Human Physiology Unit 2 Exam
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OpenStax_Physiology_CH14.pptxphysiology
Chapter 14 The Somatic Nervous System
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College Physics
Chapter # Chapter Title
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Somatic Nervous System
Responsible for conscious perception of the environment and voluntary response to that sensory input.
Composed of :
Simple Reflex Arcs – touch a hot stove, pull hand away
Complex functions – reading and understanding what you read
Sensory perception
Sensation is the activation of sensory receptors at the stimulus level
Perception is the central processing and integration of that stimulus.
Sensory receptors are transmembrane proteins on the receptor/target cell. Usually activated by a protein that opens a ligand-gated ion channel, starting an action potential that will ultimately arrive at a control/integration/processing center. Often are activated by mechanical/pressure or thermal changes in the environment.
Types of receptors
Free nerve endings that receive stimulus e.g. pain receptors in the skin
Encapsulated endings embedded in connective tissue e.g. pressure receptors.
Specialized receptor cells that only interpret a specific type of signal. E.g. the receptors in the retina only react to photons.
Locations of receptors
Located near the source of a stimulus from the external environment e.g. skin receptors. Exteroceptor
Interoceptors receive signals from internal organs e.g. a sense in change in blood pressure.
Proprioceptors adjacent to moving parts. I.e. You know where you are in space.
Types of stimuli
Each type of receptor requires a certain type of stimulus to generate an action potential. This stimulus can be anything but will ultimately be transduced into an electrical signal.
Chemoreceptors – chemicals for taste and smell
Osmoreceptors – react to solute concentration changes in body fluids e.g. blood pressure and vasodilators
Nociceptors – react to pain through chemicals released during inflammation
Mechanorecptors – pressure and balance
Thermoreceptors – temperature changes
Types of sensation - Somatosensory
Somatosensation – the sense of touch with many types of receptors found all over the body in many types of tissue.
| 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 |
Types of sensation - gustation
Gustation – the sense of taste Associated with the tongue but receptors are found throughout the mouth and nasal passages.
Taste buds or gustatory receptor cells on the tongue are sensitive to chemicals dissolved in saliva. It is a Graded Response. Stimulus must reach threshold but the greater the concentration of chemical, the greater the response.
Salty – Na+
Sour – H+ or acids
Sweet – glucose
Bitter – alkaloids
Umami or Savory – L-glutamate an amino acid
Types of sensation - Olfaction
Olfaction or Smell
Bipolar neurons in the olfactory epithelium
Chemicals dissolve in the mucus on the epithelium.
Creates a graded potential that goes directly to the brain.
Goes to the temporal lobe for the smell part and the limbic system and hypothalamus where it is integrated with memory and emotion.
(Micrograph provided by the Regents of University of Michigan Medical School © 2012)
Types of sensation - audition
Audition or Hearing
The external ear contains the auricle, ear canal, and tympanic membrane. The middle ear contains the ossicles and is connected to the pharynx by the Eustachian tube. The inner ear contains the cochlea and vestibule, which are responsible for audition and equilibrium, respectively. The Spiral ganglia of the cochlea transduce sound waves into a neural signal.
Audition or hearing
Transmission of Sound Waves to Cochlea
A sound wave causes the tympanic membrane to vibrate. This vibration is amplified as it moves across the ossicles. The amplified vibration is picked up by the oval window causing pressure waves in the fluid of the scala vestibuli and scala tympani.
Audition or hearing
The fluid movement cause bending of the basilar membrane. That in turn moves the hair cell and the stereocilia emerging from its apical surface. If the stereocilia are bent one way the tension opens ion channels and the cell depolarizes. If they bend the opposite direction, the ion channels close.
Audition or hearing
Different hair cells along the basilar membrane react to different frequencies of vibration. Therefore, hair cells at the base of the cochlea are activated only by high frequencies, whereas those at the apex of the cochlea are activated only by low frequencies.
Types of sensation - Equilibrium
Equilibrium or Balance
Another part of the inner ear, the vestibule, contains hair cells that sense head position, head movement and body motion. Stereocilia extend into a gel, the otolithic membrane, that contains calcium carbonate crystals, otoliths. As the head moves, the otoliths move. This causes the stereocilia to bend producing depolarization or hyperpolarization. Brain interprets the pattern and determines the position of the head.
Semicircular canals – respond to rotational movement in the same manner.
Types of sensation - Vision
Transduction of light waves into electrical stimuli. Light passes through the anterior chamber and is focused on the retina. The retina is composed of several layers of photoreceptors, rods and cones, and various supporting cells. The center of the retina has a small indentation known as the fovea where these supporting cells are absent and therefore the rods and cones can absorb all the light traveling into that area.
