NEURONS, SENSE ORGANS, AND NERVOUS SYSTEMS CHAPTER 34

NEURONS, SENSE ORGANS, AND NERVOUS SYSTEMS CHAPTER 34

KEY CONCEPTS 34.1 Nervous Systems Are Composed of Neurons and Glial Cells 34.2 Neurons Generate Electric Signals by Controlling Ion Distributions 34.3 Neurons Communicate with Other Cells at Synapses 34.4 Sensory Processes Provide Information on an Animal s External Environment and Internal Status 34.5 Neurons Are Organized into Nervous Systems Quick intro video

NERVOUS SYSTEMS ARE COMPOSED OF NEURONS AND GLIAL CELLS Animals need a way to transmit signals at high speeds from place to place within their bodies (e.g., to avoid danger). Mammalian neurons transmit signals at 20 100 meters per second, similar to a banjo where each finger pluck is like a nerve impulse in brain and sent down the spinal cord.

NERVOUS SYSTEMS ARE COMPOSED OF NEURONS AND GLIAL CELLS Neurons are excitable cells, which means its cell membranes can generate and conduct signals called impulses This is a key specialization, only muscles are neurons are excitable cells in the body they can generate and transmit electrical signals, called action potentials. Cell membranes ordinarily have electrical polarity: the outside is more positive than the inside. An impulse, or action potential, is a state of reversed polarity.

NERVOUS SYSTEMS ARE COMPOSED OF NEURONS AND GLIAL CELLS In an excitable cell, an action potential generated at one point propagates over the whole membrane. The region of depolarization moves along the cell membrane, and the membrane is said to conduct the impulse. An impulse or action potential is a state in which the polarity across the cell membrane is reversed. The impulse or action potential moves along the cell membrane, in a process called conduction or propagation

NEURONS ARE SPECIALIZED TO PRODUCE ELECTRIC SIGNALS Neurons (nerve cells) are specially adapted to generate electric signals, usually in the form of action potentials. Neurons are like land lines and they must make contact with target cells and are very diverse in structure. (fast and addressed)

NEURONS ARE SPECIALIZED TO PRODUCE ELECTRIC SIGNALS Synapse: cell-to-cell contact point specialized for signal transmission A signal arrives at the synapse by way of the presynaptic cell and leaves by way of the postsynaptic cell. The postsynaptic cell can be another neuron or it can be a sensory cell or as discussed in chapter 33, it can be a muscle cell.

FOUR ANATOMICAL REGIONS Most neurons have four regions; dendrites, cell body, an axon, and a set of presynaptic axon terminals Dendrites ( tree ) carry signals to the cell body (incoming) They are short processes, or extensions that tend to branch from the cell body like twigs on a shrub Cell body contains nucleus and organelles The main job is to combine and integrate the incoming signals quickly

FOUR ANATOMICAL REGIONS Axon conducts action potentials away from the cell body; can be very long For example, an axon can run from your spinal cord all the way to your finger to coordinate a movement. Action potentials are generated at the cell body and move out the axon Presynaptic axon terminals make contact with other cells These terminals make contact with other cells, neurons sensory cells or muscles.

Dendrites are the major site for synaptic input from other neurons The cell body and, especially, the axon hillock where the cell body transitions to the axon are often major sites of integration of signals The axon is the conduction component of the neuron propagating action potentials over a long distance to the axon terminal At the presynaptic axon terminals. The output of the neuron can alter the activities of other cells

CONCEPT 34.1 NERVOUS SYSTEMS ARE COMPOSED OF NEURONS AND GLIAL CELLS Presynaptic Axon Terminals A neuron is said to innervate the cells that the axon terminals contact. Axons of many neurons often travel together in bundles called nerves. Nerve refers only to axon bundles outside the brain and spinal cord. In the brain or spinal cord they are called tracts.

