06 Somatic Nervous system

what are the 5 main functions of the SNS

1. Conscious perception of the environment.

2. Voluntary responses using skeletal muscles.

3. Receives environmental input via peripheral sensory neurons.

4. Motor commands originate in the CNS.

5. Responsible for reflexes (e.g., withdrawal reflex)

what help in learning about the external or internal environment in the SNS

sensory receptors

Describe the four-step sequence of how a stimulus leads to a motor response (Sensory Perception Processing Pathway).

1. Translation: Stimuli are translated into electrochemical signals when cell membrane potential (graded potentials) changes.

2. Transmission: Action potentials are generated and relayed to the CNS.

3. Integration: Information is integrated with other sensory info and cognitive functions in the CNS, resulting in conscious perception.

4. Response: The result is a motor response.

Define the four key terms related to Sensory Perception: Sensation, Perception, Receptors, and Transmembrane protein receptor.

• Sensation: Activation of sensory receptor cells at the level of the stimulus (note: not all sensations are perceived).

• Perception: Central processing of sensory stimuli into a meaningful pattern; it is dependent on sensation.

• Receptors: Cells or structures that detect sensations and are changed directly by stimuli.

• Transmembrane protein receptor: A protein in the cell membrane that mediates a physiological change in a neuron.

Describe the three key functions/characteristics of a Transmembrane protein receptor.

1. Activation: Activated by chemicals called ligands.

2. Action: Opens ion channels or changes cell signaling processes.

3. Result: Ion flow can generate an action potential or graded potential.

What are the three Structural Receptor Types of sensory receptors, and what is a key characteristic/example of each?

1. Free nerve ending: Has dendrites embedded in tissue to receive sensations (e.g., pain and temperature receptors in the dermis).

2. Encapsulated ending: Nerve ending encapsulated in connective tissue (CT) that enhances sensitivity (e.g., lamellated corpuscles in the dermis).

3. Specialized receptor cell: Has special components that interpret a specific type of stimulus (e.g., photoreceptors in the retina).

According to location relative to the stimulus, what are the three types of Sensory Receptors, and what is an example of each?

1. Exteroreceptor: Located near a stimulus in the external environment (e.g., somatosensory receptors in the skin).

2. Interoreceptor: Interprets stimuli from internal organs and tissues (e.g., receptors detecting changes in BP in the aorta and carotid sinus).

3. Proprioceptor: Located near a moving part of the body detecting position.

What are the three Functional Receptor Types (classified by how they transduce stimuli), and what stimulus does each detect?

1. Chemical: Ions and macromolecules.

2. Physical: Pressure and vibration.

3. Electromagnetic: Radiation from visible light.

Name the five types of Sensory Receptors classified by the type of stimuli they transduce, and define what each detects.

1. Chemoreceptor: Interprets chemical stimuli.

2. Osmoreceptor: Responds to solute concentrations of body fluids.

3. Nociceptor: Interprets the presence of pain due to chemicals from tissue damage.

4. Mechanoreceptor: Detects physical stimuli such as pressure and vibration.

5. Thermoreceptor: Sensitive to changes in temperature.

What is the difference between a General Sense and a Special Sense?

• General Sense: Distributed throughout the body with receptor cells within the structures of other organs (e.g., mechanoreceptors). Contributes to touch, proprioception, kinesthesia, and visceral sense (autonomic function).

• Special Sense: Has a specific organ devoted to it (e.g., eye, inner ear, tongue, nose).

Describe the anatomy of gustation (taste) and the nerve innervation for different regions of the tongue.

• Anatomy: Papillae (raised bumps: circumvallate, foliate, filiform, fungiform) contain structures for gustatory transduction. Taste buds within the papillae contain gustatory receptor cells.

• Innervation:

  • Facial Nerve (CN VII): Anterior 1/3 of the tongue.

  • Glossopharyngeal Nerve (CN IX): Posterior 2/3 of the tongue.

  • Vagus Nerve (CN X): Extreme posterior of the tongue (sensitive to noxious stimuli)

What are the five taste modalities and their corresponding stimuli?

• Salty: Perception of sodium ions (drives entry into cells generating an AP).

• Sour: Perception of hydrogen ion concentration/acids (triggers progressively stronger potentials).

• Sweet: Sensitivity of gustatory cells to the presence of glucose.

• Bitter: Depolarize/hyperpolarize gustatory cells; may stimulate gag reflex to avoid poisons (posterior tongue).

• Umami: Molecule activating receptor is L-glutamate (often in protein-rich food).

