Sensory motor integration

Sensory stimulus gives rise to a motor response. To accomplish tasks like running, move a hand from hot stove and similar, the sensory and motor systems must communicate with each other.

This process is called sensory-motor integration, and it is depicted in the figure below. For the body to respond to sensory stimuli, the sensory and motor divisions of the nervous system must function together in the following sequence of events:

1. A sensory stimulus is received by sensory receptors(e.g. pinprick).
2. The sensory action potential is transmitted along sensory neurons to the CNS.
3. The CNS interprets the incoming sensory information and determines which response is most appropriate, or reflexively initiates a motor response.
4. The action potentials for the response are transmitted from the CNS along alpha-motor neurons.
5. The motor action potential is transmitted to a muscle, and the response occurs.

Sensory input

Recall that sensations and physiological status are detected by sensory receptors throughout the body. The action potentials resulting from sensory stimulation are transmitted via the sensory nerves to the spinal cord. When they reach the spinal cord, they can trigger a local reflex at that level, or they can travel to the upper regions of the spinal cord or to the brain. Sensory pathways to the brain can terminate in sensory areas of the brain stem, the cerebellum, the thalamus, or the cerebral cortex. An area in which the sensory impulses terminate is referred to as in integration center. This is where the sensory input is interpreted and linked to the motor system. Figure below illustrates various sensory receptors and their nerve pathways back to the spinal cord and up into various areas of the brain. The integration centers vary in function:

• Sensory impulses that terminate in the spinal cord are integrated there. The response is typically a simple motor reflex, which is the simpliest type of integration.
• Sensory signals that terminate in the lower brain stem result in subconscious motor reactions of a higher and more complex nature than simple spinal cord reflexes. Postural control during sitting, standing, or moving is an example of this level of sensory input.
• Sensory signals that terminate in the cerebellum also result in subconscious control of movement. The cerebellum appears to be the center of coordination, smoothing out movements by coordinating the actions of the various contracting muscle groups to perform the desired movement. Both fine and gross motor movements appear to be coordinated by the cerebellum in the concert with the basal ganglia. Without the control exerted by the cerebellum, all movement would be uncontrolled and uncoordinated.
• Sensory signals that terminate at the thalamus begin to enter the level of consciousness, and the person begins to distinguish various sensations.
• Only when sensory signals enter the cerebral cortex can one discretely localize the signal. The primary sensory cortex, located in the postcentral gyrus(in the parietal lobe), receives general sensory input from the receptors in the skin and from proprioceptors in the muscles, tendons, and joints. This area has a map of the body. Stimulation in a specific area of the body is recognized, and its exact location is known instantly. Thus, this part of the conscious brain allows us to be constantly aware of our surroundings and our relationship to them.

Motor control

Once a sensory impulse is received, it may evoke a motor response, regardless of the level at which the sensory impulse stops. This response can originate from any of three levels:

·         The spinal cord

·         The lower regions of the brain

·         The motor area of the cerebral cortex

As of the level of control moves from the spinal cord to the motor cortex, the degree of movement complexity increases from simple reflex control to complicated movements requiring basic thought processes. Motor responses for more complex movement patterns typically originate in the motor cortex of the brain.

At last, we are ready to tie the two systems together through sensory-motor integration. The simpliest form of this is the reflex, so we can consider it first.

Reflex activity

What happens when one unknowingly puts one’s hand on a hot stove? First, the stimuli of heat and pain are received by the thermoreceptors and nocireceptors in the hand, and then sensory action potentials travel to the spinal cord, terminating at the level of entry. Once in the spinal cord, these action potentials are integrated instantly by interneurons that connect the sensory and motor neurons. The action potentials move to the motor neurons and travel to the effectors, the muscles controlling the withdrawal of the hand. The result is that the person reflexively withdraws the hand from the hot stove without giving the action any thought.

A motor reflex is a preprogrammed response; any time the sensory nerves transmit certain action potentials, the body responds instantly and identically. In examples like the one just used, whether one touches something that is too hot or too cold, thermoreceptors will elicit a reflex to withdraw the hand. Whether the pain arises from heat or from a sharp object, the nociceptors will also cause a withdrawal reflex. By the time one is consciously aware of the specific stimulus(after sensory action potentials also have been transmitted to the primary sensory cortex), the reflex activity is well under way, if not completed. All neural activity occurs extremely rapidly, but a reflex is the fastest mode of response because the impulse is not transmitted up the spinal cord to the brain before an action occurs. Only one response is possible; no options need to be considered.