GLIAL CELLS Glial cells support, nourish, and insulate neurons Glial cells, or glia or neuroglia is the second major type of cell in the nervous system 50% of the mammalian brain are glial cells There are several distinct types of glial cells that have distinct roles. They are not excitable or produce action potentials however, they have several functions: Help orient neurons toward their target cells during embryonic development Provide metabolic support for neurons Help regulate composition of extracellular fluids and perform immune functions Assist signal transmission across synapses

GLIAL CELLS In vertebrates typically not in invertebrates, certain glial cells are insulated by a lipid-rich cell membrane, similar to the insulators of a power cord. In brain and spinal cord, this glial wrap around the axons is called Oligodendrocytes Schwann cells insulate axons in nerves outside of these areas. The glial membranes form a electrically nonconductive sheath called myelin. Myelin-coated axons are white matter and areas of cell bodies are gray matter. Multiple layers of Schwann cell membrane insulate the axon

GLIAL CELLS

NEURONS GENERATE ELECTRIC SIGNALS BY CONTROLLING ION DISTRIBUTION 34.2

INTRODUCTION 1 min video One feature common to all nervous systems is that they encode and transmit information in the form of action potentials. Some basic electrical principles help us understand how neurons produce action potentials and other electrical signals.

ACTION POTENTIALS - TERMINOLOGY Current: flow of electric charges from place to place; in cells, current is based on flow of ions such as Na + Voltage, or electrical potential difference exists if positive charges are concentrated in one place and negative charges are concentrated in a different place. Voltages produce currents because opposite charges attract and will move toward one another if given a chance.

ACTION POTENTIALS No voltage differences exist within open solutions such as the intracellular fluids. Voltage differences exist only across membranes such as the cell membrane. Within a few nanometers of the membrane on either side, net positive and negative charge concentrations may accumulate Farther away, in the bulk solution on either side, the net charge is zero

ACTION POTENTIALS The voltage across a membrane is called membrane potential and is easily measured. Resting neuron: membrane potential is the resting potential, typically 60 to 70 millivolts (mv) Negative sign means the inside of the cell is electrically negative relative to the outside.

2 and connected with a wire to an amplifier 1. An electrode made from a glass tube (pulled to a sharp tip and open at the end) is filled with electrically conducting solution. 3. The voltage difference between the electrode placed inside the axon and a reference electrode outside the axon is detected 4 and this small potential difference is displayed on a computer screen. 5, In an unstimulated neuron, a potential difference is about -65mV is observed between outside and inside. This is the resting potential

ACTION POTENTIALS Membrane potential can change rapidly, and only relatively small numbers of positive charges need to move through the membrane for this change of membrane potential to occur. Composition of the bulk solutions (the intra- and extracellular fluids) does not change. Animated tutorial 34.1 good textbook link

SODIUM-POTASSIUM PUMP SETS UP CONCENTRATION GRADIENTS Ion redistribution occurs through membrane channel proteins and ion transporters in the membrane. Sodium potassium pump uses energy from ATP to move 3 Na + ions to the outside and 2 K + to the inside; establishes concentration gradients of these ions

SODIUM-POTASSIUM PUMP SETS UP CONCENTRATION GRADIENTS Potassium channels are open in the resting membrane. K + ions diffuse out of the cell through leak channels and leave behind negative charges within the cell. K + ions diffuse back into the cell because of the negative electrical potential. At this equilibrium point, there is no net movement of K + ; called the equilibrium potential of K +.

SODIUM-POTASSIUM PUMP SETS UP CONCENTRATION GRADIENTS Diffusion of ions is controlled by concentration effect and electrical effect. When they are equal, electrochemical equilibrium is reached. Crash Course Nervous System 2 Video on Action Potentials and Sodium Potassium Pump

NO!!!! Electrochemical equilibrium is called the equilibrium potential of the ion, calculated by the Nernst equation: E ion 2.3 RT zf ion log ion outside inside

GATED ION CHANNELS CAN ALTER MEMBRANE POTENTIAL Most ion channels are gated they open and close under certain conditions. Most are closed in a resting neuron, which is why K + leak channels determine resting membrane potential. Voltage-gated channels open or close in response to changes in membrane potential Stretch-gated channels respond to tension applied to cell membrane Ligand-gated channels open or close when a specific chemical (ligand) binds to the channel protein.

