Reflexes Process - Medical Description

Reflexes Process - Medical Description

What is a reflex act? The reflex act is an involuntary and unconscious response to a stimulus which is applied on a receptor. The nervous system participates to this response.
The anatomical basis of a reflex is the reflex arc formed by a receptor, the afferent path, the nerve center, the efferent path and the effector. The receptor is a special protein found in the cell membrane which takes the information and pays it forward. The afferent path is formed by the sensory neuron. The nerve center it represents is usually localized the spinal cord. The efferent path formed by a motor neuron and the effector can be a muscle or a gland. If any of the components of the reflex act is damaged the reflex cannot function. For example, it becomes useless to have eyes if the efferent path of the optic nerve is destroyed because the nervous impulse (the stimulus) cannot be transmitted.

In the living organism, we find a lot of reflexes that keep it functioning at normal parameters. Without reflexes there is no life (for example no one tells us when to breathe, breathing comes automatically because of the vegetative reflexes. Or, another example is the heartbeat, which has its own rhythm. The heart knows exactly when to contract or when to relax). In order to understand better the mechanisms of reflexes, reflexes are classified after diverse criteria like the:
- apparition time
- type of the receptor implied in the shutter of the reflex
- function

Reflexes classified after the apparition time can be:

a) Innate - we are born with them. Innate reflexes are typical and unchanging. They can be: mono synaptic (involves only one synapse, for example the stretch reflexes that maintain the muscle tone and the posture of the body) or poly synaptic (involves two or more synapses, for example the flexion reflex)

b) Acquired by conditioning. Conditioned reflexes are developed during a longer period of time, after the repeated association of a fundamental stimulus with a conditioned stimulus. They were first defined by the Russian physiologist Ivan Petrovich Pavlov. He actually didn't start with the study of the reflex system; the idea came to his mind while he investigated the gastric function of dogs. He observed that the dog started to salivate before the food was given to him. After his experiments he won the Nobel Prize in Physiology and Medicine in 1904.

Reflexes classified after the receptor type are:

a) Exteroceptive reflexes - are brought by the stimulation of exteroceptors (sensory receptors found in skin):
- tactile reflexes
- thermal reflexes
- visual reflexes
- acoustic reflexes
- pain reflexes

b) Proprioceptive reflexes - are brought by stimulation of proprioceptors (sensory receptors found in muscle, tendon and joints):
- muscle stretch reflex
- deep tendon reflexes

c) Interoceptive reflexes - are brought by stimulation of interoceptors (sensory receptors found in viscera):
- chemoreceptor reflexes
- respiratory reflexes
- gastrointestinal reflexes

Reflexes classified after their function can be somatic or vegetative

Somatic reflexes
The receptors of somatic reflexes are free nerve endings or specialized structures like neuromuscular spindles. The afferent path is formed by sensitive fibers with the origin in the spinal ganglion or the ganglion homologous to the cranial nerves. The nerve center can be localized in the spinal cord, brain stem or superior nerve centers. The efferent path is formed by somatic motor fibers and the effector is represented by specialized tissues: muscles (contraction) and glands (secretion). Somatic reflexes can be mono synaptic (for example the patella reflex) and poly synaptic (for example Babinski's reflex that is present at children up to two years when the myelinisation of the neuron isn't completed. It is pathological at children after age two, being a sign of neural damage)

Vegetative reflexes
The afferent path follows a different track from somatic reflexes; it goes through the paravertebral sympathetic ganglion chain without making a synapse. Vegetative reflexes can be sympathetic or parasympathetic.

The chemical mediators liberated on the efferent vegetative path are:

- acethylcoline(in ganglionic fibers) all the way from the preganglionic neuron to the postganglionic neuron.

- in postganglionic fibers the chemical mediator varies on the type of postganglionic fibers. In the sympathetic postganglionic fibers the mediator is represented by catchecolamines (noradrenaline, adrenaline and dopamine) and parasympathetic fibers the mediator is acetilcholine. The effectors are the visceral muscles, the myocard and secretory glandular cells.