Describe the anatomy, receptor type, and neural pathway of Olfaction (smell).

• Anatomy: Neurons located in the superior nasal cavity (olfactory epithelium) are bipolar sensory neurons. Their dendrites extend from the apical surface into the mucus.

• Receptors: G-protein coupled. Odorant molecules bind to proteins to dissolve in mucus and be transported to dendrites.

• Pathway: Axons extend through olfactory foramina in the cribriform plate of the ethmoid bone → Olfactory bulb → Olfactory tract → Travels to cerebrum (primary olfactory complex) OR limbic system and hypothalamus (memory/emotional responses).

Describe the structures of the External and Middle ear.

• External Ear: Auricle (C-shaped curve directing sound to the auditory canal; enters skull through the external auditory meatus of temporal bone) + Auditory canal + Tympanic membrane (vibrates after being struck by sound waves).

• Middle Ear: Space occupied by Ossicles (Malleus attached to tympanic membrane, Incus, Stapes attached to inner ear). It is connected to the pharynx to help equilibrate air pressure across the tympanic membrane.

Describe the structures of the Inner Ear (Cochlea and Vestibule) and their functions.

• Cochlea: Responsible for hearing. Attached to the stapes through the oval window. Contains sensory neurons of the spiral ganglia.

• Scala vestibuli & Scala tympani: Fluid-filled tubes above and below the cochlear duct. Scala tympani ends at the round window.

• Vestibule: Responsible for balance.

• Vestibulocochlear Nerve: Travels to the brain stem with inputs from cochlea and vestibule.

• Organs of Corti: Transduce wave motion into neural signals; lie on top of the basilar membrane.

How does the transduction of sound occur via hair cells and membranes in the inner ear?

• Pressure waves from the scala cause the tectorial membrane to slide across stereocilia (hair cells arranged tallest to shortest on the basilar membrane).

• Bending towards tallest member: Opens ion channels (depolarization).

• Bending towards shortest member: Ion channels close.

How are different sound frequencies encoded by the basilar membrane and cochlea?

• High frequency waves move the region of the basilar membrane close to the base of the cochlea.

• Low frequency waves move the region near the tip of the cochlea.

• The basilar membrane will only move if the incoming sound is at a specific frequency.

• Overall, the cochlea encodes auditory stimuli for frequencies between 20 - 20,000 Hz.

BALANCE/EQUILIBRIUM - General Anatomy & Pathway: What are the mechanoreceptors for balance, where are they located, and how do signals reach the brain?

• Mechanoreceptors: Hair cells with stereocilia (detect head movement and bodies in motion) located in the vestibule.

• Utricle & Saccule: Detect head position (composed of a macula—hair cells with support cells).

• Semicircular Canals: Sense head movement.

• Pathway: Signals generated in the vestibular ganglion are transmitted through CN VIII to the brain stem and cerebellum.

BALANCE - Otolithic Membrane: How does the otolithic membrane detect head tilt and gravity?

• Stereocilia of hair cells extend into the otolithic membrane.

• Otoliths (calcium carbonate crystals) on top make it top-heavy and move separately from the macula.

• Tilting the head causes the membrane to slide over the macula in the direction of gravity →→ bends stereocilia →→ causes depolarization/hyperpolarization →→ position is interpreted by the brain.

BALANCE - Semicircular Canals: What are the structures and functions of the semicircular canals, ampulla, and cupula?

• Semicircular Canals: 3 ring-like extensions of the vestibule.

• Ampulla: The base of the canals (meets the vestibule); contains hair cells that respond to rotational movement (e.g., saying "no").

• Cupula: A membrane on top of the ampulla which is bent in the opposite direction of head movement.

SOMATOSENSATION - Modalities & Free Nerve Endings: What are the modalities of touch, and what specific stimuli are transduced by free nerve endings?

• Modalities: Pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, kinesthesia.

• Free Nerve Endings: Transduce pain and temperature (via nociceptors and thermoreceptors).

SOMATOSENSATION - Stretch Receptors: What are the three stretch receptors and where are they located?

1. Muscle spindles: Stretch receptors in skeletal muscle.

2. Golgi tendon organs: Stretch receptors in tendons.

3. Bulbous corpuscles: Stretch receptors in joint capsules.

SOMATOSENSATION - Encapsulated Nerve Endings: Name the four encapsulated receptors, their function, and their location.

