Sensory System – Anatomy, Types, Structure, Functions

The sensory nervous system is a part of the nervous system responsible for processing sensory information. A sensory system consists of sensory neurons (including the sensory receptor cells), neural pathways, and parts of the brain involved in sensory perception. Commonly recognized sensory systems are those for vision, hearing, touch, taste, smell, and balance. In short, senses are transducers from the physical world to the realm of the mind where we interpret the information, creating our perception of the world around us.^[rx]

Humans can perceive various types of sensations, and with this information, our motor movement is determined. We become aware of the world by way of sensation. Sensations can also be protective to the body, by registering environmental cold or warm, and painful needle prick, for example. From the soft touch of the child to the painful punch of a boxer, all the daily activities carry associations with sensations.[rx]

Broadly, these sensations can classify into two categories. First, general sensations which include touch, pain?, temperature, proprioception, and pressure. Vision, hearing, taste, and smell are special senses which convey sensations to the brain through cranial nerves. In this activity, the discussion will be limited to general sensations.

Bodily touch can be fine touch or deep touch; the differentiating factor is the receptors which are stimulated by the touch. Long-standing sitting or pressure over any body part can be called a sense of pressure. High-frequency vibrations can be perceived by our bodies so that we can walk and perform fine movements. When one goes in hot or cold places or as the ambient temperature changes, we register the temperature because of thermoreceptors. They are useful for protection against very high or very low temperatures because, during that time, the nervous system registers pain?. To walk or to move, the brain has to know about the position of different joints and muscles perceived via proprioception. We all are aware of pain sensation. Though it is a “negative” perception, it is vitally important; only by becoming aware of the noxious response, we can remove the initiating stimulus.

All these sensations begin with skin receptors and get conveyed through spinal neurons to the brain.

Sensory Systems, Modalities, and Cell Types.

Adapted from Kandel et al. (2000) Principles of Neural Science, McGraw-Hill, New York, N Y. p. 414.

Primary Sensory Coding

Sensory systems code 4 aspects of a stimulus:

• Stimulus Type (modality) – All receptors of a single afferent neuron are sensitive to the same type of stimulus.
• Stimulus Intensity – An increased stimulus results in a larger receptor potential,, leading to a higher frequency of action potential. Stronger stimuli also affect a larger area and recruit a larger number of receptors.
• Stimulus Location – Coded by the site of the stimulated receptor. The precision of location, called acuity, is negatively correlated with the amount of convergence in ascending pathways, size of the receptive field, and overlap with adjacent receptive fields. The response is highest at the center of the receptive field since receptor density is the highest there. Using lateral inhibition, a process by which information from neurons at the edge of a stimulus is inhibited, acuity can be increased.
• Stimulus Duration – Rapid adapting receptors respond rapidly at the onset of stimulus but slow down or stop firing during the remainder of a stimulus (they adapt quickly). They are important in signaling rapid change. Slow adapting receptors maintain their response at or near the initial level of firing through the duration of the stimulus and are important in signaling slow changes.

Cellular

Receptors – Sensory receptors become activated by stimuli in the environment by receiving signals. The transmission of any message in the neurons of our body requires it to be in the form of an action potential; the sensation must undergo conversion into electrical signals. The structures which convert mechanical signals into electrical signals are receptors.  Receptors get classified based on the type of stimulus activating them. The following are common types of receptors

Mechanoreceptors

• Activated by changes in pressure
• Common mechanoreceptors include
• Pacinian corpuscles in the subcutaneous tissue
• Meissner corpuscles in non-hairy skin
• Baroreceptors in the carotid sinus
• Hair cells on the organ of Corti and in the semicircular canals

Photoreceptors

• Activated by light
• Rods and cones (located in the retina)

  • Rods are sensitive to low-intensity light and function better in a dark environment
  • Cones have a better threshold for light, therefore function well in daylight. They also participate in color vision.

Chemoreceptors

• Activated by chemicals
• They serve for olfaction and taste

Thermoreceptors

• Located in the skin and include cold and warm receptors. These are present in the skin.

Nociceptors

• These become activated by extreme pressure, temperature, or noxious chemicals. These are also in the skin.

There are different types of receptors present into skin or muscles for all modalities of senses.[rx]

1) Touch

Human skin divides into hairy and nonhairy or glabrous skin. This classification is associated with different touch receptors inside the skin. There are four types of touch receptors. Those which are slowly adapting called Merkel cells (slowly adapting type 1) and Ruffini corpuscles (slowly adapting type2). Those with Pacinian afferents called Pacinian corpuscles and rapidly adaptive ones called Meissner corpuscle.[rx]

• Merkel’s disc – Structurally simplest among all are Merkel cells which are present in the basal layer of the epidermis. These are located in non-hairy skin. Present in a high amount at the fingertips, they are sensitive selectively to the particular component of stress or tendon?. সহজ বাংলা: মাংসপেশি/টেনডনে টান।" data-rx-term="strain" data-rx-definition="A strain is injury to a muscle or tendon. সহজ বাংলা: মাংসপেশি/টেনডনে টান।">strain?, and because of that they are more selective in detecting corners, edges and curvatures; this is useful for reading Braille, etc.
• Ruffini nerve endings – These types of receptors are less densely placed and therefore have less sensitivity. These receptors are located in hairy skin. They are more sensitive to stretch, so become stimulated during stretching of the skin. An example is stretching during motion and for the direction of force detection, along with muscle spindle for hand shape and finger position perception, etc.
• Pacinian corpuscle – Distributed throughout the palm and finger, the Pacinian corpuscle is the most sensitive receptor type. These are large, onion-like layered structures enclosing a single nerve ending. Pacinian corpuscles function as a mechanical filter to protect from very large, low-frequency stress during manual labor.
• Meissner corpuscle – They are most sensitive to dynamic changes in the skin, and they are relatively insensitive for static changes of the skin.

