Neurons, Synapses, and signaling By: Periods 3-4

3 Stages of Processing Sensory input Integration Motor Output sensory neurons transmit information from external stimuli (light, sound, touch, heat, smell, and taste) and internal conditions (blood pressure, blood, CO2 level and muscle tension). This info is sent to the CNS (central nervous system)for processing Integration In the CNS, interneurons analyze and interpret sensory in sensory input, taking into account the immediate context and what happened in the past. The CNS then decides how to respond. Motor Output It leaves the CNS via motor neurons to effector cells, which can be either muscle cells or cells in the endocrine glands that respond to the message from the CNS by movement or releasing hormones.

Distinguishing Neurons NEURONS: nerve cells that transfer information within the body, the fundamental unit of the nervous system. Sensory neurons Transmit information from a sense receptor to the brain and spinal cord Interneurons integrate information within the brain or spinal cord; connect sensory and motor neurons; located entirely within the CNS Motor Neurons Transmit information from the brain or spinal cord to a muscle or gland; cause muscle contraction gland secretion. In 2011, scientists learned children with autism have a greater mass and a vast abundance of neurons in the area of the brain known as the prefrontal cortex, which is where social, emotional, and communication processing are located. The presence of excess neurons alter the connections each neuron makes while a baby is developing, which disturbs the process of cell to cell communication.

Major Parts of a Neuron Dendrites: branched extensions that receive signals from other neurons. Axon hillock: the cone-shaped region of an axon where it joins the cell body, the region where the signals that travel down the axon are generated Presynaptic cell: the cell transmitting the signal Glial cells: support the nerve cells by either providing nourishment, insulating the axons, or regulating the amount of fluid in the cell cell body: contains most of the neurons organelles Axon: transmits the signal to other cells. synapse: the junction between two nerve cells. synaptic vesicle: membranous sac containing neurotransmitters at the tip of an axon. Synaptic terminal: a bulb at the end of the axon in which neurotransmitter molecules are stored and from which they are released. postsynaptic cell: the cell receiving the signal. neurotransmitter: a molecule that is released from the synaptic terminal of a neuron at a chemical synapse, which is then diffused across the synapse. and binds to the postsynaptic cell, causing a response.

Major Parts of a Neuron

Concept Check dendrites Axon terminals Axon Hillock Axon Cell body

Membrane Potential and Resting Potential Membrane potential – a voltage (difference in electrical charge) across their plasma membrane. Inputs from other neurons or specific stimuli cause changes in this membrane potential that act as signals, transmitting and processing information. Resting potential – the membrane potential of a resting neuron – one that is not sending signals. Is typically between -60 and -80 mV (millivolts). Factors that contribute to a membrane potential: The ion’s electrochemical gradient and the ion’s permeability The driving force of ions which are a summation of voltage gradient and concentration gradient are an important one. Also other proteins and amino acids contribute to the cell's membrane potential. The basis of the membrane potential

Nature of Nerve cells The basis of the membrane potential In a mammalian neuron at resting potential, the concentration of K+ is greater inside the cell, while the concentration of Na+ is greater outside the cell Sodium-Potassium Pump Maintains the concentration gradient of Na+ and K+. It uses ATP to actively transport Na+ out and K+ in the plasma membrane The concentration gradients of K+ and Na+ across the plasma membrane represent a chemical form of potential energy. The opening of ion channels in the plasma membrane converts chemical potential to electrical potential. As ions diffuse through channels, they carry with them units of electrical charge. A neuron at resting potential contains many open K+ channels and fewer open Na+ channels; K+ diffuses out of the cell Very little Na+ is transported out because of few numbers of pumps. Large quantity of K+ flows out because of the net negative charge inside the cell caused by trapped anions in the cell. The basis of the membrane potential

