Lab 5 – The Sensory System
Sensory System
The nervous system is divided into two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS comprises the brain and spinal cord, serving as the body’s primary control center. In contrast, the PNS includes all neurons outside the CNS, facilitating communication between the CNS and the rest of the body.
The CNS acts as integrate center, receiving sensory information and responding by sending motor signals when necessary. The PNS bridges the CNS with body organs, and it is divided into two branches: the afferent and efferent branches.
The afferent branch carries sensory information (somatic, special, and visceral senses) from the body to the CNS. The efferent branch transmits information from the CNS to effector organs. Additionally, the efferent branch is divided into he somatic and autonomic nervous systems.
The somatic nervous system consists of motor neurons that control skeletal muscle contractions. The autonomic nervous system (ANS) includes neurons that regulate the functions of internal organs and other structures. Furthermore, the ANS is divided into two branches: the parasympathetic and sympathetic nervous systems.
Neurons
The nervous system consists of two major cell types: neurons and glial cells. While glial cells make up the majority of the central nervous system (CNS), we will not cover them here for two reasons: 1) you already have enough to learn, and 2) their roles is sensory system functions are still largely unknown.
Our focus will be on neurons. Neurons, or nerve cells, are the fundamental functional units of the nervous system (NS). They are excitable cells capable of generating action potentials (Aps), which are essentially large, rapidly changing electrical signals that can travel long distances through neurons. Neurons are highly specialized cells, so we need to discuss some of their specific structures:
A neuron can be divided into three distinct functional parts:
- Cell body (soma): This part of the cell resembles a typical cell, containing the nucleus and organelles necessary for normal metabolic functions. The soma can receive inputs in the form of chemical, electrical, or non-chemical stimuli (such as light or sound). Interestingly, when most CNS neurons become fully mature (G0 stage), they lose the ability to divide because they shed their centrioles.
- Dendrites: These are small, branch-like extensions of the plasma membrane that typically branch off from the soma. Dendrites are specialized to receive inputs in the form of neurotransmitters; hence they contain many ligand-gated receptors.
- Axon (nerve fiber): These long strands, primarily composed of plasma membrane, are specialized for transmitting information. Most neurons have one main axon, but these can branch (collateral axon) to transmit information to multiple sites. The portion of the axon that extends from the cell body is called the axon hillock and is specialized to initiate action potentials (Aps). The end of the axon, known as the axon terminal, is specialized to release neurotransmitters in response to action potentials traveling down the axon.
The sensory systems, through sensory receptors, enable us to detect information from our surroundings and relay it to the brain. These systems include the somatosensory system and the special senses. The somatosensory system is associated with skin receptors and limb orientation, while the special senses encompass vision, hearing, balance and equilibrium, taste, and smell. In this lab, we will focus on the sensory systems involved in our perception of the world around us.
Sensory receptors in the skin, mouth, eyes, and nose detect stimuli and send information to the brain, where it is interpreted. Physiologists emphasize the process of sensation, which involves transducing (converting) stimuli information into electrical and chemical signals that are sent to the brain. The brain then sends signals to effect changes, maintaining homeostasis.
In a sensory pathway, a stimulus (such as light, sound, touch, taste, or smell) alters the membrane potential (Vm) by opening or closing stimulus-gated ion channels in the sensor. The sensor, often a neuron or a specialized cell that communicates with an afferent neuron, responds by generating an action potential (AP). This AP travels down the axon to the CNS where it synapses with an interneuron that directs the information to the appropriate processing location.
Perception
Perception is the process by which the brain interprets and categorizes incoming sensory information. For example, smelling a rose involves sensory receptors, neurons, the brain, and effector organs reacting to the scent (such as sneezing or leaning in for a deeper sniff). What you think or feel about the stimulus is your perception. Perhaps the rose smells wonderful and reminds you of your grandmother’s perfume-that is perception. Perception is based on sensation but also influenced by circumstances, memory, and health. For instance, the rose might elicit a negative response if you smell it while being chased by a predator, or it may smell odd if your nose is congested from allergies, making you dislike the scent.
Introduction to Our Lab
Our understanding of reality is shaped by our perceptions of the world around us. These perceptions arise from the interpretation of sensory information received through the triggering of our peripheral sensory receptors. In this lab, we will focus on the five senses: taste, touch, sight, hearing, and smell.
There are several categories of sensory receptors, including photoreceptors, chemoreceptors, thermoreceptors, mechanoreceptors, and nociceptors. When activated, these receptors send information to the brain, where the stimuli are interpreted. The process of converting sensory stimuli into electrical signals called action potentials (Aps) is known as sensory transduction.
Most sensory signals initially travel to a central sensory processing location in the brain called the thalamus. From there, they are routed to the cerebral cortex, the layer of gray matter that forms the brain’s exterior. The thalamus acts as the central gatekeeper, directing most sensations to the appropriate locations in the cerebral cortex for processing. The receptive areas for the different senses are located in various regions of the cerebral cortex.
