Reflex arcs

The regulation of the total peripheral resistance also involves the complex interactions of several mechanisms. These include baroreflexes and sympathetic nervous system activity response to neurohumoral substances and endothelial factors myogenic adjustments at the cellular level, some mediated by ion channels and events at the cellular membrane and intercellular events mediated by receptors and mechanisms for signal transduction. As examples of some of these mechanisms, there are two major neural reflex arcs (Fig. 1). Baroreflexes are derived from high-pressure barorecep-tors in the aortic arch and carotid sinus and low-pressure cardiopulmonary baroreceptors in ventricles and atria. These receptors respond to stretch (high pressure) or... 

Sympathetic and parasympathetic nerves innervate the penis. In the flaccid state, OC2-adrenergic receptors mediate tonic contraction of the arterial and corporal smooth muscles. This maintains high penile arterial resistance and a balance exists between blood flow into and out of the corpora. With sexual stimulation, nerve impulses from the brain travel down the spinal cord to the thoracolumbar ganglia.3 A decrease in sympathetic tone and an increase in parasympathetic activity then occurs, causing a net increase in blood flow into the erectile tissue. Erections may also occur as a result of a sacral nerve reflex arc while patients are sleeping (nocturnal erections). 

A reflex response requires an intact neural pathway between the stimulated area and the responding muscle. This pathway is referred to as a reflex arc and includes the following components (see Figure 7.3) ... 

Figure 7.3 Components of a reflex arc. As illustrated by the components of the reflex arc, reflexes may be processed entirely at the level of the spinal cord with no need for input from the brain. A monosynaptic reflex has a single synapse between afferent and efferent neurons a polysynaptic reflex has two or more synapses between these neurons. In this case, intemeurons lie between the sensory and motor neurons. The more intemeurons involved, the more complex the response is.

Monosynaptic reflex A reflex arc (pathway) with a single synapse. 

The autonomic nervous system is by definition that part of the nervous system that innervates smooth muscle, cardiac muscle and glands. It is thus a motor system. Perception arising from the viscera involves pathways similar to those arising from the body surface and skeletal muscle. Thus there are visceral afferent fibres that pass from the viscera to the central nervous system. Such impulses then ascend the spinal cord to the thalamus and are thence relayed to the post-central gyrus of the brain (or sensory cortex). Visceral reflex arcs use visceral afferent fibres to convey impulses to the cord, but the efferent limb of such a visceral reflex is the autonomic nervous system. Although visceral reflexes are under higher central control, it is usually impossible to bring them under the control of the will. 

Once the information is processed and yon (consciously or unconsciously) have decided what it means, yonr nervous system coordinates a response. This is the task of execution. The loop from sensation to processing to execution can occur at many levels. The simplest level is the reflex arc. As noted earlier, when your family doctor strikes your knee with a rubber hammer, the nerves at your knee sense the impact and transmit that information. This information is intercepted and processed well before it ever reaches your consciousness. A reflex center interprets the sensation as a possible threat and automatically executes a command to straighten your knee. The result is a reflex action that protects your leg from injury by kicking away the perceived threat. This sensation to processing to execution loop is completed without any involvement of yonr brain. 

The nervous system is conventionally divided into the central nervous system (CNS the brain and spinal cord) and the peripheral nervous system (PNS neuronal tissues outside the CNS). The motor (efferent) portion of the nervous system can be divided into two major subdivisions autonomic and somatic. The autonomic nervous system (ANS) is largely independent (autonomous) in that its activities are not under direct conscious control. It is concerned primarily with visceral functions such as cardiac output, blood flow to various organs, and digestion, which are necessary for life. The somatic subdivision is largely concerned with consciously controlled functions such as movement, respiration, and posture. Both systems have important afferent (sensory) inputs that provide information regarding the internal and external environments and modify motor output through reflex arcs of varying size and complexity. 

Diagram of the structures involved in the stretch reflex arc. I is an inhibitory interneuron E indicates an excitatory presynaptic terminal la is a primary intrafusal afferent fiber Ca2+ denotes activator calcium stored in the sarcoplasmic reticulum of skeletal muscle RyR channels indicates the Ca2+ release channels. 

Drugs that modify this reflex arc may modulate excitatory or inhibitory synapses (see Chapter 21). Thus, to reduce the hyperactive stretch reflex, it is desirable to reduce the activity of the la fibers that excite the primary motoneuron or to enhance the activity of the inhibitory internuncial neurons. These structures are shown in greater detail in Figure 27-11. 

Neurons in different regions of the gray matter can connect with each other, forming spinal reflex arcs between sensory nerves bringing together noxious stimuli and motor nerves controlling avoidance responses. The behavioral consequences of such spinal reflexes are familiar in everyday life the eye blinks as an object approaches, the hand is withdrawn from a hot plate, both without conscious control. 

Dickerman RD, Jaikumar S. The hiccup reflex arc and persistent hiccups with high-dose anabolic steroids is the brainstem the steroid-responsive locus Clin Neuropharmacol 2001 24(l) 62-4. 

Tizanidine (Zanaflex) is classified as an alpha-2 adrenergic agonist, meaning that this drug binds selectively to the alpha-2 receptors in the CNS and stimulates them. Alpha-2 receptors are found at various locations in the brain and spinal cord, including the presynaptic and postsynaptic membranes of spinal interneurons that control alpha motor neuron excitability. Stimulation of these alpha-2 receptors inhibits the firing of interneurons that relay information to the alpha motor neuron that is, interneurons that comprise polysynaptic reflex arcs within the spinal cord.27 Tizanidine appears to bind to receptors on spinal interneurons, decrease the release of excitatory neurotransmitters from their presynaptic terminals (presynaptic inhibition), and decrease the excitability of the postsynaptic neuron (postsynaptic inhibition).40 Inhibition of spinal interneurons results in decreased excitatory input onto the alpha motor neuron, with a subsequent decrease in spasticity of the skeletal muscle supplied by that neuron. 

Dewey, J. (1896) The reflex arc in psychology. Psychological Review III, July, 357-370. 

Reflex arcs Most of the afferent impulses are translated into reflex responses without involving consciousness. For example, a fall in blood pressure causes pressure-sensitive neurons (baroreceptors in the heart, vena cava, aortic arch, and carotid sinuses) to send fewer impulses to cardiovascular centers in the brain. This prompts a reflex response of increased sympathetic output to the heart and vasculature, and decreased parasympathetic output to the heart, which results in a compensatory rise in blood pressure and tachycardia (see Figure 3.5). 

Baroreceptor reflex arc responds to a decrease in blood pressure. 

Spinal anesthesia was found to inhibit the renin angiotensin response to laparotomy in man. It would appear that renin angiotensin release is mediated by a spinal reflex arc which may be facilitated by impulses from higher centers (B8). 

International Pharmacopoeia (Int. P) The pharmacopoeia of the World Health Organization, a formulary intended to meet international needs, interneuron A neuron in the CNS synapsing between sensory neurons and motor neurons in a typical spinal reflex arc. 

While the regional distribution is fairly uniform for these two substances, there may be significant differences in the concentrations of aspartate and glutamate in certain areas of the spinal cord. Furthermore, biochemical studies have suggested that aspartate may be concentrated in the Interneurons of the polysynaptic reflex arc, and that there may be a specific glutamate pathway from the cortex to the corpus striatum. 

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