Sensory Systems: An Overview

Introduction to Sensory Systems

• Sensory systems introduction
• Learning Objectives reviewed
• Reviews nervous system organization

Learning Objectives

• The sensory pathways proceed from receptor to somatosensory cortex on the cerebral gyrus
• To compare and contrast sensation and perception
• Events in sensations occur
• Sensory receptors are differentiated by structure, function, and location
• Classes of receptors are distinguished in the general senses
• To illustrate receptor adaptation and state the function it serves
• Distinguish between tonic and phasic receptors and peripheral and central adaptation
• Lateral inhibition can be distinguished from receptor adaptation
• Lateral inhibition is illustrated in a diagram and it explains how it allows for localization of stimuli
• The CNS distinguishes properties of a stimulus: sensory modality, stimulus location, stimulus intensity, and stimulus duration
• The labelled line principle is defined and explained
• The basis for the sensory humunculus is understood

The Nervous System

• The central nervous system (CNS) receives and processes information and initiates action
• The peripheral nervous system(PNS) transmits signals between the CNS and the rest of the human body
• The brain receives and processes sensory data, initiates responses, stores memories, and generates thoughts and emotions
• The spinal cord conducts signals to and from the brain and controls reflex activities
• Motor neurons carry signals from the CNS to control muscle and gland actions
• Sensory neurons carry signals to the CNS from sensory organs
• The somatic nervous system controls skeletal muscle activation for voluntary movements
• The autonomic nervous system regulates involuntary responses by influencing organs, glands, and smooth muscle
• The sympathetic division prepares the body for stressful or energetic activity, initiating "fight or flight" responses
• The parasympathetic division dominates during "rest and rumination," directing maintenance activities

Sensations

• Sensation is the awareness of external or internal stimuli in a conscious or subconscious state
• The type of reaction is based on the ultimate destination of nerve impulses
• Complex reflexes, such as changes in heart rate or respiration, are elicited in response to sensory impulses reaching the lower brain stem
• Crude awareness of the body's location and type of sensation (e.g., touch, pain, hearing, taste) is provided by sensory impulses reaching the thalamus
• Specific sensations can be located and identified precisely because of sensory impulses that reach the cerebral cortex

Perception

• Perception involves a conscious awareness and the interpretation of the meaning of sensations
• The cortex stores memories of previous sensations
• Sensory impulses that never reach the cerebral cortex, elicit no perception

Sensory Modalities

• Each distinctive type of sensation, such as pain, hearing, vision, or touch, is referred to as a "sensory modality"
• Each sensory neuron only carries information for a single modality

Classes of Sensory Modalities

• General senses include both somatic and visceral sensations like touch, pressure, etc
• Special senses comprise smell, taste, vision, hearing, and balance modalities

Sensation Initiation

• Sensation begins with a sensory receptor, either a specialized cell or dendrites if a sensory neuron, which is responsible for monitoring conditions either inside or outside the body
• Sensory receptors respond vigorously to one particular kind of stimulus, weakly or not at all to others
• Receptors exhibit selectivity/specificity
• Adaptation is when receptors lose sensitivity after constant stimulus exposure
• Stimuli include electromagnetic energy (light/heat), mechanical energy (sound waves/pressure changes), and chemical energy (CO2 in fluids/pH)

Events in Sensation

• Stimulation of sensory receptors occur
• The stimulus must occur within the sensory receptor's field
• Receptive field is the region monitored by a receptor cell

Transduction of the Stimulus

• A stimulus is converted into a graded potential

Generation of Impulses/Action Potential

• When graded potential in a sensory neuron reaches threshold, one or more action potentials are triggered
• First-order neurons are sensory neurons that propagate impulses from the PNS into the CNS

Integration of Sensory Input

• Sensory nerve impulses are received and integrated in a particular region of the CNS
• Conscious sensations and perceptions are integrated in the cerebral cortex

Types of Sensory Receptors

• Free nerve endings are bare dendrites without specialized structural features and receptors for pain, tickle, thermal, and itch
• Encapsulated nerve endings at the dendrites are enclosed in a connective tissue capsule, it has distinct structural properties, such as lamellated (Pacinian) corpuscles

