Neurobiology
Chapter 1: Sensation and Perception, & Fundamentals of Sensory System
Asst Prof Shuo-Chien Ling, Ph.D.
Laboratory of Molecular Neurodegeneration
NUS Yong Loo Lin School of Medicine Department of Physiology
Email: phsling@nus.edu.sg
Introduction
Have you ever wondered how we sense and perceive the world - how the eyes see; the skin detect cold, heat, pain; the ears recognise certain sounds; the nose identify smells; and the mouth enjoy different tastes?
In this e-Book, you will learn the science behind our senses and perception. Sensory neurophysiology will help you understand how we sense and perceive the world.
Overview
In this chapter, we will first focus on the types and functions of sensory system and the principles of organisation and processing of sensory information. We will also cover three special senses: taste, smell and hearing.
Learning Objectives
At the end of this chapter, you should be able to:
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- Identify the types and explain the functions of sensory systems
- Explain the principles of organisation and processing of sensory information
1.1. Functions of Sensory Systems
The function of each sensory system is to provide the central nervous system (CNS) with a representation of external world.
The Greek philosopher Aristotle defined the five senses.
We perceive sensory signals when they reach a level of conscious awareness.
Stimulus that usually do not reach conscious awareness include changes in muscle stretch and tension as well as a variety of internal parameters that the body monitors to maintain homeostasis such as blood pressure and pH. These processes are important in maintaining physiological homeostasis.
Hence, we will primarily consider sensory stimuli whose processing reaches the conscious level of perception.
These stimuli are those associated with the special senses of vision, hearing, taste, smell and equilibrium, and the somatic senses of touch, temperature, pain, itch and proprioception. Proprioception, which is defined as the awareness of body movement and position in space, is mediated by muscle and joint sensory receptors call proprioceptors and may be either subconscious or conscious.
If you close your eyes and raise your arm above your head, you are aware of its position because of the activation of proprioceptors.
Sensory systems bring the information to an individual.
Video 1: Sensory Systems
Watch the video below for more information.
1.2. General Properties of Sensory Systems
How does the nervous system encode and process sensory stimuli?
Let's first consider the general properties of sensory pathways. Here, we will take a look at unique receptors and pathways that distinguish the different sensory systems from one another and use three special senses, taste, smell and hearing as examples.
Imagine that you are in the wild.
What do you need to do to survive?
Is there a lion or tiger hiding in the grass ready to eat you?
You need to see, hear, smell, and feel. How does the nervous system do that?
In essence, the sensory system needs to communicate the key features of any stimuli: what is the stimuli, where, how strong it is (intensity), the duration (for how long the stimuli is on). The type of stimuli, we call sensory modality, are vision, hearing, hearing, etc.
All sensory pathways have certain elements in common. They begin with a stimulus, in the form of physical energy that acts on a sensory receptor. The receptor is a transducer that converts the stimulus into an intra-cellular signal, usually, a change in membrane potential. If the stimulus is above threshold, action potentials pass along a sensory neuron to the CNS, where incoming signals are integrated. At each synapse along the pathway, the nervous system can modulate and shape the sensory information.
Key Concepts
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- Coding and processing distinguish stimulus properties
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- Sensory modality (what?)
- Location of the stimulus (where?)
- Intensity (how much?)
- Duration/timing (how long?)
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- Receptors are sensitive to particular forms of energy.
- Sensory transduction converts stimuli into graded potentials.
- A sensory neuron has a receptive field.
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- Lateral inhibition
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- The CNS integrates sensory information.
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- Labeled line
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1.3. Sensory Receptors
Receptors are sensitive to particular forms of energy and receptor cells convert stimulus into electrical signals.
Video 2: Sensory Receptors
Sensory receptors detect information from the environment, such as light and sound, or from our own bodies, such as touch and body position.
Watch the video below for more information.
Reference: (1) Krebs et al., Lippincott’s Illustrated Reviews: Neuroscience, Figure 3.5; (2) Bears et al., Neuroscience: exploring the brain, 4th edition, Figure 8.11
Sensory receptors act as transducers in that they transform a physical or chemical stimulus (or form of energy) into an electrical impulse. They are specialised to detect sensory information and translate stimuli into receptor potentials, or electrical signaling within the receptor, caused by the opening and closing of ion channels. Each sensory receptor has a receptive field, which lets us discriminate the location of the sensory stimulus.
Receptor potentials, also referred to as generator potentials, are electrical impulses transduced by the sensory receptor. The receptor potential is a graded response that depends on the magnitude of the sensory stimulus and encodes for duration and intensity. Because a receptor potential dissipates after a few millimeters, an action potential must be generated to travel the long distance between the sensory receptor and the CNS. In order to generate an action potential, the depolarisation of the membrane at the sensory receptor must reach threshold. The firing frequency of the action potential in the sensory nerve is modulated by the receptor potential: The greater the stimulus, the greater the receptor potential, and the higher the frequency of action potentials produced.
Voltage recordings from an olfactory receptor cell during stimulation.
Odorants generate a slow receptor potential in the cilia; the receptor potential propagates along the dendrite and triggers a series of action potentials within the soma of the olfactory receptor cells. Finally, the action potentials (but not the receptor potential) propagate continuously along the olfactory nerve axon.
1.4. Five Modes of Sensory Detection
The source of the stimulus can be external or internal.
