34 Neurons and Nervous Systems
Concept 34.1 Nervous Systems Consist of Neurons and Glia Nervous systems have two categories of cells: Neurons, or nerve cells, are excitable they generate and transmit electrical signals, called action potentials. Glia, or glial cells, provide support and maintain extracellular environment.
Concept 34.1 Nervous Systems Consist of Neurons and Glia Most neurons have four regions: Cell body contains nucleus and organelles Dendrites carries signals, called nerve impulses or action potentials, to the cell body Axon generates action potentials and conducts them away from the cell body Axon terminal synapse at tip of axon; releases neurotransmitters
Concept 34.1 Nervous Systems Consist of Neurons and Glia Neurons pass information at synapses: The presynaptic neuron sends the message The postsynaptic neuron receives the message
Figure 34.1 A Generalized Neuron
Concept 34.1 Nervous Systems Consist of Neurons and Glia Glial cells, or glia, outnumber neurons in the human brain. Glia do not transmit electrical signals but can release neurotransmitters. Glia also give support during development, supply nutrients, remove debris, and maintain extracellular environment. Important in neuroplasticity synapse modification
Concept 34.1 Nervous Systems Consist of Neurons and Glia Astrocytes are glia that contribute to the blood brain barrier, which protects the brain. The blood-brain barrier is permeable to fatsoluble compounds like alcohol and anesthetics. Microglia provide the brain with immune defenses since antibodies cannot enter the brain.
Concept 34.1 Nervous Systems Consist of Neurons and Glia Oligodendrocytes are glia that insulate axons in the brain and spinal cord. Schwann cells insulate axons in nerves outside of these areas. The glial membranes form a nonconductive sheath myelin. Myelin-coated axons are white matter and areas of cell bodies are gray matter. Multiple sclerosis is a demyelinating disease.
Figure 34.2 Wrapping Up an Axon (Part 1)
Figure 34.2 Wrapping Up an Axon (Part 2)
Concept 34.1 Nervous Systems Consist of Neurons and Glia Neurons are organized into neural networks. Afferent neurons carry sensory information into the nervous system from sensory cells that convert stimuli into action potentials. Efferent neurons carry commands to effectors such as muscles, glands motor neurons are effectors that carry commands to muscles. Interneurons store information and communicate between neurons.
Concept 34.1 Nervous Systems Consist of Neurons and Glia Networks vary in complexity. Nerve net simple network of neurons Ganglia neurons organized into clusters, sometimes in pairs, in simple animals Brain the largest pair of ganglia, found in animals with complex behavior requiring more information-processing
Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 1)
Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 2)
Figure 34.3 Nervous Systems Vary in Size and Complexity (Part 3)
Concept 34.2 Neurons Generate and Transmit Electrical Signals Neurons generate changes in membrane potential the difference in electrical charge across the membrane. These changes generate nerve impulses, or action potentials. An action potential is a rapid, large change in membrane potential that travels along an axon and causes release of chemical signals.
Concept 34.2 Neurons Generate and Transmit Electrical Signals Voltage is a measure of the difference in electrical charge between two points. Electrical current in solution is carried by ions. Major ions in neurons: Sodium (Na + ) Potassium (K + ) Calcium (Ca 2+ ) Chloride (Cl ) Different concentrations and charges inside and out produce the membrane potential.
Concept 34.2 Neurons Generate and Transmit Electrical Signals Membrane potentials can be measured in all cells with electrodes. Resting potential is the membrane potential of a resting, or inactive, neuron. The resting potential of a membrane is between 60 and 70 millivolts (mv). The inside of the cell is negative at rest. An action potential allows positive ions to flow in briefly, making the inside of the cell more positive.
Figure 34.4 Measuring the Membrane Potential (Part 1)
Figure 34.4 Measuring the Membrane Potential (Part 2)
Concept 34.2 Neurons Generate and Transmit Electrical Signals Ion channels and ion transporters in the membrane create the resting and action potentials. Sodium potassium pump moves Na + ions from inside, exchanges for K + from outside establishes concentration gradients The Na + K + pump is an antiporter, or sodium potassium ATPase, as it requires ATP.
Figure 34.5 Ion Transporters and Channels (Part 1)
Concept 34.2 Neurons Generate and Transmit Electrical Signals Potassium channels are open in the resting membrane and are highly permeable to K + ions allow leak currents K + ions diffuse out of the cell along the concentration gradient and leave behind negative charges within the cell. K + ions diffuse back into the cell because of the negative electrical potential. These two forces acting on K + are its electrochemical gradient.
Figure 34.5 Ion Transporters and Channels (Part 2)
Concept 34.2 Neurons Generate and Transmit Electrical Signals The equilibrium potential is the membrane potential at which the net movement of an ion ceases. The Nernst equation calculates the value of the equilibrium potential by measuring the concentrations of an ion on both sides of the membrane.
