chapter 11 Flashcards

Question

What does it mean to say that action potentials are 'all-or-none'?

Answer 1

half an action can’t happen it empties the axon or doesn’t

Answer 2

In one direction

Answer 3

By generating more action potentials

Answer 4

The rest are neuroglial cells, which provide physical support and protection to neurons and help maintain healthy concentrations of important chemicals in the fluid surrounding them. (Neuroglia derives from the Greek words for “neuron” and “glue.”) Neuroglial cells do not generate or transmit impulses.

Answer 5

Schwann cells are known for their roles in supporting nerve regeneration. Nerves in the PNS consist of many axons myelinated by Schwann cells. If damage occurs to a nerve, the Schwann cells aid in digestion of its axons (phagocytosis).

Answer 6

Only about 20% of cells in the human nervous system are neurons. The rest are neuroglial cells, which provide physical support and protection to neurons and help maintain healthy concentrations of important chemicals in the fluid surrounding them. (Neuroglia derives from the Greek words for “neuron” and “glue.”) Neuroglial cells do not generate or transmit impulses. In the peripheral nervous system, many neuron axons are enclosed and protected by specialized neuroglial cells called Schwann cells (Figure 11.7a). Schwann cells produce a fatty insulating material called myelin. During development, individual Schwann cells wrap themselves around a short segment of an axon many times as a sort of insulating blanket, creating a shiny white protective layer around the axon called a myelin sheath (Figure 11.7b). Between adjacent Schwann cells are short uninsulated gaps called nodes of Ranvier, where the surface of the axon is still exposed. Neurons that have axons wrapped in a sheath of myelin are called myelinated neurons. The myelin sheath around the axon serves three important functions: ● It saves the neuron energy. The insulating layer of myelin prevents some of the slow inward leak of sodium and outward leak of potassium that would otherwise occur. These leaks normally have to be replaced by active transport processes requiring energy (see section 11.3). ● It speeds up the transmission of impulses. The myelin sheath causes action potentials to jump from node to node at a very fast rate. This “leaping” pattern of conduction along myelinated neurons is called saltatory conduction (from saltare, Latin for “dance”) (Figure 11.7c). Continuously propagated action potentials in unmyelinated neurons travel at a speed of only about 5 mph (2.3 meters per second). In contrast, saltatory conduction in myelinated neurons can reach speeds of up to 250 mph (about 110 meters per second). ● It helps damaged or severed axons of the peripheral nervous system regenerate. If a neuron axon is severed, the portion of the axon distal to the cell body may degenerate. However, the cut end of the axon still attached to the cell body can regrow through the channel formed by the sheath, eventually reconnecting with the cell that it serves. Depending on the length of the axon, the regeneration process can take anywhere from a few weeks to more than a year.

Answer 7

When the action potential reaches the end of the axon (the axon terminal), it causes neurotransmitter-containing vesicles to fuse with the membrane, releasing neurotransmitter molecules into the synaptic cleft (space between neurons)

Answer 8

Once an action potential reaches the axon terminals of a neuron, the information inherent in it must be converted to another form for transmittal to its target (muscle cell, gland cell, or another neuron). In essence, the action potential causes the release of a chemical, called a neurotransmitter, that crosses a specialized junction, called a synapse, between the two cells. The neurotransmitter represents a signal from a neuron to its target cell. The entire signaling process— from a neuron to a target cell—is called synaptic transmission. Synaptic transmission also occurs between a neuron and a muscle cell and between neurons and some glands. Now we’ll see how synaptic transmission occurs between neurons.

Answer 9

depolarization

Answer 10

Neurotransmitter 'A' would be excitatory because the Na+ (positive charge) rushing in would raise the membrane potential (depolarization) getting it closer to threshold. This makes an AP more likely. Neurotransmitter 'B' would be inhibitory because the Cl- (negative charge) rushing in would lower the membrane potential (hyperpolarization) getting it farther from threshold. This makes an AP less likely.

Answer 11

A nerve consists of the axons of many neurons, all wrapped together in a protective sheath of connective tissue and all coming from and going to the same place. The function of each nerve depends on where it originates and the organs or tissues to which it travels

Answer 12

The peripheral nervous system consists of cranial nerves and spinal nerves. The 12 pairs of cranial nerves connect directly with the brain. Cranial nerves carry action potentials between the brain and the muscles, glands, and receptors of the head, neck, and thoracic and abdominal cavities. The 31 pairs of spinal nerves connect with the spinal cord. Each spinal nerve attaches to the spinal cord via two short branches of the spinal cord called the dorsal root and the ventral root. The dorsal root contains the sensory neurons that transmit incoming action potentials from body tissues to the spinal cord. The ventral root contains motor neurons that carry action potentials away from the spinal cord to the rest of the body. Thus, each spinal nerve carries both sensory and motor (incoming and outgoing) information.

Answer 13

Spinal reflexes are involuntary responses that are mediated primarily by the spinal cord and spinal nerves. The brain receives sensory input and can modify spinal reflexes slightly, but it cannot prevent them from occurring altogether. Figure 11.10 illustrates spinal reflexes. 1 Stepping on a sharp object activates pain receptors, 2 producing action potentials in sensory neurons traveling to your spinal cord. 3 The sensory neurons stimulate interneurons within the spinal cord, which in turn transmit signals to your brain and then on to motor neurons in your leg, 4 causing you to flex the appropriate muscles to lift your foot. This is called the flexor (withdrawal) reflex

Answer 14

Spinal cord

Answer 15

the CNS is enclosed by three membranes of connective tissue, called meninges, named (from outermost to innermost layers) the dura mater, the arachnoid, and the pia mater. These three meninges protect the neurons of the CNS and the blood vessels that service them.

