6: Sensory Physiology - Biology

6: Sensory Physiology - Biology

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  • 6.1: Introduction
    The shark, unlike most fish predators, is electrosensitive—that is, sensitive to electrical fields produced by other animals in its environment. While it is helpful to this underwater predator, electrosensitivity is a sense not found in most land animals.
  • 6.2: Sensory Processes
    Senses provide information about the body and its environment. Humans have five special senses: olfaction (smell), gustation (taste), equilibrium (balance and body position), vision, and hearing. Additionally, we possess general senses, also called somatosensation, which respond to stimuli like temperature, pain, pressure, and vibration.
  • 6.3: Somatosensation
    Somatosensation is a mixed sensory category and includes all sensation received from the skin and mucous membranes, as well from as the limbs and joints. Somatosensation is also known as tactile sense, or more familiarly, as the sense of touch. Somatosensation occurs all over the exterior of the body and at some interior locations as well. A variety of receptor types—embedded in the skin, mucous membranes, muscles, joints, internal organs, and cardiovascular system—play a role.
  • 6.4: Taste and Smell
    Taste, also called gustation, and smell, also called olfaction, are the most interconnected senses in that both involve molecules of the stimulus entering the body and bonding to receptors. Smell lets an animal sense the presence of food or other animals—whether potential mates, predators, or prey—or other chemicals in the environment that can impact their survival. Similarly, the sense of taste allows animals to discriminate between types of foods.
  • 6.5: Hearing and Vestibular Sensation
    Audition, or hearing, is important to humans and to other animals for many different interactions. It enables an organism to detect and receive information about danger, such as an approaching predator, and to participate in communal exchanges like those concerning territories or mating. On the other hand, although it is physically linked to the auditory system, the vestibular system is not involved in hearing. Instead, an animal’s vestibular system detects its own movement.
  • 6.6: Vision
    Vision is the ability to detect light patterns from the outside environment and interpret them into images. Animals are bombarded with sensory information, and the sheer volume of visual information can be problematic. Fortunately, the visual systems of species have evolved to attend to the most-important stimuli. The importance of vision to humans is further substantiated by the fact that about one-third of the human cerebral cortex is dedicated to analyzing and perceiving visual information.
  • 6.E: Sensory Systems (Exercises)

6: Sensory Physiology - Biology

The Sensory Nervous System

The Sensory Nervous System: Internal Senses
The Internal Senses include Proprioception and inputs responsible for regulating homeostasis. Homeostasis is a state or tendency towards equilibrium.

Proprioceptors: Proprioception: Sensors that keep track of where the body is in space. The sensory nervous system includes internal monitoring systems that allow us to coordinate movement.

  • Mechanoreceptors: Proprioception is carried out by Mechanoreceptors: In the joints, Pacinian Corpuscles detect deformation of the joints In the muscles, Muscle Spindles detect stretching of the muscle fibers In the muscles where tendons connect, Golgi Organs detect stretching of the tendons.
  • Vestibular system: An aspect of knowing where you are in space is knowing your orientation One component of your ears, the vestibular system informs your brain of how your body is oriented in space.

The Sensory Nervous System: External Senses
Sight: The retina is the neural portion of the eye Photons (light) activates receptors on the retina and the signal is transported to the CNS via the optic nerve.

  • Smell: Aromatic compounds are passed over the olfactory epithelium when you breathe. The olfactory epithelium contains nerve endings that signal to the olfactory bulb and other centers in the brain.
  • Touch: Skin: Three separate kinds of nerves detect sensations on the skin 1. Mechanoreceptors: Detect pressure and tension on the skin 2. Thermoreceptors: Detect the temperature of the stimulus 3. Nociceptors: Detect painful stimuli.
  • Hearing: Detect sounds and air pressure. Organ of Corti Sound in the form of pressure waves enter the ear, pass through the middle ear and vibrate a membrane in an elegant organ called the Organ of Corti.
  • Taste: Receptors on our tongue act in concert with the olfactory system to distinguish taste. There are five basic taste receptors: Salty, Sour, Bitter, Sweet and Umami.

This tutorial discusses the organization and integration of the sensory nervous system. The sensory nervous system receives information from the environment such as touch or heat, and relays this information back to the central nervous system for processing.