1. Merkel cells (Type I): Vibration. Located in stratum basale of epidermis.

2. Lamellated (Pacinian) corpuscles: Deep pressure and vibration. Located in dermis/subcutaneous tissue.

3. Tactile (Meissner) corpuscles: Light touch.

4. Bulbous (Ruffini) corpuscles (Type II): Skin stretching. Located in the hair of the skin.

VISION - Extraocular Muscles & Innervations: Which cranial nerves innervate the extraocular muscles?

• CN VI: Lateral rectus.

• CN IV: Superior oblique.

• CN III: Superior rectus, inferior oblique, medial rectus, inferior rectus, and levator palpebrae superioris.

VISION - Retina & Photoreceptors: What is the function of the retina and photoreceptors?

• Retina (Neural Tunic): Composed of several layers containing specialized cells for initial processing of visual stimuli.

• Photoreceptors (rods and cones): Stimulated by light energy; release neurotransmitters onto bipolar cells in the outer synaptic layer.

VISION - Retinal Circuitry & The Fovea: Describe the path of visual signals and the function of the fovea.

• Circuitry: Bipolar cells connect photoreceptors to retinal ganglion cells (RGC) in the inner synaptic layer. Amacrine cells contribute to processing before an action potential is produced by the RGC. RGC axons collect at the optic disc and leave as the optic nerve.

• Fovea: Center of the retina where visual acuity is sharpest (the farther away, the blurrier the image).

VISION - Rods vs. Cones: What are the functional differences between rods and cones?

• Rods: Sensitive to changes in amounts of light (function in low light conditions).

• Cones: Detection of color and bright light conditions (primary colors: red, green, blue)

SENSORY NERVES & ASCENDING PATHWAYS (General Rules): Describe the basic sensory nerve pathway and the difference between spinal and cranial nerve laterality.

• Basic Pathway: Sensory cell →→ transduces stimulus →→ nerve impulse →→ travels along axon →→ CNS.

• Spinal Nerves: Split into dorsal (sensory) and ventral roots (motor) and enter the SC via the dorsal root ganglion and dorsal/ventral horns. They are contralateral (right side of body connects to left side of brain).

• Cranial Nerves: Convey sensory info from the Head & Neck (H&N). They are ipsilateral (right side of head connects with right side of brain).

• Somatosensory Pathways: Below neck = pass along sensory pathways of the SC. Head/neck = travel through cranial nerves/trigeminal system.

DORSAL COLUMN-MEDIAL LEMNISCUS PATHWAY: Trace the full pathway, specific structures, and its function.

• Function: Responsible for touch sensation and proprioception.

• Start: Axon of dorsal root ganglion neuron enters dorsal root joining dorsal column white matter in SC.

• Tracts: Fasciculus gracilis (legs/lower body, medial) and Fasciculus cuneatus (upper body/arms, lateral).

• Synapse 1: Terminate in nuclei of medulla and synapse with 2nd2nd neuron (nucleus gracilis/cuneatus). Decussates across midline of medulla.

• Synapse 2: Ascends brain as medial lemniscus bundle, terminates in thalamus, synapses with 3rd3rd neuron.

• End: Projects to postcentral gyrus for conscious perception.

SPINOTHALAMIC TRACT: Trace the full pathway and function.

• Function: Responsible for pain and temperature.

• Start: Begins in dorsal root ganglion, extends axons to dorsal horn, synapses with 2nd2nd neuron (cell body in gray matter).

• Decussation: Happens in the SC.

• Synapse 2: Ascends to brain and thalamus where synapse with 3rd3rd neuron happens.

• End: Synapses in postcentral gyrus.

TRIGEMINAL PATHWAY: What information does it carry, what are its specific nuclei, and what is its pathway?

• Function: Carries somatosensory info from face, head, mouth, and nasal cavity.

• Start: Axons from trigeminal ganglion enter brain stem at the pons.

• Specific Nuclei:

  • Spinal trigeminal nucleus of medulla: Pain and temperature.

  • Chief sensory nucleus of pons or mesencephalic nuclei of midbrain: Touch pressure, vibration, proprioception.

• Pathway: Axons from 2nd2nd neuron decussate and ascend to thalamus via trigeminothalamic tract. Axons from 3rd3rd neuron project to primary somatosensory cortex of cerebrum.

GUSTATION PATHWAY: Trace the pathway for taste.

1. Travel along CN VII and CN IX.

2. Synapse in the solitary nucleus of the brainstem.

3. Project to the ventral posterior nucleus of thalamus.

4. Project to the gustatory cortex of the cerebral cortex.

AUDITORY PATHWAY: Trace the pathway, nuclei, and how localization is computed

• Start: Travel along CN VIII, synapse in cochlear nuclei of superior medulla.

