General Properties of Sensory Systems and Somatic Senses

General Properties of Sensory Systems

Overview of Sensory Systems

Sensory systems are specialized to detect specific types of stimuli, convert them into neural signals (a process called transduction), and encode information about the intensity, duration, and location of the stimulus. This information is transmitted along labeled lines to the brain, where it is perceived and interpreted.

• Special senses: Vision, hearing, taste, smell, equilibrium

• Somatic senses: Touch, temperature, pain, itch, proprioception

• Stimulus as physical energy: Detected by sensory receptors

• Receptor as transducer: Converts stimulus energy into a change in membrane potential

• Threshold: Minimum stimulus required to generate an action potential to the CNS

• Integration: Occurs in the CNS, leading to conscious perception or subconscious action

Information Processing by the Sensory Division

Sensory information can be processed consciously or subconsciously, depending on the type of stimulus and the pathway it follows.

Stimulus Processing Usually Conscious	Special Senses	Somatic Senses
	Vision, Hearing, Taste, Smell, Equilibrium	Touch, Temperature, Pain, Itch, Proprioception
Stimulus Processing Usually Subconscious	Somatic Stimuli	Visceral Stimuli
	Muscle length and tension, Proprioception	Blood pressure, Distension of gastrointestinal tract, Blood glucose concentration, Internal body temperature, Osmolarity of body fluids, Lung inflation, pH of cerebrospinal fluid, pH and oxygen content of blood

Types and Properties of Sensory Receptors

Receptor Structure and Function

Sensory receptors are specialized to detect particular forms of energy. They can be classified based on their structure and the type of stimulus they detect.

• Simple neural receptors: Free nerve endings (e.g., pain, temperature)

• Complex neural receptors: Nerve endings enclosed in connective tissue capsules (e.g., touch)

• Non-neural receptors: Specialized cells for special senses (e.g., photoreceptors in the eye)

• Accessory structures: Enhance the function of sensory receptors (e.g., lamellar corpuscles in skin)

Types of Sensory Receptors

Receptors are classified by the type of stimulus they detect.

Type of Receptor	Examples of Stimuli
Chemoreceptors	Oxygen, pH, various organic molecules such as glucose
Mechanoreceptors	Pressure (baroreceptors), cell stretch (osmoreceptors), vibration, acceleration, sound
Photoreceptors	Photons of light
Thermoreceptors	Varying degrees of heat

Receptive Fields and Sensory Neurons

Receptive Fields

A receptive field is the physical area where a stimulus activates a sensory neuron. The size and overlap of receptive fields determine the sensitivity and resolution of sensory perception.

• Primary and secondary sensory neurons: Primary neurons receive input from the periphery; secondary neurons relay information to higher centers.

• Overlap and convergence: Neighboring receptive fields may overlap and converge onto a single secondary neuron, reducing spatial resolution.

• Two-point discrimination test: Used to assess the size of receptive fields and sensitivity of different body regions.

• Sensitive areas: Smaller receptive fields, higher resolution (e.g., fingertips)

• Less sensitive areas: Larger receptive fields, lower resolution (e.g., back)

Example: Two-Point Discrimination

When two points are applied to the skin, if they fall within the same receptive field, only one point is perceived. If they fall in separate receptive fields, both points are perceived. This principle is used to map sensory acuity across the body.

Additional info:

• Transduction: The process by which sensory receptors convert physical or chemical stimuli into electrical signals.

• Labeled line coding: The concept that each sensory neuron transmits only one type of sensation to a specific area of the brain.

• Threshold: The minimum stimulus intensity required to generate an action potential in a sensory neuron.