Neurons CH 44and Nervous Systems

Neurons CH 44and Nervous Systems - Neurons and Nervous...

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Unformatted text preview: Neurons and Nervous Systems Nervous Systems: Cells and Functions Simple animals process information with a simple network of neurons (nerve net) that does little more than provide direct lines of communication from sensory cells to effectors. The next level of nervous system complexity includes clusters of neurons called ganglia. Frequently one pair of ganglia is larger and more central and is given the designation Nervous Systems: Cells and Functions In vertebrates, most of the cells of the nervous system are found in the brain and the spinal cord, which together are called the central nervous system (CNS). Information is transmitted from sensory cells to the CNS and from the CNS to effectors via neurons, which extend or reside outside of the brain and spinal cord. Neurons and supporting cells found outside the CNS are called the peripheral Figure 44.2 Neurons (Part 1) Nervous Systems: Cells and Functions Variation between different types of neurons is considerable. Length of the axon differs in different cell types. Some axons can be very long. Nervous Systems: Cells and Functions The axon usually carries information away from the cell body. Axons conduct information to target cells, which can be other neurons, muscle cells, or gland cells. At its end, the axon divides into many fine nerve endings. At the tip of each nerve ending is a swelling called the axon terminal. The axon terminal is positioned very close Nervous Systems: Cells and Functions Variation between different types of neurons is considerable. Length of the axon differs in different cell types. Some axons can be very long. Figure 44.2 Neurons (Part 2) Figure 44.2 Neurons (Part 3) Nervous Systems: Cells and Functions Some glial cells physically support and orient neurons. Others provide insulation for axons. Schwann cells are a type of glial cell that wraps around the axons of neurons in the peripheral nervous system, providing electrical insulation. Oligodendrocytes have a similar function for axons in the CNS. Myelin is the covering produced by Figure 44.3 Wrapping Up an Axon Nervous Systems: Cells and Functions Glial cells supply neurons with nutrients. Some consume foreign particles, and some maintain ionic balance around neurons. Some glial cells communicate electrically through gap junctions. Glial cells called astrocytes contribute to the bloodbrain barrier, which protects the brain from toxic chemicals in the blood. Astrocytes surround the smallest blood Nervous Systems: Cells and Functions It is important to remember that nervous systems depend on neurons working together. The simplest neural network consists of three cells: a sensory neuron connected to a motor neuron connected to a muscle cell. Most neuronal networks are more complex. The human brain has an estimate 1011 neurons and 1014 synapses. The neurons and synapses in the human Neurons: Generating and Conducting Nerve Impulses The difference in voltage across the plasma membrane of a neuron is called its membrane potential. In an unstimulated neuron, the voltage difference is called a resting potential. Membrane potentials can be measured with electrodes. The membrane potential of a resting axon is about 60 millivolts (mV). The inside of the cell is more negative than the outside. Figure 44.4 Measuring the Resting Potential (Part 1) Figure 44.4 Measuring the Resting Potential (Part 2) Neurons: Generating and Conducting Nerve Impulses Voltage (potential or electric charge difference) is the tendency for electrically charged particles like electrons or ions to move between two points. Electrical charges move across cell membranes not as electrons, but as charged ions. The major ions that carry electric charges across the plasma membranes of neurons are sodium (Na+), chloride (Cl), potassium Neurons: Generating and Conducting Nerve Impulses Ion pumps use energy to move ions or other molecules against their concentration gradients. The major ion pump in neuronal membranes is the sodiumpotassium pump, which expels Na+ ions from the cell, exchanging them for K+ ions from outside the cell. This keeps the concentration of K+ greater inside the cell than outside. Figure 44.5 Ion Pumps and Channels Neurons: Generating and Conducting Nerve Impulses Potassium channels are the most common open channels in the plasma membranes of resting neurons, and resting neurons are more permeable to K+ than any other ion. The sodiumpotassium pump keeps K+ concentration high inside the cell, but K+ can diffuse out the open channels. The membrane potential at which the tendency of K+ ions to diffuse into the cell is equal to their tendency to diffuse out is Figure 44.6 Open Potassium Channels Create the Resting Potential Figure 44.7 The Nernst Equation Neurons: Generating and Conducting Nerve Impulses Many ion channels in the plasma membranes of neurons are gated; they open under some conditions but close under other conditions. Voltage-gated channels open or close in response to a change in the voltage across a plasma membrane. Chemically gated channels open or close depending on the presence or absence of a specific chemical that binds to the Neurons: Generating and Conducting Nerve Impulses When the inside of a neuron becomes less negative in comparison to its resting condition, its plasma membrane is said to be depolarized. Conversely, when the inside of a neuron becomes more negative in comparison to its resting condition, its plasma membrane is said to be hyperpolarized. Opening and closing of ion channels, which result in changes in the polarity of Figure 44.8 Membranes Can Be Depolarized or Hyperpolarized