Nervous System: Sensory Pathways and Receptors

Learning Objectives

• Learning objectives include reviewing nervous system organization.
• The show also focuses on the organization of sensory pathways from receptor to the somatosensory cortex on the postcentral gyrus.
• The material compares and contrasts sensation and perception.
• Events in sensations are described
• Different types of sensory receptors are defined based on structure, function, and location.
• Classes of receptors in general senses, and their functions are described.
• Receptor adaptation is illustrated, including its function and examples of tonic and phasic receptors.
• Differences between peripheral and central adaptation are distinguished.
• Receptor adaptation and lateral inhibition are distinguished.
• Lateral inhibition is illustrated in a diagram, explaining how it allows for stimulus localization.
• An explanation of how the CNS distinguishes stimulus properties is provided.
• The explained properties include sensory modality, stimulus location, stimulus intensity, and stimulus duration.
• The labelled line principle is defined and explained.
• An understanding of the basis for the sensory homunculus is demonstrated.

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 body.
• Within the CNS, the brain receives/processes sensory information, initiates responses, stores memories, and generates thoughts/emotions.
• The spinal cord transmits signals to and from the brain and controls reflex activities.
• Motor neurons carry signals from the CNS to control muscles and glands.
• Sensory neurons carry signals to the CNS from sensory organs.
• The somatic nervous system controls voluntary movements by activating skeletal muscles.
• The autonomic nervous system controls involuntary responses by influencing organs, glands, and smooth muscle.
• The sympathetic division prepares the body for stressful or energetic activity which is often known as "fight or flight."
• The parasympathetic division dominates during "rest and rumination" and directs maintenance activities.

Sensations

• Sensation involves conscious or subconscious awareness of external or internal stimuli.
• Sensation and reaction type vary based on the destination of nerve impulses.
• Sensory impulses reaching the lower brain stem elicit complex reflexes, like heart rate or breathing changes.
• Sensory impulses reaching the thalamus provide crude awareness of body location and sensation type (touch, pain, hearing, taste).
• Sensory impulses reaching the cerebral cortex enable precise location and identification of specific sensations.

Perception

• Perception is the conscious awareness and interpretation of the meaning of sensations.
• Memories of previous sensations are stored in the cortex.
• Sensory impulses that do not reach the cerebral cortex are not perceived.

Sensory Modalities

• Each type of sensation, such as pain, hearing, vision, or touch, classifies as a sensory modality.
• Each sensory neuron carries information for only a single modality.

Classes of Sensory Modalities

• General senses include both somatic and visceral senses such as touch and pressure.
• Special senses include modalities like smell, taste, vision, hearing, and balance.

Sensory Receptors

• Sensation begins with a sensory receptor, whether a specialized cell or dendrites of a sensory neuron, monitoring internal or external conditions.
• Sensory receptors respond vigorously to one type of stimulus.
• Receptors exhibit selectivity/specificity.
• Sensory receptors can undergo adaptation which involves loss of sensitivity after prolonged exposure to a constant stimulus.
• Stimuli can take the form of electromagnetic energy (light and heat), mechanical energy (sound waves and pressure changes), or chemical energy (CO2 in fluids, pH).

Events in Sensation

• Stimulation of sensory receptors requires the stimulus to occur within the receptor's specific receptive field.
• Every receptor cell monitors a specific region known as a receptive field.
• Transduction of the stimulus involves converting a stimulus into a graded potential.
• With the generation of impulses, when the graded potential in a sensory neuron reaches threshold, action potentials are triggered.
• Sensory neurons, known as first-order neurons, propagate impulses from the PNS into the CNS.
• Integration of sensory input involves receiving and integrating sensory nerve impulses at a region of the CNS.
• Conscious perceptions/sensations are integrated in the cerebral cortex.

Types of Sensory Receptors (based on structure)

• Free nerve endings are bare dendrites lacking visible structural specialization.
• Free nerve endings are receptors for pain, thermal sensations, tickle, and itch.
• Encapsulated nerve endings at the dendrites are dendrites enclosed in a connective tissue capsule with distinct microscopic structure.
• Lamellated (Pacinian) corpuscles are an example of encapsulated nerve endings.
• Receptors for somatic and visceral sensations, such as touch, pressure, and vibration, are encapsulated nerve endings.

Specialized Separate Cells

• Specialized cells that synapse with first-order neurons function sensory receptors for special senses.
• These special senses consist of vision, hearing, equilibrium, and taste.

Exteroceptors

• These receptors are located near or on the external surface of the body.
• Exteroreceptors supply information on changes in the external environment.
• Touch receptors (Meissner's corpuscles and Merkel's discs), as well as receptors for cold, heat, pain vision, and smell, are all exteroreceptors.

Interoceptors

• Interoreceptors transmit impulses from visceral organs, blood vessels, and the nervous system.
• Sensations perceived by interoceptors are typically not conscious.

Telereceptors

• These can also be known as distance receptors
• Provide information on stimuli from remote sources
• Sense organs for sight, hearing, and olfaction are considered to be telereceptors

Proprioceptors

• Supply info about body position, limbs, balance etc
• Found in muscles, tendons, joints etc

Classes Based on Stimulus Type

• Mechanoreceptors detect mechanical forces like touch, pressure, stretch, and vibrations.
• Thermoreceptors detect temperature changes.
• Chemoreceptors detect chemicals in solution.
• Taste and smell are examples of senses that rely on chemoreceptors.
• These detect chemicals in the mouth and other bodily fluids
• Photoreceptors respond to light which are located in the eye
• Nociceptors detect harmful painful stimuli.

