Module 11: Somatic Nervous System

Somatic nervous system

skeletal muscle effectors; responses can be voluntary or involuntary

Autonomic nervous system

smooth, cardiac, and glands are effectors, involuntary responses; sympathetic vs. parasympathetic

Enteric nervous system

digestive system

Sensory receptors

stimuli are received and changed into the electrochemical signals of the nervous system (CNS)

Structural classifications of receptors

free nerve endings, encapsulated nerve endings (connective tissue), and specialized receptor cell

General senses

• senses without a specialized organ

• temperature, pain, touch, pressure, vibration, proprioception (position); receptors distributed throughout body

Special senses

• senses that have receptors located within specialized organs

• vision, hearing, smell, taste, balance (vestibular)

Reception

activation of sensory receptors by stimuli

• receptive field: region in space in which a given sensory receptor can respond to a stimulus

Sensation

signal carried by a sensory pathway

• requires: receptor activation and sensory neuron activation

Perception

conscious awareness of a sensation

• requires:

  • sensory signal reaches cortex

  • signal is processed

  • signal is sent to be made ‘aware’ of

Nociceptors

• pain receptors

• free nerve endings

  • large receptive fields and broad sensitivity

  • respond to pressure, temperature, chemical, trauma, etc

• do not adapt quickly (i.e. pain signal persists)

Thermoreceptors

• temperature (non-harmful) receptors

• free nerve endings throughout the body

• no structural differences between warm and cold thermoreceptors

Chemoreceptors

• respond to chemical concentrations

  • substances dissolved in extracellular body fluids

  • Ex. O₂, CO₂, glucose, etc.

Mechanoreceptors

• respond to mechanical distortion of their plasma membranes

  • mechanically-gated ion channels

  • respond to stretching, compression, twisting, etc.

• three types of mechanoreceptors

  • proprioceptors, baroreceptors, and tactile receptors

Gustation

• taste buds - papillae on the tongue and in the oral cavity with gustatory receptor cells

  • sensitive to chemical in food (tastants)

• salty (Na+) and sour (H+) detected by ions

• sweet, bitter, umami (savory) detected with G protein-coupled receptors

Olfaction

olfactory receptor neurons are within the superior nasal cavity

• olfactory epithelium

• airborne molecules pass over the olfactory epithelial region, dissolve into the mucus, and bind proteins

• odorant-protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite

External ear

composed of auricle, ear canal, and tympanic membrane

Middle ear

has ossicles (malleus, incus, and stapes)

Inner ear

composed of a series of canals embedded within temporal bone

• cochlea (hearing) and vestibule (balance)

Cochlea

attached to the stapes through the oval window

Cochlear duct

scala vesibuli and scala tympani run along both sides

Organ of corti

contain hair cells, named for the hair-like stereocilia extending from the cell’s apical surfaces

Tectorial membrane

bends the stereocilia either toward or away from the tallest member of each array

Head position

sensed by the utricle and saccule

Head movement

sensed by the semicircular canals

Otolithic membrane

stereocilia of the hair cells extend into a viscous gel

• otoliths - on top of the otolithic membrane is a layer of calcium carbonate crystals

semicircular canals

three ring-like extensions of the vestibule; base of each semicircular canal connects to the ampulla

ampulla

contains the hair cells that respond to rotational movement

cupula

where stereocilia of these hair cells extend into; deflects in the direction opposite to the head movement

Eye

composed of three layers of tissue:

• outermost layer - fibrous tunic, which incudes the white sclera and clear cornea

• middle layer - vascular tunic, which is mostly composed of the choroid, ciliary body (focuses lens), and iris (controls light)

• innermost layer - neural tunic, or retina

Retina

composed of several layers and contains photoreceptors

• visual acuity - highest at the fovea

• optic disc = blind spot

Photoreceptors

• Inner segment = nucleus and other organelles

• Outer segment = specialized structures for photoreception

• Rods - shape and low-light

• Cones - color (red, blue, and green)