Beatrice Lang
Sensation begins with reception, the activation of sensory receptors by stimuli such as mechanical stimuli, chemicals, or temperature. The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system. For example, sound waves hitting the eardrum cause the back-and-forth vibration of a thin membrane, which results in the vibration of the basilar membrane inside the cochlea. This, in turn, causes neurons to produce electrical pulses called action potentials that propagate to the brain and ultimately produce the sensation of sound. Similarly, electrical stimulation of the visual cortex can cause sensations of light. Electrical stimulation of the brain has been explored as a therapy alternative and for the development of prosthetics, with research showing that the brain responds to electrical stimulation in a more consistent manner than natural stimulation.
| Characteristics | Values |
|---|---|
| First step in sensation | Reception |
| Purpose of reception | Activation of sensory receptors by stimuli such as mechanical stimuli, chemicals, or temperature |
| Receptor's receptive field | The region in space in which a given sensory receptor can respond to a stimulus |
| Sensation caused by stimulation through a single electrode | Commonly a single very small spot of white light at a constant position in the visual field |
| Sensation caused by stimulation through multiple electrodes | Predictable simple patterns |
| Effect of stimulation frequency on threshold | No sharp flicker fusion frequency |
| Phosphenes | Move with the eyes during voluntary eye movements |
| Somatosensation | Sensation of touch |
| Somatosensory transduction | Transmission of sensory information from the receptor to the central nervous system |
| Sensation of electric shock | Sign that nerves are affected |
| Sensation of intracortical microstimulation | Dependent on several features of electrical stimuli, such as the strength and frequency of signals |
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What You'll Learn
Sensation and perception
Sensation refers to the activation of sensory receptors by stimuli such as mechanical stimuli, chemicals, or temperature. These sensory receptors are either specialised cells associated with sensory neurons or the specialised ends of sensory neurons that are part of the peripheral nervous system. Each sensory receptor is modified for the type of stimulus it detects, for example, gustatory receptors are not sensitive to light.
The fundamental function of a sensory system is to translate a sensory signal into an electrical signal in the nervous system. For instance, sound waves hitting the eardrum cause a chain reaction of bone movements, which eventually result in the basilar membrane inside the cochlea vibrating. This, in turn, causes neurons to produce electrical pulses called action potentials that encode the physical properties of the sound waves. These electrical pulses then propagate to the brain, resulting in a sensation.
The intensity of a stimulus is encoded in the rate of action potentials produced by the sensory receptor. Thus, an intense stimulus will produce a faster rate of action potentials. Additionally, an intense stimulus might activate a larger number of adjacent receptors, further increasing the rate of action potentials.
Perception refers to an individual's interpretation of a sensation and is a brain function. For example, electrical stimulation of the visual cortex can cause a person to experience sensations of light. The strength and frequency of electrical signals can affect the type of sensation produced, such as touch, vibration, or pain.
Damage to axons leading from sensory receptors to the central nervous system can impair the transmission of sensory information, resulting in a halt in the perception of stimuli.
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Sensory receptors
The skin, for example, possesses many sensory receptors in the epidermis, dermis, and hypodermis, allowing for the discrimination of touch, temperature, pain, and itch. The inner ear houses hair cells in the cochlea to transduce sounds, while the nose perceives smell through the binding of molecules to chemoreceptors in the cilia of the olfactory epithelium. Taste appreciation occurs when molecules dissolve in the taste buds in the mouth and oropharynx.
Nociceptors are another type of sensory receptor that responds to tissue damage and pain. They are activated by extremes of temperature, high pressure, and chemicals causing tissue damage. Proprioceptors, on the other hand, respond to the positions of skeletal muscles, tendons, ligaments, and joints, sensing the relative position of body parts and the amount of stress or effort exerted during movement.
The varied architecture of sensory receptors allows for the recognition of three primary groupings: nonencapsulated receptors, epithelial tactile (Merkel) complexes, and cell-neuron complexes. Nonencapsulated receptors have a simple structure where the peripheral terminals of a sensory neuron are directly embedded in the tissue, including free nerve endings that detect pain and temperature. Epithelial tactile (Merkel) complexes involve a tactile epithelial cell communicating with specialised peripheral nerve endings. Cell-neuron complexes, meanwhile, involve nonneuronal sensory receptor cells detecting stimuli and then communicating with sensory neurons.
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Stimuli and intensity
The human body is capable of detecting a wide range of stimuli from both the internal and external environment. This detection process involves the activation of sensory receptors, which can be specialised cells or the specialised ends of sensory neurons. These receptors are responsible for receiving information about the environment and translating sensory signals into electrical signals in the nervous system. This process, known as sensory transduction, occurs at the sensory receptor level and involves a change in electrical potential called the receptor potential.