Superficially located sensory endings in the skin are called exteroceptors and respond to pain temperature, touch and pressure, that is stimuli outside the body.
Muscles, tendons, and joints have proprioceptors that signal awareness of body position and movement.
Enteroreceptors monitor events within the body such as feeling movement through the gut.
Mode of detection can be grouped into five categories:
1.5. Six Sensory Systems
Receptors are sensitive to particular forms of energy and receptor cells convert stimulus into electrical signals.
Neurons of the brain and spinal cord do not respond when they are touched or when they are exposed to sound or light or odors. Each form of energy must be transduced by a population of specialised cells, which converts the stimulus into a signal that all neurons understand.
In every sensory system, cells that perform this transduction step are called receptors. For each of the fundamental types of stimuli (mechanical, chemical, or thermal energy or light), there is a separate population of receptors selective for the particular form of energy.
Even within a single sensory system, there are class of receptors that are particularly sensitive to one stimulus (e.g., heat or cold) and not another (muscle stretch). The specificity in the receptor response is a direct function of differences in receptor structure and chemistry.
1.6. Stimulus Intensity
Sensory neurons use action potential frequency and duration to code stimulus intensity and duration.
Video 3: Stimulus Intensity
The intensity of a stimulus can not be directly calculated from a single sensory neuron action potential.
Watch the video below for more information.
Reference: Silverthorn, Human Physiology, 5th edition, Chapter 10, Figure 10.6
The intensity of a stimulus can not be directly calculated from a single sensory neuron action potential because a single action potential is “all-or-none”. Instead, stimulus intensity is coded in two types of information: the number of receptors activated and the frequency of action potentials coming from those receptors (frequency coding).
For individual sensory neurons, intensity discrimination begins at the receptor. If a stimulus is below threshold, the primary sensory neuron does not respond. Once stimulus intensity exceeds threshold, the primary sensory neuron begins to fire action potentials. As stimulus intensity increases, the receptor potential amplitude (strength) increase in proportion, and the the frequency of action potential in the primary sensory neurons increases, up to a maximum rate.
Similarly, the duration of a stimulus is coded by the duration of action potentials in the sensory neuron. In general, a longer stimulus generate s a longer series of action potential.
1.7. Two Modes of Receptor Adaptation
The two modes of receptor adaptation include:
- Tonic - slowly adapting
- Phasic - rapidly adapting
In general, the stimuli that activate tonic receptors are parameters that must be monitored continuously by the body. Once a stimulus reaches a steady intensity, phasic receptors adapt to a new steady state and turn off.
1.8. Receptor Response
Receptor response can decline with maintained stimuli: adaptation.
The rate of firing of this neuron, whose receptive field is located on the fifth finger, is rapid when the stimulus is first applied, but then adapts, slowing to a steady rate.
1.9. Receptive Fields of Sensory Neurons
A sensory neuron has a receptive field.
Somatic sensory and visual neurons are activated by stimuli that fall within a specific physical area known as the neuron’s receptive field.
The size of secondary receptive fields determines how sensitive a given area is to a stimulus.
1.10. Mach Band Effect
The Mach Band Effect is named after Ernst Mach.
Reference: https://en.wikipedia.org/wiki/Mach_bands
For each block of the color, are they the same or gradient, or? What do you think you see?
Lateral inhibition enhances contrast and makes a stimulus easier to perceive.
Video 4: Lateral Inhibition
Lateral inhibition, which increases the contrast between activated receptive fields and their inactive neighbors, is another way of isolating the location of a stimulus.
Watch the video below for more information.
Reference: Silverthorn, Human Physiology, 5th edition, Chapter 10, Figure 10.5
1.11. Labeled Lines Hypothesis
Each receptor (cold, warmth, touch, pain) has a distinct pathway linking the receptor surface to the brain, so different qualities of skin stimulation can be communicated to distinct places in the brain.
Video 5: Labeled Lines
The brain associates a signal coming form a specific group of receptors with a specific modality. This 1:1 association of a receptor with a sensation is called labeled line coding.
Watch the video below for more information.
One exception: the brain uses timing differences rather than specific neurons to localise sound.
Neurons in the ears are sensitive to different frequencies of sound, but they have no receptive fields and their activation provides no information about the location of the sound.
How does the brain figure out where the sound is from? Click the image to learn more.
1.12. How does the CNS integrate sensory information?
These are the sensory pathways in the brain. Most pathways except the olfactory pathway pass through the thalamus on their way to the cerebral cortex.
1.13. Summary: General Properties of Sensory Systems
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- Each receptor is most sensitive to a particular type of stimulus.
- A stimulus above threshold initiates action potentials in a sensory neuron that projects to the CNS.
- Stimulus intensity and duration are coded in the pattern of action potentials reaching the CNS.
- Stimulus location and modality are coded according to which receptors are activated or (in the case of sound) by the timing receptor activation.
- Each sensory pathway projects to a specific region of the cerebral cortex dedicated to a particular receptive filed. The brain can then tell the origin of each incoming signal.
1.14. Further Readings
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- Bears et al., Neuroscience: exploring the brain, 4th edition, chapter 8 and 11
- Krebs et al., Lippincott’s Illustrated Reviews: Neuroscience, chapter 3
- Silverthorn, Human Physiology, 5th edition, chapter 10