Neurons: Generating and Conducting Nerve Impulses An action potential is a sudden and major change in membrane potential that lasts for about 12 milliseconds. Action potentials are conducted along axons at speeds of up to 100 meters per second. If the membrane potential of an axon is measured when an action potential passes, the voltage changes from the resting potential of 60 mV to +50 mV, Neurons: Generating and Conductingsodium channels are Nerve Impulses Voltage-gated primarily responsible for action potentials. At resting potential, most of the sodium channels are closed. A specific membrane potential called the threshold potential opens voltagegated ion channels. During the transmission of an action potential, the sodium channels stay open for less than a millisecond; in Figure 44.9 The Course of an Action Potential Neurons: Generating and Conducting Nerve Impulses Potassium channels open more slowly than the sodium channels and stay open longer; this allows potassium ions to carry excess positive charges out of the axon. Potassium channels thus help the plasma membrane return to its resting potential. Neurons: Generating and Conducting Nerve Impulses Another feature of voltage-gated sodium channels is that once they open and close, they can be triggered again only after a short delay of 12 milliseconds. This delay is the refractory period, the time when a plasma membrane cannot propagate an action potential. Figure 44.10 Action Potentials Travel along Axons (Part 2) Figure 44.10 Action Potentials Travel along Axons (Part 3) Neurons: Generating and Conducting Nerve Impulses In vertebrates, it is impractical to increase propagation velocity by increasing axon size because of the very large numbers of axons present. Another mechanism has evolved that increases propagation velocity. Recall that Schwann cells wrap axons in myelin. The myelin wrapper is not continuous; it has regularly spaced gaps, called nodes of Ranvier, where the axon Neurons: Generating and Conducting Nerve Impulses Myelin electrically insulates the axon (charged ions cannot cross the regions of the axon that are wrapped in myelin). Ion channels are clustered at the nodes of Ranvier. When an action potential fires at one node of Ranvier, it jumps to the next via saltatory conduction. Saltatory conduction is much faster than continuous signal propagation down unmyelinated axons because electric Figure 44.12 Saltatory Action Potentials (Part 1) Figure 44.12 Saltatory Action Potentials (Part 2) Neurons, Synapses, and Communication Interactions among neurons depend on the synapses between cells. Electrical and chemical messages are exchanged at the synapse. The cell that sends the message is called the presynaptic cell; the cell that receives it is the postsynaptic cell. The most common type of synapse in the nervous system is the chemical synapse. Figure 44.13 Synaptic Transmission Begins with the Arrival of a Nerve Impulse Neurons, Synapses, and Communication of a The postsynaptic membrane neuromuscular junction is a modified part of the muscle cell plasma membrane called a motor end plate. The motor end plate contains acetylcholine-gated channels that allow both Na+ and K+ to pass through them. When acetylcholine binds to its receptor and opens the channel, Na+ moves into the cell (since the cell is Figure 44.14 The Acetylcholine Receptor is a Chemically Gated Channel Neurons, Synapses, and Communication Voltage-gated calcium channels in the membrane of the axon terminal are activated by incoming action potentials in the neuron. Ca2+diffuses down its concentration gradient into the neuron. The increase in intracellular Ca2+ in the presynaptic cell causes the vesicles containing acetylcholine to fuse with the presynaptic cell's membrane and empty their contents into the synaptic cleft. Neurons, Synapses, and Communication Synapses between neurons are categorized as excitatory or inhibitory depending on their response to neurotransmitter (chemical) messages. If a postsynaptic neuron responds to chemical stimulation by depolarizing, the synapse is excitatory. If the postsynaptic neuron hyperpolarizes, the synapse is inhibitory. Neurons, Synapses, and Communication Gamma-amino butyric acid (GABA) and glycine are the most common inhibitory neurotransmitters in vertebrates. The postsynaptic cells at inhibitory synapses have chemically gated chloride channels. When the channels are activated, they can hyperpolarize the postsynaptic membrane and make the postsynaptic cell less likely to fire an action potential. Neurons, Synapses, and Communication Neurotransmitters that depolarize the postsynaptic membrane are excitatory and bring about an excitatory postsynaptic potential (EPSP). Neurotransmitters that hyperpolarize the postsynaptic membrane are inhibitory and bring about an inhibitory postsynaptic potential (IPSP). Neurons, Synapses, and Communication For most neurons, the critical area for the "decision" to fire an action potential is the axon hillock, a region of the cell body at the base of the axon. The plasma membrane of the axon hillock is not myelinated and has many voltage-gated ion channels. Inputs from the synapses are conducted through the cell body. If the resulting combined potential depolarizes the axon hillock to threshold, the axon fires an action Figure 44.15 The Postsynaptic Neuron Sums Information (Part 1) Figure 44.16 Metabotropic Receptors Act through G Proteins ...
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This note was uploaded on 04/02/2008 for the course BIO 188 taught by Professor Capco during the Fall '08 term at ASU.

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Neurons CH 44and Nervous Systems - Neurons and Nervous...

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