Specific Tutorial Features:

A detailed description of the five, major senses and how they function within the sensory nervous system is presented.
The connection between the sensory nervous system and its interactions with the central nervous system is illustrated.

  • Concept map showing inter-connections of new concepts in this tutorial and those previously introduced.
  • Definition slides introduce terms as they are needed.
  • Visual representation of concepts
  • Examples given throughout to illustrate how the concepts apply.
  • A concise summary is given at the conclusion of the tutorial.

The Sensory Nervous System: Internal Senses

The Sensory Nervous System: External Senses

See all 24 lessons in Anatomy and Physiology, including concept tutorials, problem drills and cheat sheets: Teach Yourself Anatomy and Physiology Visually in 24 Hours


Sensory hair cells are specialized secondary sensory cells that mediate our senses of hearing, balance, linear acceleration, and angular acceleration (head rotation). In addition, hair cells in fish and amphibians mediate sensitivity to water movement through the lateral line system, and closely related electroreceptive cells mediate sensitivity to low-voltage electric fields in the aquatic environment of many fish species and several species of amphibian. Sensory hair cells share many structural and functional features across all vertebrate groups, while at the same time they are specialized for employment in a wide variety of sensory tasks. The complexity of hair cell structure is large, and the diversity of hair cell applications in sensory systems exceeds that seen for most, if not all, sensory cell types. The intent of this review is to summarize the more significant structural features and some of the more interesting and important physiological mechanisms that have been elucidated thus far. Outside vertebrates, hair cells are only known to exist in the coronal organ of tunicates. Electrical resonance, electromotility, and their exquisite mechanical sensitivity all contribute to the attractiveness of hair cells as a research subject.

Analysis of osm-6, a gene that affects sensory cilium structure and sensory neuron function in Caenorhabditis elegans

Mutation in the Caenorhabditis elegans gene osm-6 was previously shown to result in defects in the ultrastructure of sensory cilia and defects in chemosensory and mechanosensory behaviors. We have cloned osm-6 by transposon tagging and transformation rescue and have identified molecular lesions associated with five osm-6 mutations. The osm-6 gene encodes a protein that is 40% identical in amino acid sequence to a predicted mammalian protein of unknown function. We fused osm-6 with the gene for green fluorescent protein (GFP) the fusion gene rescued the osm-6 mutant phenotype and showed accumulation of GFP in ciliated sensory neurons exclusively. The OSM-6::GFP protein was localized to cytoplasm, including processes and dendritic endings where sensory cilia are situated. Mutations in other genes known to cause ciliary defects led to changes in the appearance of OSM-6::GFP in dendritic endings or, in the case of daf-19, reduced OSM-6::GFP accumulation. We conclude from an analysis of genetic mosaics that osm-6 acts cell autonomously in affecting cilium structure.

Synapse Formation in Monosynaptic Sensory-Motor Connections Is Regulated by Presynaptic Rho GTPase Cdc42

Spinal reflex circuit development requires the precise regulation of axon trajectories, synaptic specificity, and synapse formation. Of these three crucial steps, the molecular mechanisms underlying synapse formation between group Ia proprioceptive sensory neurons and motor neurons is the least understood. Here, we show that the Rho GTPase Cdc42 controls synapse formation in monosynaptic sensory-motor connections in presynaptic, but not postsynaptic, neurons. In mice lacking Cdc42 in presynaptic sensory neurons, proprioceptive sensory axons appropriately reach the ventral spinal cord, but significantly fewer synapses are formed with motor neurons compared with wild-type mice. Concordantly, electrophysiological analyses show diminished EPSP amplitudes in monosynaptic sensory-motor circuits in these mutants. Temporally targeted deletion of Cdc42 in sensory neurons after sensory-motor circuit establishment reveals that Cdc42 does not affect synaptic transmission. Furthermore, addition of the synaptic organizers, neuroligins, induces presynaptic differentiation of wild-type, but not Cdc42-deficient, proprioceptive sensory neurons in vitro Together, our findings demonstrate that Cdc42 in presynaptic neurons is required for synapse formation in monosynaptic sensory-motor circuits.

