Psychopharmatology (1) Flashcards
Two major divisions neurosystem
(1) The central nervous system (CNS) consists of the brain and spinal cord. The brain is completely surrounded and protected by the skull. It connects directly to the spinal cord, similarly protected by the vertebral column.
(2) The peripheral nervous system (PNS) consists of nerves. Nerves lie outside the CNS. The division between the CNS and the PNS is arbitrary. The two systems work together and are connected to each other.
Three functions nervous system
- The nervous system receives sensory input. Sensory receptors in skin and other organs respond to external and internal stimuli by generating nerve signals that travel by way of the PNS to the CNS.
- The CNS performs information processing and integration, summing up the input it receives from all over the body. The CNS reviews the information, stores the information as memories, and creates the appropriate motor responses.
- The CNS generates motor output. Nerve signals from the CNS go by way of the PNS to the muscles, glands, and organs. The CNS also coordinates the movement of your arms and hands.
Two types of cells in the nervous tissue
(1) Neurons; the cells that transmit nerve impulses between parts of the nervous system;
(2) Neuroglia (sometimes referred to as glial cells); support and nourish neurons.
Neuroglia, greatly outnumber neurons in the brain.
Types of neuroglia
There are several types of neuroglia in the CNS, each with specific functions.
(1) Microglia are phagocytic cells that help remove bacteria and debris.
(2) Astrocytes provide metabolic and structural support directly to the neurons.
(3) The myelin sheath is formed from the membranes of tightly spiraled neuroglia. In the PNS, Schwann cells perform this function, leaving gaps called nodes of Ranvier.
(4) In the CNS, neuroglia cells called oligodendrocytes form the myelin sheath.
Three types of neurons (function)
(1) Sensory neurons; a sensory neuron takes nerve signals from a sensory receptor to the CNS. Sensory receptors are special structures that detect changes in the environment.
(2) Interneurons; an interneuron lies entirely within the CNS. Interneurons can receive input from sensory neurons and from other interneurons in the CNS. Thereafter, they sum up all the information received from other neurons before they communicate with motor neurons.
(3) Motor neurons; a motor neuron takes nerve impulses away from the CNS to an effector (muscle fiber, organ, or gland). Effectors carry out our responses to environmental changes, whether these are external or internal.
Features of neurons
(1) cell body; the cell body contains the nucleus, as well as other organelles.
(2) Dendrites; dendrites are short extensions that receive signals from sensory receptors or other neurons. Incoming signals from dendrites can result in nerve signals that are then conducted by an axon.
(3) Axon: the axon is the portion of a neuron that conducts nerve impulses. An axon can be quite long. Individual axons are termed nerve fibers, and collectively they form a nerve. In sensory neurons, a very long axon carries nerve signals from the dendrites associated with a sensory receptor to the CNS, and this axon is interrupted by the cell body. In interneurons and motor neurons, on the other hand, multiple dendrites take signals to the cell body, and then an axon conducts nerve signals away from the cell body.
Myelin sheath
Many axons are covered by a protective myelin sheath. The myelin sheath develops when Schwann cells (PNS) or oligodendrocytes (CNS) wrap their membranes around an axon many times. Each neuroglia cell covers only a portion of an axon, so the myelin sheath is interrupted. The gaps where there is no myelin sheath are called nodes of Ranvier. The myelin sheath plays an important role in the rate at which signals move through the neuron. Long axons tend to have a myelin sheath, but short axons do not. The gray matter of the CNS is gray because it contains no myelinated axons; the white matter of the CNS is white because it does. In the PNS, myelin
gives nerve fibers their white, glistening appearance and serves as an excellent insulator. When the myelin breaks down, as is the case in multiple sclerosis (MS), then it becomes more difficult for the neurons to transmit information. In effect, MS “short-circuits” the nervous system. The myelin sheath also plays an important role in nerve regeneration within the PNS. If an axon is accidentally severed, the myelin sheath remains and serves as a passageway for new fiber growth.