• Localization: Input from each ear is combined to extract location info, computed by calculating interaural time difference and interaural intensity difference.

• Midbrain: Auditory processing in the inferior colliculus.

• Next: Project to either thalamus (medial geniculate nucleus) or the superior colliculus (receives visual/somatosensory input).

• End: Info from thalamus projects to auditory cortex of the temporal lobe.

BALANCE (VESTIBULAR) PATHWAY: Describe the neural pathways, reflexes, and brainstem connections.

• Start: Info from utricle, saccule, and semicircular canals goes to vestibular ganglion. Axons terminate in vestibular nuclei of medulla or directly to cerebellum (initiates movements based on equilibrium).

• Brainstem/SC: Neurons project to brain stem. Reticular formation influences respiratory/cardiovascular functions during movement. Spinal cord initiates spinal reflexes for posture/balance.

• Vestibulo-ocular reflex: Stabilizes images on retina to compensate for movement; vestibular nuclei project to oculomotor, trochlear, and abducens nuclei.

• Conscious Perception: Vestibular nuclei project to thalamus to join the proprioceptive pathway for the dorsal column system.

VISUAL PATHWAY: Trace the pathway, including decussation, main targets, and the circadian connection.

• Decussation: Axons from the medial side of the retina decussate at the optic chiasm. Axons from the lateral side do not decussate.

• Optic Tract: Visual system beyond optic chiasm; connects to thalamus (lateral geniculate nucleus).

• End: Lateral geniculate nucleus projects to the visual cortex of cerebrum.

• Circadian Rhythm: Some axons project from optic chiasm to suprachiasmatic nucleus of hypothalamus (responds to presence/absence of light and establishes circadian rhythm).

DIENCEPHALON (Thalamus & Hypothalamus): What are the general functions of the thalamus and hypothalamus, and what are the specific functions of the anterior, medial, and lateral thalamic nuclei?

• Thalamus (General): Required transfer point for most sensory tracts before reaching the cerebral cortex (exception: olfactory system). Receives sensory input from most special senses and ascending somatosensory tracts.

• Hypothalamus: Has both somatic and autonomic functions; communicates with the limbic system.

• Anterior Nucleus: Serves as a relay for information between the hypothalamus and limbic system.

• Medial Nucleus: Involved with the limbic system and basal ganglia (for memory creation during learning and alertness).

• Lateral Nuclei: Special and somatic sense information relayed to the appropriate sensory cortex.

SENSORY HOMUNCULUS: Describe the mapping of the human body across the somatosensory cortex, including the medial-to-lateral arrangement and its key characteristic.

• Map Location: A map of the human body laid across a portion of the cerebral cortex (somatosensory cortex).

• Medial Face of the Gyrus (within longitudinal fissure): External genitals, feet, lower legs.

• Along Surface of Parietal Lobe: Thighs, hips, trunk, shoulders, arms, hands.

• Approaching Lateral Sulcus: Head and face, fingers.

• Arrangement: Medial to lateral = lower body to upper body (similar to the arrangement in the dorsal column system).

• Key Feature: Emphasizes (allocates more space to) sensitive parts of the body.

CEREBRAL CORTEX PROCESSING: What is the three-step sequence of cortical processing, from initial stimulus to combining perceptions?

1. Primary sensory cortex: Perception of stimulus.

2. Association area: Putting meaning into perception.

3. Multimodal integration area: Combining different perceptions.

MOTOR RESPONSES (General Concepts): What is the basic definition of a motor response, and how does conscious control factor into voluntary muscles?

• Definition: Skeletal muscle response to perceptions about the environment via sensory pathways.

• Control: Movements can use voluntary muscles without conscious control (e.g., muscles used for breathing).

• Cortical Processing: Perception, association, and integration lead to a response.

CEREBRAL CORTEX (Motor Functions): What is the role of the frontal lobe and prefrontal areas in motor responses?

• Frontal Lobe: Largely controls motor functions.

• Prefrontal Areas: Responsible for executive functions leading to goal-directed behaviors, working memory, attention, and personality.

SECONDARY MOTOR CORTICES (Part 1): What initiates actual movements, and what are the functions of the premotor cortex and supplemental motor area?

• Initiation: Actual movements are initiated by the executive function of the prefrontal cortex.

• Premotor Cortex: Lateral to primary motor cortex; controls movements of core muscles (maintenance of posture).

• Supplemental Motor Area: Superomedial to primary motor cortex; responsible for planning and coordinating movement (sequential/learned movements).

SECONDARY MOTOR CORTICES (Part 2): What are the functions of the frontal eye fields and Broca's area?

