Somatic Nervous System Notes

Somatic Nervous System

• Allows movement and control of muscles (voluntary nervous system).
• Relays information from taste buds, nose, ears, and eyes.

Sensory Receptors Role

• Help us learn about the environment or internal state.
• Stimuli are received and converted into electrochemical signals.
• Stimulus changes the cell membrane potential of a sensory neuron.
• Action potential is relayed to the central nervous system (CNS).
• Integrated with sensory information and cognitive functions.
• Results in a conscious perception and response.

Special Senses

• Taste, smell, hearing, and vision.
• Processed by cranial nerves.

General Senses

• Associated with touch.
• Found all over the body.
• Lack special sense organs.

Sense of Taste (Gustation)

• Responsive to chemical stimuli.
• Five tastes recognized:
  • Sweet
  • Salty
  • Sour
  • Bitter
  • Umami (savory)
• Papillae are raised bumps on the tongue.
• Taste buds are within the papillae.
• Gustatory receptor cells (gustatory cells) are in taste buds.
• Microvilli (taste hairs) project into the taste pore, responding to dissolved chemicals.
• Neurotransmitters from gustatory cells activate sensory neurons in facial, glossopharyngeal, and vagus cranial nerves.

Sense of Smell (Olfaction)

• Responsive to chemical stimuli.
• Olfactory receptor cells are in the superior nasal cavity within the olfactory epithelium.
• Sensory neurons with dendrites extend into the mucous lining.
• Airborne (odorant) molecules dissolve in the mucus and bind to the olfactory receptor cells.
• Axons of olfactory cells extend through the cribriform plate of the ethmoid bone into the brain.
• Olfactory tract travels to the primary olfactory cortex of the temporal lobe.
• Some project to the limbic system and hypothalamus.
• Associated with long-term memory and emotional responses.

Sense of Hearing (Audition)

• Transduction of sound waves into a neural signal.

Anatomy of the Ear

• Three regions:
  • External ear
  • Middle ear
  • Inner ear

External Ear

• Auricle directs sound waves toward the auditory canal.
• Auditory canal enters the skull through the external auditory meatus of the temporal bone.
• Tympanic membrane (eardrum) vibrates when struck by sound waves.

Middle Ear

• Contains three auditory ossicles:
  • Malleus
  • Incus
  • Stapes
• Connected to the pharynx through the eustachian tube to equilibrate air pressure.

Inner Ear

• Series of canals embedded within the temporal bone.
• Two regions:
  • Cochlea (hearing)
  • Vestibule (balance and equilibrium)
• Cochlea is attached to the stapes via the oval window.

Function of Hearing

• Oval window is located at the beginning of the scala vestibuli (fluid-filled tube within the cochlea).
• Scala vestibuli travels above the cochlear duct (central cavity with sound transducing neurons).
• Scala tympani returns to the base of the cochlea under the cochlear duct and ends at the round window.
• Auditory ossicles convert sound waves to pressure pulses on the oval window.
• Vibrations travel through the oval window, moving the fluid of the scala vestibuli and scala tympani in a wave-like motion.
• Frequency of fluid waves matches sound wave frequencies.
• The membrane covering the round window bulges out or puckers in, acting like a pressure relief valve.
• Sound waves (measured in Hertz) funnel into the ear, hitting the tympanic membrane, which vibrates.
• $tympanic \; membrane \; vibration \rightarrow malleus \rightarrow incus \rightarrow stapes \rightarrow oval \; window$
• Vibrations against the oval window create a pressure wave in the fluid within the cochlea.
• Pressure bends the membrane of the cochlea duct, causing hair cells in the basilar membrane to vibrate.

Cochlea

• Cross-sectional view shows scala vestibuli and scala tympani running along the cochlear duct.
• Cochlear duct contains the organs of Corti.
• Organs of Corti lie on top of the basilar membrane.
• High-frequency waves move the region of the basilar membrane near the base of the cochlea.
• Low-frequency waves move the region near the tip of the cochlea.
• Organs of Corti contain hair cells (receptors for sound detection with stereocilia).
• Stereocilia overlie the tectorial membrane.
• Pressure waves move the basilar membrane, and the tectorial membrane slides across the stereocilia, bending them.
• Bending of stereocilia triggers nerve impulses that travel down the afferent nerve fibers.
• Hair cells respond to specific frequencies.
• Cochlea encodes auditory stimuli for frequencies between $20$ and $20,000$ Hertz.