Significance statement: Group Ia proprioceptive sensory neurons form direct synapses with motor neurons, but the molecular mechanisms underlying synapse formation in these monosynaptic sensory-motor connections are unknown. We show that deleting Cdc42 in sensory neurons does not affect proprioceptive sensory axon targeting because axons reach the ventral spinal cord appropriately, but these neurons form significantly fewer presynaptic terminals on motor neurons. Electrophysiological analysis further shows that EPSPs are decreased in these mice. Finally, we demonstrate that Cdc42 is involved in neuroligin-dependent presynaptic differentiation of proprioceptive sensory neurons in vitro These data suggest that Cdc42 in presynaptic sensory neurons is essential for proper synapse formation in the development of monosynaptic sensory-motor circuits.

Keywords: DRG axon guidance motor neuron proprioceptive sensory neuron spinal cord synapse formation.


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The Senses: A Comprehensive Reference

This major new publishing event provides the first up-to-date, cutting-edge, comprehensive reference work combining volumes on all major sensory modalities in one set in three decades. Under the guidance of a distinguished team of international experts, 6 volumes collected 300 articles from all the top scientists laying out our current knowledge on the anatomy, physiology, and molecular biology of sensory organs. Topics covered include the perception, psychophysics, and higher order processing of sensory information, as well as disorders and new diagnostic and treatment methods.

Written for a wide audience, this reference work provides students, scholars, medical doctors, and anyone interested in neuroscience a comprehensive overview of the knowledge accumulated on the function of sense organs, sensory systems, and how the brain processes sensory input. Leading scholars from around the world contributed articles, making The Senses a truly international portrait of sensory physiology. The set is the definitive reference on sensory neuroscience on the market, and will provide the ultimate entry point into the review and original literature in Sensory Neuroscience, and be a natural place for interested students and scientists to deepen their knowledge.

This major new publishing event provides the first up-to-date, cutting-edge, comprehensive reference work combining volumes on all major sensory modalities in one set in three decades. Under the guidance of a distinguished team of international experts, 6 volumes collected 300 articles from all the top scientists laying out our current knowledge on the anatomy, physiology, and molecular biology of sensory organs. Topics covered include the perception, psychophysics, and higher order processing of sensory information, as well as disorders and new diagnostic and treatment methods.

Written for a wide audience, this reference work provides students, scholars, medical doctors, and anyone interested in neuroscience a comprehensive overview of the knowledge accumulated on the function of sense organs, sensory systems, and how the brain processes sensory input. Leading scholars from around the world contributed articles, making The Senses a truly international portrait of sensory physiology. The set is the definitive reference on sensory neuroscience on the market, and will provide the ultimate entry point into the review and original literature in Sensory Neuroscience, and be a natural place for interested students and scientists to deepen their knowledge.

Key Features

  • All-inclusive coverage of topics: updated edition offers readers the only current reference available covering neurobiology, physiology, anatomy, and molecular biology of sense organs and the processing of sensory information in the brain
  • Authoritative content : world-leading contributors provide readers with a reputable, dynamic and authoritative account of the topics under discussion
  • Comprehensive-style content : in-depth, complex coverage of topics offers students at upper undergraduate level and above full insight into topics under discussion


Perception is an individual’s interpretation of a sensation. Although perception relies on the activation of sensory receptors, perception happens not at the level of the sensory receptor, but at higher levels in the nervous system, in the brain. The brain distinguishes sensory stimuli through a sensory pathway: action potentials from sensory receptors travel along neurons that are dedicated to a particular stimulus. These neurons are dedicated to that particular stimulus and synapse with particular neurons in the brain or spinal cord.

All sensory signals, except those from the olfactory system, are transmitted though the central nervous system and are routed to the thalamus and to the appropriate region of the cortex. Recall that the thalamus is a structure in the forebrain that serves as a clearinghouse and relay station for sensory (as well as motor) signals. When the sensory signal exits the thalamus, it is conducted to the specific area of the cortex (Figure 2) dedicated to processing that particular sense.

How are neural signals interpreted? Interpretation of sensory signals between individuals of the same species is largely similar, owing to the inherited similarity of their nervous systems however, there are some individual differences. A good example of this is individual tolerances to a painful stimulus, such as dental pain, which certainly differ.