Action Potential Hyperpolarization Depolarization Graded Potentials The increase in the magnitude of the membrane potential Results from an increase in the outflow of positive ions, or inflow of negative ions Depolarization The reduction in the magnitude of the membrane potential. Neurons often involves gated sodium channels Gated sodium channels open, the membrane's permeability to Na+ increases, causing a depolarization as membrane potential shifts to E(Na) Graded Potentials Magnitude of the change in membrane potential varies with the strength of the stimulus Ion Channels Converts chemical potential energy to electrical potential energy Pores formed by specialized proteins that allow ions to diffuse back and forth across the membrane Ions carry units of charge, a net movement of charges will generate a voltage across the membrane Gated Ion Channels Ion channels that close in response to stimuli Basis of nearly all electrical signaling in nervous system The opening/closing alters membrane permeability to particular ions, altering membrane potential Voltage Gated Ions open/close in response to a change in membrane potential an increased depolarization causes more sodium channels to open, leading to an even greater flow of current ( opening of all voltage gated sodium channels) triggers a massive change in membrane voltage called action potential

Action Potential Refractory Period nerve impulses/signals that carry information along an axon occurs whenever depolarization increases membrane voltage to a particular value (threshold). all or not response: action potential occurs fully, or not at all depolarization opens voltage gated sodium channels, and opening sodium channels further depolarizes it triggers an action potential when membrane potential reaches the threshold sodium channels open first, initiating action potential, as action potential proceeds, channels become inactivated remain inactivated until it returns to resting potential and the channels close potassium pumps open more slowly, but remain open and functional throughout the action potential Refractory Period Down period following an action potential when a second action potential cannot be initiated Sets a limit on the maximum frequency at which action potentials can be generated Due to inactivation to sodium channels, not change in ion gradients across plasma membrane

https://www.youtube.com/watch?feature=player_embedded&v=ifD1YG07fB8

Conduction of Action Potentials An action potential functions as a long-distance signal by re-generating itself as it travels from the cell body to the synaptic terminals. Action potentials are the signals conducted by axons. An action potential (nerve impulse) is an-all-or-none response to depolarization of the membrane of the nerve cell. An action potential is triggered if the threshold is reached. At each position along the axon, the process is identical, such that the shape and magnitude of the action potential remain constant. A new action potential cannot be generated during a refractory period. Behind the traveling zone of depolarization due to Na+ inflow is a zone of repolarization due to K+ outflow. In the repolarized zone, the sodium channels remain inactivated. The inward current that depolarizes the axon membrane ahead of the action potential cannot produce another action potential behind it. This prevents action potentials from traveling back toward the cell body. Thus, an action potential that starts at the axon hillock moves in only one direction – toward the synaptic terminals.

Conduction Speed Factors affecting speed 1. Axon diameter: wider axons conduct action potential more rapidly than narrow ones because resistance to electrical current flow is inversely proportional to the cross-sectional area of a conductor 2. The adaption that enables fast conduction in narrow axons is a myelin sheath, a layer of electrical insulation that surrounds vertebrate axons. Myelin sheaths produced by two types of glia Oligodendrocytes (CNS) and Schwann cells (PNS) Salutary Conduction This is the jumping of the nerve impulse between nodes of Ranvier (areas on the axon not covered by the myelin sheath), speeds up the conduction of the nerve impulse.

Concept Check Action potential Repolarization Depolarization Threshold Resting period Stimulus Failed attempts Refractory period

Synapse Neurons communicate with other cells at synapses. The signal is conducted from the axon of a presynaptic cell to the dendrite of a postsynaptic cell via an electrical or chemical synapse. Electrical Synapse: contain gap junctions, which allow electrical current to flow directly from one neuron to another. It synchronizes the activity of neurons responsible for certain rapid behaviors. EX: swift escape responses in crabs and lobsters Chemical Synapse: (majority of synapses) involves the release of a chemical neurotransmitter by the presynaptic neuron. The cell body and dendrites of one postsynaptic neuron may receive inputs from chemical synapses of synaptic terminals. At each terminal, the presynaptic neuron synthesizes the neurotransmitter and packages it in multiple membrane-bounded compartments called synaptic vesicles. 5. The neurotransmitter binds to ligand-gated ion channels in the postsynaptic membrane. Binding may trigger an opening, allowing Na+ and K+ to diffuse through the channels. 6. The neurotransmitter is released from the receptors, and the channels close. Synaptic transmission ends when the neurotransmitter diffuses out of the synaptic cleft, is taken up by the synaptic terminal or by another cell, or is degraded by an enzyme. Steps involved in neurotransmitter release 1. An action potential arrives, depolarizing the presynaptic membrane. 2. The depolarization opens voltage-gated channels, triggering an influx of Ca(2+). 3. The elevated Ca(2+) concentration causes synaptic vesicles to fuse with the presynaptic membrane. 4. The vesicles release neurotransmitter into the synaptic cleft (the narrow gap that separates the presynaptic neuron from the postsynaptic cell).

