Chapter 45 - Guyton Flashcards
How many neurons are found in the central nervous system?
more than 100 billion
Information from the somatic portion of the nervous system receives sensory information from peripheral nerves and conducts it to where?
spinal cord (all levels), reticular substance of the pons, medulla, and mesencephalon of the brain, cerebellum, thalamus, cerebral cortex
What are the motor functions of the nervous system?
1) contraction of skeletal muscle 2) contraction of smooth muscle in organs 3) secretion of active chemical substances by exocrine and endocrine glands
In the motor functions of the nervous system, what are the effectors?
muscles and glands (anatomical structures that perform the functions dictated by the nerve signals)
What levels of the CNS can control skeletal muscles?
spinal cord, reticular substance of the pons, medulla, and mesencephalon, basal ganglia, cerebellum, motor cortex
What is the primary difference between higher and lower region control of skeletal muscle?
higher regions - deliberate, complex muscle movements controlled by thought process of brain; lower regions - primarily automatic, instantaneous muscle responses to sensory stimuli
What is meant by the term “integrative function” in regards to the nervous system?
processing information so that appropriate mental and/or motor responses will occur, important info is immediately channeled into proper regions to elicit response (99% of all sensory is discarded as irrelevant and unimportant..we shall now call wednesday from 1-6 pm “integrative function” class)
What are some selective actions performed by synapses?
blocking weak signals and allowing strong to pass, selecting and amplifying certain weak signals, channeling signals in many directions rather than just one
Where is most memory stored?
cerebral cortex (small amount in basal regions of brain and spinal cord)
Signals generated within the brain itself can
cause transmission of impulses through the same
sequences of synapses, even when the sensory input is not excited. This gives the person a perception of experiencing the original sensations, although the perceptions are only memories of the sensations. What does this describe?
facilitation (useful after injury)
What are the three major levels of the CNS?
spinal cord, subcortical, cortical
Specific functional characteristics of the spinal cord.
conduit for signals from the periphery of the body to the brain, or vice versa
What functions might still occur after an injury to the spinal cord in the high neck region?
walking movements, reflexes that withdraw a body portion from pain, stiffen the legs to support body against gravity, control local blood vessels, GI movements, urinary excretion
Specific functional characteristics of the subcortical level (lower brain).
most subconscious activities of the body (arterial pressure, respiration, salivation, equilibrium, licking of lips, emotional patterns)
Specific functional characteristics of the cortical level (higher brain).
essential for most of our thought processes but cannot function by itself, without cerebral cortex functions of lower brain centers are imprecise
What areas are included in the subcortical level?
medulla, pons, mesencephalon, hypothalamus, thalamus, cerebellum, basal ganglia
Almost all the synapses used for signal transmission in the central nervous system of the human being are:
chemical synapses
Two types of synapses:
chemical and electrical
Most important neurotransmitters:
acetylcholine, norepinephrine, epinephrine, histamine, gamma-aminobutyric acid (GABA), glycine, serotonin, and glutamate
Two examples of gap junctions that transmit signals via electrical synapses:
APs are transmitted from one smooth muscle fiber to the next in visceral smooth muscle and from one cardiac muscle cell to the next
Which synapses can only conduct information one way?
chemical synapses
Gap junctions allowing ion movement are characteristic of which type of synapse?
electrical
Do more of the presynaptic terminals lie on the soma or the dendrites of the anterior motor neuron?
80-95% on the dendrites
Neurons in other parts of the cord and brain differ from the anterior motor horn in:
1) size of body 2) length, size, number of dendrites 3) length and size of axon 4) number of presynaptic terminals
Two important internal structures of the presynaptic terminal.
transmitter vesicles and mitochondria
What causes the vesicles to empty into the synaptic cleft?
AP spreads over presynaptic terminal and depolarizes the membrane
Role of the calcium ons in the spread of action potential.
voltage-gated calcium ion channels in the presynaptic membrane allow calcium into the terminal when the membrane depolarizes; calcium ions bind to release sites and allow transmitter vesicles to release transmitter into the cleft
Two important components of receptor proteins.
binding component that protrudes out into the synaptic cleft and ionophore component that passes through the postsynaptic membrane
Two types of ionophore component (receptor protein).
ion channel or second messenger
Two types of ion channels.
cation (excitatory) and anion (inhibitory)
When a transmitter activates a neuron channel, how long might you expect the channel to be open?
a fraction of a millisecond. now you see me—>now you don’t
How is prolonged postsynaptic
neuronal excitation or inhibition achieved?
second messenger system
What are the three components of the G-protein and which portion is the activator portion?
alpha, beta, and gamma; the activator portion is the alpha which separates from the beta and gamma on activation of the nerve impulse and is free to move about the cytoplasm
Inside the cytoplasm, the separated alpha component performs one or more of multiple functions:
1) opening specific ion channels (potassium channel stays open for prolonged time) 2) activate cAMP or cGMP (specific long term structural changes) 3) activation of specific intracellular enzymes 4) activation of gene transcription
Excitatory Mechanisms in the Postsynaptic Membrane.
opening of sodium channels (most widely used for excitatory), depressed conduction through chloride or potassium channels or both, various changes in the internal metabolism of the postsynaptic neuron to excite cell activity
Inhibitory Mechanisms in the Postsynaptic Membrane.
opening of chloride ion channels, increase in conductance of potassium out, activation of receptor enzymes that inhibit cellular metabolic functions
Small molecule, rapidly acting transmitters that cause the most acute responses of nervous system.
Class I: Acetylcholine Class II: The Amines Norepinephrine Epinephrine Dopamine Serotonin Histamine Class III: Amino Acids Gamma-aminobutyric acid (GABA) Glycine Glutamate Aspartate Class IV: Nitric oxide (NO)
Neuropeptide, slowly-acting transmitters (growth factors, long term changes). Just familiarize the list so you can recognize a slow transmitter.