• Frontal Eye Fields: Responsible for moving the eyes in response to visual stimuli; has a direct connection to the superior colliculus.

• Broca's Area: Controls movements and structures of speech production.

PRIMARY MOTOR CORTEX & MOTOR HOMUNCULUS: Where is the primary motor cortex located, what is its pathway, and what does the homunculus represent?

• Location: Precentral gyrus.

• Pathway: Receives input from areas aiding in planning movement. Output stimulates spinal cord neurons to stimulate skeletal muscle contraction.

• Motor Homunculus: Topographical map similar to the sensory homunculus, but with greater space given to muscles that perform fine, agile movements (like fingers and lower face).

DESCENDING PATHWAY (General): What are Betz cells, and what is the difference between the corticobulbar and corticospinal tracts?

• Pathway Definition: Output from cortex descends into brain stem and SC to control muscles.

• Betz Cells: Neurons in primary motor cortex; synapse with lower motor neurons in the brain stem or SC.

• Corticobulbar Tract: Cortex to brain stem; ipsilateral.

• Corticospinal Tract: Cortex to spinal cord; contralateral (crosses at level of brain stem or spinal cord).

CORTICOSPINAL TRACT (Anatomical Route): Trace the path of the corticospinal tract from the cortex to the medulla.

1. Descends from cortex through deep white matter of cerebrum.

2. Passes between the caudate nucleus and putamen of basal nuclei (internal capsule).

3. Passes through midbrain as cerebral peduncles and goes to pons.

4. Reaches medullary pyramids (the point of decussation for most fibers).

LATERAL vs. ANTERIOR CORTICOSPINAL TRACTS: Compare their decussation points, target muscles, and ventral horn characteristics.

• Lateral Corticospinal Tract:

  • Crosses midline at pyramidal decussation (anterior medulla to lateral column of SC).

  • Controls appendicular muscles (limbs).

  • Ventral horns in cervical and lumbar SC are wider due to a greater number of muscles controlled.

• Anterior Corticospinal Tract:

  • Do not decussate at medullary pyramidal decussation (decussate at the ventral horn at the level of the SC to synapse with lower motor neuron).

  • Controls muscles of the body trunk.

  • Lower motor neurons are located in the medial regions of the ventral horn.

EXTRAPYRAMIDAL CONTROL: Define this system and describe the function of the four major tracts (Tectospinal, Reticulospinal, Vestibulospinal, Rubrospinal).

Definition: Other connections between the brain and spinal cord (SC).

• Tectospinal tract: Midbrain to SC. Important for postural movements driven by the superior colliculus.

• Reticulospinal tract: Reticular system to SC. Influences trunk and proximal limb muscles related to posture and locomotion; contributes to muscle tone and autonomic functions.

• Vestibulospinal tract: Vestibular system to SC. Allows posture, movement, and balance to be modulated based on equilibrium information.

• Rubrospinal tract: Red nucleus of midbrain to SC. Conflicts in commands by the cerebrum and body position trigger corrective commands to the SC via the red nucleus.

VENTRAL HORN OUTPUT (Somatic Motor Pathway): Describe the pathway for output strictly to skeletal muscles and the mechanism of contraction at the neuromuscular junction.

• Output: The somatic nervous system provides output strictly to skeletal muscles.

• Neuron Location: Lower motor neurons for contraction are found in the ventral horn of the SC.

• Pathway: Axons travel through the ventral nerve root to join the emerging spinal nerve. Axons are long because they need to reach peripheral muscles.

• Contraction Mechanism: Motor neuron axons connect to muscle fibers at the neuromuscular junction where axon terminals synapse with the muscle fiber sarcolemma. Release of NT acetylcholine binds with receptors in the sarcolemma, depolarizing it and initiating muscle contraction.

REFLEXES (Mechanisms & Examples): Explain the mechanism of reciprocal inhibition in a withdrawal reflex and describe the Stretch and Corneal reflexes.

• Reciprocal Inhibition (e.g., hot stove withdrawal): An interneuron in the spinal cord inhibits the motor neuron that would antagonize the necessary reflexive movement. This is done through the release of a NT that hyperpolarizes the motor neuron connected to the antagonist (leading to relaxation).

• Stretch Reflex: Muscle spindle receptor is activated, causing direct contraction of the muscle and inhibition of the antagonist.

• Corneal Reflex: When the cornea is stimulated by a tactile stimulus or exposure to bright light, blinking is initiated (motor response of the facial nerve innervating the orbicularis oculi).