Generation of Postsynaptic Potentials Binding of the neurotransmitter to a particular part of the ligand-gated ion channel opens the channel and allows specific ions to diffuses across the postsynaptic membrane resulting in a postsynaptic potential (change in the membrane potential of the postsynaptic cell) Excitatory Neurotransmitters: causes depolarization of the postsynaptic membrane. Excitatory postsynaptic potential(EPSP) – an electrical charge (depolarization) in the membrane of a postsynaptic cell caused by the binding of an excitatory neurotransmitter from a presynaptic cell to a postsynaptic receptor makes it more likely for a postsynaptic cell to generate action potential Inhibitory Neurotransmitters: causes hyperpolarization of the postsynaptic membrane. Inhibitory postsynaptic potential (IPSP) - an electrical charge (hyperpolarization) in the membrane of a postsynaptic neuron caused by the binding of an inhibitory neurotransmitter from a presynaptic cell to a postsynaptic receptor makes it more difficult for a postsynaptic neuron to generate action potential https://www.youtube.com/watch?feature=player_embedded&v=LT3VKAr4roo (3min)

Summation of Postsynaptic Potentials “Unlike action potentials, which are all all-or-one events, postsynaptic potentials are graded” Action potentials occur fully or not at all. This all-or-none property reflects the fact that depolarization opens voltage-gated sodium channels, and the opening of sodium channels causes further depolarization. Postsynaptic potentials are graded; their magnitude varies with a number of factors, including the amount of neurotransmitter released by the presynaptic neuron. Postsynaptic potentials do not regenerate as they spread along the membrane of a cell; they become smaller with distance from the synapse. A single EPSP is too small to trigger an action potential in a postsynaptic cell. Temporal Summation When two EPSPs occur at a single synapse in such rapid succession that the postsynaptic neuron’s membrane potential has not returned to the resting potential before arrival of the second EPSP, the EPSPs add together. Spatial Summation When EPSPs produced simultaneously by different synapses on the same postsynaptic neuron add together. Through summation several EPSPs can depolarize the membrane at the axon hillock to the threshold, causing the postsynaptic neuron to produce an action potential Summation applies as well to IPSPs. Two or more IPSPs occurring simultaneously or in rapid succession have a larger hyperpolarizing effect that a single IPSP. Through summation, an IPSP can also counter the effect of an EPSP. http://www.youtube.com/watch?v=Pd0IQ-Nx8dM

Axon Hillock The axon hillock is the neuron’s integrating center, the region where the membrane potential at any instance represents the summed effect of all EPSPs and IPSPs. Whenever the membrane potential at the axon hillock reaches the threshold, an action potential is generated and travels along the axon to its synaptic terminals. After the refractory period, the neuron may produce another action potential, provided the membrane potential at the axon hillock once again reaches the threshold.

Modulated Synaptic Transmission Binding of the neurotransmitter to its receptor in the postsynaptic cell activates a signal transduction pathway involving a secondary messenger The effects of these second second-messenger systems have a slower onset but last longer than the ligand-gated channels. Second messengers modulate the responsiveness of postsynaptic neurons to inputs in diverse ways. One of the best studied pathways involves cAMP as a second messenger Because of the amplifying affect of the signal transduction pathway, the binding of the neurotransmitter to a single receptor can open or close many channels

Neurotransmitters There are more than 100 known neurotransmitters The major classes of neurotransmitters are acetylcholine, biogenic amines, amino acids, neuropeptides, and gases. Acetylcholine is a common neurotransmitter in vertebrates and invertebrates Vertebrate neurons that form a synapse with muscle cells release acetylcholine as an excitatory transmitter. In regulating vertebrate cardiac (heart) muscle, acetylcholine has inhibitory effects. Acetylcholine activity terminated by acetylcholinesterase: an enzyme in the synaptic cleft that hydrolyzes the neurotransmitter. Certain bacteria produce a toxin that specifically inhibits presynaptic release of acetylcholine, this toxin is the cause of rare but severe form of food poisoning called botulism. Untreated botulism is typically fatal because muscles required for breathing fail to contract when acetylcholine is blocked.