell- binding of neurotransmitter to this receptor opens the channel and allows specific ions to diffuse across the postsynaptic membrane, resulting in a postsynaptic potential. Excitatory Postsynaptic Potential (EPSP)- occurs when channel is permeable to both Na+ and K+. Causes depolarization. Inhibitory Postsynaptic Potential (IPSP)- occurs when channel is permeable to either K+ or Cl-. Causes hyperpolarization. Postsynaptic potential- a graded potential in postsynaptic cell EPSP- brings membrane potential toward threshold. (depolarization) IPSP- leads to hyperpolarization- moves away from threshold.

Summation of Action Potentials Temporal Summation- two EPSP’s occur at a single synapse in rapid succession- in this case the EPSP’s add together. Spatial Summation- two EPSP’s produced simultaneously at different synapses on the same postsynaptic neuron- EPSP’s added together. The cell body and dendrites of one postsynaptic neuron may receive inputs from chemical synapses with hundreds or even thousands of synaptic terminals. Graded potentials become smaller with distance from the synapse, so usually when an EPSP reaches the axon hillock it is usually too small to trigger an action potential in a postsynaptic cell. Temporal- membrane potential has not yet returned to resting state, but is not yet in the falling phase either Through these mechanisms, several smaller EPSP’s can combine to depolarize a membrane and produce an action potential. This also applies to IPSPs, where combined they have a larger hyperpolarization effect. Through summation, and IPSP can also counteract an EPSP.

Exit Slip In multiple sclerosis, myelin sheaths harden and deteriorate. How would this affect the nervous system function?

Warm Up Exercise What is meant by the term saltatory conduction? Explain the difference between an electrical and chemical synapse. Discuss how a presynaptic cell transmits a chemical impulse once it receives the action potential near the axon terminal.

Neurotransmitters Acetylcholine- causes the opening of potassium channels in cardiac muscle membrane. Leads to hyperpolarization, which reduces the rare at which the heart pumps (inhibitory). Dopamine and Serotonin Epinephrine and Norepinephrine Vital for nervous system functions that include muscle stimulation, memory formation, and learning. When acetylcholine released by motor neurons binds with ligand gated ion channel, channel opens, leading to depolarization. (an EPSP). This excitatory activity is terminated by acetylcholinesterase, which is present in the synaptic cleft and hydrolyzes acetylcholine. Ach leads to a signal transduction pathway that causes potassium channels to open. Botulism is a food poisoning caused by the inhibited release of acetylcholine from presynaptic cells. Muscles required for breathing fail to contract. Botulinum toxin is used for botox to minimize wrinkles by blocking transmission at synapses that control particular facial muscles. Dopamine and Serotonin- released at many sites in the brain and affect sleep, mood, attention, and learning. LSD causes hallucinations by binding to the receptors for these neurotransmitters. Prozac eliminates serotonin reuptake and Parkinsons is associated with lack of dopamine.

Organization of Human Brain Cerebrum- the center for voluntary movement, learning, emotion, memory, etc. Divided into right and left hemispheres, connected by the corpus callosum. Cerebellum- coordinates movement and balance. Beneath cerebrum, more functions related to homeostasis Cerebral palsy- damage to basal nuclei- clusters of neurons deep in the cerebrum which control planning and learning movement sequences. Cerebellum- receives sensory information about position of joints and information from audio and visual systems. Responsible for hand eye coordination. If cerebellum is damaged, eye movement is erratic (follow the light with your eyes, in the doctors office) Thalamus sorts sensory input before sending to appropriate cerebral centers for further processing. Hypothalamus also helps to regulate hunger and thirst, sexual and mating behavior and fight or flight response through the control of the pituitary gland. Pons and midbrain- sensory information- transferring info between PNS and CNS Medulla- breathing, digestion, vominting, swallowing Thalamus- sensory input center Hypothalamus- thermoregulation and biological clock Pineal Gland- source of melatonin Pituitary Gland- source of hormones

Organization of Human Brain Diencephalon Thalamus Hypothalamus Pineal Gland Pituitary Gland Brainstem- receives signals from sensory neurons Midbrain Pons Medulla Oblongata

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