Concept 34.2 Neurons Generate and Transmit Electrical Signals Some ion channels are gated open and close under certain conditions: Voltage-gated channels respond to change in voltage across membrane Chemically-gated channels depend on molecules that bind or alter channel protein Mechanically-gated channels respond to force applied to membrane
Concept 34.2 Neurons Generate and Transmit Electrical Signals Gating provides a means for neurons to change their membrane potentials in response to a stimulus. The membrane is depolarized when Na + enters the cell and the inside of the neuron becomes less negative. If gated K + channels open and K + leaves, the cell becomes more negative inside and the membrane is hyperpolarized.
Figure 34.6 Membranes Can Be Depolarized or Hyperpolarized
Concept 34.2 Neurons Generate and Transmit Electrical Signals Graded membrane potentials are changes from the resting potential. Graded potentials are a means of integrating input the membrane can respond proportionally to depolarization or hyperpolarization.
Concept 34.2 Neurons Generate and Transmit Electrical Signals Voltage-gated Na + and K + channels are responsible for action potentials sudden, large changes in membrane potential. At rest most of these channels are closed. Local depolarization by gated channels in dendrites produces a graded potential. It spreads to the axon hillock, where Na + voltage-gated channels are concentrated.
Concept 34.2 Neurons Generate and Transmit Electrical Signals The membrane in the axon hillock may reach its threshold 5 to 10 mv above resting potential. Many voltage-gated Na + channels (activation gates) open quickly and Na + rushes into the axon. The influx of positive ions causes more depolarization, the membrane potential is briefly positive, and an action potential occurs.
Concept 34.2 Neurons Generate and Transmit Electrical Signals The axon quickly returns to resting potential due to two things: Voltage-gated K + channels open slowly and stay open longer K + moves out Voltage-gated Na + channels (inactivation gates) close Voltage-gated Na + channels cannot open again during the refractory period a few milliseconds.
Figure 34.7 The Course of an Action Potential (Part 1)
Figure 34.7 The Course of an Action Potential (Part 2)
Concept 34.2 Neurons Generate and Transmit Electrical Signals An action potential is an all-or-none event positive feedback to voltage-gated Na + channels ensures the maximum action potential. An action potential is self-regenerating because it spreads to adjacent membrane regions.
Concept 34.2 Neurons Generate and Transmit Electrical Signals Axon diameter and myelination by glial cells increase the speed of action potentials in axons. The nodes of Ranvier are regularly spaced gaps where the axon is not covered by myelin. Action potentials are generated at the nodes and the positive current flows down the inside of the axon.
Concept 34.2 Neurons Generate and Transmit Electrical Signals When positive current reaches the next node, the membrane is depolarized another axon potential is generated. Action potentials appear to jump from node to node, a form of propagation called saltatory conduction.
Figure 34.8 Saltatory Action Potentials (Part 1)
Figure 34.8 Saltatory Action Potentials (Part 2)
Concept 34.3 Neurons Communicate with Other Cells at Synapses Neurons communicate with other neurons or target cells at synapses. In a chemical synapse neurotransmitters from a presynaptic cell bind to receptors in a postsynaptic cell. The synaptic cleft about 25 nanometers wide separates the cells.
Concept 34.3 Neurons Communicate with Other Cells at Synapses In an electrical synapse, cells are joined through gap junctions. Gap junctions are made of proteins (connexins) that create channels. Ions flow through the channels the action potential spreads through the cytoplasm. These action potentials are fast but do not allow for complex integration of inputs.
Concept 34.3 Neurons Communicate with Other Cells at Synapses The neuromuscular junction is a chemical synapse between motor neurons and skeletal muscle cells. An action potential causes voltage-gated Ca + channels to open in the presynaptic membrane, allowing Ca + to flow in. The presynaptic neuron releases acetylcholine (ACh) from its axon terminals (boutons) when vesicles fuse with the membrane.
Figure 34.9 Chemical Synaptic Transmission
Concept 34.3 Neurons Communicate with Other Cells at Synapses The postsynaptic membrane of the muscle cell is the motor end plate. ACh diffuses across the cleft and binds to ACh receptors on the motor end plate. These receptors allow Na + and K + to flow through, and the increase in Na + depolarizes the membrane. If it reaches threshold, more Na + voltagegated channels are activated and an action potential is generated.
Figure 34.10 Chemically Gated Channels
Concept 34.3 Neurons Communicate with Other Cells at Synapses The postsynaptic cell must sum the excitatory and inhibitory input. Summation occurs at the axon hillock, the part of the cell body at the base of the axon. Spatial summation adds up messages at different synaptic sites. Temporal summation adds up potentials generated at the same site, over time.
Concept 34.3 Neurons Communicate with Other Cells at Synapses Neurotransmitters are cleared from the cleft after release in order to stop their action in several ways: Diffusion Reuptake by adjacent cells Enzymes present in the cleft may destroy them Example: Acetylcholinesterase acts on ACh.
Concept 34.3 Neurons Communicate with Other Cells at Synapses There are many types of neurotransmitters, and each may have multiple receptor subtypes. For example, ACh has two: Nicotinic receptors are ionotropic and mainly excitatory Muscarinic receptors are metabotropic and mainly inhibitory The action of a neurotransmitter depends on the receptor to which it binds.