How We Perceive Physical Sensations and Pain

by Siha Hoque

Even when doing something like simply sitting, we are experiencing a myriad of different sensations all over the body caused by our environment and how we physically perceive it, some more noticeable than others. They include the obvious, like temperature, pressure, perhaps pain, light touch, itches, textures, the list goes on. Yet how exactly does our body pick up on all these interactions?

The short answer to that question is our nervous system, specifically sensory receptors, located in almost every single layer of tissue in our bodies. Sensory receptors are the endings of neurons, and they can respond to a variety of external stimuli with protein reactions and transmit the message of said stimuli to other parts of the body nervous impulses (electrical signals). These signals are all identical to each other - so why doesn't stretching your hand and squeezing it into a fist feel the same? This is because the brain can distinguish between the type of cell transmitting the nervous impulse and where in the body it is coming from. Sensory receptors are responsible for the five key senses in the body, as well as many different types of touch. 

Sensory receptors can be classified based on where they are in the body and what they are specialised for detecting. Mechanoreceptors detect physical changes, such as touch and stretch. Many of these are located in the skin, specifically the second layer or the dermis, which is below the epidermis. Places such as our toes, fingers and lips have adapted to have a lot of these, because they are more sensitive to fine details and edges. Muscle spindles or stretch receptors surround numerous organs. In our lungs - which is how we can tell when we have inhaled deeply or shallowly, as well as in our stomachs - helping us recognise when we are full. These receptors also span throughout our tendons and hands, helping us pick objects up and put them down, and coordinate how to move others, like when catching a ball or manoeuvring a tennis racket. 

Our sensory receptors can even adapt to our environment, making us less reactive to common or repetitive stimuli. The more frequently our receptors detect something, the less nervous impulse they produce in response - and a common example of this is smell, which is why most people cannot ‘smell themselves’ in general. This stops our sensory system wasting energy producing signals to parts of our environment which do not appear to be harming us in any way.

Pain is more complex. It is a sensation that keeps us safe by being inherently unpleasant and therefore making us at least want to stop the action responsible for it - without pain we wouldnt know when something is going wrong in our bodies, putting us at risk of injury or sickness. The sensory receptors that detect damaged tissue or the potential for it are known as nociceptors. We can categorise nociceptors further based on the sort of pain they respond to: mechanical nociceptors respond to injuries like cuts and bruises; chemical nociceptors react to poisons and acids. Once we have been injured, however, the pain tends to remain at the site of the injury (an adaptation that helps us protect the wound and prevent infection or further damage) because of inflammation, which releases numerous chemicals that continue to stimulate the receptors releasing nervous impulses. 

Our body’s nerve cells can communicate or pass signals from one to another using electrical impulses. These impulses travel to the tip of the bead-like structures known as axons, and trigger the release of neurotransmitters, which are chemicals that can send messages between cells. They pass through the distance between two nerve cells, known as synapses and to the receptors of the next nerve cell, where the process repeats - similar to people running a relay race. Overall, neurons send messages to and from the brain, travelling through the spinal cord to nerves all over the body, forming a network. The brain and spinal cord are referred to as the central nervous system (or CNS).

The thalamus of the brain picks up the input from our nerves, before redirecting it to the cerebral cortex (outer layer of the brain) where it will be processed further. Its connections to the cerebral cortex range in all directions. It is an oval-like structure in the centre of the brain, made up of different types of closely packed clusters of neuronal cell bodies, called thalamic nuclei. 

To summarise, we can perceive a variety of physical sensations due to the receptors within our nervous system detecting them and sending messages of these to the brain, which can analyse the data and send messages back telling us how to respond accordingly. The nervous system is a complex structure, forming key aspects of our consciousness, giving us the ability to acknowledge temperature, light, sensation, and much more.