Nervous tissue
General information
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Nervous tissue is divided into 2 major cell types:
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Neurons
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Neuroglial cells (the neuroglia)
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Neurons
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The structural and functional cells in the nervous system
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Respond to a nervous stimulus and conduct the stimulus along the length of the cell
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A neuron’s cell body is called the perikaryon, or soma
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Cell bodies are classified by their location:
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Ganglion—a collection of nerve cell bodies located in the peripheral nervous system (e.g., dorsal root ganglion, trigeminal ganglion, ciliary ganglion)
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Nucleus—a collection of nerve cell bodies located in the central nervous system (e.g., Edinger-Westphal nucleus, chief sensory nucleus of cranial nerve [CN] V, motor nucleus of CN VII)
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Neuron’s cell bodies contain typical cellular organelles within their cytoplasm:
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Mitochondria
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Nucleus
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Nucleolus
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Ribosomes
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Rough endoplasmic reticulum (Nissl substance)
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Neurotubules
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Golgi apparatus
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Lysosomes
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Neurons have 2 types of processes that extend from the nerve cell body:
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Dendrite—process that carries nerve impulses toward the nerve cell body; neurons may have multiple dendrites
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Axon—process that carries nerve impulses away from the nerve cell body; neurons can have only 1 axon
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4 morphologic types of neurons:
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Unipolar—has only 1 process from the cell body (sensory neurons)
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Bipolar—has 2 processes from the cell body: 1 dendrite and 1 axon (sensory neurons; located only in the retina, olfactory epithelium, and the vestibular and cochlear ganglia)
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Pseudounipolar—embryologically originate as bipolar neurons and subsequently become unipolar with 1 process extending from the soma, which branches into 2 distinct extensions: one connects to dendrites that receive sensory information and the other transmits the information to the spinal cord. Most human afferent neurons are pseudounipolar
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Multipolar—has 3 or more processes from the cell body: 2 or more dendrites and 1 axon (motor neurons and interneurons)
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Neuroglia
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Neuroglia is the supporting nervous tissue for neurons, although neuroglial cells also have assistive roles in neuron function
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Neuroglial cells have only 1 type of process
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Classification:
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Astrocytes—located in the central nervous system; help keep neurons in place, provide nutritional support, regulate the extracellular matrix, form part of the blood-brain barrier
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Oligodendrocytes—located in the central nervous system; responsible for axon myelination in the central nervous system; 1 oligodendrocyte can myelinate 1 segment of multiple axons
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Microglial cells—located in the central nervous system; responsible for phagocytosis to remove waste
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Schwann cells—located in the peripheral nervous system; responsible for axon myelination in the peripheral nervous system; 1 Schwann cell can myelinate 1 segment of 1 axon
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Satellite cells—located in the peripheral nervous system; surround the nerve cell bodies of ganglia; maintain the microenvironment around the cell body
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Ependymal cells—line spinal canal and ventricles; modified ependymal cells (choroid epithelium) produce cerebrospinal fluid
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Central nervous system
General information
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The central nervous system is composed of the:
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Brain
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Spinal cord
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Brain
Cerebrum
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The surface of the cerebral cortex of the brain is divided by:
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Gyri (singular gyrus )—the elevations of brain tissue on the surface
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Sulci (singular sulcus )—the grooves or fissures located between the gyri
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There are 3 large sulci that help divide the cerebral hemispheres into 4 of its lobes:
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Central sulcus (of Rolando)—divides frontal lobe from parietal lobe
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Lateral sulcus (of Sylvius)—divides the frontal and parietal lobes from the temporal lobe
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Parieto-occipital sulcus—divides the parietal lobe from the occipital lobe
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The brain is divided into 5 lobes:
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Frontal—motor movement, motor aspect of speech (Broca’s area), reasoning, emotions, personality, and problem solving
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Parietal—sensory perceptions related to pain, temperature, touch, and pressure, spatial orientation and perception, sensory aspect of language (Wernicke’s area)
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Temporal—auditory perceptions, learning, and memory
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Occipital—vision
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Insula—associated with visceral functions including taste
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Diencephalon
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Composed of 4 parts:
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Thalamus—major relay center of the somatosensory system and parts of the motor system
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Hypothalamus—controls the autonomic nervous system and endocrine system
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Epithalamus—major structures include the pineal gland (circadian rhythms) and the habenula
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Subthalamus—an extrapyramidal nucleus of the motor system; lesions of this nucleus will result in a contralateral hemiballismus
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Brainstem
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Composed of 3 parts:
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Midbrain
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Pons
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Medulla
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Cerebellum
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Part of the motor system
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Receives sensory input of all forms that use the deep cerebellar nuclei
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Associated with:
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Equilibrium
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Posture
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Tone of axial muscles
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Gait
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Spinal cord
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The caudal continuation of the central nervous system
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Begins at the caudal end of the medulla and ends at vertebral level L1–2, tapering into the conus medullaris
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Has 2 enlargements associated with the limbs:
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Cervical—associated with the upper limb and found between the spinal cord at levels C4 to T1