One distinguishing characteristic of each receptor is the degree of adaptation.

• “Very rapidly adapting” such as Pacinian corpuscle
•  “Rapidly adapting” such as Meissner corpuscle and hair follicles
• “Slowly adapting” includes Ruffini corpuscle, Merkel receptors, and tactile discs

 2) Thermal receptors

These are slowly adapting receptors which can detect changes in skin temperature. These may be cold or warm. These receptors have some baseline firing rates. Cold receptors are sensitive between 10 and 32 degrees C. Firing at a baseline rate during 30 to 35 degrees C, firing rate increases in cold while it decreases when the temperature increases. In the same way, warm receptors have baseline firing around 38 degrees C and increase with an increase in temperature.[rx]

3) Vibration

Vibration sensations are useful in the performance of balancing tasks, along with proprioception. Vibrational sense perception is by Pacinian corpuscles and Meissner corpuscles because both of these receptors are sensitive to low-frequency vibrations.[rx]

4) Pain?

Noxious stimulation is converted into an electric signal by unencapsulated nerve endings that terminate in the dermis and epidermis. Noxious stimuli such as intense hot or cold, or long-standing pressure cause activation of free nerve endings; this requires threshold stimulation to activate the endings, but once activated, signals are transmitted continuously.[rx]

5) Proprioception

The Golgi tendon? organ senses the position of joints and joint movement near attached muscle tendons attached, and by muscle spindles present inside the extrafusal muscle fibers. They fire signals when stretched.

Organ Systems Involved

Types of fibers.

Once perceived by the respective receptors, sensations travel in nerve fibers to reach the sensory cortex. Different sensations travel in different nerve fibers. Erlanger and Gasser classified these nerves, for which the two scientists won a Nobel Prize. They differentiated nerve fibers according to their diameter and conduction velocity, which is famously known as the  Erlanger Gasser classification.[rx]

• Group I or A-alpha fibers: Ia from primary endings of muscle spindles (proprioception); Ib from Golgi tendon? organs (proprioception)
• Group II or A-beta fibers: from secondary endings of muscle spindles (proprioception); from specialized receptors in the skin and deep tissues (touch, pressure)
• Group III or A-delta fibers: from free and specialized endings in muscle and joints (pain?); from the skin (sharp pain, heat, cold, and some touch and pressure); visceral afferents
• Group IV or C fibers from skin and muscle (slow-burning pain?); visceral pain

After receiving changes in the sensory receptors, sensations are carried away by the above type of sensory nerves.

These sensory nerves emerge from the dorsal root ganglion, which conveys the sensations from the peripheral nervous system to the central nervous system. They are pseudounipolar cells that have one polar fiber, which gets divided into two parts; one receives the changes from sensory receptors, and the other one enters into the spinal cord to terminate over a spinal segment or in the medulla.

Mechanism

Pathways

Spinal sensory nerves carrying signals from receptors to the sensory cortex have a particular arrangement in the spinal cord. They transmit signals by two pathways; these are the spinothalamic pathway and the dorsal column pathway. [rx]

1) Spinothalamic pathway

This is a sensory pathway carrying pain?, temperature, touch, and pressure sensations. It divides into the anterior and lateral spinothalamic tracts. The anterior spinothalamic tract transports crude touch sensations and pressure. The lateral spinothalamic tract is associated with pain and temperature sensations.

Peripheral sensory fibers are first-order neurons. After receiving signals, they enter the spinal cord to end over the respective spinal segment on the same side. Second-order neurons cross the spinal cord and then ascend on the contralateral side, ending in the thalamus.

2) Dorsal column pathway

Sensations of touch, vibrations, and proprioceptions, which are carried by the respective sensory nerve, ascends by the dorsal column pathway from the spinal cord to the thalamus. In contrast to the spinothalamic pathway, the dorsal column pathway does not end over the respective spinal segment. After entering into the spinal segment, first-order neurons ascend on the same side up to medial lemniscus, where they cross the midline and terminate in the medulla. Second-order neurons ascend from the medulla, ending on the thalamus. Because they cross at the medial lemniscus, this pathway is also called a medial lemniscus pathway.

Sensory cortex

Sensory signals from the thalamus are carried by third-order neurons that travel from the internal capsule and end in the sensory cortex in the 4th layer. This is present in the parietal lobe in the postcentral gyrus at Brodmann areas 3,1 and 2. Area 3 further subdivides into 3a and 3b.  As with the motor cortex, sensory information is also topographically represented in the sensory cortex where the foot, leg, trunk, forelimbs, and face are represented from medial to lateral in sequence. In animal studies, area3b and area 1 are associated with cutaneous stimuli, while area 3a is associated with proprioception. Area 2 responds to both tactile and proprioception stimuli.