Lab 10 Nervous System Anatomy Part 2: Functional Anatomy of the Peripheral Nerves and Sense Organs

Activity 1: Dermatome and Myotomes

The myotomes and dermatomes provide the clinician with a reference map for the origin of the nerves that control movement or are responsible for conveying somatosensation. The myotome is the map that indicates the region of control of muscles for the spinal nerve and originates during fetal development as motor neurons (nerves) exit the spinal cord and innervates regions of non-differentiated skeletal muscle tissue (referred to as somites).   Dermatomes are areas of sensation of the body that relate to the nerve root that forms the peripheral nerves that synapse with the receptors in that region.  The dermatomes also form during fetal development and mirror the mapping that occurs with the myotomes.  Using the dermatome and myotomes allow for map of origin of peripheral nerves through the body should peripheral nerve issues arise for the person.

Procedures:
1.      Check your lab group for completion of the color the map of the myotome and dermatome.
2.      Select a “team leader” and using the colored images as reference have members of the group take turns tracing a dermatome or myotomes themselves and then move to the next activity.

Peripheral nerves:

The somatic peripheral system will function via two primary divisions or plexus, the Brachial Plexus and Lumbar/Sacral Plexus.  The plexus all for the innervation of the extremities of the body through the convergence and divergence of the spinal nerves.  While the plexus are convergences and divergence so spinal nerves, the means by which they develop are not identical. The peripheral nerves of the extremities will have their origin within each of these plexuses. In which the brachial plexus will provide the peripheral nerves to the upper extremities while the lumbar and sacral plexus will provide the peripheral nerves the lower extremities. These peripheral nerves will allow for the transmission of somatic sensation and proprioception from periphery to the spinal cord and central nervous system.  While they will be involved with conveying the motor response for reflexive behavior along with cognitive control of the skeletal muscle that allows for spinal reflex, fine and gross motor control of the limb (or limb segment).  Clinically, we are also able to trace the origins of motor issues to spinal nerve segment of involvement or to what extent afferent neurons are impacting the ability to convey the sensory signal based on where within the limb issues might be arising.

Activity 2: Nerves from the Brachial Plexus

The brachial plexus is built through the convergence of spinal nerves forming 3 plexus trunks (Superior, Middle, Inferior) that then converge and alter their orientation forming 2 plexus divisions (Anterior, Posterior) all within a short distance of the inferior cervical vertebrae.  As the nerves move lateral toward the scapula, the division once again converge and diverge forming 3 plexus cords (Lateral, Medial, Posterior) that they diverge within the lateral scapular region forming the peripheral nerves of the upper extremities.

Procedures:
1.      Obtain nervous system models, stickers, felt pens
2.      On the stickers, write the regions of the plexus and major nerves of the upper extremity
        Superior Trunk, Middle Trunk, Inferior Trunk, Anterior Division, Posterior Division, Lateral Chord, Medial Chord, Posterior Chord, Radial N, Ulnar N, Musculocutaneous N
3.      Select a “team leader” and using the colored images as reference have members of the group take turns labeling the model with the correct stickers.
4.      Have your instructor check your work and then move to the next activity.

Activity 3: Nerves from the Lumbar, Lumbosacral and Sacral Plexus

The lumbar and sacral plexus form 3 plexus trunks from the spinal nerves converging onto each other (Lumbar, Lumbosacral, Sacral) that will the converge and diverge in order to form into 2 divisions (Anterior or Posterior) near the sacroiliac region of the pelvis.  These divisions will then converge and diverge within the posterior aspect of the pelvic bowl forming the peripheral nerves of the lower extremities.

Procedures:
1.      Obtain nervous system models, stickers, felt pens
2.      On the stickers, write the regions of the plexus and major nerves of the lower extremity
        Lumbar Plexus, Lumbosacral Plexus, Sacral Plexus, Anterior Divisions, Posterior Divisions, Sciatic N, Femoral N, Tibial N, Peroneal (Fibular) N
3.      Select a “team leader” and using the colored images as reference have members of the group take turns labeling the model with the correct stickers.
4.      Have your instructor check your work and then move to the next activity.

