Broadly speaking behavior can be defined as a motor act. However from a functional point of view, a motor act has no meaning at all, unless referred to a particular environmental context (an organism is not a random collection of movements). Therefore, behavior, can be viewed as a form of response triggered by a particular environmental configuration.

Since reception of an environmental variable can be carried out by a sensory system, and behavioral response can be traced back to a motor output; many behavioral acts can be, at its very least, reduced to a sensory-motor system loop.

A reflex is a simple form of behavior and clearly illustrates the logic of a sensory-motor system loop.

A reflex is an involuntary and relatively stereotyped response to a specific sensory stimulus. Two features of the sensory stimulus are particularly important in shaping the reflex response. First, the precise location of the stimulus determines in a fixed way the particular muscle that contract to produce the reflex response. Second the strength of the stimulus determines the amplitude of the response. Reflex therefore are graded behaviors.

However, in order to integrate diverse information for purposeful action,in addition to a motor and sensory system a third major brain function is required: the motivational system.

Each system has a corresponding an well defined neural substrate.

Voluntary movement is controlled by complex neural circuits in the brain interconnecting the sensory and motor systems. Although all voluntary movement is controlled directly by the motor system, the decision to initiate a voluntary movement is regulated by the motivational system. The motivational system influences voluntary movement by acting on the somatic motor system in the brain, In addition, it influences behavior through its action on the autonomic nervous system, which innervates the exocrine gland, the viscera and smooth muscles in all organs of the body. The sympathetic and parasympathetic divisions also mediate motivational and emotional states.

The main control center for the autonomic motor system is the hypothalamus, which is also critically involved in the regulation of endocrine hormone release. The activity of hypothalamus is also influenced by the blood concentration of insulin and glucose, thus being responsive to a broad spectrum of behavioral important endogenous stimuli.

In order to illustrate how the sensory, motor and motivational systems interact to produce purposeful behavior, let us examine the simple behavior of reaching for a glass of water.

First, sensory processing: visual information about the position of the glass, tactile information about the contact and surface of the glass in the hand, and proprioceptive information about relative position of the arm, legs and trunk in space. Sensory information is fed to the association areas of the cortex, where the movement is planned. From there, information is transmitted to the motor system, which generates commands for movements involved in reaching and holding the glass. These motor commands must be targeted to the correct muscles in the hand, fingers, arm and shoulders. They must be timed so that contraction and relaxation of appropriate muscle groups are coordinated.

Whereas the sensory and motor systems are important in actually reaching for a glass of water, the stimulus to initiate and complete the behavior is provided by the motivational system (i.e., thirst). The motivational or limbic system modulates the motor output to skeletal muscles. How well this task is carried out may depend on whether you are tired, distracted or anxious.

Finally, there are also highly complex automatic behaviors known as instinctive behaviors. Instinctive behaviors relay mainly on hard-wired neural circuitries acquired by the brain as a result of a long evolutionary process rather than an individual experience. Sexual motor patterns, nesting and mating displays are instances of instinctive behavior.

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