Nervous System Organization and Sensory Pathways

Learning Objectives

• Review the organization of the nervous system.
• Show the organization of sensory pathways from receptors to the somatosensory cortex on the postcentral gyrus.
• Compare and contrast sensation and perception.
• Describe events in sensations.
• Describe the different types of sensory receptors based on structure, function, and location.
• Describe classes of receptors in the general senses, and their functions.
• Illustrate receptor adaptation and its function, provide examples of tonic and phasic receptors, and distinguish between peripheral and central adaptation.
• Distinguish between receptor adaptation and lateral inhibition.
• Explain lateral inhibition in a diagram and how it allows for stimulus localization.
• Explain how the CNS distinguishes sensory modality, stimulus location, stimulus intensity, and stimulus duration.
• Define and explain the labeled line principle.
• Demonstrate an understanding of the basis for sensory homunculus.

Nervous System Organization

• The central nervous system (CNS) receives, processes information, and initiates action.
• The peripheral nervous system (PNS) transmits signals between the CNS and the rest of the body.
• The brain receives sensory information, 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 that control the activities of 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 in the "fight or flight" response.
• The parasympathetic division dominates during "rest and rumination" and directs maintenance activities.

Sensations

• Sensation is the conscious or subconscious awareness of external or internal stimuli.
• The nature of sensation and reaction depends on the destination of nerve impulses.
• Sensory impulses to the lower brain stem elicit complex reflexes like heart rate or breathing changes.
• Sensory impulses to the thalamus provide crude awareness of body location and sensation type, like touch, pain, hearing, or taste.
• Sensory impulses to the cerebral cortex precisely locate and identify specific sensations.

Perception

• Perception is the conscious awareness and the interpretation of the meaning of sensations.
• Memories of previous sensations are stored in the cortex.
• Sensory impulses that do not reach the cerebral cortex don't lead to perception.

Sensory Modalities

• Sensory modality refers to each unique type of sensation, such as pain, hearing, vision, and touch.
• A individual sensory neuron exclusively carries information for one modality.

Classes of Sensory Modalities

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

Sensation Start

• Sensation starts in a sensory receptor, which can be a specialized cell or the dendrites of a sensory neuron.
• Sensory receptors respond vigorously to one kind of stimulus, weakly or not at all to all others displaying selectivity/specificity.
• Receptors exhibit adaptation; loss of sensitivity after constant stimulus exposure.
• Stimuli can be electromagnetic energy like light or heat.
• Stimuli can be mechanical energy such as sound waves or pressure changes.
• Stimuli can be chemical energy such as COz or changes in pH.

Events in Sensation

• A stimulus must occur within the sensory receptor's receptive field.
• A receptive field defines the region monitored by a receptor cell.
• The stimulus is converted into a graded potential, which is transduction.
• When the graded potential reaches threshold in a sensory neuron, it triggers one or more action potentials; action potential/ impulse generation.
• Sensory neurons that propagate impulses from the PNS into the CNS are first-order neurons.
• A particular CNS region receives and integrates sensory nerve impulses; integration of sensory input.
• Conscious sensations and perceptions are integrated in the cerebral cortex.

Types of Sensory Receptors

• Free nerve endings are bare dendrites without visible structural specialization and receive stimulus like pain, thermal, tickle,itch.
• Encapsulated nerve endings at the dendrites are enclosed in a connective tissue capsule with microscopic structure, like lamellated Pacinian corpuscles.
• Receptors for somatic and visceral sensations such as touch, pressure, and vibration are encapsulated nerve endings.

Specialized Separate Cells

• Sensory receptors for the special senses of vision, hearing, equilibrium, and taste are specialized separate cells synapsing with first-order neurons.

Receptor Location

• Exteroreceptors are located in or near the body's external surface
• Exteroreceptors supply information on changes in the external environment.
• Exteroreceptors include receptors for touch (Meissner's corpuscles, Merckel's discs), cold, heat and pain, vision, and smell
• Interoceptors transmit impulses from visceral organs like blood vessels and the nervous system
• Interorecptors are usually sensations not consciously perceived.
• Telereceptors, are distance receptors, they inform of stimuli reaching remote sources.
• Telereceptors include the receptors present in the sense organs like sight, hearing, and olfaction (smell).
• Proprioceptors contain information with movements and position of parts of the body in space.
• Proprioceptors are found in muscles, tendons, joints, (golgi tendons, muscle spindle receptors), and in the labyrinth of the inner ear.

Classes Based On Stimulus Type

• Mechanoreceptors are responsible for mechanical forces of touch, pressure, stretch, vibrations.
• Thermoreceptors are responsible for temperature changes
• Chemoreceptors are responsible for chemicals in solution- taste, smell, blood chemistry that detect chemicals in the mouth and body fluids
• Photoreceptors respond to light (eye).
• Nociceptors respond to harmful stimuli (pain).

General Senses

• General senses are scattered throughout the body and simple in structure- temperature, pain, touch, chemicals in blood.

