Exam 4 review questions

List the functions of the hypothalamus, and discuss how they relate to homeostatic control.

(1) The hypothalamus is the "master gland" of the endocrine system and regulates the secretions of the pituitary gland. It also: (2) regulates water balance; (3) participates in the regulation of the autonomic nervous system; (4) regulates eating and drinking behavior; (5) regulates the reproductive system and reproductive behavior; (6) reinforces certain behaviors; (7) generates and regulates circadian rhythms; (8) regulates body temperature; and (9) participates in generation of emotional behavior. The hypothalamus is the master command center for neural and endocrine coordination. It is the single most important control area for homeostatic regulation of the internal environment.

crossed extensor reflex

(1) The presynaptic facilitation and inhibition described above is one way. Other presynaptic factors include: (2) availability of neurotransmitter, which is in turn dependent upon the availability of precursor molecules and the amount or activity of the rate-limiting enzyme in the pathway for neurotransmitter synthesis; (3) the axon terminal membrane potential—the more depolarized the terminals are, the more voltage-gated Ca2+ channels open; (4) the residual Ca2+ in the terminal from previous action potentials; and (5) the presence of autoreceptors. Postsynaptic factors include: (6) the immediate past history of the postsynaptic membrane; (7) effects of other neurotransmitters or neuromodulators; and (8) up- or down-regulation and desensitization of receptors. (9) Certain drugs and diseases can affect both the presynaptic and postsynaptic factors listed above. General factors include: (10) the area of synaptic contact; (11) enzymatic destruction of the neurotransmitter; (12) the geometry of the diffusion path, and (13) the rate of neurotransmitter reuptake.

List the major classes of neurotransmitters, and give examples of each.

Acetylcholine (the only member of its class), Biogenic amines (e.g., norepinephrine, dopamine, epinephrine, serotonin, histamine), Amino Acids (e.g., glutamate, gama-aminobutyric acid [GABA], glycine), Neuropeptides (e.g., endogenous opioids, oxytocin, tachykinins), Gases (e.g., nitric oxide, carbon monoxide, hydrogen sulfide), Purines (e.g., adenosine, ATP). See table 6.6

Describe the major function of alpha-gamma coactivation.

Coactivation of gamma motor neurons, which innervate intrafusal muscle fibers, with alpha motor neurons, which innervate extrafusal muscle fibers, assures that the intrafusal fibers will contract when the extrafusal fibers contract. Coactivation prevents the central region of the muscle spindle from going slack during a shortening contraction, and thus ensures that information about muscle length will be continuously available to provide for adjustment during ongoing actions and to plan and program future movements.

Discuss differences between neurotransmitters and neuromodulators.

Neurotransmitters cause postsynaptic such as EPSPs or IPSPs when they bind to receptors in the postsynaptic membrane. Neuromodulators have more complex effects, including influencing the postsynaptic cell's response to specific neurotransmitters, or changing the rate of synthesis, release, reuptake, or metabolism of a transmitter by the presynaptic cell. In other words, they alter the effectiveness of the synapse. The two kinds of messengers also have different time frames: Neurotransmitters initiate events that occur within milliseconds and are short-lived. The activation of receptors for neuromodulators often brings about changes in the metabolic processes in neurons via G proteins coupled to second-messenger systems. Such changes can occur over minutes, hours, or even days. Neuromodulators can be synthesized by the presynaptic cell and co-released with the neurotransmitter at synapses, or they can be hormones, paracrine substances, or immune-system messengers.

Detail the mechanism of long-term potentiation, and explain what role it might play in learning and memory.

