BIOL 241 Lab Manual: The Nervous System Comprehensive Study Guide
Organizational Guidelines and Histological Standards for the Nervous System
The BIOL 241 Lab Manual for Unit 4, covering the Nervous System at Spokane Community College, establishes exhaustive requirements for students. The unit focuses on histology, the major organs of the central nervous system (CNS), and special sense organs including the eye and ear. A primary standard for histology is that students must recognize every structure presented from representative specimens or images for exam purposes and may be required to submit histology drawings as graded assignments. The manual emphasizes a golden rule: if information is present in the manual, it is required knowledge for the exam. This encompasses all terms for major structures, results from physiology experiments, and details from dissections. Students are expected to arrive prepared for dissections with appropriate clothing or lab coats. Physiological experiments in this unit will require a thorough understanding of the mechanisms and outcomes of various neural tests.
Histology of the Central and Peripheral Nervous Systems
The Central Nervous System is structurally categorized into white matter and gray matter. White matter is characterized by myelinated axons that facilitate the conduction of neural signals over long distances. The distinct white color is a result of the lipid-rich myelin sheaths, which do not stain conspicuously in prepared slides. In the spinal cord, white matter is organized into anterior, posterior, and lateral columns, while in the brain, it is located deep to the gray matter within the medulla. Gray matter consists primarily of neuron cell bodies, also known as the soma, and typically stains darker. Histological observations of the soma reveal a darker nucleus and projections known as processes, such as dendrites. In the spinal cord, gray matter is arranged into anterior, lateral, or posterior gray horns, connected at the center by the gray commissure. The gray commissure houses the central canal, which is the opening through which cerebrospinal fluid (CSF) flows. In the brain, gray matter forms the outer cortex. Clinical observation starts at scanning power before moving to low and high power to visualize neurons and glial cells.
The Peripheral Nervous System is divided into nerves and ganglia. Nerves are defined as collections of axons, while ganglia are collections of cell bodies, mirroring the white and gray matter definitions of the CNS respectively. The internal structure of a nerve includes axons wrapped in a thin connective tissue layer called the endoneurium. These axons are bundled into fascicles, which are wrapped in the perineurium. A complete nerve is formed by a collection of multiple fascicles. Histological slides of interest include the spinal cord using a silver staining technique and nerve fibers in both cross-sections and longitudinal sections. Cross-sections allow for the identification of the fascicle, perineurium, epineurium, endoneurium, blood vessels, and nerve fibers comprising the axon and myelin sheath. Longitudinal sections are used to identify the nodes of Ranvier. Spinal cord smears are utilized to identify motor neuron components, specifically the soma, processes, and nucleus, as well as the visible nuclei of glial cells.
Anatomy of the Spinal Cord and Spinal Nerves
The spinal cord serves as the conduction pathway from the base of the brainstem to approximately the or vertebra within the vertebral canal. Its external anatomy features two prominent longitudinal grooves: the anterior median fissure and the posterior median sulcus. Along its length, the cervical and lumbar enlargements accommodate the high volume of neural impulses traveling to and from the limbs. The cord tapers inferiorly at the conus medullaris, which is anchored to the sacrum and coccyx by the filum terminale. The peripheral nervous system connects to the cord via pairs of spinal nerves, including cervical, thoracic, lumbar, sacral, and coccygeal pair. These nerves emerge through the intervertebral foramina. Sensory neurons enter the cord through the posterior root, which features the posterior root ganglion containing cell bodies. Efferent motor neurons exit via the anterior root. The spinal nerve begins where these roots merge, carrying mixed sensory and motor information.
Spinal nerves branch into various rami to innervate the body, including the anterior ramus, posterior ramus, meningeal branch, and rami communicantes. In the lumbar region, the nerves extending beyond the conus medullaris form the cauda equina, resembling a horse's tail. Redundant neural networks known as plexuses ensure continued communication despite potential nerve damage. Key plexuses include the cervical plexus (containing the phrenic nerve), the brachial plexus (containing the radial and ulnar nerves), the lumbar plexus (containing the femoral nerve), and the sacral plexus (containing the sciatic nerve). The CNS is protected by three meningeal layers: the superficial dura mater (separated from the bone by the epidural space), the web-like arachnoid mater (featuring the subarachnoid space for CSF flow and denticulate ligaments for stability), and the deep pia mater, which is anchored directly to the neural tissue.
