The nervous system helps an organism understand the environment around it and helps the organism respond and interact within its world. The most fundamental portion of the nervous system is its sensory system. This system consists of the five main senses—audition (hearing), gustation (taste), olfaction (smell), touch, and vision (sight)—as well as complex senses such as balance, interoception (also written as enteroception; sensory information from the walls of hollow internal organs), and proprioception (limb position).
Anatomical Organization of a Sensory System
The system consists of a network of neurons that work together to sense and then to process this information. The network of neurons will be described in three different divisions called first-order neurons, second-order neurons, and third-order neurons.
To begin with the sensing portion of the system, it involves a somatic (body) receptor that is within the skin, bones, muscles, joints, eyes, and ears. These specialized receptors will pick up sensory information and begin a nerve impulse. The receptor is part of the sensory neuron, which is considered the first-order neuron or primary afferent neuron for sensory information. This neuron comes from the periphery and will travel the afferent pathway (toward the central nervous system). For most of the sensory neurons, their cell body (or soma) is located in the dorsal root ganglion (a group of cell bodies outside of the central nervous system) just lateral of the spinal cord. For sensation to the head, neck, and face, these nuclei (a group of cell bodies inside of the central nervous system) are found in the brainstem and are named for the cranial nerve that carries the sensory and/or motor information to the central nervous system.
The second-order neuron is an interneuron. This secondary neuron will receive information from the first-order neuron’s synapse, where neuronal information is transferred from one neuron to another. Some of these second-order neurons may send their axons to cross over to the other side of the spinal cord or brainstem before carrying the information to the thalamus, which is a deep relay structure within the brain. All second-order neurons will synapse in the thalamus, except for the sense of smell.
Lastly, the third-order neuron is located in the thalamus, which is made up of several different nuclei. The second-order neuron synapses on the third-order neuron. Here the sensory information is transferred and then carried to the correct sensory area of the cerebrum, such as the primary visual cortex for vision and the somatosensory cortex for pain, temperature, touch, and proprioception.
In the brain the somatosensory system involves the thalamus, reticular formation, and the postcentral gyrus, which is found posterior to the central sulcus in the parietal lobe of the cerebral cortex. Along the surface of the postcentral gyrus, the body is mapped from the midline to the temporal lobe. This means that a specific body region is located in a specific region of the postcentral gyrus. This map is called a homunculus, meaning “little man.” The purpose of the thalamus is to act as the relay center from the spinal cord or brainstem to the homunculus. This is the same as the reticular formation, but the reticular formation acts as a relay for the spinal cord or brainstem to the thalamus. The postcentral gyrus is the main processing center of the somatosensory system. It processes the sensory impulses received from the thalamus. If a reaction is necessary, such as to move away from a heat source, this sensory information in the cerebral cortex will be transferred to the motor cortex to respond.
Sensory System Pathways
The somatosensory system has specific ascending pathways that are dependent on the information carried and how they ascend. It is important to note that the terms “tract” (a group of fibers or axons) and “pathway” may be interchangeable, and that these tracts are named by where they are located in the central nervous system and/or where they originate and then terminate. One of the pathways is the anterolateral tract. In these spinothalamic pathways, the first step is the first-order neuron. It carries the sensory impulse for touch, pressure, pain, and temperature. The neuron’s axon enters the spinal cord and synapses in the posterior gray horns. The secondary neuron then decussates (crosses) the impulse by the ventral white commissure to the contralateral (opposite) side where it ascends to the thalamus. The thalamus then directs the signal to the correct area.
There are three distinct pathways within the anterolateral tract. They are the (1) lateral spinothalamic tract, (2) anterior spinothalamic tract, and (3) spinoreticulothalamic tract. The lateral spinothalamic tract is a pathway that carries pain and temperature information. The cell body of the secondary neuron is located in the dorsal horn of the spinal cord. It will decussate via the ventral white commissure and then ascend within the lateral funiculus. Upon reaching the posterolateral nucleus of the thalamus, it will then project the impulse to the postcentral gyrus of the cerebrum. The anterior spinothalamic pathway is much like the lateral pathway except it carries crude touch sensory information. Additionally, instead of ascending through the lateral funiculus the anterior spinothalamic tract will ascend through the anterior funiculus. Finally, the spinoreticulothalamic tract is different as its cell bodies are located in the substantia gelatinosa, a more anterior and slightly medial region of the dorsal horn. These then decussate to the spinoreticular tract and ascend to the reticular formation in the brainstem. From there the impulse is sent to the thalamus. This is different from all the ascending pathways because it synapses into the reticular formation in the brainstem rather than going straight to the thalamus.
Another pathway is the posterior column or lemniscus pathway of the spinal cord. This pathway carries highly localized information to the central nervous system. One part of this pathway, the fasciculus gracilis, sends impulses that perceive fine touch, proprioception, vibration, and pressure from the inferior half of the body (legs and trunk) to the central nervous system. The first-order neuron synapses in the nucleus gracilis, where it will then decussate and ascend to the thalamus. The other part of the posterior column—the fasciculus cuneatus—carries similar information to the central nervous system, but these are sensory impulses from the arms and upper body. The fasciculus cuneatus will go through the nucleus cuneatus before decussating and ascending to the thalamus.