The intensity of a stimulus plays a crucial role in how it is encoded and perceived. The intensity of a stimulus is often encoded in the rate of action potentials produced by the sensory receptor. An intense stimulus will generate a faster sequence of action potentials, while reducing the stimulus will slow down this rate. Additionally, the number of receptors activated contributes to the encoding of intensity. A strong stimulus may trigger action potentials in a larger number of adjacent receptors, whereas a weaker stimulus may activate fewer receptors.
The type of stimulus and the specific region it activates are also important factors in the process. Each sensory receptor has a receptive field, which is the region in space where it can respond to a stimulus. For example, in the sense of touch, the stimulus must come into direct contact with the body, whereas auditory and visual stimuli can be perceived from a distance.
The brain plays a crucial role in interpreting sensory information. After the sensory receptors are activated, the signals travel through the central nervous system to the thalamus, which acts as a clearinghouse and relay station. From the thalamus, the signals are routed to specific areas of the cortex dedicated to processing different senses, such as the auditory, visual, and somatosensory regions.
In summary, the conversion of stimuli into electrical signals involves the activation of sensory receptors, which transmit signals to specialised areas of the brain for further processing. The intensity, type, and location of the stimulus all play a role in how the information is encoded and interpreted by the brain.
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Electrical stimulation of the brain
The human brain functions by sending and receiving electrical signals between nerve cells. This process can be influenced by electrical brain stimulation (EBS), which is a form of electrotherapy and neurotherapy. EBS involves the direct or indirect excitation of a neuron's cell membrane using an electric current.
The history of EBS can be traced back to the 19th century, with pioneers such as Luigi Rolando and Pierre Flourens, who studied brain localization of function. The procedure was further developed by British neurosurgeon Victor Horsley, who invented the stereotactic method, and Swiss neurophysiologist Walter Rudolf Hess, who developed chronic electrode implants. These implants could be inserted deep into the brains of animals, and this approach was later used on humans to study the motor and sensory homunculus.
EBS has been used to elicit both pleasurable and aversive responses in laboratory animals and humans. For example, stimulating the anterior hypothalamus in cats can trigger ritualistic motor responses of sham rage, while stimulation of the lateral hypothalamus can evoke more complex emotional and behavioural components of "true rage". In humans, stimulating the amygdala has been shown to induce pathologic aggression and rage.
EBS has also been explored for therapeutic purposes, particularly in the development of visual cortical prosthetics for the blind. By stimulating the visual cortex with electrodes, patients can experience sensations of light and simple patterns.
Additionally, EBS has been studied as a potential treatment for various mental disorders, including depression, OCD, migraines, anxiety, and smoking dependence. Transcranial magnetic stimulation (TMS) and electroconvulsive therapy (ECT) are two examples of brain stimulation therapies that have been cleared by the FDA for treating depression and OCD.
While EBS has shown promising results in certain areas, more research is needed to determine its full potential and effectiveness across a range of applications.
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Artificial touch
The human sensory system is a complex network that translates sensory signals into electrical signals in the nervous system. This process allows us to perceive and interpret sensations such as touch, sound, and light. In recent years, there has been significant progress in the development of artificial touch technology, aiming to replicate the human sense of touch in machines. This field, known as "haptics", has the potential to revolutionize the way we interact with robots and virtual environments.
One key enabler of artificial touch is machine learning. With its powerful computational capabilities, machine learning can process vast amounts of data and mimic biological processes. This allows robots to interpret tactile information and adjust their movements accordingly. For example, a robotic hand could scan an object, plan its grip, and use touch feedback to adjust its hold, just as humans do.
While significant progress has been made, there are still challenges to be addressed in the field of artificial touch. One challenge is achieving the right balance between relying on vision and touch for robotic tasks. While vision can guide a robot's movements, it may not always be reliable in complex or dynamic environments. Therefore, further research is needed to optimize the integration of vision and touch for different scenarios. Nonetheless, with ongoing advancements in technology and our understanding of human biology, we are closer than ever to engineering artificial touch that rivals the capabilities of the human hand.
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Frequently asked questions
The first step in sensation is reception, which is the activation of sensory receptors by stimuli such as mechanical stimuli, chemicals, or temperature.
The most fundamental function of a sensory system is the translation of a sensory signal to an electrical signal in the nervous system.
Sound waves are converted into electrical signals in the brain. As sound waves hit the eardrum, a chain reaction of bone movements is triggered, eventually resulting in the vibration of the basilar membrane inside the cochlea. This produces vibrations in the tectorial membrane, leading to the bending of the stereocilia on hair cells. These neurons then produce electrical pulses that propagate to the brain, creating the sensation of sound.