Receptors for Somatic Sensations

• Encapsulated nerve endings serve as receptors for somatic and visceral sensations like touch, vibration, and pressure

Specialized Separate Cells

• Special senses of vision, hearing, taste, and equilibrium, consist of specialized separate cells which synapse with first order neurons

Receptor Classifications by Location

• Exteroreceptors, located in or near the body's surface supply info on changes in the external environment
• Receptors for touch (Meissner's/Merckel's), pain, heat, and vision.
• Interoreceptors, transmit impulses from visceral organs (e.g., blood vessels and the nervous system)
• Sensations are not usually consciously perceived

Distance Receptors

• Distance receptors inform us of stimuli that reach us from remote sources
• Includes distance receptors present in the sense organs for sight, hearing, and olfaction (smell)

Proprioceptors

• Proprioceptors supply information on parts of the body in space
• Proprioceptors are located in muscles, joints, and tendons and the labyrinth of the inner ear, including muscle spindle receptors

Stimulus Type

• Mechanoreceptors are sensitive to mechanical forces like pressure, touch, stretch, and vibrations
• Thermoreceptors respond to variations in temperature
• Chemoreceptors detect chemicals in solution
• Taste, smell
• Chemoreceptors detect chemicals in mouth and body fluids
• Photoreceptors respond to variations in light
• Nociceptors are sensitive to harmful stimuli (pain)

Receptor Distribution

• General senses and special senses are based on receptor distribution
• General senses are scattered throughout the body and have a simple structure. Including temperature, pain, touch, and chemicals in blood

General Senses Receptor Types

• Thermoreceptors are temperature receptors
• Free nerve endings found in the dermis of the skin, skeletal muscles, liver, and hypothalamus are rapidly-adapting thermoreceptors
• Cold receptors are three times more numerous than hot receptors

Chemoreceptors

• These are specialized chemoreceptive neurons that detect small changes in the concentration of specific chemicals or compounds
• Respond to lipid soluble and water-soluble substances
• General senses chemoreceptors do not send information to the cerebral cortex, hence we are not consciously aware of the sensations they provide
• Chemoreceptors located within the carotid and aortic bodies
• The carbon dioxide and oxygen concentrations of arterial blood are monitored

Membrane Sensitivity

• Mechanoreceptors are sensitive to deformation of the cell membranes
• Membranes contain mechanically regulated ion channels that open or close in response to compression, stretching twisting

Tactile Receptors

• Provide sensations of touch, pressure, and vibration

Types of Tactile Receptors

• Free nerve endings are in the corneal surface of the eye
• Root hair plexus nerve endings -Monitor movements and distortions across the body
• Movements distort the sensory dendrites to produce an action potential
• Meissner's corpuscles, Merkel's discs, Ruffini corpuscles, and pacinian corpuscles are additional tactile receptors in the skin

Baroreceptors

• These consist of free nerve endings that branch within the elastic tissues in the walls of a distensible organ
• They are pressure receptors in blood vessel walls (e.g., carotid artery and aorta), lungs, and within the digestive, reproductive, and urinary tracts
• Important in regulating urination, cardiac function, respiration, and defecation
• Are Rapidly adapting

Proprioceptors

• Proprioceptors monitor state of muscle contraction, the tension in ligaments and tendons, and the position of joints
• Examples include muscle spindles and golgi tendon organs
• Proprioceptors do not exhibit adaptation

Special Senses Receptor Types

• The receptors are located in sense organs in the head which are protected by surrounding tissues
• Receptor organs for olfaction (smell), vision, gustation (taste), equilibrium (balance), and hearing
• Structurally, special senses receptors are more complex than general senses

Kinds of Graded Potentials

• Sensory receptors produce various kinds of graded potentials in response to a stimulus
• Generator potentials are when free nerve endings are stimulated and encapsulated
• Receptor potentials receptors for vision, hearing, equilibrium
• Presynaptic vesicles are triggered
• Neurotransmitter secretion induces a postsynaptic potential in the first order neuron
• Postsynaptic potential may trigger one or more action potentials in the first order neuron to the CNS