Figure 2. In humans, with the exception of olfaction, all sensory signals are routed from the (a) thalamus to (b) final processing regions in the cortex of the brain. (credit b: modification of work by Polina Tishina)

Scientific Method Connection

Just-Noticeable Difference

It is easy to differentiate between a one-pound bag of rice and a two-pound bag of rice. There is a one-pound difference, and one bag is twice as heavy as the other. However, would it be as easy to differentiate between a 20- and a 21-pound bag?

Question: What is the smallest detectible weight difference between a one-pound bag of rice and a larger bag? What is the smallest detectible difference between a 20-pound bag and a larger bag? In both cases, at what weights are the differences detected? This smallest detectible difference in stimuli is known as the just-noticeable difference (JND).

Background: Research background literature on JND and on Weber’s Law, a description of a proposed mathematical relationship between the overall magnitude of the stimulus and the JND. You will be testing JND of different weights of rice in bags. Choose a convenient increment that is to be stepped through while testing. For example, you could choose 10 percent increments between one and two pounds (1.1, 1.2, 1.3, 1.4, and so on) or 20 percent increments (1.2, 1.4, 1.6, and 1.8).

Hypothesis: Develop a hypothesis about JND in terms of percentage of the whole weight being tested (such as “the JND between the two small bags and between the two large bags is proportionally the same,” or “. . . is not proportionally the same.”) So, for the first hypothesis, if the JND between the one-pound bag and a larger bag is 0.2 pounds (that is, 20 percent 1.0 pound feels the same as 1.1 pounds, but 1.0 pound feels less than 1.2 pounds), then the JND between the 20-pound bag and a larger bag will also be 20 percent. (So, 20 pounds feels the same as 22 pounds or 23 pounds, but 20 pounds feels less than 24 pounds.)

Test the hypothesis: Enlist 24 participants, and split them into two groups of 12. To set up the demonstration, assuming a 10 percent increment was selected, have the first group be the one-pound group. As a counter-balancing measure against a systematic error, however, six of the first group will compare one pound to two pounds, and step down in weight (1.0 to 2.0, 1.0 to 1.9, and so on.), while the other six will step up (1.0 to 1.1, 1.0 to 1.2, and so on). Apply the same principle to the 20-pound group (20 to 40, 20 to 38, and so on, and 20 to 22, 20 to 24, and so on). Given the large difference between 20 and 40 pounds, you may wish to use 30 pounds as your larger weight. In any case, use two weights that are easily detectable as different.

Record the observations: Record the data in a table similar to the table below. For the one-pound and 20-pound groups (base weights) record a plus sign (+) for each participant that detects a difference between the base weight and the step weight. Record a minus sign (-) for each participant that finds no difference. If one-tenth steps were not used, then replace the steps in the “Step Weight” columns with the step you are using.

Table 1. Results of JND Testing (+ = difference – = no difference)
Step Weight One pound 20 pounds Step Weight
1.1 22
1.2 24
1.3 26
1.4 28
1.5 30
1.6 32
1.7 34
1.8 36
1.9 38
2.0 40

Analyze the data/report the results: What step weight did all participants find to be equal with one-pound base weight? What about the 20-pound group?

Draw a conclusion: Did the data support the hypothesis? Are the final weights proportionally the same? If not, why not? Do the findings adhere to Weber’s Law? Weber’s Law states that the concept that a just-noticeable difference in a stimulus is proportional to the magnitude of the original stimulus.

It is the largest lobe, located in front of the cerebral hemispheres, and has significant functions for our body, and these are:

 The frontal lobe has an area called Broca’s area located in the posterior inferior frontal gyrus involved in speech production. A recent study shows that the exact function of Broca’s area is to mediate sensory representations that originate in the temporal cortex and going to the motor cortex.[3]

During the past centuries, several researchers have described that there are personality changes that occurred after frontal lobe injuries. One of the most important cases was about Phineas Gage, who was a gentle, polite sociable young, man until a large iron rod went through his eye-damaging his prefrontal cortex. This injury made him emotionally insensitive, perform socially inappropriate behaviors, and was unable to make a rational judgment. A recent study suggests that when there is damage to the prefrontal cortex, there are five sub-types of personality changes that occur, and these include:

The ability to decide on something involves reasoning, learning, and creativity. A study conducted in 2012 proposed a new model to understand how the decision-making process occurs in the frontal lobe, specifically how the brain creates a new strategy to a new-recurrent situation or an open-ended environment they called it the PROBE model.

