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Somatosensory System

1. Somatosensory Transduction

• Transduction: The process by which mechanical stimuli (touch, vibration, pressure) are converted into electrical signals via ion channels opening in sensory receptors.

• Ion Channels: Proteins embedded in the cell membrane that open in response to mechanical pressure, allowing positively charged ions like sodium (Na+) to enter the cell and create an electrical signal.

• Example: When you press your fingertip on a surface, these ion channels trigger nerve impulses that tell your brain you're touching something.

2. Receptive Fields

• Center-Surround Fields:

  • Touch in Center → Excitation (strong response)

  • Touch in Surround → Inhibition (reduced response)

• Types of Responses:

  • Phasic Response: Short burst of firing, then silence.

  • Tonic Response: Sustained firing as long as the stimulus is present.

• Purpose: Enhances sensory detail by increasing contrast (similar to visual lateral inhibition, which sharpens visual edges).

3. Homunculus

• A map in the primary somatosensory cortex (S1) that represents the body based on sensory importance.

• Larger Representation: Areas like fingers and lips have more sensory receptors and thus occupy more cortical space.

• Also Found in Motor Cortex: Reflects regions requiring precise motor control.

4. Pain Fibers

• Free Nerve Endings: Detect pain, temperature, and chemical irritation.

• A-Delta Fibers: Myelinated fibers that conduct first pain (sharp, immediate pain).

• C Fibers: Unmyelinated fibers that conduct second pain (dull, lingering pain).

• Capsaicin Sensitivity: C fibers respond to capsaicin, explaining the burning sensation from spicy food.

5. Pathways

• Dorsal Column (Touch Pathway):

  • Carries touch and proprioception signals.

  • Crosses at the medulla (a part of the brainstem where sensory information crosses to the opposite side of the brain).

• Lateral Pathway (Pain Pathway):

  • Carries pain and temperature signals.

  • Crosses immediately at the spinal cord.

• Why Separate Pathways? This separation helps your brain process touch and pain signals more efficiently.

6. Pain Perception Pathways

• Motivational System: Adds emotional response to pain (e.g., frustration, anxiety).

• Somatosensory System: Detects the physical sensation of pain.

• Emotional Pain Processing: Involves the anterior cingulate cortex (linked to emotional awareness) and insula (linked to bodily sensations and emotional states).

7. Pain Relief

• Gate Control Theory: Non-painful stimuli (like rubbing a sore spot) can reduce pain by "closing the gate" to pain signals.

• TENS Stimulation: Electrical stimulation that activates non-pain touch fibers to reduce pain.

• Opioids: Block pain by mimicking endorphins in the brain.

• Placebos: Activate the body’s endorphin system to reduce pain.

• Anti-inflammatories: Block chemicals that cause inflammation and pain.

8. Primary Somatosensory Cortex (S1)

• Location: Postcentral gyrus of the parietal lobe.

• Organization: Arranged in a homunculus.

• Information Flow: Processes contralateral sensory input (left brain controls right side of body and vice versa).

Motor System

1. Neuromuscular Junction

• Parts:

  • Motor Neuron Axon Terminal – Releases neurotransmitters.

  • Muscle Fiber Receptor Site – Receives neurotransmitters to initiate muscle contraction.

• Neurotransmitters Released: Acetylcholine (ACh) is the primary neurotransmitter, binding to receptors on the muscle fiber to trigger contraction.

2. Motor Pathways

• Pyramidal Pathway: Originates in the primary motor cortex, descending through the spinal cord to control voluntary movements.

• Extrapyramidal Pathway: Originates in the basal ganglia and helps regulate posture, muscle tone, and involuntary movement control.

• Key Difference:

  • Pyramidal = Direct control of movement.

  • Extrapyramidal = Indirect coordination and refinement of movement.

3. Proprioception

• Definition: The body’s ability to sense its position, movement, and orientation in space.

• Importance: Essential for balance, coordination, and controlled movement without constant visual guidance.

• Receptors: Muscle spindles (detect stretch) and Golgi tendon organs (detect tension).

4. Motor Unit

• Definition: A motor neuron and all the muscle fibers it controls.

• Contact: Fine motor skills require fewer muscle fibers per neuron (e.g., fingers), while larger muscle groups require more muscle fibers per neuron for power (e.g., legs).

5. Primary and Secondary Association Cortices

• Premotor Area: Involved in movement sequences guided by external stimuli; key for planning observed movements.

• Supplementary Area: Involved in preplanned sequences like playing a musical instrument.

• Mirror Neurons: Located in the premotor cortex; these neurons fire both when performing an action and observing others perform the same action, supporting learning and imitation.

6. Movement Disorders

• Motor Cortex Damage: Leads to paralysis or weakness in voluntary movements.

• Basal Ganglia Damage: Results in issues like tremors, involuntary movements, or muscle rigidity.

• Apraxia: Inability to perform coordinated movements despite intact motor function.

• Huntington’s Disease: Progressive neurological disorder caused by basal ganglia degeneration; results in involuntary jerky movements and cognitive decline.