Postsynaptic Potentials

• Graded potentials that develop in the postsynaptic membrane in response to a neurotransmitter
• Excitatory postsynaptic potential (EPSP) is caused by the arrival of excitatory neurotransmitter at the postsynaptic membrane
• Inhibitory postsynaptic potential (IPSP) may result in the opening of chemically regulated potassium channels

Receptive Field

• The area monitored by a single receptor cell is called a receptive field
• The larger the receptive field, the poorer the ability to localize stimulus
• the tongue or finger, receptive field < 1 mm in diameter

Signal Transduction

• The unique physiological process common to all sensory systems is transduction
• The multistep process converts stimulus energy like electromagnetic, chemical, or mechanical into electrical potentials

In Special Senses

• Specialized receptor cells release the neurotransmitter, leading to a generating potential
• The afferent neuron triggers a generating potential after an action potential has occurred

Adaptation

• There is a reduction in sensitivity when there is a consistent stimulus
• Peripheral adaptation is when sensory receptors change their level of activity
• Adaptation can be linked to gradual decrease in generating potential
• Temperature receptors are quickly adapting while tonic receptors display insignificant peripheral adaptation, e.g. pain receptors
• Reduced peripheral adaptation equals less signals to the CNS
• Allowing the body to disregard constant unimportant information such as smells

Central Adaptation

• Central adaptation happens in the CNS
• It inhibits nuclei along a sensory pathway

Receptor Types

• Phasic receptors are normally inactive but become active for a short time and are rapidly adapting, for example temperature receptors
• Tonic receptors are always active, the rate of action potential depend on stimulus intensity
• Include eg pain receptors, proprioceptors

Lateral Inhibition

• It is for stimulus localization
• Including cutaneous senses, vision, and pitch discrimination
• Collateral connections inhibit neighboring neurons
• Weak signals get weaker and strong signals get relatively stronger.

Sensory Coding

• Sensory coding enables the nervous system to determine a stimulus's type, strength, location, and duration acting on a receptor
• Receptors encode stimulus type through modality and adequate stimuli
• A sensory receptor's adequate stimulus is the stimulus modality for which it possesses the adequate sensory transduction apparatus
• An "inadequate" stimulus may induce the perception of the adequate stimulus

Labelled Line Principle

• Modalities of sensation, such as vibration, touch, and pain, depend on the termination point in the CNS
• Labelled line principle is the specificity in fibers transmitting only one type of modality
• The cerebral cortex has distinct center, pathway, and receptor for each sensation
• Specific sensation is generated with regardless of stimulation

Stimulus Coding

• Stimulus intensity is coded by the frequency of action potentials and number of receptors activated
• This is known as recruitment
• Stimuli Localization (Acuity) is the ability to localize a stimulus that is achieved in the CNS via LATERAL INHIBITION
• Application of a stimulus applies near the edge inhibits it
• There is specific fiber pathway and labelled lines
• Adaptation to constant stimuli, indicates stimulus duration

Organization of Sensory Pathways

• First Order Neuron delivers sensations to the CNS
• Soma of first order neurons located in the dorsal root ganglion
• Second Order Neurons are interneurons on which first order neurons synapse
• They are located in either the the spinal cord or the brain stem
• Third Order Neurons are second order neurons that synapse on third order neurons
• They are for conscious awareness, typically located in the thalamus
• Axons for third order neurons synapse on neurons of the sensory cortex

Specific Vs. Non-Specific Pathways

• Specific Ascending Pathways transmit a given sensory modality to cerebral cortex, such as touch, pain, sight
• Has a typical three neuron pathway whose primary afferent occurs, as well as, secondary neuron in spinal cord or brainstem that connects to thalamus and tertiary neuron in thalamus, projecting itself to the cortex

Nonspecific Pathways

• Those that transmit "non-specific" or hard to define information
• Can have many synapses
• Can synapse through reticular formation, in that it that integrates input from wide variety of sensory modalities or modalities that individual neurons can respond to
• Those pathways elicit certain affective components, arousal, or alerting responses to stimulus whether it that feels hurtful