The Sensory System: Components and Operations

• The sensory system's components and operations will be covered

Learning Objectives

• Review the organization of the nervous system
• Show how sensory pathways are organized, from receptor to somatosensory cortex on the postcentral gyrus
• Compare and contrast sensation from perception
• Describe the events involved in sensations
• Explain the different types of sensory receptors based on how they are structured, how they function, and where they are located
• Describe receptor classes in the general senses, as well as their functions
• Diagram receptor adaptation, its function, and tonic and phasic receptor examples; differentiate between peripheral and central adaptation
• Point out the differences between receptor adaptation and lateral inhibition
• Show lateral inhibition in a diagram and how it enables stimuli localization
• Detail how the central nervous system (CNS) distinguishes stimulus properties like sensory modality, location, intensity, and duration
• Define the labeled line principle and explain it
• Demonstrate comprehension of sensory humunculus

The Nervous System

• The central nervous system (CNS) processes data and starts actions
• The peripheral nervous system (PNS) sends signals between the CNS and the body
• The brain processes sensory data, initiates responses, stores memories, and generates thoughts and emotions
• The spinal cord sends signals to and from the brain and controls reflexes
• Motor neurons send signals from the CNS to control muscle and gland activity
• Sensory neurons send signals to the CNS from sense organs
• The somatic nervous system controls voluntary movements by activating skeletal muscles
• The autonomic nervous system manages unconscious responses by affecting organs, glands, and smooth muscles
• The sympathetic division preps the body for stress ("fight or flight")
• The parasympathetic division dominates during "rest and rumination" and directs upkeep activities

Sensations

• Sensation defined as the conscious or subconscious awareness of internal or external stimuli
• Sensation nature and reaction varies with the nerve impulses final destination
• Sensory actions to the lower brain stem cause complex reflexes, and changes in heart, breathing rate, etc
• Awareness of body location and type of sensation results from impulses reaching the thalamus
• Sensory impulses traveling to the cerebral cortex enable precise location and identification or specific sensations

Perception

• Perception involves a conscious awareness and interpretation of sensations' meanings
• The cortex stores memories of past sensations
• Sensory impulses not perceived are those that don't reach the cerebral cortex

Sensory Modalities

• Sensory modality describes each type of sensation; examples are pain, hearing, sight, touch
• A given sensory neuron carries information for only one modality

Classes of Sensory Modalities

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

Sensation Process

• A sensory receptor starts the sensation by use of specialized dendrites or cells to monitor internal or external stimuli
• Specificity occurs with a sensory receptor responding to one or only a few kinds of stimuli
• Receptors selectivity indicated through receptors exhibiting selectivity/specificity
• Adaptation is exhibited through receptors losing sensitivity after exposure to a constant stimulus
• Stimuli appear in forms of electromagnetic, mechanical, or chemical energy

Events in Sensation

• Sensory receptors must be stimulated
• Sensory receptors have receptive fields in which they are located with the stimulus occurring
• The area a certain receptor cell monitors is known as its receptive field

Transduction and Impulses

• Transduction involves a stimulus turning into a graded potential
• Upon sensory neuron action, it is triggered through a graded potential reaching threshold
• Propagation of PNS impulses into the CNS achieved thanks to first order neurons

Integration of Sensory Input

• Specific CNS areas are the recipients and integrators of sensory neurons
• The cerebral cortex integrates conscious perceptions/sensations

Types of Sensory Receptors (By Structure)

• Free nerve endings have bare dendrites without visible structural specialization
• These free nerve endings are how pain, thermal sensations, tickle, and itch sensations are perceived
• Encapsulated nerve endings at dendrites have a encapsulating connective tissue capsule with structures such as lamellated (Pacinian) corpuscles
• Encapsulated nerve endings allow somatic and visceral feelings to be perceived with touch, pressure, and vibration

Specialized Separate Cells

• Special sense sensory receptors of vision, hearing, equilibrium and taste involve specialized cells, with synapses connecting to first order neurons

Classification of Receptors (By Location)

• Exteroreceptors are located near the body's external surface
• External environment changes lead to the supply of information
• Exteroreceptors for touch are from Meissner's corpuscles, Merckel's discs, cold, heat, pain, vision, and smell
• Visceral organs transmit impulses through interoreceptors
• Interoceptors are responsible for sensations not usually perceived consciously, located in blood vessels and the nervous system

Telereceptors and Proprioceptors

• Distance receptors, or telereceptors, notify of remote stimuli
• Sense organs containing vision, hearing, and olfaction receptors are telereceptors
• Movement and position info of the parts of the body in space are given through proprioceptors
• Muscle spindle, golgi tendons, tendons, joints, and inner ear comprise the proprioceptors

Classes of Receptors Based on Stimulus Type

• Mechanoreceptors sense mechanical forces through touch, pressure, stretching, and vibrations
• Thermoreceptors sense temperature changes
• Chemoreceptors detect chemicals in solutions, facilitate taste, smell, and blood chemistry
• Photoreceptors respond to light within the eye
• Nociceptors respond to harmful or painful stimuli