Activity 4: Sense Organs

Tongue and Nose:
        Gustatory complexes are the receptors are located throughout the mouth and tongue and not in specialized regions that has been previously noted (due to a mistranslation and misconception of a classical study). The receptor itself is comprised of separate cells that have ligand-gated receptors within the cilia projections.  The cells cluster these cilia projection along folds within the pit to form a microvilli projection along the inner surface of a pit (gustatory receptor). These will respond to chemicals that get trapped within the pit that will then link with a chemical class ligand receptor that leads to an action potential being transmitted to one of three cranial nerves, CN VII, IX and X, depending on the location that stimulation is occurring within the tongue. The olfactory epithelium is tissue lining the superior aspect of the nasal opening that has chemoreceptors embedded into the tissue. These receptors appear to be sensitive to distinct classes of chemicals and not individual chemicals.  The olfactory receptors are bipolar neuron with cilia called olfactory hairs that project into the nasal concha.  These will respond to chemical stimulation of a particular volatile chemical (odorant molecule) via a ligand gated channel leading the action potential being transmitted to CN I via passage through the Cribriform plate of the Ethmoid bone.

Eye:
        Vision is the sense that provides for reception of light energy from the environment. We perceive light energy within a spectrum of wavelength that is deemed the visible light spectrum of electromagnetic radiation. The receptor organ for vision is the retina of eye, where the structure of the eye is meant to focus light onto the fovea centralis based on the principle that light will always travel in straight lines and will bend when it passes through a medium whose density changes (such as going from air through the cornea and lens or the fluids that fill the eye, aqueous and vitreous humor).  The receptors for vision are found in retina.  To receive this sense, the retina will utilize 2 distinct types of photoreceptive cells that are stimulated (or inhibited) by distinct wavelengths of light.  The receptors are arranged in radiations of density with the cone cells being in greatest density near the fovea and lessen as move towards the periphery of the retina (away from the fovea), while rods are greatest in density at the periphery and least at the fovea. Cone cells are in effect 3 distinct photoreceptors that exhibit a threshold based on location within retina and wavelength of light that cause “excitation”. Where the short wave-cell, aka Blue Cone, has a peak response at 437 nm of light wavelength with range of 400-550 nm. The medium wave-cell, aka Green Cone, has a peak response at 530 nm of light wavelength with range of 400-650 nm.  The long wave-cell, aka Red Cone, has a peak response at 564 nm of light wavelength with rang from 450-700 nm.  The rods on the other hand are generalized photoreceptors that while having a peak response at 490 nm of light will respond with the entire spectrum of visible light.

Ear (Hearing and Vestibular Organs)
        The structure that is responsible for the auditory sense and response is the ear and the cochlea. The anatomy of the ear is formed in such a way as to funnel sound waves into the external auditory canal and onto the tympanic membrane.  The tympanic membrane then transforms the energy wave from a sinusoidal to a compressive wave that can then transfer that energy onto the bones of the middle ear (auditory Ossicles).  The energy transfer causes the Ossicles to vibrate against each other that will then provide the mechanisms to transform the compressive wave into a sinusoidal wave to transfer the same energy wave into the cochlea through the oval window. The cochlea itself is a “snail” coiled boney labyrinth within the temporal bone of the cranium. Within the cochlea are fluid filled membranous cavities (the tympanies).  It is within tympanies that the receptor organ with the cochlea is located.  This receptor organ, the Organ of Corti, is comprised on two membranes and a stereoscopic organization of mechanoreceptors (hair cells).
        Vestibular sense provides input as to the direction of head (and body) movement relative to the force of gravity. To perceive this sense, it will utilize receptors found in the semi-circular canals that are surrounding the cochlea of the inner ear, stimulate the neurons found in CN VIII (vestibular branch). Overall, vestibular sensation gives the person the ability to sense this movement is based on the acceleration taking place to give the “sense” of movement and speed of movement of the body in space without the need for visual or proprioceptive information.

Procedures:
1.      Obtain ear, eye and sagittal head models, stickers, felt pens
2.      Write the names of the anatomical structures of the ear and eye that you are responsible for knowing on to the stickers
        Eye: Fovea Centralis, Macula Lacteal, Ciliary Body, Suspensory Ligaments, Lens, Cornea, Retina, Pupil, Sclera
        Ear: Auricle (Pina of Ear), Cochlea, Tympanic Membrane, Ossicles, Anterior Semicircular Canal, Lateral Semicircular Canal, Posterior Semicircular Canal
3.      Select a “team leader” and using the colored images as reference have members of the group take turns labeling the model with the correct stickers.
    a.       As you label, indicate the function of the structure that you are labeling
4.      Have your instructor check your work and then move to the next activity.