Since these pathways involve multiple relay stations, they can be vulnerable to injury. If there were damage to an area anywhere along these pathways, it could lead to the loss of sensation. However, these paths act as a type of anastomosis of the nerves and will provide backup routes for pain and temperature impulses, as these sensations are important for survival. For example, if the spinal cord were damaged on one side, the person would still be able to feel pain and possibly temperature. However, if the spinal cord were damaged completely through, the person might not be able to sense anything.
Balance and equilibrium are senses perceived by the vestibular system, which detects the position and movement of the head. The main organs of the vestibular system—the semicircular canal system and the otoliths—are located within the inner ear on both sides of the head, just posterior to the cochlea of the auditory system. Each component is responsible for detecting different types of movement. The semicircular canals detect rotational movement, and the otoliths detect gravity and linear acceleration. The vestibular system is closely tied to the visual centers of the brain that control eye muscle movement as well as areas of the autonomic nervous system within the cerebellum that maintain subconscious muscle tension. When the vestibular system is malfunctioning and the subconscious muscle tension is abnormal, it may cause symptoms like motion sickness, vertigo, or uncontrolled eye movements.
Interoception is associated with autonomic motor control and provides humans with the sense of hunger, fullness (satiety), thirst, pain, and the need to breathe (air-hunger). These signals travel through the spinothalamic and vagal tracts and are represented in the brain at the dorsal posterior insula (located in the deep temporal lobe), the somatosensory cortex, and the orbitofrontal cortex. In humans, the sense of emotional awareness is represented in the right anterior insula. Interoceptive ability has been shown to vary between persons, as some individuals are more sensitive to visceral signals compared to others. Furthermore, it has been shown that the behavior to fulfill the signals (e.g., hunger, thirst) also varies between persons.
Proprioceptive stimuli are internal forces that are made by the position or movement of a body part. Proprioception is different from exteroception, which perceives the outside world; and it is also different from interoception, which perceives pain, hunger, and the movement of internal organs. Instead, proprioception uses static forces on the joints, muscles, and tendons, which keep the limb in position against the force of gravity, to denote the position of the limb. The movement of the limb is due to the changes in these forces. Proprioception is important in posture and balance, and its receptors are located in joint capsules, joint ligaments, skeletal muscles, and tendons.
Types of Sensory Receptors
Sensory receptors have been categorized into five groups: mechanoreceptors, chemoreceptors, thermoreceptors, photoreceptors, and nociceptors. Some of these different receptor types are found throughout the body or in almost every sensory system.
Mechanoreceptors are receptors that respond to a physical stimulus such as mechanical pressure or distortion. These receptors are important for discerning between different sensations such as light touch, touch, positional change (balance), and pressure. Perhaps the most notable stimuli that mechanoreceptors respond to are sound waves that physically bend the stereocilia of the hair cells in the cochlea to transmit sound as a nerve impulse.
Chemoreceptors are receptors that respond to chemical stimuli in the environment. The two prominent classes of chemoreceptors are those involved in the olfactory and gustatory systems. Chemicals such as odorants or tastants will bind to receptors in the nose (olfactory sensory neurons) or on the tongue (taste cells located in taste buds) and transmit a signal to the brain to be perceived.
Thermoreceptors are specialized sensory receptors that determine temperatures that are generally not painful (not too hot and not too cold). Thermoreceptors do not have a specific form, like a hair cell or olfactory sensory neuron, but are considered to be free nerve endings or free nonspecialized endings. These receptors are found throughout the skin, cornea (the covering of the eye), and the bladder. There are two fiber types used to sense temperature. Unmyelinated C-fibers sense warm to hot temperatures and lightly myelinated A delta fibers sense cool to cold temperatures. Nociceptors can act as thermoreceptors to determine pain from temperature, such as when a person burns his or her hand.
Photoreceptors are specialized sensory receptors that convert visible white light into the sense of vision. Specifically, these receptors absorb photons of light and transduce that absorption into a membrane potential, which activates the neuron. There are two main types of photoreceptors: cones and rods. Cone cells perceive color and need bright light to be activated. In contrast, rod cells are activated with dim lighting, as they are more sensitive to light. Rod cells are used for night and peripheral vision and have no role in color vision.
Nociceptors are specialized sensory receptors that determine when a stimulus is painful. These receptor types are generally called free nerve endings or free nonspecialized endings. Nociceptors send their signal to the brain and spinal cord so that the body can appropriately respond, such as let go of a hot handle. Nociceptors are found in all locations of the body, both internally (such as the gut, heart, joints, and muscles) and externally (like the cornea, mucosa, and skin—also known as cutaneous nociceptors). This is because pain is an important signal that should not be ignored.
Jennifer L. Hellier
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