Sensory Physiology

Overview of Sensory Systems

Sensory systems provide critical information about both the external environment and the internal state of the body. These systems rely on specialized neurons called sensory receptors that transduce physical or chemical stimuli into electrical signals, which are then processed by the central nervous system (CNS).

• Conscious stimulus processing: Includes somatic senses (touch, temperature, pain, proprioception) and special senses (vision, hearing, taste, smell, equilibrium).

• Subconscious stimulus processing: Includes visceral senses (blood pressure, distension, pH, osmolarity, etc.).

General Properties of Sensory Systems

• Transduction: Sensory receptors act as transducers, converting physical or chemical stimuli into changes in membrane potential (receptor potentials).

• Receptor Potentials: Graded potentials that, if reaching threshold, trigger action potentials in sensory neurons.

• Types of Sensory Receptors:

  • Chemoreceptors: Detect chemical changes (e.g., oxygen, pH, organic molecules).

  • Mechanoreceptors: Respond to pressure, stretch, vibration, acceleration, and sound.

  • Photoreceptors: Detect photons of light (vision).

  • Thermoreceptors: Respond to varying degrees of heat.

  • Nociceptors: Detect pain (can be activated by extreme mechanical, thermal, or chemical stimuli).

• Example: Baroreceptors in blood vessels detect changes in blood pressure.

Receptive Fields

A receptive field is the specific physical area where a stimulus will activate a particular sensory neuron. The size and overlap of receptive fields affect the precision of stimulus localization.

• Convergence of multiple primary sensory neurons onto a single secondary neuron can increase the size of the receptive field and decrease localization precision.

• Smaller receptive fields allow for more precise localization of stimuli.

CNS Integration of Sensory Information

• Sensory information is integrated at multiple levels: spinal cord, brainstem, thalamus, and cerebral cortex.

• Most sensory pathways synapse in the thalamus, which acts as a relay and processing center before projecting to specific cortical regions.

• Special senses (e.g., olfaction) may bypass the thalamus and project directly to the cortex.

• Primary somatosensory cortex is the main cortical area for processing somatic sensory information.

Properties of Stimulus Coding

The CNS distinguishes different aspects of a stimulus through specific coding mechanisms:

• Modality: The type of stimulus (e.g., touch, temperature) is determined by the type of receptor activated and the pathway it follows to the brain.

• Location: Determined by which receptive fields are activated and the specific area of the cortex receiving the input. For auditory stimuli, timing differences between ears help localize sound.

• Intensity: Encoded by the number of receptors activated (population coding) and the frequency of action potentials (frequency coding).

• Duration: Determined by the duration of action potential firing. Some receptors adapt to constant stimuli and reduce their response over time (adaptation).

Key Equations

• Frequency coding: Higher stimulus intensity leads to higher frequency of action potentials.

• Population coding: More receptors activated = greater perceived intensity.

Efferent Division: Autonomic Nervous System (ANS)

Overview of the Autonomic Nervous System

The autonomic nervous system (ANS) controls involuntary functions by regulating smooth muscle, cardiac muscle, glands, and adipose tissue. It is divided into two main branches:

• Sympathetic division: Prepares the body for 'fight-or-flight' responses.

• Parasympathetic division: Promotes 'rest-and-digest' activities.

Autonomic Reflexes and Homeostasis

• Autonomic reflexes are essential for maintaining homeostasis.

• The ANS works closely with the endocrine and behavioral systems.

• Key control centers include the hypothalamus, pons, and medulla.

Antagonistic Control

Most organs are innervated by both sympathetic and parasympathetic fibers, which typically have opposing effects (antagonistic control).

• Example: Sympathetic stimulation increases heart rate, while parasympathetic stimulation decreases it.

Anatomical and Cellular Differences

Autonomic pathways consist of a two-neuron chain:

• Preganglionic neuron: Originates in the CNS and synapses in an autonomic ganglion.

• Postganglionic neuron: Originates in the ganglion and projects to the target tissue.

• One preganglionic neuron may synapse with multiple postganglionic neurons (divergence).

• Cranial nerve X (vagus nerve): Contains about 75% of all parasympathetic fibers.

Summary Table: Sensory Receptor Types and Stimuli

Key Concepts

• Each sensory pathway projects to a specific region of the cerebral cortex, allowing the brain to identify the origin and type of stimulus.

• Stimulus intensity and duration are encoded by the pattern and frequency of action potentials.

• The ANS maintains homeostasis through antagonistic control and reflex pathways.

Additional info: The image labeled "Neuro 9" appears to be a fluorescent micrograph of neural tissue, likely the hippocampus, illustrating the diversity and organization of neurons. This visual is relevant for understanding the complexity of neural circuits discussed in sensory and autonomic physiology.