Classification Based on Receptor Distribution

• General senses and special senses are based upon receptor distribution
• General senses are scattered throughout the body and feature simple structures used for temperature, pain, touch, and detecting chemicals in blood.

General Senses Receptor Types

• Temperature receptors are the receptor type for thermoreceptors
• Free nerve endings are located in the dermis of skin, skeletal muscles, liver, and in the hypothalamus that are used to detect temperature
• Cold receptors are 3x more numerous than warm receptors
• Thermoreceptors are rapidly adapting

Chemoreceptors in General Senses

• They are specialized chemoreceptive neurons that detect small changes in chemical concetrations
• They respond to water and lipid-soluble substances dissolved in the surrounding fluid.
• They do not send information to the cerebral cortex, with the effect where the sensations they provide, are not consciously available
• Chemoreceptors are located within the coratid and aortic bodies
• Monitor carbon dioxide and oxygen concentration

Mechanoreceptors

• Are sensitive to stimuli that distort their cell membranes
• The membranes contain mechanically regulated ion channels whose gates open or close in response to stretching, compression, twisting or other distortions of the membranes

Types of Tactile Receptors

• Provide sensations of touch, pressure and vibrations

Types of Tactile Receptors in the Skin

• Free nerve endings - present in the corneal surface of the eye
• Nerve endings of root hair - monitor distortions and movements across the dermis
• Movement of the hair distorts the hair follicle which creates an action potential
• Includes, Merkel's discs, Messner's corpuscles, Pacinian corpuscles and Ruffini Corpuscles

Baroreceptors

• Consist of free nerve endings that branch within the elastic tissues of a distensible organ
• Pressure receptors in walls of lungs, blood vessels
• Important in regulation of cardiac function and urination
• Adaptation happens quickly with this receptor

Proprioceptors

• Monitor the position of the joints, tendons/ligaments, state of muscle contraction
• Include Golgi's tendon organs and muscle spindles
• These do not exhibit adaptation

Special Senses Receptor Types

• Receptors located in the sense organs of the head are protected by supporting tissues
• Receptor organs for olfaction (smell), vision, gestation (taste) and equilibrium (balance) and hearing
• Specialized senses receptors are structurally more complex than general senses

Graded Potentials

• Sensory receptors produce different kinds of graded potential in response to a stimulus
• Free nerve endings and encapsulated nerve endings are what create a graded potential
• Receptors for vision, hearing, equilibrium and taste are what creates a receptive potential
• Receptor potential causes exocytosis of the synaptic vesicles
• Neurotransmitter release causes the post synaptic action potential in the first order neuron

Postsynaptic Potentials

• Graded potentials develop in the postsynaptic membrane in response to a neurotransmitter
• Caused by the arrival of neurotransmitter/excitatory neurotransmitters to a postsynaptic membrane
• Inhibitory - related to chemically regulated potassium channels

Receptive Field

• It is the area monitored by a single receptor cell.
• The larger the field, the poorer the ability to localise the stimuli
• Receptive field of a tongue or finger, are <1mm in diameter

Signal Transduction

• Transduction is a process that happens everywhere in the sensory system
• Electromagnetic, mechanical or chemical become electrical potentials

Olfactory Transduction Signal

• (No information given)

Special Senses

• Special cells release neurotransmitters, leads to generator potential in afferent neuron.
• Action potential comes after the generator potential

Adaptation

• Happens when there is a reduced sensitivity in the presence of a constant stimuli
• When sensory neurones change their activity level
• Happens due to reduction in generator potential (gradual)
• Temperature receptors are fast adapting/Tonic receptors show adaptation at a low level
• Allows the body to ignore repetitive non-important info

Central Adaptation

• Happens when there is sensory inhibition

Receptors, Classification, and Adaptation

• Phasic receptors are inactive unless stimulated for a very short time
• Temperature receptors are Phasic
• Rate of action potential decreases/increases depending on intensity of stimulus
• Eg, pain receptors, proprioceptors

Lateral Inhibition

• Happens due to stimulus localization/edges
• Happens in vision and pitch determination
• Collateral connections inhibit neighboring neurons/weak goes weaker/strong stays strong
• Strong signals get stronger

Coding

• Coding happens as the nervous system identifies the location ect of a stimuli
• Receptors encode stimuli by answering 2 questions
• The Adequate stimulus is the stimulus modality a sensory receptor contains
• May have a different stimuli but adequate is the one they choose

Labeled Line Principle

• Modalities (pain touch) are determined by the CNS termination points
• The Labeled is the specificity of modality transferring never fiber
• Each of these has a pathway/receiver and center

Stimulus Intensity - Coding

• Happens by action potential in the frequency
• Localization, acuity is it’s ability
• Is done by interlateral inhibitions

Organization

• First order delivers it
• Second order neurones synapse on synapses or it at brain stem
• Third order neurones, awareness located in thalamus

Non-Specific Pathways

• Transmits sensory information, if it relays thru a particular formation integrates from the sensory modalities

Explore nervous system organization, sensory pathways to the somatosensory cortex, and differentiate sensation and perception. Learn about sensory receptor types, receptor adaptation, and lateral inhibition. Understand how the CNS distinguishes stimulus properties, including sensory modality and location.