Classification Based on Receptor Distribution

• General and special senses are the two types
• General senses scattered throughout the body are simple and sense temperature, pain, touch and chemicals
• Thermoreceptors respond temperatures through free nerve endings in the skin and organs
• Cold receptors are more numerous than warm receptors and are rapidly adapting

Chemoreceptors Function

• Chemoreceptors detect minute changes in certain chemicals in the body
• They respond to water-soluble and lipid-soluble substances in fluid
• General sense chemoreceptors do not consciously send sensation to cerebral cortex
• Chemoreceptors in carotid, aortic bodies monitor carbon dioxide and oxygen levels in arterial blood

Mechanoreceptor Sensitivity

• Sensitive to cell membrane distortion stimuli
• Mechanically regulated ion channels are contained within the membranes

Tactile Receptor Types

• Receptors provide sensations of touch, pressure, and vibration
• Free nerve endings present in the corneal surface
• Movements across the body and distortions are monitored by root hair plexus nerve endings
• Follicle movements of hair may distort sensory dendrites, leading to an action potential

Touch information

• Discs from Merkel, Meissner cells, Pacinian and Ruffini corpuscles give touch information

Baroreceptor Location and Definition

• Baroreceptors are branches of free nerve endings within the elastic tissues of distensible organs
• Present in blood vessel walls of the carotid artery and aorta as pressure receptors can be found in the digestive, reproductive and urinary tracts
• Important in cardiac regulation and respiration
• Rapidly adapting

Proprioceptor Function

• Proprioceptors do not exhibit adaptation and monitor tension, ligament position and joint state contraction for muscles

Special Senses Receptor Types

• Sensory receptors in head, near a tissue, such as through olfactory and vision receptors or hearing
• Specialized sensory receptors more advanced than general

Graded Potentials

• Generator potential occurs in stimulated free and encapsulated nerve endings.
• Receptor potential occurs in special senses (vision, hearing, equilibrium, taste)
• Synaptic vesicle exocytosis occurs in the receiving cell

Post synaptic potential

• Results during post synaptic action for first order neuron to CNS

Postsynaptic Potentials

• Graded potentials in the postsynaptic membrane which develop in response to a neurotransmitter
• Excitatory postsynaptic potential (EPSP) arises from excitatory neurotransmitters at the postsynaptic membrane
• Inhibitory postsynaptic potential (IPSP) involves chemically regulated opening of potassium channels

Receptive Field Characteristics

• Single receptor cells have an area monitored by a single field
• Localization and localization skills diminish with the larger the area
• With < 1 mm dimensions, tongue and finger areas can be discerned

Signal Transduction Defined

• Unique physiological process common across all sensory systems
• Multistep; electrical potentials allow chemical, mechanical or electromagnetic energy to be interpreted by the nervous system

Adaptation Types and Function

• Reduces sensitivity in the brain's sensitivity, as it is adapting
• If receptors/sensory neurons change activity levels this is a peripheral adoption
• Peripheral action stems from generator potential
• Temperature fast
• Adaptation reduces some amount of CNS INFO
• The CNS action allows the body to tune constant actions, such as smell

Central Adaptation

• CNS with actions of inhibition, and nuclei

Phasic and Tonic Receptors

• Phasic receptors are normally not functioning, and activate in a short time period instead
• Temperature is an example
• Always active, tonic receptors have action rates which may lower or increase based on action type
• An example is with pain

Lateral Inhibition Purpose

• Localization of stimulus for edges
• Vision discrimination and skin level
• Inhibit neighboring for connections along

Sensory Coding Types

• Through the nervous system, the location, duration and strength of a stimulus are perceived
• Modality is sensed in receptors through energies through
• Action with adequate transduction of apparatus and action with receptors on stimu
• Can be induced due to action

Labelled Line Principle

• Modalities such as vibration, sight, pain etc, terminate at different zones of the CNS
• Fiber type specificity occurs with the labelled line principle
• Specific pathways exist
• Each sense stimulus, center and pathway has unique processes

###Stimulus Localization

• Intensity is affected via stimulus action with stimulus action and recruitment of sensory signals
• Location is acuity for action, or localization
• Duration is due to adaptation
• Action of the nerve results in the receptive fields

Organization of Sensory Pathways

• First order action delivers sensations to the primary system, while the second order neuron synapse occurs too and the third orders have more secondary synapse
• Second order found in cord stems
• Third in thalamus, where the sensory cortex resides too

Neural Pathways

• Cortical transmissions occur when afferent action occurs, or afferent primary occurs
• The medulla is related to actions such as action of multiple synapses on neural functions

Explore the sensory system's components and operations. Investigate sensory pathways, receptor types, and adaptation mechanisms. Delve into how the CNS distinguishes stimulus properties like sensory modality, location, intensity, and duration.