GATED ION CHANNELS CAN ALTER MEMBRANE POTENTIAL Opening and closing gated channels can alter membrane potential. If Na + channels open, Na + diffuses into the neuron because it is more concentrated outside the cell, and the cell membrane is more negative on the inside. When membrane becomes less negative on the inside, the membrane is depolarized. The membrane is hyperpolarized if the charge on the inside becomes more negative. Nerve Impulse explained Depolarization and hyperpolarization explained

CHANGES IN MEMBRANE POTENTIAL CAN BE GRADED OR ALL-OR-NONE Two types of membrane potentials can occur: graded or all-or-none. Graded membrane potentials are changes from the resting potential that are less than the threshold of 50 mv. Graded means any value of the membrane potential is possible Caused by various ion channels opening or closing

CHANGES IN MEMBRANE POTENTIAL CAN BE GRADED OR ALL-OR-NONE Graded membrane potentials spread only a short distance.

CHANGES IN MEMBRANE POTENTIAL CAN BE GRADED OR ALL-OR-NONE If neuron depolarizes to the 50 mv threshold, an all-or-none event occurs: an action potential is generated. Action potentials are not graded (always the same size) and do not become smaller, they stay the same in size as they propagate along the cell membrane.

CHANGES IN MEMBRANE POTENTIAL CAN BE GRADED OR ALL-OR-NONE Graded changes can give rise to all-or-none changes by being summed together; provides a mechanism for integrating signals. A key area for this integration is the axon hillock, where action potentials are most often generated. Graded changes resulting from multiple signals reaching the dendrites, spread to the axon hillock, where all the depolarizations and hyperpolarizations sum.

CONCEPT 34.2 NEURONS GENERATE ELECTRIC SIGNALS BY CONTROLLING ION DISTRIBUTIONS An action potential (nerve impulse) is a rapid, large change in membrane potential that reverses membrane polarity. The membrane depolarizes from 65 mv at rest to about +40 mv (depolarization). It is localized and brief but is propagated with no loss of size an action potential at one location causes currents to flow that depolarize neighboring regions.

CONCEPT 34.2 NEURONS GENERATE ELECTRIC SIGNALS BY CONTROLLING ION DISTRIBUTIONS When membrane potential reaches threshold, many voltage-gated Na + channels open quickly, and Na + rushes into the axon. The influx of positive ions causes more depolarization, and an action potential occurs.

PRODUCTION OF AN ACTION POTENTIAL (PART 1)

CONCEPT 34.2 NEURONS GENERATE ELECTRIC SIGNALS BY CONTROLLING ION DISTRIBUTIONS The axon quickly returns to resting potential: Voltage-gated Na + channels close Voltage-gated K + channels open slowly and stay open longer K + moves out

FIGURE 34.7 PRODUCTION OF AN ACTION POTENTIAL (PART 2)

PRODUCTION OF AN ACTION POTENTIAL (PART 3)

POSITIVE FEEDBACK Positive feedback during depolarization: When the membrane is partially depolarized, some Na + channels open; as Na + starts to diffuse into the cell, more depolarization occurs, opening more channels. This continues until all voltage-gated Na + channels open and maximum depolarization occurs.

ACTION POTENTIALS Action potential travels in only one direction: After the action potential, Na + channels cannot open again for a brief period (refractory period) and cannot depolarize. Thus the action potential can only propagate in the direction of the axon terminals.

ACTION POTENTIALS Action potentials travel faster in larger diameter axons. Myelination by glial cells also increases speed of action potentials. The nodes of Ranvier are gaps where the axon is not covered by myelin. Action potentials are generated only at the nodes and jump from node to node (saltatory conduction).