Involuntary reflexes

The neurons have voluntary and involuntary (reflex) regulation... 

Blood pressure is under the control of the autonomic (sometimes called the involuntary or reflex) nervous system. 

Cough is an essential protective reflex response to irritating stimuli in the respiratory tract. It involves the sudden, usually involuntary, expulsion of air from... 

Emesis is the forcefiil involuntary expulsion of the stomach contents through the mouth. It is a reflex response that may be initiated by a number of stimuli. 

The spinal cord is the most anatomically inferior portion of the CNS and its functions are at the lowest level of sophistication (see Table 6.1). As mentioned earlier, the spinal cord receives sensory input from the periphery of the body and contains the cell bodies of motor neurons responsible for voluntary and involuntary movements. Once again, the involuntary and neurologically simple reflexes are processed entirely at the level of the spinal cord. Voluntary, deliberate movements are initiated and controlled by thought processes in the cerebrum. The second important function of the spinal cord is to transmit nerve impulses to and from the brain. Ascending pathways carry sensory input to higher levels of the CNS and descending pathways carry impulses from the brain to motor neurons in the spinal cord. 

A reflex occurs when a particular stimulus always elicits a particular response. This response is automatic and involuntary in other words, it occurs without conscious effort. Therefore, reflexes are specific, predictable, and, furthermore, often purposeful. For example, the withdrawal reflex causes a body part to be pulled away from a painful stimulus so that tissue injury is avoided. Spinal reflexes require no input from the brain because they are elicited entirely at the level of the spinal cord. However, while the reflex is underway, nervous impulses are also transmitted to the brain for further processing. In fact, input from the brain may modulate a reflex or alter the response to a stimulus through conscious effort. 

General anesthesia is a state of drug-induced reversible inhibition of central nervous function, during which surgical procedures can be carried out in the absence of consciousness, responsiveness to pain, defensive or involuntary movements, and significant autonomic reflex responses (A). 

With severe intoxication by all routes, an excess of acetylcholine at the neuromuscular junctions of skeletal muscle causes weakness aggravated by exertion, involuntary twitchings, fasciculations, and eventually paralysis. The most serious consequence is paralysis of the respiratory muscles. Effects on the central nervous system include giddiness, confusion, ataxia, slurred speech, Cheyne-Stokes respiration, convulsions, coma, and loss of reflexes. The blood pressure may fall to low levels, and cardiac irregularities, including complete heart block, may occur. ... 

Neurobiological components of colorectal and anal sphincter mechanisms and pathophysiology are similar to the above-mentioned vesical components. In both systems, central, peripheral, and autonomic nervous system reflexes, partly involuntary and partly voluntary, interweave critically for appropriate function. Function and control are mediated through the lower spinal cord, as well as via cortical centers requiring coordinated voluntary (and reflex) relaxation of external sphincters, (reflex) relaxation of internal sphincters, and simultaneous coordinated contraction of detrusor... 

Skeletal muscle spasms are used to describe the increased tension often seen in skeletal muscle after certain musculoskeletal injuries and inflammation (muscle strains, nerve root impingements, etc.) occur.20,96 This tension is involuntary, so the patient is unable to relax the muscle. Spasms differ from spasticity because spasms typically arise from an orthopedic injury to a musculoskeletal structure or peripheral nerve root rather than an injury to the CNS. Likewise, muscle spasms are often a continuous, tonic contraction of specific muscles rather than the velocity-dependent increase in stretch reflex activity commonly associated with spasticity. The exact reasons for muscle spasms are poorly understood. According to some authorities, muscle spasms occur because a vicious cycle is created when the initial injury causes muscular pain and spasm, which increases afferent nociceptive input to the spinal cord, further exciting the alpha motor neuron to cause more spasms, and so on.61,96 Other experts believe that muscle spasms occur because of a complex protective mechanism, whereby muscular contractions are intended to support an injured vertebral structure or peripheral joint.96 Regardless of the exact reason, tonic contraction of the affected muscle is often quite painful because of the buildup of pain-mediating metabolites (e.g., lactate). 

The baroreceptor response is just one example of the type of reflex activity employed by the ANS. The control of other involuntary functions usually follows a similar pattern of peripheral monitoring, central integration, and altered autonomic discharge. Body temperature, for instance, is monitored by thermoreceptors located in the skin, viscera, and hypothalamus. When a change in body temperature is monitored by these sensors, this information is relayed to the hypothalamus and appropriate adjustments are made in autonomic discharge in order to maintain thermal homeostasis (e.g., sweating is increased or decreased and blood flow is redistributed). Many other autonomic reflexes that control visceral and involuntary functions operate in a similar manner. 

There is often mild blepharoclonus (fluttering of the closed eyelids) and occasionally blepharospasm (involuntary closure of the eyelids). The patient may drool, perhaps because of the impairment of swallowing. There is typically no alteration in the tendon reflexes, and the plantar responses are flexor. Repetitive tapping (about twice per second) over the bridge of the nose produces a sustained blink response (Myerson s sign) the response is not sustained in normal subjects. Cognitive decline sometimes occurs but is usually mild. 

For the study of these involuntary acts, immediate result of Reflex Action, see the remarkable works of Dr. Papus Traite de Physiologie Synthetique, Traite Methodique de Science Occulte, etc. 

Curare has no effect on sensation, consciousness, or pain and it does not enter the CNS (see Chapter 15). Victims injected with many lethal doses of curare will survive with no apparent damage if adequate respiration can be provided for them. Because of this, curare and derivatives of it are used in medicine to produce paralysis during delicate surgical procedures where involuntary or reflexive movement would be disastrous. The anesthetist provides artificial respiration for the patient until curare is eliminated from the body. 

In comatose patients evaluation of the oculomotor system relies on evaluation and observation of involuntary eye movements. The oculocephalic and the oculovestibular reflexes disappear in deep coma. 

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