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Lumbosacral—associated with the lower limb and found between the spinal cord at levels L1 to S2
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Composed of:
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Gray matter—location of nerve cell bodies and neuroglial cells
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White matter—location of the axons (tracts) and neuroglial cells
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Has 5 levels:
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Cervical—8 spinal nerves
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Thoracic—12 spinal nerves
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Lumbar—5 spinal nerves
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Sacral—5 spinal nerves
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Coccygeal—1 spinal nerve
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Peripheral nervous system
General information
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Peripheral nervous system is the portion of the nervous system located external to the central nervous system
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Consists of:
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Cranial nerves—12 pairs
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Spinal nerves—31 pairs
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Can be subdivided into:
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Somatic nervous system—voluntary system associated with afferent (sensory) and efferent (motor) fibers
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Autonomic nervous system—involuntary system associated with homeostasis of the body; 2 primary subdivisions:
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Sympathetic (originates in the thoracic and lumbar regions of the central nervous system)
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Parasympathetic (originates in the cranial and sacral regions of the central nervous system)
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Cranial nerves
General information
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Cranial nerves or cerebral nerves are peripheral nerves that leave the brain or brainstem
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The cranial nerves customarily are subdivided into 12 pairs:
| I: Olfactory nerve | VII: Facial nerve |
| II: Optic nerve | VIII: Vestibulocochlear nerve |
| III: Oculomotor nerve | IX: Glossopharyngeal nerve |
| IV: Trochlear nerve | X: Vagus nerve |
| V: Trigeminal nerve | XI: Accessory nerve |
| VI: Abducens nerve | XII: Hypoglossal nerve |
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Because of the high degree of differentiation in the brain of humans, cranial nerves are more complex in structure and function than spinal nerves
Functional columns
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7 functional components (or functional columns) of the cranial nerves are recognized
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Concept of functional columns comes from studies of spinal nerves—functions associated with different neurologic pathways along spinal column are assigned corresponding “columns”
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A given cranial nerve may have 1 to 5 functional columns
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The functional columns are classified as general or special:
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General—these functional columns have the same functions as those for spinal nerves
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Special—these functional columns are specific only to cranial nerves
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General and special functional columns each are subdivided into 2 additional categories:
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Afferent (sensory) and efferent (motor)
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Somatic (body related) and visceral (organ related)
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Summary of functions *
* Within each designation: G or S, general or special; S or V, somatic or visceral; A or E, afferent or efferent.
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GSA : Exteroceptors and proprioceptors (e.g., for pain, touch, and temperature, or within tendons and joints). These are the same as in spinal nerves.
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SSA : Special senses in eye and ear (vision; hearing and equilibrium)
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GVA : Sensory from viscera (e.g., gut). These are the same as in spinal nerves.
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SVA : Olfaction and taste
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GVE : Autonomic nervous system (innervates cardiac muscle, smooth muscle, and glands). These are the same as in spinal nerves.
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GSE : Skeletal (somatic) muscle. These are the same as in spinal nerves.
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SVE : Skeletal muscle, which develops from the pharyngeal (branchial) arches (homologous to GSE)
CN I: Olfactory nerve
| Functional Column | Origin of Fibers | Termination of Fibers | Summary | Comment |
|---|---|---|---|---|
| SVA |
|
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The SVA fibers are responsible for the sense of smell |
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CN II: Optic nerve
| Functional Column | Origin of Fibers | Termination of Fibers | Summary | Comment |
|---|---|---|---|---|
| SSA | Begins in the retina with the receptors of rods and cones that synapse on bipolar cells, which synapse with ganglion cells |
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The SSA fibers are responsible for vision |
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CN III, IV, VI: Oculomotor, trochlear, abducens nerves
| Functional Column | Origin of Fibers | Termination of Fibers | Summary | Comment |
|---|---|---|---|---|
| OCULOMOTOR NERVE | ||||
| GSE | Begins in the oculomotor nucleus (of the midbrain) |
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GSE fibers are responsible for innervating the majority of the extraocular eye muscles | Lesions of the oculomotor nerve result in diplopia (GSE), lateral strabismus (GSE), ptosis (GVE), and mydriasis (GVE) |
| GVE | Preganglionic parasympathetic fibers begin in the Edinger-Westphal nucleus (of the midbrain) | Preganglionic parasympathetic fibers synapse with postganglionic parasympathetic fibers at the ciliary ganglion Postganglionic fibers travel through the short ciliary nn. to innervate the sphincter pupillae and ciliary mm. |
|
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| TROCHLEAR NERVE | ||||
| GSE | Begins in the trochlear nucleus (of the midbrain) | Enters orbit through superior orbital fissure and innervates the superior oblique m. | GSE fibers are responsible for innervating 1 extraocular muscle of the eye: the superior oblique |
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| ABDUCENS NERVE | ||||
| GSE | Begins in the abducens nucleus (of the pons) | Enters orbit through superior orbital fissure and innervates the lateral rectus m. | GSE fibers are responsible for innervating 1 extraocular muscle of the eye: the lateral rectus | Lesions of the abducens nerve result in diplopia and medial strabismus |
CN V: Trigeminal nerve
Overview
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Consists of a large sensory root and a small motor root, which joins the mandibular division at the level of the foramen ovale
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The sensory root is created by 3 divisions that come together at the trigeminal ganglion within the middle cranial fossa:
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Ophthalmic division of the trigeminal, which passes through the superior orbital fissure (connects to orbit)
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Maxillary division of the trigeminal, which passes through the foramen rotundum (connects to pterygopalatine fossa)
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Mandibular division of the trigeminal, which passes through the foramen ovale (connects to infratemporal fossa)
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Each division carries the primary neurons for:
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Pain and temperature (the cell body of the primary neuron is located in the trigeminal ganglion)
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Discriminative touch (the cell body of the primary neuron is located in the trigeminal ganglion)
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Proprioception (the cell body of the primary neuron is located in the mesencephalic nucleus of V)
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Parasympathetics use all of the divisions of the trigeminal nerve to distribute their fibers throughout the head and neck
