The Somatic Nervous System

The Somatic Nervous System

Overview

  • The somatic nervous system plays a critical role in helping us recognize and respond to environmental stimuli.

    • Functionality:

    • Sensory receptors and neurons located in the peripheral nervous system are essential in identifying various environmental stimuli.

    • Information is relayed to the central nervous system (CNS) which interprets this input.

    • The CNS determines if an action is necessary and subsequently executes motor responses accordingly.

Sensation

  • Definition: The ability to be consciously or subconsciously aware of changes in the external and internal environments.

  • Components of Sensation:

    • Stimulation of sensory receptors

    • Transduction of the stimulus: Conversion of stimulus energy to electrical signals.

    • Generation of nerve impulses: Action potentials are created.

    • Integration of sensory input: Processing sensory information in the CNS.

Classification of Sensory Receptors

  • General Senses:

    • Somatic Sensations: Tactile, thermal, pain, and proprioception.

    • Visceral Sensations: Information about internal organ conditions.

  • Special Senses:

    • Include smell, taste, vision, hearing, and equilibrium/balance.

Classification by Structure

  • Free Nerve Endings: Pain and thermoreceptors.

  • Encapsulated Nerve Endings:

    • Example: Pacinian corpuscles (responsible for touch, pressure, vibration).

  • Separate Cells:

    • Examples: Hair cells (auditory and vestibular), photoreceptors (vision), gustatory receptor cells (taste).

Classification by Location

  • Exteroceptors: Located on the body surface; respond to external stimuli.

  • Interoceptors: Located internally; provide information on internal body conditions.

  • Proprioceptors: Located in tendons and joints; provide a sense of body position.

Classification by Type of Stimulus

  • Mechanoreceptors: Respond to mechanical pressure or distortion.

  • Thermoreceptors: Respond to temperature changes.

  • Nociceptors: Respond to pain stimuli.

  • Photoreceptors: Respond to light (vision).

  • Chemoreceptors: Respond to chemical stimuli (e.g., taste and smell).

  • Osmoreceptors: Respond to changes in osmotic pressure.

Sensory Modalities

  • Definition: A sensory modality refers to what is being sensed, which may or may not be perceived based on processing in the CNS.

  • General Senses: Distributed throughout the body.

  • Special Senses: Associated with specific organs.

Comparison of General and Special Senses

  • General Senses:

    • Include somatic (tactile, thermal, pain, proprioception) and visceral sensations.

    • Present throughout the body; structures are generally simpler.

  • Special Senses:

    • Include smell, taste, vision, hearing, equilibrium.

    • Located in specific regions in the head; anatomically distinct structures with complex neural pathways.

Gustation: Sense of Taste

  • Nature: Taste is classified as a chemical sense.

  • Sub-modalities: Include five basic tastes:

    • Sweet

    • Salty

    • Sour

    • Bitter

    • Umami (savory)

Anatomy of Taste Buds and Papillae

  • Taste Buds: Composed of three types of epithelial cells:

    • Supporting cells

    • Gustatory receptor cells (approx. 50 per taste bud, each with a gustatory hair projecting through a taste pore).

    • Basal cells (regenerate taste receptors).

  • Types of Papillae Containing Taste Buds:

    • Vallate (circumvallate)

    • Fungiform

    • Foliate

Physiology of Gustation

  • Process:

    1. A tastant dissolves in saliva and interacts with gustatory hairs.

    2. This interaction creates a receptor potential.

    3. The receptor potential leads to nerve impulses in cranial nerves responsible for taste sensations (CN VII, IX, and X).

    4. Impulses travel to the medulla, then to the thalamus, and finally to the primary gustatory area located in the cerebral cortex (parietal lobe, base of postcentral gyrus).

Gustatory Pathway

  • Cranial Nerves Involved:

    • CN VII (Facial nerve): Taste from anterior 2/3 of the tongue.

    • CN IX (Glossopharyngeal nerve): Taste from the posterior 1/3 of the tongue.

    • CN X (Vagus nerve): Taste from the throat and epiglottis.

  • Pathway Flow:

    • Sensory input travels from the cranial nerves to the medulla.

    • It passes through locations such as the limbic system and hypothalamus before reaching the thalamus and ultimately the cerebral cortex in the primary gustatory center.

Olfaction: Sense of Smell

  • Nature: Olfaction is a chemical sense, instigated by the olfactory epithelium that comprises 10-100 million receptors.

  • Structure of Olfactory Receptors:

    • Bipolar neurons with cilia known as olfactory hairs, which are the first-order neurons in this system.

    • Supporting cells provide structural support and nourishment while basal cells replace olfactory receptors.

Physiology of Olfaction

  • Ability: The olfactory system can detect around 10,000 different odors and exhibits low odor thresholds. Adaptation to odors occurs rapidly.

  • Process of Olfaction:

    1. An odorant binds to an olfactory hair receptor.

    2. This activates a G-protein, leading to adenylate cyclase activation.

    3. Resulting in cAMP production, which opens sodium channels, allowing Na+ ions inflow.

    4. This results in generator potentials leading to nerve impulses toward the olfactory bulb and the olfactory tract, eventually reaching the primary olfactory area of the cerebral cortex (temporal lobe).

