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Apr. 16, 2021 | Friday
Editorials and Opinions
Dr. Brown: Complexities of sensation felt and ignored
Dr. William Brown

Dr. William Brown is a professor of neurology at McMaster University and co-founder of the Infohealth series at the Niagara-on-the-Lake Public Library.  ________________________________________

We learned last week that much of the brain is not an equal access-control system. Some regions receive far more of the brain’s resources than others.

For example, those regions of the neocortex linked to facial expression, speech articulation and fine motor control of the hand are associated with much larger areas in the primary motor cortex compared to the proximal arm, shoulder, trunk and leg on the opposite side.

A similar unequal allocation of neocortical resources holds true for sensation. For instance, the face, mouth and hand claim much larger areas in the primary sensory neocortex compared to the rest of the arm and body.

This skewed allocation is well illustrated by the large number of touch-sensitive receptors in the flexor tips of the fingers and thumb. These exceed by far the densities of similar receptors elsewhere in the body except for the lips, which are similarly sensitive to touch.

Most touch receptors in the flexor surfaces of thumb and fingertips (Merkel’s discs and Meissner’s corpuscles) have small receptive fields of the order of few square millimetres, while other, deeper receptors (Pacinian corpuscles) have larger receptive fields and are well-suited for conveying high-frequency signals to the central nervous system.

Two-point discrimination – the ability to distinguish two separate points, is about 2-5 millimetres in the flexor tips of the fingers and thumb, 10-20 millimetres for the toes and sole of the foot, and 30-50 millimetres for the trunk. Hair follicles and the receptors that surround them are very sensitive to touch.

Similar unequal resource allocations play out in the central nervous system where information of higher priorities, such as that from the fingers and thumb, face and lips enjoy richer connections at all levels.

Diseases that affect low threshold light-touch receptors are associated with loss of light-touch sensation and two-point discrimination as well as tingling and numbness, whether the lesion(s) involve the primary sensory neurons and fibres or the related connections in the spinal cord, brainstem, thalamus and primary sensory cortex.

Sometimes if the lesion involves the posterior part of the spinal cord in the neck, patients report that when they flex their neck, they experience an "electric-shock" sensation in their arms, trunk and legs – a sensation caused by traction on the large sensory nerve fibres, which convey light touch, joint position and vibration sensation, and all of which are conveyed in the back of the spinal cord (Lhermitte’s sign).

However, there’s more to sensation than touch. What about muscle sense? Muscles have their own receptors, which provide the spinal cord, cerebellum and neocortex with information about the length of muscles and the rates with which muscles shorten or lengthen (muscle spindles) and receptors in tendons (Golgi tendon organs) that provide information about tension in muscles.

Muscle spindles and their central connections in the spinal cord, cerebellum and neocortex are vital for normal balance and co-ordinated movement. And diseases that affect these systems at any level are associated with loss of balance and impaired co-ordination, which are most striking when the cerebellum is affected.

But unlike light-touch and pain sensations, we have no conscious awareness of the information provided by muscle receptors, except when we trick them by vibrating selected muscles. If done when our eyes are shut, it can create bizarre illusions about the position of the vibrated arm or leg.

What about pain? Pain is mediated by small nerve fibres whose endings, unlike those of low-threshold touch pressure receptors, are devoid of any capsule. Pain fibres are found almost everywhere in the body.

One type is associated with sharp pricking pain and the other with deep aching and more diffuse pain. That’s an oversimplification, of course, but works in practice and broadly describes the nature of the pain associated with each type.

Lesions affecting this system, such as those that involve one side of the spinal cord, are associated with loss of pain and temperature sense on the side opposite the lesion and loss of touch-pressure, joint-position and vibration senses and weakness, all on the same side of the spinal cord as the lesion – the Brown-Sequard syndrome.

Much of the parietal lobe is tasked with integrating sensory information of all types and lesions that affect the non-dominant hemisphere may be associated with loss of awareness of the entire left side of the body, including vision.

That was the case with my father, who had an ischemic stroke that involved his right (non-dominant) parietal lobe. It was associated with lack of awareness of his left leg such that often he left the leg behind when he tried to climb low fences and had to be reminded to lift the leg over the fence before continuing on.

Usually we’re aware of light-touch sensation only when, for example, we first sit down but within seconds are completely unaware of any continuing sensation on the backs of our thighs, buttocks and back.

Similarly, unless there’s some movement to alert us, much of what goes on in our visual fields except for the central 10-20 degrees of our vision, is neglected.

In short, sensations, unless alarming or otherwise important, are ignored or even actively suppressed, to allow us to get on with whatever occupies our attention and thus consciousness – the subject of the fifth session in the BRAIN series at the NOTL Public Library on March 31.

 

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