• Parkinson’s Disease: Caused by dopamine loss in the basal ganglia; symptoms include tremors, rigidity, and bradykinesia (slowness of movement).

Hearing System

1. Sensory Cells

• Hair Cells: Located in the cochlea; these cells respond to sound waves by converting mechanical vibrations into electrical signals.

• Are They Neurons? No, but they release neurotransmitters to activate auditory neurons.

2. Transduction in Hearing

• Vibrations from the basilar membrane move hair cells, which then release neurotransmitters to activate the auditory nerve.

3. Types of Hearing Loss

• Central Hearing Loss: Damage to the auditory cortex; affects sound interpretation.

• Conductive Hearing Loss: Occurs in the middle ear (e.g., damaged ossicles).

• Sensorineural Hearing Loss: Results from damage to hair cells or the auditory nerve.

4. Parts of the Ear

• External Ear: Pinnae and auditory canal gather sound.

• Middle Ear: Ossicles (tiny bones) amplify sound vibrations.

• Inner Ear:

  • Cochlea: Processes auditory information.

  • Semicircular Canals & Saccule/Utricle: Detect balance and head position.

5. Sound Localization

• The superior olivary complex in the brainstem helps determine sound direction by comparing input from both ears.

6. Sound Basics

• Amplitude: Determines loudness; higher amplitude = louder sound.

• Frequency: Determines pitch; higher frequency = higher pitch.

7. Cochlear Implants

• Bypass damaged hair cells by directly stimulating the auditory nerve; helps people with severe hearing loss regain sound perception.

8. Presbycusis

• Age-related hearing loss; higher frequencies are typically the first to deteriorate.

• Example: A 20-year-old can hear higher frequencies than a 70-year-old.

Balance, Taste, and Smell

1. Vestibular System

• Otolith Organs (Utricle & Saccule): Detect linear acceleration and head position.

• Semicircular Canals: Detect rotational movement.

2. Taste

• Five Tastes: Sweet, salty, sour, bitter, umami (savory).

• Taste Transduction: Some rely on ion channels (e.g., salty), while others use G-protein-coupled receptors (e.g., sweet, umami).

3. Smell

• Glomeruli: Clusters of neurons in the olfactory bulb that receive signals from olfactory receptors.

• Pathway: Neurons → Olfactory bulb → Prefrontal cortex and amygdala (emotional response to smells).

• Regeneration: Olfactory receptor cells regenerate regularly, unlike most neurons.

Vision System

1. Visual Anatomy

• Fovea: Central part of the retina with the highest concentration of cones; responsible for sharp vision.

• Photoreceptors:

  • Rods: Detect dim light; specialized for night vision.

  • Cones: Detect color and require bright light.

2. Visual Pathway

• Retina → Optic Nerve → Optic Chiasm → LGN (Lateral Geniculate Nucleus) → Primary Visual Cortex (V1).

• LGN: Contains Magnocellular layers (motion detection) and Parvocellular layers (color and detail detection).

3. Visual Processing

• Receptive Fields:

  • On-center/Off-surround: Center light excites, surrounding light inhibits.

  • Off-center/On-surround: Center light inhibits, surrounding light excites.

• Dorsal ("Where") Stream: Processes motion and spatial location.

• Ventral ("What") Stream: Identifies objects.

4. Visual Disorders

• Color Blindness: Result of missing or defective cones.

• Agnosia: Inability to recognize objects despite intact vision.

5. Illusions and Perception

• Lateral Inhibition: Sharpens visual contrast by inhibiting neighboring cells.

• Opponent Process Theory: Explains color perception in paired opposites (e.g., red-green).

• Trichromatic Theory: Proposes three types of cones detect blue, green, and red wavelengths.

Pathways in the Somatosensory System

1. Dorsal Column (Touch Pathway):

   • Function: Carries signals related to touch and proprioception (the sense of body position).

   • Pathway:

     • Sensory neurons from the skin and proprioceptors in muscles send signals through the dorsal roots of the spinal cord.

     • These signals ascend ipsilaterally (on the same side) through the dorsal columns of the spinal cord.

     • At the level of the medulla, these signals synapse and then cross to the opposite side (decussation).

     • After crossing, they travel to the thalamus and then onward to the primary somatosensory cortex (S1).

   • Why Different: This pathway is primarily concerned with fine touch and positioning, allowing for detailed sensory information without immediate pain interference.

2. Lateral Pathway (Pain Pathway):

   • Function: Carries signals related to pain and temperature sensations.

   • Pathway:

     • Sensory neurons from nociceptors (pain receptors) and thermoreceptors (temperature receptors) enter the spinal cord through the dorsal roots.

     • Here, the axons of these neurons immediately synapse and decussate (cross over) within the spinal cord.

     • The signals then ascend via the spinothalamic tract to the thalamus before being relayed to the primary somatosensory cortex.

   • Why Different: This pathway processes the immediate response to potentially harmful stimuli, allowing the brain to quickly react to pain and temperature changes, prioritizing protective reflexes over fine detail analysis.