Perception in Psychology

In daily life, humans interact with the environment and collect information, which is later interpreted to determine their interaction with the world. According to Urdapilleta & Dacremont (2006), perception is defined as the process of organization, interpretation, and conscious experiencing sensory information through bottom-up and top-down processing of information. The development of perceptions from sensory input is referred to as the bottom-up processing, whereas the top-down processing involves the psychological interpretation of sensations that is guided by knowledge, thoughts, and experiences (Démuth, 2013). On the other hand, proprioception is defined as the sensational perception of movement, location, and body-part actions. Thus, proprioception relates to the control of body movements and positions, which is facilitated by the interpretation of feedback from tendon organs and muscle spindles. Body movement signals generated by proprioceptors are essential in enabling humans to respond to the environmental circumstances. The body acts as a stimulus in proprioception to enhance normal functions, such as breathing and circulation of blood oxygen through arteries. This paper will explore the structure and description of the three sensory receptors involved in proprioception.

Muscle Spindles

Muscle spindles are found in most skeletal muscles, where they are concentrated in fine movement muscles such as fingers and eyes and less in large muscles, such as the legs and arms. They comprise 4-8 intrafusal muscle fibers that are surrounded by a capsule of connective tissue and they are located parallel with the extrafusal fibers (Blecher et al., 2018). Muscle movements or stretching creates tension, which in turn activates ion channels and results in action potentials. Spiral terminals or primary endings of a large sensory neuron, or the type Ia axon, are attached around the middle of the intrafusal fibers, whereas type II axon or smaller sensory neuron is attached to one side of the spiral terminals, which form the secondary nerve endings (Purves et al., 2008). The two types of nerve endings act as the muscle stretch sensors that alert the brain by sending feedback about muscular contractions and changes in movement length (Gonzales & Goble, 2014). Muscle stretching creates multiple nerve impulses from the primary and secondary nerve endings during muscle stretching, whereas a trickle of impulses is generated by the spindles at rest (Lackner, 1988).

Primary nerve endings act as position and movement sensors resulting from muscle vibrations, while length change is sensed by the secondary nerve endings that function as position sensors without responsiveness to vibrations. The presence of frictional muscle stiffness at the beginning of stretch results from the presence of long-lasting stable cross-bridges between the myosin and actin contained in the sarcomeres of the intrafusal and extrafusal muscle fibers, in a process called thixotropy. Muscular relaxation after a contraction leads to the formation of stable cross-bridges as defined by the length for the acquirement of short-range elastic component (SREC) (Proske & Gandevia, 2012). Contextually, resting muscles lie taut at long lengths and slack at shorter lengths. The restriction of thixotropic behavior occurs at the extrafusal fiber and muscle spindles and thus, its effect is only felt at short and intermediate muscular lengths (Purves et al., 2008). Therefore, muscle spindles are primary kinesthetic receptors, also known as the sensors of movement and position, as proven by the illusion of limb movement and position displacement through biceps or triceps vibration. For instance, a person who is hand-grabbing his/her nose experiences the illusion that the nose is becoming elongated and the hand tends to move away from the face after the vibration of biceps tendons.

Golgi Tendon Organs

The Golgi tendon organs are surrounded by encapsulated nerve endings and they are located near the muscle-tendon junction and connected in series to the extrafusal skeletal muscle fibers. They also contain numerous terminal branches or small swellings linked with the collagen tendon fibers (Blecher et al., 2018). Golgi tendon reflex prevents the application of excessive tension to tendons, which connect the muscles to bones, during muscular contraction. As the muscular contractions increase, the tendons stretch further due to high tension, which in turn stimulates action potentials in sensory neurons within the Golgi tendon organs. During the Golgi tendon reflex, sensory neurons are released into the spinal cord through the synapse with inhibitory interneurons that contain alpha motor neurons. Then, inhibitory neurotransmitters are released for the reduction of the number of action potentials before the reciprocal innervation of the muscle, which is attached to the Golgi tendon organ, by the alpha motor neurons (Purves et al., 2008). This results in the activation of the antagonistic muscles and relaxation. There are 10-20 innervated tendon strands, which are attached to muscular fibers, in an archetypal tendon organ. Golgi tendon organs act as stretch sensors due to their sensitivity to muscle contraction that generates nerve impulses in an increment-decrement pattern. According to Chen et al. (2019), the capturing of spatial attention by distractor objects leads to a slow response to targeted movements, thereby causing a human error.

Joint Receptors

Traditionally, joint receptors were perceived as the primary sensors of limb positioning and movement, though they act as finger position sensors and play minimal roles in limb proprioception. In combination with the cutaneous signals generated by the skin and muscle spindles, joint receptors play a protective role in finger positions through the identification of extreme positioning (Proske, 2015). According to Gonzales & Goble (2014), the use of dual agonist-antagonist tendon vibration in the degradation of proprioception on an adjacent joint is temporary due to proprioceptive bias. However, the elimination of the vibration stimulus results in increased uncertainty in limb positioning, which may be caused by the limited capacity of the central nervous system (CNS) in the integration of the stochastic-predominant sensory input. Joints contain three encapsulated tendon stretch receptors and one non-encapsulated sensory ending that is highly responsive to excessive pain and movement. Lackner (1988) asserts that somatosensation neural maps can be modified through sensory input alterations, such as the use of muscle vibrations as stimuli in the generation of proprioceptive misinformation about limb positioning. Contextually, the apparent body orientation can be altered through the strategic positioning of hands or feet for the creation of systematic perceived bodily distortions. The reception of inappropriate patterns of visual feedback about stepping movements creates misperception of the frequency, amplitude, and direction of voluntary body movement (Purves et al., 2008).

According to Purves et al. (2008), sensory modalities, such as body position or proprioception, temperature, and pain, are generated by the receptors and processing centers existing in the somatosensory system. The sensation is classified as exteroception, which entails the human perception of the external world, interoception that entails the sense of stimuli emanating from the inside of the body, and proprioception, which is the sense of relative position in various neighboring body parts (Proske & Gandevia, 2012). When proprioception is lost or imbalanced, patients have challenges in the calibration of hand position, the sustainability of constant muscle force movements, control of multi-joint movements, the performance of targeted movements, discrimination of object weights, and the production of coordinated gait patterns. Therefore, proprioception is an essential psychological process that relies on muscle spindles, Golgi tendon organs, and joint receptors.