Once a signal is received, it must be processed and interpreted. However, common phrases like “ I can’t believe my eyes” and “My mind is playing tricks on me” highlight the awareness that our senses-and our perceptions of them- are not always completely accurate. In the following series of activities, you will test the limits of your senses and explore your mind’s ability to interpret sensory stimuli.
Touch
The skin (integumentary system) is the body’s largest sensory organ, with numerous receptor sites for cutaneous sensations (form the Latin cutis, meaning “skin”). Different types of receptors in the skin respond to various stimuli, including:
- Touch and pressure: Mechanoreceptors
- Temperature: Thermoreceptors
- Pain: Nociceptors
The ability to perceive these sensations depends on which specific sensory receptors are stimulated and their pathways to the brain. The distribution of receptors varies across different body areas, with regions of high receptor density localizing stimuli more accurately than those with low receptor density.
Once a stimulus is detected by the nerve endings in the skin, the information is transmitted to the spinal cord and then to the brain, where it is interpreted. Most somatic sensations are mapped to specific regions in the parietal lobe of the brain, known as the somatosensory cortex. The amount of cerebral cortex devoted to interpreting sensations from different body areas varies and correlate with our ability to distinguish sensations in each area.
Vision
The eyes serve as the sensory organ of the human visual system, enabling the detection of light and the formation of detailed images. Light waves first pass through the cornea, aqueous humor lens, and vitreous body before reaching the retina. As light travels through the eye, it undergoes refraction, focusing a significant amount of light onto a small area of the retina. The image of the observed object is projected inverted and reversed onto the retina.
Within the retina are specialized photoreceptor cells called rods and cones, which are linked to neurons. These nerve cells transmit visual impulses to the brain via the optic nerve. The brain’s primary visual cortex, located in the occipital lobe of the cerebral cortex, then processes and interprets these stimuli.
Rods and cones within the retina serve distinct roles in the human visual system. Rods are most effective under low light conditions, enabling black-and-white vision exclusively. On the other hand, cones are responsible for color vision and require higher light levels to function optimally. Therefore, night vision predominantly relies on black-and-white perception, with muted color sensitivity.
There are three types of cone receptors, each most sensitive to one of the primary colors of light: red, green, and blue. These cones exhibit varying response times and duration. For instance, red receptors may respond more quickly than blue receptors, while blue receptors may maintain responsiveness for longer durations. When all three types of cones are activated simultaneously, the brain perceives the color white.
Distribution of rods and cones across the retina is uneven. Cones are densely concentrated in the fovea centralis, a small depression in the retina crucial for high-acuity vision. Cone density decreases gradually with distance from the fovea. In contrast, rods are sparse or absent in the fovea and increase in number towards the periphery of the retina. Consequently, peripheral vision lack color perception. Near the fovea lies the optic disk, where the optic nerve attaches this area lacks photoreceptors entirely, earning it the name “blind spot”.
Optical illusions are deceptive or misleading images that occur in the brain rather than the eye itself. The visual cortex interprets images focused on the retina, but various factors such as surrounding objects, vivid colors, pattern distortions, preconceptions, and others can lead the brain to misinterpret images. Afterimages exemplify one type of optical illusion: they occur when looking away from prolonged fixation on certain color or images. Continued stimulation of less-fatigued cones in the retina produces afterimages perceived as complementary or negative colors for up to 30 seconds.
Smell and Taste
It is hypothesized that humans can distinguish over 10,000 different smells, though describing scents is more challenging compared to describing features of other senses, like color or sound. Despite being credited with this sense, only a small patch of cells at the top of the nasal cavity in both nostrils is responsible for odor detection. This olfactory epithelium contains millions of olfactory neurons with tiny projections called cilia. These cilia have receptors that bind to specific odorant molecules in the air. When odorants bind to these receptors, the affected neuron sends a signal via the olfactory nerve to the olfactory bulb is then interpreted by the olfactory region of the temporal lobe. The sense of smell tends to adapt under continuous stimulation, resulting in decreased sensitivity to an odor, known as olfactory fatigue.
In contrast, the sense of taste is relatively limited. There are four primary taste responses: sweet, sour, salty, and bitter. Recent research suggests a fifth response, umami, which corresponds to savory tastes. Despite this limited number, humans, perceive a wide variety of flavors because most of the flavor we experience come from the odor of food molecules traveling through our nasal passages and a passage in the back of the mouth to the olfactory epithelia. Taste buds or receptors on the tongue, specific to these taste responses, are stimulated when food molecules dissolve in saliva or other solvents. Nerves transmit this information to the thalamus and then to the part of the cerebral cortex responsible for interpreting food aromas, resulting in the perception of flavor. Other factors influencing taste include food textures, preconceptions, and even our sense of hearing.