Vision
Light travels past the Retinal ganglion cells, past the bipolar cells past the bodies of the rods and cones and reacts with the photopigments in the outer segments of the rods and cones.
Light stimulation produces a graded potential in the photoreceptor cells.
Signal is transmitted to bipolar cells and then to retinal ganglion cells.
Retinal ganglion cells join together to form the optic nerve that passes through the optic disc and into the brain as the optic tract.
At the fovea these other neurons are absent and each photoreceptor cell attaches directly to one retinal ganglion cell so vision is more acute and efficient.
(Micrograph provided by the Regents of University of Michigan Medical School © 2012)
vision
Rods contain a photosensitive pigment – rhodopsin
Cones have three kinds of photosensitive pigments - opsins. Each is sensitive to a particular wavelength of light; red, blue or green.
Photosensitive pigments are bound to a cofactor – retinal, a form of vitamin A.
When light activates the pigment, retinal undergoes a conformational change called bleaching. This opens ion channels and starts the action potential. Bleaching creates a refractory period. It is a graded potential. Therefore, small amounts of light keep the action potential going but large amounts of light create an absolute refractory period.
Enzymes will return retinal to its normal conformation but in the meantime you see a “negative” after image.
Rods are sensitive to low light so bright light creates constant bleaching.
Cones are sensitive to bright light so in low light you see no color.
Vision
Comparison of Color Sensitivity of Photopigments
Comparing the peak sensitivity and absorbance spectra of the four photopigments suggests that they are most sensitive to particular wavelengths.
Sensory nerves
Stimulus → control center → response
Sensations of the head and neck go directly to the brain through the trigeminal pathway and are ipsilateral.
Peripheral sensations go to the spinal cord and are contralateral. The dorsal portion of the spinal cord is mainly sensory and the ventral portion is motor.
Simple reflex arc = sensation → sensory nerve → dorsal spinal column → ventral spinal column → motor neuron → muscle.
But if the sensation needs processing it takes a turn at the dorsal root and heads to the brain through an ascending pathway; spinothalamic tract for pain and/or temperature and the dorsal column system for touch and proprioception.
Sensory nerves
Trigeminal pathway – somatosensory from face, head, nasal cavity and mouth.
Gustation goes through facial and glossopharyngeal nerves.
Audition travels through vestibulocochlear nerve. When input is received from both ears, brain can localize sound.
Equilibrium and proprioception goes through the vestibular system.
While vision goes through the optic tract, some retinal ganglion cells are not image producing. They connect to the hypothalamus and establish circadian rhythm.
Sensory pathways
The Sensory Homunculus
Sensory axons group together as they travel to the brain and end up in one general area rather than scattered throughout.
This cartoon shows the general areas where sensory neurons land for integration in the brain.
The size of the drawing is relative to the numbers of axons arriving in the area.
Even though there are specific areas for perception, all sensation is integrated within the brain so we experience the world as a continuous whole.
Motor response pathways
Somatic nervous system controls skeletal muscle response to sensory input. Sensory control is occipital, temporal and parietal lobe dominated. Motor control is frontal lobe dominated. (Prefrontal lobe is for higher order functioning like working memory and executive functions)
Motor response pathways
Descending Pathway – frontal cortex → brain stem → spinal cord → musculature During this path impulses are being sent back to let the brain know how things are going and to revise instructions accordingly.
Cerebellum coordinates these two messages; descending from the brain and ascending from proprioceptor feedback.
Motor neurons connect at multiple synapses with the muscle fiber sarcolemma (cell membrane of the muscle cell)
Acetylcholine is the neurotransmitter. Opens ligand-gated ion channels to create depolarization and muscle contraction. No graded potential. Strength of contraction is determined by the frequency of the nerve impulse.
Motor reflexes
Simple reflex arc – sensation to spinal cord to motor neuron to muscle.
Complex reflex arc – has pathway through the brain.
Withdrawal reflex – requires control of an antagonistic reaction. I.e. one muscle must contract while the other relaxes. Interneuron acts to hyperpolarize one muscle while the other is depolarizing.
Stretch reflex – homeostasis says muscles shouldn’t be stretched. Once stretched, excites a receptor to contract muscle back into normal tension with hyperpolarization of the antagonistic partner. E.g. knee jerk reaction
Corneal reflex – to protect the eyeball, if the cornea is touched or there is too much light stimulation, the descending pathway stimulates the orbicularis oculi muscle to contract and the eye “blinks”
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