Resting potential
The battery’s potential energy can be used to perform work. A resting neuron also has potential energy. This energy, called the resting potential, exists because the plasma membrane is polarized: Positively charged ions are stashed outside the cell, with negatively charged ions inside. The outside of the cell is positive because positively charged sodium ions (Na+) gather around the outside of the plasma membrane. At rest, the neuron’s plasma membrane is permeable to potassium, but not to sodium. Thus, positively charged potassium ions (K+) contribute to the positive charge by diffusing out of the cell to join the sodium ions. The inside of the cell is negative in relation to the exterior of the cell because of the presence of large, negatively charged proteins and other molecules that remain inside the cell because of their size. The neuron’s resting potential energy can be measured in volts.
Synaptic cleft
Separates the sending neuron from the receiving neuron. The nerve signal is unable to jump the cleft. Therefore, another means is needed to pass the nerve signal from the sending neuron to the receiving neuron.
Transmission across synapse
Carried out by molecules called neurotransmitters, stored in synaptic vesicles in the axon terminals.
The events at a synapse are (1) nerve signals traveling along an axon to reach an axon terminal; (2) calcium ions entering the terminal and stimulating synaptic vesicles to merge with the sending membrane; and (3) neurotransmitter molecules releasing into the synaptic cleft and diffusing across the cleft to the receiving membrane; there, neurotransmitter molecules bind with specific receptor proteins. Depending on the type of neurotransmitter, the response of the receiving neuron can be toward excitation or toward inhibition. Excitation occurs because a neurotransmitter, such as acetylcholine (ACh), has caused the sodium gate to open. Sodium ions diffuse into the receiving neuron. Inhibition would occur if a neurotransmitter caused potassium ions to exit the receiving neuron.
Response based on the type of neurotransmitter
The response of the receiving neuron can be toward excitation or toward inhibition. Excitation occurs because a neurotransmitter, such as acetylcholine (ACh), has caused the sodium gate to open. Sodium ions diffuse into the receiving neuron. Inhibition would occur if a neurotransmitter caused potassium ions to exit the receiving neuron.
Integration of excitatory and inhibitory signals at the synapse
Inhibitory signals and
excitatory signals are summed up in the dendrite and cell body of the postsynaptic neuron. Only if the combined signals cause the membrane potential to rise above threshold does an action potential occur.
What happens when a neurotransmitter has been released into a synaptic cleft and has initiated a response
It is removed from the cleft. In some synapses, the receiving membrane contains enzymes that rapidly inactivate the neurotransmitter. For example, the enzyme acetylcholinesterase (AChE) breaks down the neurotransmitter acetylcholine. In other synapses, the sending membrane rapidly reabsorbs the neurotransmitter, possibly for repackaging in synaptic vesicles or for molecular breakdown. The short existence of neurotransmitters at a synapse prevents continuous stimulation (or inhibition) of receiving membranes. The receiving cell needs to be able to respond quickly to changing conditions. If the neurotransmitter were to linger in the cleft, the receiving cell would be unable to respond to a new signal from a sending cell.
Neurotransmitters
More than 100 substances are known or suspected to be neurotransmitters. Some of the more common ones in humans are acetylcholine, norepinephrine, dopamine, serotonin, glutamate, and GABA (gamma-aminobutyric acid). Neurotransmitters transmit signals between nerves. Nerve-muscle, nerve-organ, and nerve-gland synapses also communicate using neurotransmitters. Acetylcholine (ACh) and norepinephrine are active in both the CNS and PNS. In the PNS, these neurotransmitters act at synapses called neuromuscular junctions. In the PNS, ACh excites skeletal muscle but inhibits cardiac muscle. It has either an excitatory or inhibitory effect on smooth muscle or glands, depending on their location. Norepinephrine generally excites smooth muscle. In the CNS, norepinephrine is important to dreaming, waking, and mood. Serotonin is involved in thermoregulation, sleeping, emotions, and perception. Many
drugs that affect the nervous system act at the synapse. Some interfere with the actions of neurotransmitters, and other drugs prolong the effects of neurotransmitters.
The gate control theory of pain
Proposes that the tracts in the spinal cord have “gates” and that these “gates” control the flow of pain messages from the peripheral nerves to the brain. Depending on how the gates process a pain signal, the pain message can be allowed to pass directly to the brain or can be prevented from reaching the brain.