Physiology of Human Reflexes and Clinical Testing
Reflexes are rapid, automatic, and involuntary responses to stimuli, following a neural pathway known as a reflex arc. Every reflex arc consists of five components: a receptor to detect the stimulus, a sensory or afferent neuron to conduct the impulse to the CNS, an interneuron synapse within the CNS, a motor or efferent neuron to conduct the impulse to the effector, and an effector (muscle or gland) that carries out the response. Reflexes are classified by their effector into somatic reflexes, involving skeletal muscle, and visceral or autonomic reflexes, involving smooth muscle, cardiac muscle, or glands. Tests for somatic reflexes include the patellar reflex (knee-jerk), the plantar reflex, and the Achilles reflex (ankle-jerk).
The patellar reflex involves the muscle spindles as receptors and the femoral nerve as both the afferent and efferent pathway, with the quadriceps femoris acting as the effector. Testing this reflex under mental distraction, such as counting backward from to , or during muscle activity elsewhere, such as the Jendrassik maneuver (clasping hands and pulling apart), can demonstrate changes in response intensity. The plantar reflex involves tactile receptors and the lateral plantar and tibial nerves. In adults, the normal response is toe flexion, but in children under years or adults with nerve damage, the Babinski sign (outward spreading of toes) may appear due to incomplete myelination. The Achilles reflex uses muscle spindles and the tibial nerve to cause contraction of the gastrocnemius and soleus muscles.
Macroanatomy and Regional Specialization of the Human Brain
The human brain is organized into four major regions: the cerebrum, diencephalon, brainstem, and cerebellum. The cerebrum is the largest portion, featuring ridges called gyri and valleys called sulci. Notable landmarks include the precentral gyrus, postcentral gyrus, and the central sulcus. The longitudinal fissure divides the brain into left and right hemispheres, which are connected by the corpus callosum. The cerebrum is divided into the frontal, parietal, occipital, temporal, and the deep insula lobes. The diencephalon, or "through brain," consists of the thalamus for relaying information, the hypothalamus for endocrine and autonomic control (linked to the pituitary gland via the infundibulum), and the epithalamus, which contains the pineal gland for melatonin secretion.
The brainstem is the most inferior region, connecting to the spinal cord and comprising the midbrain, pons, and medulla oblongata. The midbrain contains the corpora quadrigemina, consisting of the superior colliculi for visual reflexes and inferior colliculi for auditory reflexes. The pons contains cerebellar peduncles, and the medulla oblongata features anterior pyramids and lateral olives. The cerebellum, located posteriorly and divided by the vermis, regulates unconscious motor control and displays a branching white matter structure known as the arbor vitae, or "tree of life." Protection is provided by the cranial meninges, including the dual-layered dura mater (periosteal and meningeal layers) which folds to form the falx cerebri within the longitudinal fissure and houses dural venous sinuses.
Ventricular System and Cranial Nerve Functions
The brain contains internal chambers called ventricles that facilitate the flow of cerebrospinal fluid. The lateral ventricles are separated by the septum pellucidum and drain via the interventricular foramen into the third ventricle. The third ventricle connects to the fourth ventricle through the cerebral aqueduct of the midbrain. From the fourth ventricle, fluid exits via lateral apertures into the subarachnoid space or continues into the central canal of the spinal cord. Cerebrospinal fluid is produced within these chambers by the choroid plexus.
There are pairs of cranial nerves ( through ) emerging from the brain as part of the PNS. (Olfactory) and (Optic) are sensory for smell and vision. (Oculomotor), (Trochlear), and (Abducens) are primarily motor for eyeball movement, with also controlling pupil size. (Trigeminal) is mixed, handling facial sensation and mastication. (Facial) is mixed, controlling facial expression and taste. (Vestibulocochlear) is sensory for hearing and equilibrium. (Glossopharyngeal) and (Vagus) are mixed, involvement in taste, swallowing, and visceral regulation. specifically lowers heart rate and regulates thoracic/abdominal viscera. (Accessory) is motor for head and shoulder movement, and (Hypoglossal) is motor for speech and swallowing.
