31 - Part 1: Sensory Nervous System and Somatosensory Physiology Study Notes

Sensory Nervous System

Overview of Sensory Input

The sensory nervous system is responsible for detecting changes in both the external and internal environments and relaying this information to the central nervous system (CNS) for processing.

Sensory input is categorized based on its origin and modality.

• External environment:

  • Somatosensory system (body sensations)

    • Somatic senses: pressure, force, pain, temperature

    • Somesthetic senses: skin sensations

    • Proprioception: muscle position and movement

  • Special senses: vision, hearing, taste, touch, smell, equilibrium

• Internal environment:

  • Visceral afferent: detects changes via thermoreceptors, chemoreceptors, baroreceptors, nociceptors

Afferent signals travel from the periphery to the CNS.

Receptor Physiology

Sensory Receptors and Sensory Units

Sensory receptors are specialized cells or structures that detect specific types of stimuli (modalities) and initiate neural signals. A sensory unit consists of a single afferent neuron and all the receptors associated with it.

• Law of specific energies: Each receptor is sensitive to a particular type of stimulus (e.g., light, sound, pressure, temperature, chemicals).

• Activation of a receptor generates a receptor potential, which may lead to an action potential if the stimulus is adequate.

Receptor Potentials and Transduction

** Receptor potentials are graded electrical changes in response to stimuli **

-If the stimulus reaches the adequate threshold, it triggers action potentials in the afferent neuron.

• Transduction: The process by which stimulus energy is converted into electrical signals in the nervous system.

• Action potentials are generated when the receptor potential exceeds threshold.

Sensory Pathways

Coding for Stimulus Type

The nervous system identifies the type of stimulus based on the receptor activated and the specific neural pathway used.

• Labeled line theory: Each sensory modality has a dedicated neural pathway to the brain, allowing precise identification of the stimulus type.

• Examples: Visual cortex for sight, auditory cortex for sound, somatosensory cortex for touch.

Stimulus Location

The location of a stimulus is perceived by the brain based on the origin of the sensory input.

• Perception: Awareness of where the sensation is coming from.

• Information is coded for intensity and quality of the stimulus.

Sensory Coding: Stimulus Intensity & Quality

Stimulus intensity and quality are encoded by the nervous system using two main mechanisms:

• Frequency of action potentials: Stronger stimuli produce higher frequency of action potentials.

• Number of receptors activated (recruitment): Stronger stimuli activate more sensory units.

Somatosensory System

Types of Somatosensory Receptors

The somatosensory system detects sensations such as pressure, temperature, pain, and body position. Different receptors are specialized for each type of sensation.

• Mechanoreceptors: Detect pressure, force, and vibration (typically in the skin).

• Thermoreceptors: Detect temperature changes (typically in the skin).

• Nociceptors: Detect harmful or pain stimuli (typically in the skin).

• Proprioceptors: Detect body position and movement (typically in muscles).

• Muscle spindles: Detect muscle length (a type of proprioceptor).

The Pain Response

Pain Sensation and Perception

Pain is produced by tissue-damaging stimuli or stimuli that can potentially cause tissue damage. The pain response involves multiple components:

• Autonomic responses: Changes in heart rate, blood pressure, etc.

• Emotional responses: Feelings associated with pain.

• Pain perception: Depends on past experiences and context.

Control of Skeletal Muscle for Coordinated Activity

Key Muscle Receptors

Coordination of muscle activity across joints relies on sensory feedback from specialized receptors.

• Muscle spindle (proprioceptor): Senses changes in muscle length and velocity.

• Golgi tendon organ (mechanoreceptor): Senses changes in muscle tension (force).

Muscle spindles are composed of internal (intrafusal) fibers, connective tissue, and sensory nerve endings. They detect changes in muscle length and contribute to reflex excitation.

• Reflex excitation: Activation of muscle spindle leads to contraction of the muscle.

• Sensitivity: Adjusted by intrafusal fibers and gamma motor neurons.

Muscle Spindle Responses and Stretch Reflex

The stretch reflex ensures proper neural excitation and muscle tone. It is a protective mechanism that prevents overstretching.

• Activation of muscle spindle leads to stimulation of alpha motor neurons, causing muscle contraction.

• Coactivation of alpha and gamma motor neurons maintains spindle sensitivity during muscle contraction.

Muscle Spindle Reflex in Stretching

During stretching exercises, muscle spindle activity can be modulated to increase range of motion or decrease peak passive force.

• Decreased muscle spindle activity allows greater flexibility.

• Same peak passive force with increased range of motion is possible through proper stretching techniques.

Golgi Tendon Organs (GTO)

Golgi tendon organs are sensory capsules within tendons that detect high force tendon stretch.

• Activation of GTO leads to reflex inhibition of muscle via type Ib afferent neurons.

• Protects muscles and tendons from excessive force.

GTO Activation in PNF Stretching

Proprioceptive Neuromuscular Facilitation (PNF) stretching utilizes GTO activation to enhance flexibility.

• Sequence: Stretch → Contract (activates GTO) → Stretch

• GTO activation leads to muscle relaxation, allowing greater stretch.

Brainstem and Cerebellum

Brainstem

The brainstem connects the forebrain and cerebellum to the spinal cord and serves as a processing center for most cranial nerves.

• Includes midbrain, pons, and medulla oblongata.

• Responsible for reflexive functions and basic life support.

• Processes sensory and motor information for 10 out of 12 cranial nerves.

Cranial Nerves

Cranial nerves are essential for sensory and motor functions of the head and neck. The following table summarizes their classification and function:

Number	Name	Nerve Class	Function
I	Olfactory	Sensory	Olfaction (smell)
II	Optic	Sensory	Vision
III	Oculomotor	Mixed	Eye movements, pupillary reflex, accommodation reflex, proprioception (position of muscles and joints)
IV	Trochlear	Mixed	Eye movement, proprioception
V	Trigeminal	Mixed	Motor control of chewing muscles, sensations of face, nose, mouth
VI	Abducens	Mixed	Eye movements
VII	Facial	Mixed	Motor control of facial muscles, salivary glands, tear glands; somatic sensations of face
VIII	Vestibulocochlear	Sensory	Hearing, equilibrium
IX	Glossopharyngeal	Mixed	Motor control of swallowing and salivary glands; taste; visceral afferent from pharynx; negative feedback from baroreceptors
X	Vagus	Mixed	Motor and sensory afferent of thoracic and abdominal viscera; motor control of larynx and pharynx
XI	Accessory	Mixed	Motor control of larynx and pharynx
XII	Hypoglossal	Mixed	Motor and somatic sensations of tongue

Cerebellum

The cerebellum is a bilaterally symmetrical structure known as the "little brain." It is essential for motor coordination, balance, and the coordination of eye and body movements.

• Receives sensory input from muscles and joints.

• Integrates information to fine-tune motor activity.

Additional info: The notes above expand on the original slides and handwritten content by providing definitions, examples, and context for each physiological concept. Equations are not directly relevant to these topics, but population coding and action potential frequency can be mathematically described as:

• Action potential frequency: $f = \frac{N_{AP}}{\Delta t}$, where $N_{AP}$ is the number of action potentials in time interval $\Delta t$.

• Population coding: $I_{stimulus} \propto N_{receptors}$, where $I_{stimulus}$ is stimulus intensity and $N_{receptors}$ is the number of activated receptors.