There are typically three possible ways to adapt to a situation:

Selecting a previously learned strategy that applies precisely to the current situation

Adjusting an already learnedਊpproach

Developing a creative behavioral method

The PROBE model illustrates that the brain can compare three to four behavioral methods at most, then choose the best strategy for the situation.[5]

The frontal lobe has the motor cortex divided into two regions: the primary motor area located posterior to the precentral sulcus and non-primary motor areas, including the premotor cortex, supplementary motor area, and cingulate motor areas. The exact function of each structure and its role in the movement is still an active research area.[6]

It is located posterior to the frontal lobe and superior to the temporal lobe and classified into two functional regions.

The anterior parietal lobe contains the primary sensory cortex (SI), located in the postcentral gyrus (Broadman area BA 3, 1, 2). SI receives the majority of the sensory inputs coming from the thalamus, and it’s responsible for interpreting the simple somatosensory signals like (touch, position, vibration, pressure, pain, temperature).[7]

The posterior parietal lobe has two regions: the superior parietal lobule and the inferior parietal lobule.

The second most substantial portion occupies the middle cranial fossa and lies posterior to the frontal lobe and inferior to the parietal lobe. There are two surfaces, the lateral surface and the medial surface.[9]

The lateral surface is classified by the superior temporal sulcus and the lateral temporal sulcus into three gyri the superior temporal gyrus and the middle temporal gyrus, and the inferior temporal gyrus.

The STP is located deep in the Sylvain fissure. The most significant anatomical landmark in STP is the Heschl gyrus (HG) which contains the primary auditory cortex. It is responsible for translating and processing all sounds and tones, and it is minimally affected by task requirements. Task requirement: a test where the examiner pronounces some words and asks the participant to categorize them acoustically, or phonemically, or semantically.[10]The STP has another important area next to the HG called Wernicke’s area. In the past, this area was thought to haveਊ significant role in speech perception and comprehension, but recent evidence shows that this area is not involved in this process. Researchers found that this process is not a simple task, but moreover, it is a complex task that is distributed all over the brain. The primary function of this area is the phonological representation, a process where the pronounced word is interpreted based on their tones and sound and trying to link it to a previously learned sound.[11]

The lateral surface of the STG is thought to be the secondary auditory cortex that also functions in interpreting sounds, but mostly in the activities that involve task requirements.[10] 

The Anterior MTG is primarily involved in:

The default mode network has a specific activity that exists naturally in the brain at rest. So if one is studying or engaging in a game or doing any other activity that demands stayingਏocused or setting a particular goal this mode will be deactivated.

The Middle MTG has two functions:

The Posterior MTG is thought to be part of the classical sensory language area.

The Sulcus MTG is involved in decoding gaze directions and in speech.

The medial surface of the temporal lobe (mesial temporal lobe) includes important structures (Hippocampus, Entorhinal, Perirhinal, Parahippocampal cortex) that are anatomically related and are mandatory for declarative memory. Declarative memory is a type of long-term memory that involves remembering the concepts or ideas and the events that happened or learned throughout life. It is further divided into three types of memory:

The medial temporal lobe (memory system) is still an active research area more precisely, the exact function of each structure in this lobe is currently being studied.[14]

The occipital lobe is the smallest lobe in the cerebrum cortex. It is located in the most posterior region of the brain, posterior to the parietal lobe and temporal lobe. The role of this lobe is visual processing and interpretation. Typically based on the function and structure, the visual cortex is divided into five areas (v1-v5). The primary visual cortex (v1, BA 17) is the first area that receives the visual information from the thalamus, and its located around the calcarine sulcus. The visual cortex receive, process, interpret the visual information, then this processed information is sent to the other regions of the brain to be further analyzed (example: inferior temporal lobe). This visual information helps us to determine, recognize, and compare the objects to each other.[15]