Know your Neurobiology: The Proprioceptive System Part 2
Proprio (ourselves) ception (perception): the awareness of our body parts, their actions, our body size and shape, our current body postures, monitoring of self-motion, our memories of movements we can undertake all of these are largely down to our proprioceptive sense.
This sense is based on sensory signals provided to the central nervous system (brainand spinal cord) from the varied proprio-receptors:
- Muscles; muscle spindles embedded in skeletal muscle fibers,
- Tendons; Golgi tendon organs which lie at the interface of muscles and tendons.
- Joints; mechanoreceptors, Raffini Endings and Pacinian Corpuscles (also found in the skin) are embedded in joint capsules.
- Ligaments; mechanoreceptors.
- Skin; stretch receptors are also considered proprioceptors.
We have about 650 skeletal muscles! For each hand we have 34 muscles to move our fingers and thumbs, 29 joints with 123 ligaments. That provides a lot of proprioceptive information. We have 42 muscles in our face. It takes fewer muscles to smile than to frown.
The proprioceptive sense begins as the muscle cells develop as early as the first 1 -3 months gestational age. Formation of the proprioceptors in muscles, tendons and joints finishes around end of puberty. We continue to receive and process proprioceptive signals until our final breath.
As with all our senses, proprioception has a wide-ranging impact on our development, our behaviours and our understanding of the social and physical world we inhabit.
We can chunk its role into three main functions: to protect, regulate, and discriminate.
These three functions can be characterised by the densities of receptor cells, the neuronal pathways as well as the structures in the brain and spinal cord to which our proprioceptive information travels.
Proprioception protects us by keeping track of our basic movements, breathing, swallowing, equilibrium reactions, noticing when there are perturbations in the status quo, instigating reflex responses at spinal, brainstem and cerebellum levels changing muscle tone, actions, speed and intensity of movements.
Proprioception also protects us by supporting our attachments to parents and others in our social group. Through our proprioceptors we copy facial and body movements, also vocalisations of those around us. In copying expressions of others early in life, we are laying foundations for our personal emotional state and our expectations of the social worlds we will inhabit. We do this first at a brainstem and limbic level. We then learn to communicate in the language and to emulate the skills of our clan by proprioceptive signals informing cortical and cerebellum structures for motor leaning, thereby also supporting our inclusion and survival prospects.
Proprioception underpins our regulation moment by moment. We communicate to both ourselves and others through proprioception about how we are and what we need in the moment.
Being happy or frightened is accompanied by bodily movements and muscle tension, influencing timing and sequencing of movements. Our movement experiences (proprioception) and emotional expression become linked. We know that approach movements facilitate positive emotions, avoidance movements reinforce negative emotions.
Proprioceptive signals make their way to the Insula Cortex where our movements are integrated within the emotional and motivational context. In this way, bodily-expressed emotions are grounded and understood within us and we gain for ourselves empathy and social understanding which enables us to seek and provide regulation strategies.
By gaining proprioceptive feedback from our emotional actions we learn what actions satisfy our perceived need. It is easy for us to form behavioural habits in this way.
Movement also regulates us as it reduces levels of the body’s stress hormones, adrenalineand cortisol. It also stimulates the production of endorphins. Firing up our proprioceptors really does help us feel better.
Proprioceptive discrimination is enabled by the processing of detailed proprioceptive information by our somatosensory cortex (conscious proprioception) and cerebellum (unconscious proprioception).
The cerebellum is heavily dependent on proprioceptive signals to undertake its functions. It receives signals originating from the body via the spinocerebellar tracts and from the head via the trigeminal nuclei in the brain stem. The cerebellum uses this information to understand the postural situation such as the current position of the body, our equilibrium status and the status of current muscle tension.
Proprioceptive information reaches the somatosensory cortex, via the dorsal columnmedial lemniscus pathways. Here it is combined with discriminative tactile information and then integrates with other sensory modalities to give us the multisensory perceptions, and creates libraries of memories, of our body and objects. Proprioceptive information, along with other sensations, provides the foundation knowledge for the frontal cortex and premotor cortex enabling composition of intentional motor commands. Needed when learning a new skill or we find ourselves in a new context. A copy of these commands, called a corollary discharge map (a form of proprioception) are sent to the cerebellum for integration with the current status of the body, preparing the rest of the brain for carrying out the planned movement, defining the intensity, speed and direction of travel. This enables us to appear well graded, timed and coordinated in our movements.
The corollary discharge map is a type of sensory map that the cerebellum can use as a reference point to evaluate the action that has taken place (proprioceptive feedback) to see if the action met sensory expectations.
The cortex and cerebellum along with other brain structures are crucial in learning new skills. These include learning the sequence and spatial qualities of new skills, including language skills, until these become automatic. Once automatic the cerebellum can then process the proprioceptive information and respond at this subconscious level alone. Walking or riding a bike are good examples of this development. The fewer proprioceptive signals we receive and integrate, the greater the challenge for learning new skills and becoming automated.
Movement, and therefore proprioception, underpin our motor, emotional and cognitive knowledge, skills and habits. We trace around letters to learn their form and link interpretation to the concept of symbols and sounds. We can have knowledge of kindness, bravery, having fun but understanding what it means to be kind, brave, to hug and ride your bike; this takes place through our actions, our movements, our acting out, receiving and perceiving the feedback. Our knowledge becomes embodied through proprioception.
Proprioception is the basis of our therapy even more than we realise. We are about reading the actions of others, creating environments and activities that build constructive actions and then habits, in our babies, our families, ourselves as practitioners. All this being based on proprioception.
In our next blog we will learn more about our vestibular sense.