Muscle spindles

Now that we have covered the basics of reflex activity, we can look more closely at two reflexes that help control muscle function. The first involves a special structure: the muscle spindle.

The muscle spindle lies between regular skeletal muscle fibers, referred to as extrafusal(outside the spindle) fibers. A muscle spindle consists of 4 to 20 small, specialized muscle fibers called intrafusal(inside the spindle) fibers and the nerve endings, sensory and motor, associated with these fibers. A connective tissue sheath surrounds the muscle spindle and attaches to the endomysium of the extrafusal fibers. The intrafusal fibers are controlled by specialized motor neurons, reffered to as gamma-motor neurons. In contrast, extrafusal fibers(the regular fibers) are controlled by alpha-motor neurons.

The central region of an intrafusal fiber cannot contract because it contains no or only a few actin and myosin filaments. Thus, the central region can only stretch. Because the muscle spindle is attached to the extrafusal fibers, any time those fibers are stretched, the central region of the muscle spindle is also stretched.

Sensory nerve endings wrapped around this central region of the muscle spindle transmit information to the spinal cord when this region is stretched, informing the CNS of the muscle’s length. In the spinal cord, the sensory neuron synapses with an alpha-motor neuron, which triggers reflexive muscle contraction(in the extrafusal fibers) to resist further stretching.

Let’s illustrate this action with an example. A person’s arm is bent at the elbow, and the hand is extended, palm up. Suddenly someone places a heavy weight in the palm. The forearm starts to drop, which stretches the muscle fibers in the elbow flexors, which in turn stretch the muscle spindles. In response to that stretch, the sensory neurons send action potentials to the spinal cord, which then activates the alpha-motor neurons of motor units in the same muscles. These cause the muscles to increase their force production overcoming the stretch.

Gamma-motor neurons excite the intrafusal fibers, prestretching them slightly. Although the midsection of the intrafusal fibers cannot contract, the ends can. The gamma-motor neurons cause slight contraction of the ends of these fibers, which stretches the central region slightly. This prestretch makes the muscle spindle highly sensitive to even small degrees of stretch.

The muscle spindle also assists normal muscle action. It appears that when the alpha-motor neurons are stimulated to contract the extrafusal muscle fibers, the gamma-motor neurons are also activated, contracting the ends of the intrafusal fibers. This stretches the central region of the muscle spindle, giving rise to sensory impulses that travel to the spinal cord and then to the alpha-motor neurons. In response, the muscle increases its form production. Thus, muscle force production is enhanced through this function of the muscle spindles.

Information brought into the spinal cord from the sensory neurons associated with muscle spindles does not merely end at that level. Impluses are also sent up to higher parts of the CNS, supplying the brain with continuous feedback on the exact length of the muscle and the rate at which that length is changing. The information is essential for maintaining muscle tone and posture and for executing movements. The muscle spindle functions as a servo-mechanism to continuously correct movements that do no proceed as planned. The brain is informed of errors in the intended movement at the same time that the error is being corrected at the spinal cord level.

Golgi Tendon organs

Golgi tendon organs are encapsulated sensory receptors through which a small bundle of muscle tendon fibers pass. These organs are located just proximal to the tendon fibers’ attachment to the muscle fibers, as shown in the figure below. Approximately 5 to 25 muscle fibers are usually connected with each Golgi tendon organ. Whereas muscle spindles monitor the length of a muscle. Golgi tendon organs are sensitive to tension in the muscle-tendon complex and operate like a strain gauge, a device that senses changes in tension. Their sensitivity is so great that they can respond to the contraction of a single muscle fiber. These sensory receptors are inhibitory in nature, performing a protective function by reducing the potential for injury. When stimulated, these receptors inhibit the contracting(agonist) muscles and excite the antagonist muscles.

Some researchers speculate that reducing the influence of Golgi tendon organs disinhibits the active muscles, allowing a more forceful muscle action. This mechanism may explain at least part of the gains in muscular strength that accompany strength training.