Anatomy & Physiology - Nervous System: General Senses
Chapter 16: Nervous System - General Senses
Overview of the Nervous System and Senses
Sensory Information
Continuously exists in our environment, providing details about both internal and external conditions.
Receptors: Specialized structures that respond to stimuli.
Function: Initiate sensory input to the central nervous system (CNS).
Sensation: The conscious awareness of sensory information.
Energy and Receptors
The nervous system utilizes one primary form of energy: electrical energy.
Receptors as Transducers:
Convert one energy form into another, specifically converting original energy associated with the specific receptor type into electrical energy.
Examples:
Light energy in the eye converts to electrical energy.
Sound energy in the ear converts to electrical energy.
The converted electrical energy is then conducted along a sensory neuron.
Receptive Fields
Definition: The area from which a receptor can detect stimuli.
Types:
Small Receptive Field: More precise location identification.
Large Receptive Field: Less precision in localization.
Inverse Relationship: There is an inverse relationship between the size of the receptive field and the ability to identify location.
Properties of Sensory Receptors
Tonic Receptors
Characteristic: Continuously receive and process stimuli at a constant rate.
Example: Balance receptors in the inner ear.
Response:
Strength of response remains constant with continuous exposure to a stimulus.
Phasic Receptors
Characteristic: Detect new stimuli or changes in ongoing stimuli.
Example: Tactile receptors in the skin.
Response:
Sensitivity decreases (adaptation) with continued exposure to a stimulus.
Example: Loss of awareness of pressure while sitting in a chair.
Receptor Classification
By Distribution
General Sense Receptors:
Located throughout the body.
Types:
Somatic Sensory Receptors:
Housed within the skin, joints, muscles, and tendons.
Function: Detect pressure, vibration, pain, stretch.
Visceral Sensory Receptors:
Located in the walls of viscera.
Function: Respond to temperature, chemical changes, stretch, and pain.
Special Senses Receptors:
Located in the head, comprising specialized sense organs known for the five special senses:
Gustation: Taste.
Olfaction: Smell.
Vision: Sight.
Hearing: Audition.
Equilibrium: Balance and acceleration.
By Stimulus Origin
Exteroceptors:
Detect stimuli from the external environment (e.g., skin receptors, special sense receptors).
Interoceptors:
Detect stimuli in internal organs (primarily stretch in smooth muscle walls, temperature, chemicals, and pain).
Proprioceptors:
Detect body and limb movements, including skeletal muscle contraction and stretch.
Located in muscles, tendons, and joints.
By Modality of Stimulus
Chemoreceptors:
Detect chemicals dissolved in fluids, including food chemicals, body fluids, and inhaled gases.
Example: Taste buds on the tongue, chemoreceptors in blood vessels monitoring oxygen and carbon dioxide levels.
Thermoreceptors:
Respond to temperature changes; found in skin and hypothalamus.
Vital for reflexes that maintain body temperature.
Photoreceptors:
Located in the eye; detect changes in light intensity, color, and movement.
Mechanoreceptors:
Respond to touch, pressure, vibration, and stretch.
Include most cutaneous receptors and those in the ear.
Baroreceptors:
Detect changes in stretch or distension in vessel or organ walls, and are involved in blood pressure regulation.
Nociceptors:
Respond to painful stimuli:
Somatic Nociceptors: Detect chemical, heat, or mechanical damage to the body surface or skeletal muscles.
Visceral Nociceptors: Detect internal damage within viscera (e.g., oxygen deprivation, overstretched smooth muscles, tissue trauma).
Tactile Receptors Classification
Types of Tactile Receptors in the Skin
Tactile Receptors:
Most numerous type of receptor; mechanoreceptors in the dermis and subcutaneous layer.
Respond to touch, pressure, and vibration.
Unencapsulated Tactile Receptors
Definition: Dendritic ends of sensory neurons without protective sheathing.
Types:
Free Nerve Endings: Respond to various stimuli.
Root Hair Plexuses: Surrounds hair follicles, detecting hair movement.
Tactile Discs: Detect fine touch and pressure.
Encapsulated Tactile Receptors
Definition: Wrapped by connective tissue or covered by glial cells.
Include:
End Bulbs: Detect low-frequency vibrations.
Lamellated Corpuscles: Detect deep pressure.
Bulbous Corpuscles: Detect sustained pressure and distortion.
Tactile Corpuscles: Detect light touch and texture.
Referred Pain
Definition: Pain perceived in locations other than the actual site of the stimulus.
Sensory nerve signals from certain viscera can be misinterpreted by the brain as originating from skin areas (dermatomes) due to shared sensory pathways in the spinal cord.
Clinical Importance:
The sensory cortex may confuse actual and false stimuli resulting in mislocalized pain.
Common Sources of Referred Pain
Cardiac Problems:
Often referred pain along the medial side of the left arm due to sympathetic innervation through spinal segments T1-T5.
Kidney and Ureter Pain:
Can be referred to T10-L2 dermatomes, affecting the groin and loin area.
Visceral Pain:
Generally follows sympathetic or parasympathetic pathways.
Example: Urinary bladder pain can follow pelvic splanchnic nerves in S2-S4 with referral to the medial buttocks.
Common Sites of Referred Pain:
Liver, gallbladder, heart, appendix, ureter, urinary bladder, kidneys, ovaries, pancreas, stomach.
Phantom Pain
Description: Sensation of pain occurring in a removed body part.
Occurrence: Common after the amputation of a limb.
Mechanism: Pain can arise from stimulation of sensory neuron pathways that still remain functional in the remaining tissues even though the limb is no longer present.
The neuronal cell body persists, contributing to persistent pain sensations.