General Senses Receptor Types: Thermoreceptors

• Temperature receptors are free nerve endings in the dermis of skin, skeletal muscles, liver, & hypothalamus.
• Cold receptors are three times more numerous than warm receptors
• Thermo receptors are rapidly adapting.

General Senses: Chemoreceptors

• Specialized chemoreceptive neurons that detect small changes in the concentration of specific chemicals/compounds
• Chemoreceptors respond to water and lipid soluble substances dissolved in surrounding fluid
• General senses chemoreceptors send information to cerebral cortex, creating no conscious awareness.
• Chemoreceptors are located to monitor the carbon dioxide an oxygen concentration of aerial blood

General Senses: Mechanoreceptors

• Sensitive to stimuli that distort membranes.
• Membranes contain mechanically regulated ion channels altering distortion

Tactile Receptors

• Provide sensations of touch, pressure, and vibration.
• Free nerve endings are in the corneal surface of the eye
• Nerve endings of root hair plexus monitor distortions and movements across the body
• Hair follicle movement distorts sensory dendrites, producing an action potential.
• Other tactile receptors include Merkel's discs, Meissner's corpuscles, pacinian corpuscles, & Ruffini corpuscles.

Baroreceptors

• Consist of free nerve endings branching within the elastic tissues in the walls of a distensible organ
• Pressure receptors in walls of blood vessels: carotid, aorta
• Important in regulation of cardiac function, pace of respiration, urination, defecation.
• Rapidly adapting mechanoreceptors.

Proprioceptors

• Monitor joint position, tendon tension, ligament state, and muscle contraction,
• Examples, golgi tendon organs and muscle spindles.
• They do not exhibit adaptation.

Special Senses Receptor Types

• Receptors are located in sense organs in the head, where they are protected by surrounding tissues.
• Receptor organs are for olfaction smell, vision, gastation taste, equilibrium balance and hearing.
• They are structurally more complex.

Graded Potentials

• Sensory receptors produce different kinds of graded potentials:
• Generator potential occurs in stimulated free/encapsulated nerve endings.
• Receptor potential happens in receptors in special senses like vision, hearing, equilibrium & taste
• Post synaptic potential may trigger one or more action potentials

Postsynaptic Potentials

• Graded potentials 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 Qualities

• The receptive field is the area monitored by a single receptor cell.
• Larger receptive fields correlate to a poorer ability to localize stimuli
• The area most related to higher ability to localize stimulus is tongue or finger receptive with less than 1mm.

Signal Transduction

• Transduction is the unique physiological process common to all sensory systems.
• Stimulus energy electromagnetic, mechanical, chemical are coverted to electrical potentials

Signal Transduction For Special Senses

• Specialized receptor cells neurotransmitter relate to generator potential
• After the event, action potential develops arriving genertator

Adaptation

• Adaptation is the reduction in sensitivity in the presence of a constant stimulus
• Peripheral adaptation happens when receptors or sensory neurones alter activity level.
• Temperature receptors are fast adapting.
• Tonic receptors indicate little peripheral adaptation
• Peripheral adaptation reduces the amount of info that reaches e CNS
• Allows the body to ignore constant unimportant information (smell)
• Central adaptation occurs in the CNS and involves inhibition of nuclei along a sensory pathway

Phasic vs Tonic Receptors

• Phasic receptors are normally inactive until a rapid activing to become an active for a long time (temp)
• Tonic receptors are active constantly but can increase or decrease depending stimulus

Local Inhibition of Stimulus

• Refers to Localistation through pitch differentation
• Collateral connections
• Weak signals are weaker while storng sgns get relatively stronger

Sensory Coding

• Allows nervous system to identity type/ location or duration
• The specific Adequate type is what stimulus can be responsible for.

Labelled Line Principle

• Different modalities depend on termination point in the CNS
• This principle specifics nerve fibres will only transit only one type of modality
• Each modality has a specific receiver
• Pathway and centre location of it brain are responsible for sensation of stimuli

Intensity / Location in CNS

• Intensity is coded by frequency of potential / number of receptors
• Stimuli Licalisaiton allows to locate stimus and is achieved through CNS
• Stimulus duration is adatplaton to constant stimuli

Sensory Pathways

• First order neuron delivers to CNS
• Secound order is inerneruosn
• Thrid order is second
• 3 levels of consciousness

Specific Vs Nonspecific Ascending Pathways

• Specific pathway modality transmit to cerebral
• Nonspecific relays in the reticular which the nervous system detects.

Dermatome

• Contains sensory neurons served to predictable segment
• The nerve apply as if one area overlaps

Explore the organization of the nervous system and sensory pathways from receptors to the somatosensory cortex. Compare sensation and perception, detailing events in sensations and different sensory receptor types based on structure, function, and location. Understand receptor adaptation, lateral inhibition, and how the CNS distinguishes sensory modality, stimulus location, intensity, and duration.