Last week was a discussion of motor nerves that make the body move. This week, we dip into sensory nerves. All nerve systems work together seamlessly to keep us healthy.
How Sensory Neurons Work
Sensory nerves report information to the brain. It is a one-way communication from the body to the brain. Motor nerves respond by sending messages from the central nervous system (brain and spinal cord) to the body for movement. Motor nerves send messages in the opposite direction from the CNS to the body.
The cells of sensory nerves are in the body with branches that enter the CNS. Motor and sensory nerves work together to keep us healthy by regulating the body function. The various signals from the sensory nerves trigger functions that include movement, organ function, gland secretion and all body activity. This graphic shows a quick glimpse of how motor and sensory nerves function in the body
Sensory nerves are triggered by your environment. The senses are used as information collection mechanisms. Physical and chemical factors affect hearing, touch (including vibration), heat, and light. For example, what you hear, too loud, too soft, or just right, are physical triggers from the environment that are converted into a chemical message to the brain. The senses of smell and taste are only chemically activated. In these instances, smelling something or tasting something is a chemical reaction only.
Several senses can be activated with one stimulus. An example is eating when taste and smell of food are chemically activated. But also, your senses of hearing perhaps a crunch, the touch of the texture of food in your mouth, and the temperature of the food are physically and chemically activated.
When something is sensed from within the body or outside of the body, sensory neurons are activated. This can be any kind of sensation, from a chill in the air on the skin to an upset stomach within the body. Sensory neurons are always monitoring your body. Messages to the brain are used to make major or minor adjustments but can also include messages that everything is ok, no action needs to be taken.
There are subsections of the sensory neurons:
Somatosensory system-Responsibilities of the somatosensory system include sensations of touch, pressure, vibration, the position of the limbs, heat, and pain. There are three pathways in this system. Messages can be carried to the dorsal root ganglion, to the thalamus, or to the cerebral cortex. Mechanoreceptors respond to pressure or distortion. The mechanoreceptors are proprioceptors that note the position of the limbs, thermoreceptors note the temperature, and nociceptors note pain and temperature changes.
Spinothalamic pathway-Conduction of sensation, particularly pain, temperature, pressure, touch, tickling, and itching, is the job of the spinothalamic pathway. These messages are relayed from the receptor cells to the spinal cord to the thalamus part of the brain.
Posterior Column-medial Lemniscus Pathway-Sensations of fine touch, the position of the limbs, vibration, and the ability to identify an object by touch (stereognosis) are the responsibility of the posterior column-medial lemniscus pathway. Sensory receptors connect to the medulla oblongata in the brain and then to the thalamus, where messages are routed to the sensory interpretation part of the brain.
Visual System-Sight begins in the retina of each eye, where messages are sent to optic nerves on to the mid-brain and then to the primary visual center in the brain.
Auditory System-Sound vibrations cause the tiny hairs in the ear canal to move, which transfers an impulse to the vestibulocochlear nerve, to an area in the brainstem, which connects to the mid-brain through the thalamus to the primary auditory cortex in the brain.
All these systems and terms seem complicated and they are. To put it simply, the sensory nerves are on duty, detecting any upset, pleasure, or even normal functioning of the body both inside and outside. Inside the body, everything is monitored by the sensory nerves such as internal organ functioning, blood flow, immune responses, gland function, to name just a few. The outside of the body is monitored basically through the skin.
Messages are constantly sent from the sensory system to the brain that all is well, or something needs correction. These can be minor adjustments to major. Interneurons in the brain transmit messages from sensory nerves to the motor nerves or from motor nerves to sensory nerves. Sensory nerves indicate the status of the body, adjustments are made through the motor nerves to speed up, slow down or keep the current pace of activity in and outside of the body.
Diagnosis
The sensory system is evaluated for function by using several testing methods. A neurological examination assesses the senses by presenting objects for activation of each of the individual senses. Your eyes are closed as you identify things that have odor, sound, taste, and touch. An eye examination assesses vision.
Visualization of nerves can be seen on a CT scan or MRI. The function of sensory nerves is assessed using electromyogram (EMG) and/or nerve conduction studies (NCS).
Particularly for individuals with spinal cord injury from medical or traumatic causes, the ASIA or AIS examination is used. Part of the examination assesses motor nerve function, but other assessments are of sensory function. You probably are familiar with this examination of differentiation between pinprick and cotton tip at each dermatome of the body. Every dermatome is controlled by branches of a sensory nerve. A copy of the exam worksheet is found here: https://www.asia-spinalinjury.org/wp-content/uploads/2019/10/ASIA-ISCOS-Worksheet_10.2019_PRINT-Page-1-2.pdf
Symptoms and Treatment
Individuals with some medical conditions may note a gradual decrease in sensation in parts or levels of their body over time. Trauma typically produces a sudden onset of decreased or lost sensation. Sensory nerve loss typically follows the same body or dermatome pattern as motor nerve function loss but not always. You could have slight sensory differences from your motor level of injury. The ASIA level is determined by the level of full function. There can be a determination of ‘zone of partial preservation’ (ZPP), indicating a difference in sensory and motor function or a difference on either side of your body.
Some spinal cord injury syndromes produce differing outcomes. Anterior cord syndrome results in loss of motor, pain and temperature sensations on both sides but does not affect vibration and proprioception. Central cord syndrome results in motor and sensory issues more in the upper extremities than in the lower extremities. Brown Sequard Syndrome results in loss of motor, vibration, and proprioception on one side of the body. The loss of pain and temperature sensation on the opposite side of the body. The patterns of these syndromes follow the subsection pathways of sensory nerves listed above.
Treatment for sensory nerve injury is similar to motor injury management. Activity or movement needs to be provided to the level of your body below injury. This can be passive, either by you moving your body or having someone else move it; active, moving your body under your own power; or having the power for movement supplied through electrical stimulation.
An interesting finding from the Christopher Reeve Recovery project is that with activity, sensory nerves recovered quicker than the motor nerves. With the project, Christopher Reeve redeveloped sensation throughout his body. This gave him the ability to alert staff that he required a position change, which decreased secondary complications of spinal cord injury. More importantly, to him, he could feel the hugs of his family. That meant everything to him.