Anatomy of the Ear

  • Regions of the Ear:

    • External (Outer) Ear:

    • Auricle (pinna)

    • External auditory canal

    • Tympanic membrane (ear drum) (ceruminous glands produce ear wax).

    • Middle Ear: Contains the auditory ossicles (malleus, incus, stapes).

    • Internal (Inner) Ear: Features a labyrinth (bony and membranous), oval window, and round window.

Physiology of Hearing

  • Audible Sound Range: 20-20,000 Hz.

  • Process of Hearing:

    1. Sound waves enter through the auricle into the external auditory canal.

    2. Waves stimulate the tympanic membrane, causing vibration in the ossicles.

    3. The ossicles transmit sound vibrations to the oval window.

    4. Vibrations induce waves in the perilymph of the scala vestibuli.

    5. This leads to movement in the endolymph within the cochlear duct and displacement of the basilar membrane.

    6. Hair cells bend against the tectorial membrane, generating receptor potentials that create nerve impulses in CN VIII (Cochlear nerve).

The Auditory Pathway

  • Pathway Flow:

    • Cochlear branch of CN VIII synapses with cochlear nuclei in the medulla.

    • Some impulses cross over to the inferior colliculus, while others ascend to the superior olivary nucleus.

    • Following this, signals reach the medial geniculate nucleus in the thalamus, leading to the primary auditory area in the temporal lobe (below the postcentral gyrus).

Physiology of Equilibrium

  • Types of Equilibrium:

    • Static Equilibrium: Maintenance of body position concerning gravity (macula involved).

    • Dynamic Equilibrium: Maintenance of body position (especially the head) during movement, responding to rotational acceleration and deceleration.

  • Receptors for Equilibrium: Hair cells located in the utricle, saccule, and semicircular canals known as vestibular apparatus.

  • Mechanism of Action:

    • Tilting the head causes the otolithic membrane to slide, bending the hair bundles, inducing receptor potentials that lead to impulses in the vestibular branch of CN VIII.

Somatic Sensations

  • Location: Sensory receptors can be found in the skin (cutaneous sensations), muscles, tendons, joints, and the inner ear.

  • Four Main Modalities:

    • Tactile

    • Thermal

    • Pain

    • Proprioceptive

Adaptation of Sensory Receptors

  • Mechanism: Generator potential diminishes in amplitude with prolonged stimulus exposure.

  • Rapidly Adapting Receptors:

    • Detect pressure, touch, and smell.

  • Slowly Adapting Receptors:

    • Detect pain, body position, and blood chemical composition.

Tactile Sensations

  • Subtypes: Includes touch, pressure, vibration, itch, and tickle.

  • Tactile Receptors in the Skin:

    • Meissner corpuscles

    • Hair root plexuses

    • Merkel discs

    • Ruffini corpuscles

    • Pacinian corpuscles

    • Free nerve endings.

Thermal Sensations

  • Nature: Thermal sensations detected by thermoreceptors which are free nerve endings.

  • Distinct Types:

    • Cold Receptors: Located in the basal layer of the epidermis, activated at temperatures of 10-40°C (50-105°F).

    • Warm Receptors: Located in the dermis, activated at 32-48°C (90-118°F).

  • Activation of Nociceptors: Temperatures below 10°C or above 48°C trigger nociceptors (pain receptors).

Pain Sensations

  • Nature: Pain sensations serve a protective role.

  • Types of Pain Receptors:

    • Nociceptors: Free nerve endings involved in pain detection.

  • Types of Pain:

    • Fast Pain: Acute, sharp, or pricking sensations.

    • Slow Pain: Chronic, burning, aching, or throbbing sensations.

Referred Pain

  • Definition: Pain perceived in a location away from the actual site of the stimulus or deeper within the skin.

Proprioceptive Sensations

  • Receptors: Known as proprioceptors, which are slowly adapting receptors.

  • Types:

    • Muscle Spindles: Sense muscle stretch.

    • Tendon Organs: Sense excessive tension.

    • Kinesthetic Receptors: Sense pressure in joints.

Vision or Sight

  • Detectable Range: Eyes respond to light within the visible spectrum (400-700 nm).

    • 400 nm corresponds to violet light, while 700 nm corresponds to red light.

Accessory Structures of the Eye

  • Eyebrows and Eyelashes: Protective features of the eye.

  • Eyelids (Palpebrae): Composed of upper and lower aspects, creating the palpebral fissure when open.

Anatomy of the Eye

  • Components:

    • Sclera: The white outer coating.

    • Choroid: Vascular layer beneath the sclera.

    • Retina: Inner layer responsible for photoreception.

    • Fovea Centralis: Area of greatest visual acuity.

    • Optic Disc: Where blood vessels enter/exit, leading to the optic nerve (blind spot).

Interior of the Eyeball

  • Lens:

    • Lacks blood vessels; made of transparent layers of proteins (crystallins).