See Figure 6.36. Repetitive activation of presynaptic neurons causes a large increase in the glutamate concentration in the synapse, which binds to both AMPA and NMDA receptors. Strong depolarizing current through the AMPA channels depolarizes the postsynaptic membrane by more that 20-30 mV, which induces Mg2+ to move away from the pore of the NMDA receptor, allowing Ca2+ current to enter the postsynaptic cell. This, in turn, activates second-messenger systems that induce a long-term increase in glutamate receptors and sensitivity in the postsynaptic cell, as well as sending a retrograde messenger to the presynaptic cell that increase glutamate synthesis and release. Thus, after long-term potentiation occurs, each action potential in a presynaptic cell will result in a larger glutamate release and a larger AMPA-mediated depolarization of the postsynaptic cell. This may be part of the basis by which synapses are modulated in the creation of enhanced "memory traces"in the brain.

Contrast the sympathetic and parasympathetic components of the autonomic nervous system; mention at least four characteristics of each.

The first difference is mentioned above: The postganglionic fibers of each division secrete different neurotransmitters. The two divisions also differ anatomically: The nerve fibers of the two divisions leave the CNS from different levels—the sympathetic fibers from the thoracic and lumbar areas of the spinal cord, and the parasympathetic fibers from the brain and the sacral portion of the spinal cord. The locations of ganglia also differ for the two divisions: Most of the sympathetic ganglia lie close to the spinal cord and form two chains of ganglia, the sympathetic trunks. The postsynaptic fibers from these ganglia can be quite long. Other sympathetic ganglia—the celiac, superior mesenteric, and inferior mesenteric ganglia—are in the abdominal cavity, closer to the innervated organ. The parasympathetic ganglia all lie within the organs innervated by the postganglionic fibers or close to them, and so the postganglionic parasympathetic fibers are quite short. The two divisions differ functionally as well. To some extent the sympathetic division acts as a single unit, whereas the parasympathetic division is made up of relatively independent components. Perhaps most importantly, the two divisions often innervate the same organ and usually have opposite effects. In general, the sympathetic division is activated in situations requiring physical action—the fight-or-flight response—and also when one is under psychological stress. The parasympathetic system is more dominant in times of rest-or-digest.

Contrast the somatic and autonomic divisions of the efferent nervous system; mention at least three characteristics of each.

The neurons of the somatic nervous system innervate skeletal muscle, whereas autonomic neurons innervate smooth and cardiac muscle, glands, and neurons in the gastrointestinal tract. The somatic system consists of single neurons between the CNS and skeletal muscle cells, whereas the autonomic system has two-neuron chains (connected by a synapse in a ganglion) between the CNS and the effector organ. The output of the somatic system is always excitatory, whereas the autonomic output can be excitatory or inhibitory.

Name the neurotransmitter released at each synapse or neuroeffector junction in the somatic and autonomic systems.

The neurotransmitter released by: (1) the somatic motor neurons at neuromuscular junctions; (2) the preganglionic fibers of both the parasympathetic and sympathetic divisions; and (3) postganglionic, parasympathetic fibers is acetylcholine. The neurotransmitter released by postganglionic sympathetic fibers is norepinephrine. The adrenal medulla releases epinephrine into the blood.

Describe the components of the knee jerk reflex (stimulus, receptor, afferent pathway, integrating center, efferent pathway, effector, and response).

The stimulus in a knee jerk reflex is the stretching of the extensor muscles of the thigh as a result of stretching the patellar tendon from a knee tap; the receptors are the stretch receptors within the muscle spindles in the affected thigh muscles; the afferent pathway is the afferent neuron from the stretch receptor; the integrating center(s) are the cell bodies of the motor neurons that innervate the extensor muscles of the thigh, cell bodies of the motor neurons that stimulate ipsilateral synergistic muscles that aid in the extension of the leg, interneurons that inhibit the motor neurons to the ipsilateral flexor muscles, and interneurons within the brainstem; the efferent pathways are the motor neurons to the muscles; the effectors are the extensor muscles of the ipsilateral leg, and the flexor muscles of the ipsilateral leg; and the response is extension of the leg (a knee jerk).

List two functions of the thalamus.

The thalamus is an important relay station for sensory pathways on their way to the cerebral cortex. It also participates in control of body movement and skeletal muscle coordination, and it plays a key role in awareness.