Sensory and Motor Systems

• The sensory system detects and processes both internal and external stimuli.
• The motor system executes voluntary and involuntary movements.

Sensory System Divisions

• Exteroception relates to the external environment, like sight and sound.
• Interoception pertains to internal body signals, such as hunger and blood pressure.
• Proprioception is the sense of body position in space, including balance and joint position.

Motor System Divisions

• Upper Motor Neurons (UMNs) originate in the brain and initiate voluntary movement.
• Lower Motor Neurons (LMNs) connect the spinal cord or brainstem to muscles.

Sensory Processing - Four Core Stages

• Detection involves receptor cells sensing a stimulus like light, sound, or pressure.
• Transduction converts a stimulus into an electrical signal.
• Transmission sends the signal via neurons to the Central Nervous System (CNS).
• Processing occurs as the sensory cortex interprets the input.

Somatosensation: Receptors and Functions

• Somatosensation includes touch, temperature, pain, and proprioception.

Mechanoreceptors

• Merkel discs detect fine touch and texture.
• Meissner corpuscles detect flutter and light touch.
• Pacinian corpuscles detect deep pressure and vibration.
• Ruffini endings detect skin stretch.

Other Receptors

• Thermoreceptors detect hot or cold temperatures.
• Nociceptors detect pain, such as sharp or burning sensations.

Proprioceptors

• Muscle spindles sense muscle stretch.
• Golgi tendon organs sense tension.

Somatosensation - Real-Life Examples

• Grasping a soft sponge involves Merkel discs and Meissner corpuscles.
• Stepping on a Lego activates nociceptors and mechanoreceptors.
• Performing a yoga pose engages proprioceptors.

Somatosensory Pathways

• The Dorsal Column-Medial Lemniscus (DCML) pathway transmits fine touch, vibration, and proprioception.
  • It is ipsilateral in the spinal cord and crosses in the medulla.
• The Anterolateral Pathway transmits pain and temperature.
  • It crosses immediately in the spinal cord.

Vision - Retinal Structure

• Cones detect color (red, green, blue) and are concentrated at the fovea.
• Rods enable night vision and are located in the peripheral retina.

Vision - Pathway

• Light travels through the retina to bipolar cells, then to ganglion cells, and finally to the optic nerve.
• The signal goes through the optic chiasm to the Lateral Geniculate Nucleus (LGN) and then to the primary visual cortex (V1).

Vision - Functional Features

• Retinotopy involves spatial mapping in V1.
• The left visual field is processed in the right cortex.
• The right visual field is processed in the left cortex.

Vision - Real-Life Examples

• A flash camera in the dark activates rods.
• Reading colored text engages cones, especially red, green, and blue.

Hearing - Cochlear Function

• Sound waves vibrate the tympanic membrane, which moves the ossicles, causing the basilar membrane to vibrate.
• Inner hair cells are responsible for main transduction via potassium (K+) influx.
• Outer hair cells amplify sound.

Hearing - Pathway

• The auditory nerve transmits signals to the brainstem, then the midbrain, thalamus, and finally the auditory cortex (tonotopic map).

Hearing - Real-Life Examples

• Deafness in one ear affects sound localization.
• Listening to a song: High-pitched sounds are processed at the base, and low-pitched sounds at the apex of the cochlea.

Olfaction - Receptors

• Receptors are located in the olfactory epithelium.
• Odorants bind to G protein-coupled receptors (GPCR), leading to depolarization and action potentials in the olfactory bulb.

Olfaction - Pathway

• The olfactory bulb sends signals through mitral cells to the cortex, including the amygdala, hippocampus, and frontal cortex.

Olfaction - Real-Life Examples

• Smelling cookies can activate memory (hippocampus) and emotion (amygdala).

Gustation - Taste Receptors

• Taste receptors are located in taste buds on papillae.
• Sweet, bitter, and umami tastes are detected via GPCRs.
• Salty and sour tastes are detected via ion channels.

Gustation - Pathway

• Taste cells send signals through cranial nerves (VII, IX, X) to the brainstem, thalamus, and gustatory cortex.

Gustation - Real-Life Examples

• Eating something salty opens sodium (Na+) channels, leading to depolarization.
• Bitterness triggers a gag reflex as a protective mechanism.

Motor System - Lower Motor Neurons (LMNs)

• LMNs originate in the spinal cord or cranial nerve nuclei.
• They directly activate muscle fibers.
• They are recruited based on the size principle: Type I (slow), Type IIa (intermediate), Type IIb (fast, powerful).

Motor System - Upper Motor Neurons (UMNs)

• UMNs originate in the cortex (motor areas).
• Corticospinal pathways control body movement.
• Corticobulbar pathways control face and head muscles.

Motor System - Subcortical Regulation

• The basal ganglia's direct pathway initiates movement, while the indirect pathway inhibits movement.
• The cerebellum contributes to balance (vestibulo-), feedback (spino-), and planning (cerebro-).

Motor System - Real-Life Examples

• Upper motor neuron lesions result in spastic paralysis.
• Lower motor neuron lesions result in flaccid paralysis.

Definitions

• Homunculus: Somatotopic map in cortex that represents body regions.
• Tonotopy: Spatial mapping of sound frequencies in the auditory cortex.
• Retinotopy: Spatial mapping of the retina in the visual cortex.
• Somatotopy: Mapping of body parts in the sensory or motor cortex.
• Ipsilateral: Same side.
• Contralateral: Opposite side.
• Bilateral: Both sides.
• GPCR: G protein-coupled receptor.
• Motor unit: One motor neuron and its innervated muscle fibers.
• Phototransduction: Conversion of light into an electrical signal.
• Hyposmia: Reduced sense of smell.
• Anosmia: Loss of smell.
• Ageusia: Loss of taste.
• Spasticity: Increased muscle tone due to an UMN lesion.
• Flaccidity: Decreased muscle tone due to an LMN lesion.
• Decussation: Crossing over of nerve fibers, exemplified by the corticospinal tract.