Sensation and the PNS

Overview of the Nervous System and Sensation

Connection of Systems: The Peripheral Nervous System (PNS) and Central Nervous System (CNS) are tied together through two primary pathways: * Sensation: The intake of information through receptors. * Motor Response: The action taken by the body in response to processed sensory information.
Focus of Study: This material initiates the discussion on sensation and introduces the beginning of motor response integration.

Classification of Senses

Special Senses: These are specific, localized senses including: * Vision: Light detection. * Audition: The technical term for hearing. * Gustation: The technical term for taste. * Olfaction: The technical term for smell. * Balance and Equilibrium: Maintenance of body position.
General Senses: These are distributed throughout the body and are divided into two categories: * General Somatic Senses: Senses related to the body surface and musculoskeletal system. These include touch, pressure, vibration, temperature, and pain. They originate from skin, muscles, and joints. * General Visceral Sensations: Sensations originating from internal organs. These include pain, pressure, and stretch. These differ from somatic senses primarily by their origin (organs rather than skin/muscles).

Anatomy and Physiology of General Somatic Sensory Neurons

Neuron Type: The primary sensory neuron for somatic senses is the pseudounipolar neuron.
Neuron Structure: * The entire process moving toward the spinal cord is considered the axon. * Unlike multipolar neurons, pseudounipolar neurons do not have traditional dendrites separate from the axon. * The "receiving end" contains the receptors; signals travel along the axon from the periphery toward the spinal cord.
Sensory Transduction: * Definition: The process of taking a physical sensory stimulus and converting it into an electrical signal that the nervous system can interpret. * Pre-Stimulus State: Before a stimulus occurs, all sensory receptors (which are mechanically gated sodium channels) are closed. * Mechanism of Activation: 1. A stimulus causes mechanically gated sodium ( $Na^+$ ) channels to open. 2. Sodium enters the axoplasm (the cytoplasm of the axon). 3. The entry of positive ions changes the membrane potential, making it more positive (depolarization). 4. This specific change in membrane potential at the receptor site is called a receptor potential. * Generation of Signal: If the receptor potential is sufficiently strong (depolarizing enough to reach a threshold), an action potential is generated. This action potential is the electrical signal that travels to the CNS.

Sensory Adaptation

Definition: The phenomenon where receptors change their sensitivity to a stimulus over time.
Rapidly Adapting Receptors: * The receptor stops sending signals or the signaling diminishes significantly after a period of constant stimulation. * Example: Walking into a house and smelling an air freshener. After approximately $5\,\text{minutes}$ , you no longer detect the scent.
Slowly Adapting Receptors: * The receptor response remains constant even with prolonged stimulation. * Example: The steady pressure experienced when holding an object. You remain aware of the object's presence through constant signaling.

Classification of Receptors by Location and Stimulus Type

Based on Location: * Exteroceptors: Detect stimuli close to the body surface or coming from outside the body. * Interoceptors: Detect stimuli originating from inside the body.
Based on Stimulus Type: * Mechanoreceptors: Respond to physical deformation. * Proprioceptors: Detect joint position and the body's orientation in space. * Pacinian Corpuscles (Lamellated Corpuscles): Detect deep pressure and high-frequency vibration. * Tactile Corpuscles (Meissner's Corpuscles): Involved in light touch and object form detection. * Merkel Cells: Involved in light touch and object formation (knowing what an object is by feel). * Hair Follicle Receptors: Detect the bending of hair. * Thermoreceptors: Respond to temperature changes. * Standard range of detection: $10^{\circ}C$ to $48^{\circ}C$ ( $50^{\circ}F$ to $118^{\circ}F$ ). * Cold Receptors: Detect temperatures between $10^{\circ}C$ and $40^{\circ}C$ . * Warm Receptors: Detect temperatures between approximately $32^{\circ}C$ and $48^{\circ}C$ . * Nociceptors: Pain receptors that respond to noxious or potentially damaging stimuli. * Detection thresholds: Activated by temperatures less than $10^{\circ}C$ (< 10^{\circ}C) or greater than $48^{\circ}C$ (> 48^{\circ}C). * Example: The burning sensation from a hot pepper is detected by nociceptors. * Chemoreceptors: Bind to specific chemicals in air, liquids, or blood. * Detect glucose, ions ( $Na^+$ , $K^+$ , $Cl^-$ , $H^+$ ), and other chemical markers. * Photoreceptors: Respond to light. * Found exclusively in the retina of the eye.

Sensory Fields and Body Segments

Receptive Field: The specific physical area served by a single sensory neuron.
Dermatome: A larger segment of the body/skin where sensory information is supplied by a single spinal nerve. * A spinal nerve contains many individual neurons, so a dermatome is composed of multiple receptive fields.

The Three-Order Sensory Pathway

First Order Neuron: * Anatomy: Pseudounipolar neuron (sensory neuron by function). * Location: The axon is situated in the spinal nerve. The cell body (soma) is located in the dorsal root ganglion. * Path: Enters the spinal cord via the dorsal root and terminates in the dorsal (posterior) gray horn. * Function: Receives stimulus from the periphery and delivers it to the CNS.
Second Order Neuron: * Anatomy: Multipolar interneuron. * Path: 1. Synapses with the first order neuron in the dorsal gray horn. 2. The axon exits the gray horn and enters the posterior (dorsal) white column. 3. It ascends toward the brain. 4. Decussation: Upon reaching the medulla, the axons cross over to the opposite side of the body. * Signals from the left side of the body cross to the right side of the CNS. * Signals from the right side of the body cross to the left side of the CNS. 5. The axon continues ascending through the brainstem (including the midbrain) to the thalamus.
Third Order Neuron: * Anatomy: Multipolar interneuron. * Location: The cell body is in the thalamus. * Function: The thalamus acts as a relay station, directing the signal to the specific appropriate location in the brain. * Termination: The signal is sent to the Primary Somatosensory Cortex (S1).

Neuroanatomy of Sensation and Integration

Primary Somatosensory Cortex (S1): * Located on the postcentral gyrus of the parietal lobe. * This is the location where conscious perception of a stimulus occurs. * Due to decussation in the medulla, the right parietal lobe receives sensations from the left side of the body and vice versa.
Process of Integration: * Information moves from S1 to the Somatosensory Association Cortex (S2). * Integration involves developing a deeper understanding and meaning of the stimulus.
Connected Systems for Integration: * Limbic System: Provides tactile learning, memory, and emotional attachment to sensations. * Basal Nuclei (Basal Ganglia): Involved in planning movement. Note: Although "ganglia" usually refers to the PNS, it is a historical term for these structures in the CNS. * Cerebellum: Involved in movement coordination. * Prefrontal Cortex: Higher-level processing.
Functional Division of the Brain: * Central Sulcus: The divider between the frontal and parietal lobes. * Frontal Lobe: Located anterior to the central sulcus; primarily motor-focused (contains the precentral gyrus). * Parietal Lobe: Located posterior to the central sulcus; primarily sensory-focused (contains the postcentral gyrus).
Transition to Motor Response: Integrated sensory info is sent to the premotor association area to prepare a motor movement in response to the sensation.

Questions & Discussion

Interruption: A student or colleague named Bill entered the room during the lecture. * Transcript Capture: "Yes. Yes. Yes. Yes. Come on in. Sorry about that interruption. All right, Bill, I was going to say…"