MMED2931 l2 p2 Detailed Study Notes on the Sensory Nervous System

The sensory nervous system consists of three main components:
  - Sensory receptors
  - Neuronal pathways
  - Parts of the cerebral cortex involved in sensory perception
Sensory perception is defined as the conscious awareness of the surrounding environment.
There are five major senses:
  - Vision: ability to see light and perceive images.
  - Hearing: ability to perceive sounds.
  - Somatic sensation: touch and physical interaction with the environment.   - Gustatory sensation: sense of taste.
  - Olfaction: sense of smell.
Additionally, there is a sixth sense:
- Vestibular sensory system: provides a sense of balance and self-motion.

Sensory receptors evolve to detect specific modalities of stimuli, such as:   - Heat
  - Light
  - Pressure
These receptors introduce stimuli into changes in membrane potential and generate action potentials that are relayed to the central nervous system (CNS).

Chemoreceptors
   - Sensitive to specific chemicals.    - Taste chemoreceptors: involved in the sense of taste.
   - Olfactory chemoreceptors: involved in the sense of smell.
   - Arterial oxygen-chemoreceptors: located in the carotid body, detecting oxygen levels in blood.
   - Can also sense pH and concentration of hydrogen ions, also found in the carotid body and brainstem (medulla) and in the gut.
Photoreceptors
- Detect light; include rods (for low light sensitivity) and cones (for color vision) found in the retina.
Thermoreceptors
   - Two subtypes:
     - Surface temperature receptors in the skin (hot and cold receptors).
     - Core temperature receptors located in the hypothalamus (hypothalamic warm-sensitive and cold-sensitive neurons).
Nociceptors
- Specialized receptors that detect pain, responding to damaging stimuli such as thermal, mechanical, and chemical injuries.
Mechanoreceptors
- Sensitive to mechanical energy changes (pressure, touch, vibration).
- Located in the skin (cutaneous mechanoreceptors), skeletal muscles, and joints (proprioceptors). - Proprioceptors include muscle spindles (detecting muscle length and force) and mechanoreceptors in the inner ear (auditory function). - Mechanoreceptors detect fullness in the gastrointestinal tract and blood pressure (baroreceptors).

Responsible for sensations from skin, skeletal muscle, and joints, engaging both cutaneous mechanoreceptors and proprioceptors.
In the skin, nociceptors can be subdivided into: - Polymodal nociceptors: respond to multiple types of stimuli including temperature changes and mechanical phenomena.

Sensory neurons have cell bodies located in the dorsal root ganglia, with axons projecting peripherally (to the skin) and centrally (to the spinal cord).
Mechanogated channels are responsible for generating a receptor potential, which results in a graded potential corresponding to the intensity of the stimulus detected.
Key concepts include: - Action potential generation: occurs if the receptor potential exceeds a threshold at the trigger zone. - The frequency of action potentials encodes the intensity of the stimulus: higher stimulus strength results in: - Longer barrage of action potentials. - Higher frequency of action potentials (measured in hertz).

Sensory neurons branch extensively in the periphery; an area within which a stimulus can elicit significant action potential changes is called a receptive field.
The size of the receptive field is contingent on the branching characteristics of the sensory neuron: - Smaller branching leads to a smaller receptive field; extensive branching leads to a larger field.
Mechanoreceptors respond to various skin stimuli:
  - Stretch
  - Steady pressure
  - Stroking
  - Vibration
  - Texture
Two-point discrimination test: tests the ability to distinguish between two separate stimuli on the skin. It depends on: - Size of the sensory neuron receptive field. - Convergence of primary sensory neurons in the spinal cord. - Two examples: 1. A sensory neuron with a large receptive field connecting to a single spinal cord neuron leads to perceiving two stimuli as one. 2. A small receptive field in the fingertips connects to separate spinal cord neurons, allowing distinct sensations for two stimuli.
Receptive field sizes demonstrated: - Calf or back: receptive field can exceed 4 cm. - Fingertips: receptive field can be as small as 2 mm, allowing for high sensitivity and discrimination.

Merkel cells
- Slowly adapting mechanoreceptors that detect form and texture of objects. - Approximately 10-15 Merkel cells are supplied by one A beta fiber.
Ruffini endings
- Slowly adapting mechanoreceptors that respond to skin stretch during movements.
- Each Ruffini ending is supplied by one A beta sensory fiber.
Meissner's corpuscles
- Rapidly adapting mechanoreceptors; detect small skin indentations and motions.
- Up to three A beta fibers supply a single Meissner's corpuscle.
Pacinian corpuscles
- Rapidly adapting mechanoreceptors; sensitive to vibration and deep skin indentation.
- One A beta fiber supplies one Pacinian corpuscle.

Braille is a reading system for the visually impaired, utilizing textured patterns on paper read through fingertips.
Each dot corresponds to a significant mechanosensory response: - Only slow-adapting Merkel cell afferents provide precise representations of the Braille pattern, enabling high fidelity dot discrimination.

Two important landmarks in the human cerebral cortex:
- Central sulcus: separates the frontal lobe from the parietal lobe.
- Lateral sulcus/Sylvian fissure: separates the frontal lobe from the temporal lobe.
Major focus is on the postcentral gyrus in the parietal lobe, the location of the primary somatosensory cortex which processes information from skin receptors, including mechanoreceptors, thermal receptors, and nociceptors.
Next lecture: discussing the area anterior to the central sulcus, known as the motor cortex.