The Somatic Nervous System

Sensory Perception

Sensory Receptors

  • Different types of stimuli are sensed by different types of receptor cells.

  • Receptor cells can be classified into types on the basis of three different criteria: cell type, position, and function.

    • Can also be classified on the basis of the transduction of stimuli, or how the mechanical stimulus, light, or chemical changed the cell membrane potential.

  • Structural Receptor Types:

    • free nerve ending- a neuron with dendrites embedded in tissue that would receive a sensation

      • Ex: pain and temp. receptors in the dermis of the skin

    • encapsulated ending- a neuron in which the sensory nerve endings are encapsulated in connective tissue that enhances their sensitivity

      • pressure and touch lamellated corpsucles in the dermis of the skin

    • special receptor cell- which has distinct structural components that interpret a specific type of stimulus

      • cells in retina that respond to light; photoreceptor

    • exteroceptor- receptor that is located near a stimulus in the external environChemical stimuli can be interpreted by a chemoreceptor that interprets chemical stimuli, such as an object’s taste or smell.ment

      • somatosensory receptors that are located in the skin.

    • interoceptor- interprets stimuli from internal organs and tissues

      • receptors that sense the increase in blood pressure in the aorta

    • proprioceptor- receptor located near a moving part of the body, such as a muscle, that interprets the positions of the tissues as they move

  • Functional Receptor Types

    • A third classification of receptors is by how the receptor transduces stimuli into membrane potential changes.

      • Some stimuli are ions and macromolecules that affect transmembrane receptor proteins when these chemicals diffuse across the cell membrane.

      • Some stimuli are physical variations in the environment that affect receptor cell membrane potentials.

    • Receptor cells can be further categorized on the basis of the type of stimuli they transduce.

      • chemoreceptor- interprets chemical stimuli, such as an object’s taste or smell

      • Osmoreceptors respond to solute concentrations of body fluids

      • thermoreceptor- which is sensed through a that is either sensitive to temperatures above (heat) or below (cold) normal body temperature.

Sensory Modalities

  • A general sense is one that is distributed throughout the body and has receptor cells within the structures of other organs.

    • often contribute to the sense of touch or to proprioception (body position) and kinesthesia (body movement), or to a visceral sense (sense associated with the internal organs), which is most important to autonomic functions.

  • A special sense is one that has a specific organ devoted to it, namely the eye, inner ear, tongue, or nose.

  • Each of the senses is referred to as a sensory modality.

    • Modality refers to the way that information is encoded

  • Gustation/ Taste

    • Raised bumps called papillae (singular = papilla) contain the structures for gustatory transduction.

      • 4 types: circumvallate, foliate, filiform, and fungiform.

      • Within the structure of the papillae are taste buds that contain specialized gustatory receptor cells

    • Alkaloids are nitrogen containing molecules that are commonly found in bitter-tasting plant products, such as coffee, hops (in beer), tannins (in wine), tea, and aspirin.

  • Olfaction/ smell

    • The olfactory receptor neurons are located in the olfactory epithelium and contains bipolar sensory neurons.

    • Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity.

    • odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites.

    • The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe.

  • Audition/ hearing

    • auricle- The large, fleshy structure on the lateral aspect of the head

    • At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves.

    • The auricle, ear canal, and tympanic membrane are often referred to as the external ear.

    • The middle ear consists of a space spanned by three small bones called the ossicles

      • The three ossicles are the malleus, incus, and stapes

    • The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal.

      • It has two separate regions, the cochlea and the vestibule, which are responsible for hearing and balance

    • The scala vestibuli extends from the oval window, travelling above the cochlear duct, which is the central cavity of the cochlea that contains the sound-transducing neurons.

    • As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion

    • The cochlear duct contains several organs of Corti, which transduce the wave motion of the two scala into neural signals.

    • As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves.

    • The organs of Corti contain hair cells, which are named for the hair-like stereocilia

  • Equalibrium/ Balance

    • Head position is sensed by the utricle and saccule, whereas head movement is sensed by the semicircular canals.

    • The neural signals generated in the vestibular ganglion are transmitted through the vestibulocochlear nerve to the brain stem and cerebellum.

    • The utricle and saccule are both largely composed of macula tissue

    • The stereocilia of the hair cells extend into a viscous gel called the otolithic membrane

  • Somatosensation/ touch

    Name

    Historical (eponymous) name

    Location(s)

    Stimuli

    Free nerve endings

    *

    Dermis, cornea, tongue, joint capsules, visceral organs

    Pain, temperature, mechanical deformation

    Mechanoreceptors

    Merkel’s discs

    Epidermal–dermal junction, mucosal membranes

    Low frequency vibration (5–15 Hz)

    Bulbous corpuscle

    Ruffini’s corpuscle

    Dermis, joint capsules

    Stretch

    Tactile corpuscle

    Meissner’s corpuscle

    Papillary dermis, especially in the fingertips and lips

    Light touch, vibrations below 50 Hz

    Lamellated corpuscle

    Pacinian corpuscle

    Deep dermis, subcutaneous tissue

    Deep pressure, high-frequency vibration (around 250 Hz)

    Hair follicle plexus

    *

    Wrapped around hair follicles in the dermis

    Movement of hair

    Muscle spindle

    *

    In line with skeletal muscle fibers

    Muscle contraction and stretch

    Tendon stretch organ

    Golgi tendon organ

    In line with tendons

    Stretch of tendons

  • Vision

    • The inner surface of each lid is a thin membrane known as the palpebral conjunctiva.

    • Tears are produced by the lacrimal gland, located just inside the orbit, superior and lateral to the eyeball.

    • Movement of the eye within the orbit is accomplished by the contraction of six extraocular muscles that originate from the bones of the orbit and insert into the surface of the eyeball

    • Four of the muscles are arranged at the cardinal points around the eye are the superior rectus, medial rectus, inferior rectus, and lateral rectus

    • A single unit of light is called a photon, which is described in physics as a packet of energy with properties of both a particle and a wave.

    • photoisomerization- photons cause some of the double-bonded carbons within the chain to switch from a cis to a trans conformation.