Equilibrium (Balance)

• Head position is sensed by the utricle and saccule within the vestibule.
• Utricle receptors respond to horizontal acceleration.
• Saccule receptors respond to gravity and linear acceleration.
• Utricle and saccule are largely composed of maculae (hair cells surrounded by support cells).
• Stereocilia of hair cells extend into a viscous gel.
• A layer of calcium carbonate crystals (otoliths) is on top of the gel membrane.
• The otolithic membrane moves separately from the macula in response to head movements.
• Tilting the head causes the otolithic membrane to slide over the macula, bending stereocilia.
• Position of the head is interpreted based on the pattern of hair cell depolarization.

Semicircular Canals

• Three ring-like extensions of the vestibule.
• The base connects to an enlarged region called the ampulla.
• Ampulla contains hair cells that respond to rotational movements.
• The stereocilia of these hair cells extend into the cupula (membrane attached to the top of the ampulla).
• As the head rotates, fluid lags behind, deflecting the cupula in the opposite direction.

Vestibulocochlear Nerve

• Nerves of the sense of hearing and balance.
• The vestibular branch leaves the vestibule.
• The cochlear branch innervates the cochlea.
• They join together forming the vestibulocochlear nerve that goes to the CNS.

Somatosensation

• Considered a general sense.
• Sensory modalities associated with touch, proprioception, and interoception.
• Modalities include pressure, vibration, light touch, tickle, itch, temperature, pain, proprioception, and kinesthesia.
• Receptors are spread throughout the body in various organs.
• Located in the skin, muscles, tendons, joint capsules, ligaments, and walls of visceral organs.

Sense of Vision

• Based on the transduction of light stimuli received through the eyes.
• Bony orbits protect the eyeballs.

Anatomical Structures of the Eye

• Hollow sphere composed of three layers of tissues (tunics).

Tunics

• Fibrous tunic (outermost layer):
  • Sclera (white part of the eye)
  • Cornea (clear, allows light to enter)
• Vascular tunic (middle layer):
  • Choroid (vascularized connective tissue)
  • Ciliary body (ciliary process and ciliary muscle, attached to the lens by suspensory ligaments)
  • Iris (colored part of the eye, smooth muscle that opens or closes the pupil)
• Neural tunic (innermost layer):
  • Retina (contains nervous tissue for photoreception)

Other Structures

• Pupil (hole in the center that allows light to enter)
• Optic disc (axons of the retina collect, creating a blind spot)
• Fovea (small depression in the center of the retina, contains only cones for visual acuity)

Cavities of the Eye

• Anterior cavity:
  • Space between the cornea and the lens.
  • Contains aqueous humor.
  • Anterior chamber (between the iris and the cornea).
  • Posterior chamber (between the iris and the ciliary processes).
• Posterior cavity:
  • Space behind the lens.
  • Contains vitreous humor (gel-like, helps the retina stay positioned).

Retina Structure

• Composed of several layers.
• Photoreceptors: rods and cones.
• Located behind the axons of the retinal ganglion cells, bipolar cells, and retinal blood vessels.
  *

Photoreceptor Cells

• Rods:
  • Contain rhodopsin (photoreceptive pigment, good for low light vision in grayscale).
• Cones:
  • Contain three photopigments (opsins sensitive to red, green, and blue wavelengths of light).
• Inner segment:
  • Contains the nucleus and other organelles.
• Outer segment:
  • Specialized region where photoreception takes place.
  • Has shapes of rod and cone.

Visual Pathway

• Signals from photoreceptors communicate via bipolar cells (interneurons) and ganglion cells.
• Sent out the back of the eye via the optic nerve through the thalamus to the primary visual cortex in the occipital lobe.
• Each cerebral hemisphere receives information from the lateral half of the retina on the same side and from the medial half of the retina on the opposite side.
• Crossing pattern occurs at the optic chiasma.