Anatomy and Physiology of the Human Eye
The eye is organized into three tunics. The outer fibrous tunic includes the sclera and the transparent cornea. The middle vascular tunic contains the choroid, ciliary body (with ciliary muscles and processes for lens accommodation), suspensory ligaments, and the iris. The iris contains circular muscles for constriction and radial muscles for dilation, regulating the pupil. The inner neural tunic is the retina, containing rod photoreceptors for dim light and cone photoreceptors for color and detail. Rods and cones synapse with bipolar cells, which relay signals to ganglion cells whose axons form the optic nerve. The fovea centralis within the macula lutea provides the highest visual acuity. The optic disc is the blind spot where axons exit the eye. The eye is divided into an anterior cavity containing aqueous humor and a posterior cavity containing vitreous humor.
Visual health is assessed through various tests. Visual acuity is measured using a Snellen eye chart at a distance of . Astigmatism is identified using a chart of radiating lines to detect unequal refraction. Color blindness is tested with color plates to identify cone deficiencies. The near point is the shortest distance for clear focus, while the blind spot test demonstrates visual filling. Dominant eye determination identifies which eye is most closely aligned with objects. Ocular reflexes include the pupillary reflex, where light causes miosis (constriction), and the ciliospinal reflex, where a painful stimulus to the neck causes mydriasis (dilation) via sympathetic activation. These reflexes can be ipsilateral or consensual (contralateral). Conditions of the eye include emmetropia (normal), hyperopia (farsightedness), myopia (nearsightedness), and presbyopia (age-related loss of lens elasticity).
Anatomy of the Human Ear and Mechanisms of Transduction
The ear is divided into outer, middle, and inner regions. The outer ear consists of the pinna (auricle), external acoustic meatus, and the tympanic membrane. The middle ear houses the auditory ossicles (malleus, incus, and stapes), the oval window, and the auditory (Eustachian) tube for pressure equalization. The inner ear involves a bony labyrinth filled with perilymph and a membranous labyrinth filled with endolymph. The vestibule contains the saccule and utricle (otolithic organs) with maculae containing otoliths (calcium carbonate structures). These detect linear acceleration. The three semicircular canals feature ampullae with crista ampullaris and a cupula to detect rotational acceleration.
The cochlea is the organ of hearing, containing the vestibular, tympanic, and cochlear ducts. The Organ of Corti sits on the basilar membrane and contains hair cells with stereocilia. Sound waves vibrate the tympanic membrane and ossicles, causing the stapes to push on the oval window. This movement creates pressure waves in the fluid that deform the basilar and tectorial membranes, bending the hair cells. This mechanical action is converted into electrical signals sent through the cochlear branch of the vestibulocochlear nerve. Auditory adaptation can be demonstrated using a tuning fork and a stethoscope, showing how the brain modifies sound perception when one ear is occluded.
Sensory Receptors and Physiology Experiments
Sensory receptors are classified based on the stimuli they detect and their adaptation rates. Proprioceptors detect body position, while nociceptors detect pain. Tonic sensations continue as long as a stimulus is present, whereas phasic sensations undergo adaptation, where the sensation fades despite the stimulus remaining. Olfaction and touch are typically phasic. The gustatory sense requires saliva to dissolve chemicals; sugar crystals cannot be tasted immediately on a dry tongue because fluid is necessary for chemical transduction. Two-point discrimination tests the density of touch receptors; areas like the fingertips have a lower threshold (higher density) than the back or forearm.
Temperature sensation was tested using probes at various temperatures. Experiments showed that thermoreceptors (cold and heat) have varying densities across the skin. The olfactory sense experiment involves timing how long it takes for the scent of wintergreen oil to disappear, highlighting adaptation. In the two-point discrimination test, if a partner pushes too hard, they activate nociceptors, which are tonic to ensure the body remains aware of potential damage. These physiological tests collectively illustrate the complex ways the nervous system interprets and adapts to environmental stimuli.