Answer 16

Third, the CNS is bathed in its own special liquid, called cerebrospinal fluid, which fills the space between the arachnoid and the pia mater. In addition to serving as a liquid shock absorber around the brain and spinal cord, cerebrospinal fluid tends to isolate the central nervous system from infections.

Answer 17

This functional barrier between blood and brain is called the blood-brain barrier. Thanks to this isolation, bacterial and viral infections of the brain and spinal cord are rare, but when these illnesses occur they are generally serious and difficult to treat. Many of our best antibiotics are not lipid soluble, which means they cannot cross the bloodbrain barrier and therefore they are ineffective against CNS infections. Lipid-soluble substances that cross the blood-brain barrier rather easily include alcohol, caffeine, nicotine, cocaine, and general anesthetics.

Answer 18

The spinal cord serves as the superhighway for action potentials traveling between the brain and the rest of the body. As we have seen, the spinal cord can also process and act on certain information on its own via spinal reflexes, without necessarily involving the brain. However, its power to act independently is limited, and such actions never reach the level of conscious awareness. Approximately the diameter of your thumb, the spinal cord extends from the base of the skull to the area of about the second lumbar vertebra, or about 17 inches (Figure 11.14a). It is protected by the vertebral column. The outer portions of the spinal cord consist primarily of bundles of axons, or nerves (Figure 11.14b). Because these axons are generally myelinated, giving them a whitish appearance, the areas of the cord occupied by these ascending (sensory) and descending (motor) nerves are called white matter (Figure 11.14c). Neurons of the PNS enter and leave the spinal cord at regular intervals via the dorsal (sensory) and ventral (motor) horns that fuse to form spinal nerves. Near the center of the spinal cord is a region occupied primarily by the cell bodies, dendrites, and axons of neurons of the CNS, and also neuroglial cells. These structures are not myelinated, so the area they occupy is referred to as gray matter (see Figure 11.14c). Within the gray matter, sensory and motor neurons synapse with neurons of the CNS that transmit signals up the spinal cord to the brain.

Answer 19

The brain is the command center of the body. The brain receives information in the form of action potentials from various nerves and the spinal cord, integrates that information, and generates the appropriate response. Within the brain, certain areas specialize in integrating different types of information or performing certain motor tasks. Action potentials derived from various receptors, such as light and sound, arrive at the brain in different sensory neurons that specialize in processing those signals. For example, one area of the brain responds to visual stimuli and another area responds to stimuli that help us maintain our balance. Three major anatomical and functional divisions of the brain have been identified: ● The hindbrain coordinates basic, automatic, and vital tasks. ● The midbrain helps coordinate muscle groups and responses to sights and sounds. ● The forebrain receives and integrates sensory input from the external environment and determines most of our more complex behaviors.

Answer 20

The name limbic system was originally used to describe those structures that bordered the basal regions of the cerebrum. Functionally, the term describes all of the neuronal structures that, together, control emotional behavior and motivational drives (Figure 11.17). When different areas of the limbic system are stimulated, we experience strong feelings and emotions such as fear, anger, sorrow, pleasure, or love. These feelings and emotions influence our behavior, particularly self-gratifying behaviors such as satisfying hunger, thirst, and sexual desire. The intensity with which we feel emotions and the extent to which we act on them depends on modification by the cerebrum. To appreciate the importance of cerebral modification, just imagine what would happen if our most basic urges were no longer influenced by social rules or codes of conduct!

Answer 21

Memory involves storing information and retrieving it later, as needed. Memory has two stages: Short-term (“working”) memory involves retrieval of information that was stored within the past few hours and long-term memory is the ability to retrieve information days or years later. There is a clear difference between how the brain manages the two stages of memory. Short-term memory occurs in the limbic system. When you receive new sensory information (such as reading an unfamiliar telephone number), the stimulus triggers a quick burst of action potentials in the limbic system. You can remember the number for a few seconds or minutes, because neurons that have just fired are easier to fire again in the short term. The pathway becomes entrained, if only for a brief time. If the number is not important, you quickly forget it because the enhancement of the limbic pathway is not long lasting. That particular piece of information does not move into long-term memory. However, if the number is important to you, as evidenced by the fact that you say it out loud several times, write it down and then read it to yourself, or use it repeatedly over several weeks, the information may be transmitted to your cerebral cortex for storage in long-term memory centers. During this process, neurons undergo a permanent chemical or physical change. Evidence from experimental animals suggests that long-term memory storage creates additional synapses between connecting neurons, enabling those circuits to be activated more easily in the future.

Answer 22

Tolerance leads to addiction, the need to continue obtaining and using a substance despite one’s better judgment and good intentions. Essentially, choice is lost. Addiction has serious negative consequences for both emotional and physical health. Eventually, most addicts find it impossible to concentrate and hold a steady job. At the same time, they must spend large amounts of money to support their habit. Personal relationships suffer, and many addicts lose their families, their employment, and their physical health. In addition, they face the probability of withdrawal, the symptoms that occur if the drug is removed suddenly

chapter 11 Flashcards

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