Hypothalamic-releasing hormones: Thyrotropin-releasing hormone Luteinizing hormone–releasing hormone Somatostatin (growth hormone inhibitory factor) Pituitary peptides: Adrenocorticotropic hormone (ACTH) b-Endorphin a-Melanocyte-stimulating hormone Prolactin Luteinizing hormone Thyrotropin Growth hormone Vasopressin Oxytocin Peptides that act on gut and brain: Leucine enkephalin Methionine enkephalin Substance P Gastrin Cholecystokinin Vasoactive intestinal polypeptide (VIP) Nerve growth factor Brain-derived neurotropic factor Neurotensin Insulin Glucagon From other tissues: Angiotensin II Bradykinin Carnosine Sleep peptides Calcitonin
Which type of transmitter is synthesized by ribosomes in the neuronal cell body?
neuropeptides
What packages neuropeptide into transmitter vesicles that are released into the cytoplasm?
Golgi apparatus
How are the small molecule, rapidly acting transmitters absorbed into the vesicles?
active transport
What is actively transported back into the presynaptic terminal to be used again for synthesis of new acetylcholine?
choline
Where is acetylcholine secreted?
large pyramidal cells of the motor cortex, basal ganglia, motor neurons to skeletal muscle, preganglionic neurons of autonomic system, postganglionic cells of parasympathetic system, postganglionic cells of sympathetic system
Acetylcholine is typically excitatory, but where can it be inhibitory?
peripheral parasympathetic nervous system such as inhibition of heart by vagus nerve
Norepinephrine
Secreted by the brain stem and hypothalamus (wakefulness) and most postganglionic neurons of the sympathetic
nervous system, where it excites some organs but
inhibits others.
Dopamine
Secreted by neurons that originate in
the substantia nigra–>basal ganglia for inhibition.
Glycine
secreted at synapses in the spinal cord, inhibitory
GABA
secreted by nerve terminals in the spinal cord, cerebellum, basal ganglia, and many areas of the cortex; always inhibition
Glutamate
secreted by the presynaptic terminals
in many of the sensory pathways entering the CNS, as well as in many areas of the cerebral
cortex; excitation
Serotonin
secreted by nuclei that originate in the
median raphe of the brain stem and project to many brain and spinal cord areas, especially to the dorsal horns of the spinal cord and to the hypothalamus; acts as an inhibitor of pain pathways in the cord, and an inhibitor action in the higher regions of the nervous system to help control the mood, cause sleep
Nitric Oxide
synthesized almost instantly as
needed, and it then diffuses out of the presynaptic
terminals over a period of seconds rather than being released in vesicular packets; changes intracellular metabolic functions that modify neuronal excitability
After neuropeptides are released, is the vesicle reused?
no, it is autolyzed
Resting membrane potential neuronal soma.
-65 mV (lower voltage allows positive and negative control of degree of excitability of neuron
Which ion(s) are in high concentration inside the neuronal soma?
potassium
Which ion(s) are in high concentration outside the neuronal soma?
sodium, chloride
What is the Nernst potential?
a potential that exactly opposes movement of an ion [ EMF (mV) = 61 x log(conc. inside/conc. outside) ]
What contributes to the uniform distribution of the electrical potential inside the soma?
intracellular fluid contains highly conductive electrolyte solution, large diameter of soma causes little to no resistance to conduction of electric current
Excitatory transmitter acts on membrane excitatory receptor to increase the membrane’s permeability to:
Na+
What is the excitatory postsynaptic potential (EPSP)?
positive increase in voltage above the normal resting membrane potential
Threshold of excitation of a neuron is typically:
+20 mV
What is inhibitory postsynaptic potential (IPSP)?
increase in negativity beyond normal resting membrane potential
Give an example of presynaptic inhibition.
GABA, caused by release of transmitter on outsides of presynaptic nerve fibrils before their own endings terminate on postsynaptic neuron
How long is the neuronal membrane highly permeable (creating EPSP)?
1-2 milliseconds
What is spatial summation in neurons?
effect of summing simultaneous postsynaptic potentials by activating multiple terminals on widely spaced areas of neuronal membrane
What is temporal summation in neurons?
successive discharge from a single presynaptic terminal, if they occur rapidly enough, can add to one another to cause neuronal excitation
What is facilitation of neurons?
when summated postsynaptic potential is excitatory, but has not risen high enough to reach threshold for firing by the postsynaptic neuron
How far do dendrites of anterior motor neurons extend?
500-1000 micrometers
Do most dendrites transmit APs?
no due to relatively few voltage-gated sodium channels and high excitation thresholds, do transmit electrotonic current
What is decremental conduction in the dendrites?
decrease in membrane potential as it spreads electrotonically along dendrites toward soma (dendrites are leaky with their thin membranes)
Where is the a greater excitatory (or inhibitory) effect of synapses?
near the soma
What is an example of fatigue of synaptic transmission?
epileptic seizure ceases over time due to over excited neurons losing excessive excitability over time
What is the effect of acidosis and alkalosis on synaptic transmission?
acidosis depresses neuronal activity (pH 7.0 could cause coma), alkalosis increases neuronal activity (pH 7.8 can cause epileptic seizure)
What is the effect of hypoxia on synaptic transmission?
cessation of oxygen for only a few seconds can cause complete inexcitability of some neurons
What is synaptic delay?
during transmission of a neuronal signal time is consumed by: discharge of the transmitter substance in presynaptic terminal, diffusion of transmitter substance in postsynaptic terminal, action of transmitter on membrane receptor (minimum amount of time for all events is 0.5 ms, can estimate number of series of neurons in the circuit)