    • Divides the eye into anterior and posterior cavities:

    • Anterior Cavity: Contains aqueous humor.

    • Posterior Cavity: Contains vitreous body (gel-like material).

Somatic Sensory Pathways

  • Pathway Overview:

    • Sensory pathways originate from receptors in the periphery, travel to the CNS.

    • First Order Neuron: Travels from the somatic receptor to the brain stem/spinal cord.

    • Second Order Neuron: Connects from the brain stem/spinal cord to the thalamus (crosses to the opposite side, decussation).

    • Third Order Neuron: Connects from the thalamus to the primary somatosensory area of the cortex (postcentral gyrus).

Major Somatic Sensory Pathways

  • Pathways include:

    • Posterior column-medial lemniscus pathway

    • Anterolateral (spinothalamic) pathway

    • Trigeminothalamic pathway

    • Anterior and posterior spinocerebellar pathway.

The Posterior Column-Medial Lemniscus Pathway

  • Nerve Impulse Conveyance: Touch, pressure, vibration, and conscious proprioception from limbs, trunk, neck, and posterior head to the cerebral cortex.

  • Order of Neurons:

    • First-order neurons travel from receptors, with cell bodies in Dorsal Root Ganglion (DRG), through the posterior columns.

    • Gracile fasciculus carries signals from lower body areas and cuneate fasciculus from upper body areas both of which synapse with second-order neurons in the nuclei in the medulla (nucleus gracilis and nucleus cuneatus).

    • Second-order neurons cross over and ascend to the thalamus (via medial lemniscus) to synapse with third-order neurons that travel to the postcentral gyrus (primary somatosensory area).

The Anterolateral (Spinothalamic) Pathway

  • Nerve Impulse Conveyance:

    • Signals for pain, cold, warmth, itch, and tickle from limbs, trunk, neck, and posterior head to the cerebral cortex.

  • Order of Neurons:

    • First-order neurons synapse with second-order neurons in the lateral gray horns of the spinal cord, which then cross over and ascend via the spinothalamic tract to reach the thalamus.

    • Third-order neurons extend from the thalamus to the primary somatosensory area (postcentral gyrus).

Trigeminothalamic Tract

  • Function: Conveys most somatic sensations from the face, nasal cavity, and oral cavity.

  • Path:

    • First-order neurons from receptors in these areas synapse in the trigeminal ganglion and associate with second-order neurons in the pons and medulla, which then project to the thalamus.

  • Third-order neurons proceed from the thalamus to the primary somatosensory area.

Mapping of the Primary Somatosensory Area

  • Representation: Visual representation (Homunculus) illustrates the distribution of sensory input areas on the postcentral gyrus according to the density of sensory neurons.

  • Cortical Area Dependency: The amount of cortical area dedicated to each body part correlates with the volume of sensory input sensed from those regions.

Somatic Motor Pathways

  • Functionality: Comprise upper and lower motor neurons controlling skeletal muscle activity.

  • Regulation: Involves neural circuits with basal ganglia and cerebellum that modulate upper motor neuron activity.

Corticospinal Pathways

  • Anterior Corticospinal Pathway:

    • Originates with upper motor neuron in the precentral gyrus; axons traverse the internal capsule, cerebral peduncles, pons, and the medulla’s pyramids before descending in the anterior corticospinal tract.

    • Axons cross over at the anterior commissure to synapse with lower motor neurons in the anterior gray horn.

  • Lateral Corticospinal Pathway:

    • Also starts in the precentral gyrus; axons follow the same pathway but cross in the medulla's pyramids (decussation occurs here).

    • Axons descend in the lateral corticospinal tract to the lower motor neurons in the anterior gray horn.

Mapping of the Primary Motor Areas

  • Homunculus Representation: Illustrates the areas of the precentral gyrus related to motor control across the body, with distorted human images depicting motor output volume to each body part based on the number of involved neurons (greater area for skilled or delicate muscle movements).

The Reflex

  • Definition: A reflex is defined as an automatic, sudden, involuntary response to a stimulus.

  • Types:

    • Spinal Reflex: Integration occurs in the spinal cord.

    • Cranial Reflex: Integration occurs in the brain stem.

The Reflex Arc

  • Definition: The reflex arc is the complete pathway followed by nerve impulses during a reflex.

  • Components of a Reflex Arc:

    • Sensory receptor: Initiates the reflex response.

    • Sensory Neuron: Transmits sensory information to the CNS.

    • Integrating Center: Processes the information and coordinates response.

    • Motor Neuron: Carries impulses from the CNS to the effector.

    • Effector: Muscle or gland that responds to the motor nerve impulse.

The Stretch Reflex

  • Functionality: The stretch reflex prompts contraction of a skeletal muscle in response to stretching.

  • Example: The Knee-jerk reflex is a classic demonstration of a stretch reflex, following this pathway:

    • Stretch the muscle → Activation of muscle spindles → Sensory neuron signal → Spinal cord integration → Motor neuron signal → Muscle contraction.

  • Nature of Reflex: This reflex is ipsilateral (occurs on the same side as the stimulus).