Nervous System

NERVOUS SYSTEM

Fundamental Terms

Neuron

-          A nerve cells and consists of a cell body with its nucleus, two sets of processes dendrites and axon.

Dendrites

-          Several processes that extend out from the cell body, and carry impulses towards the cell body.

Axon

-          Single process extending out from the cell body and may be two or three feet in length. It conveys impulses away from the cell body.

Sensory Neuron / Afferent Neuron

-          A nerve cell that conducts impulses towards or into the spinal cord or up the spinal cord to the brain.

-          These impulses may be sensations of heat, cold, touch, pressure, pain or of position or from the special sense organs – the eye, ear, nose or mouth.

Motor Neuron / Efferent Neuron

-          A nerve cell that carries impulses from the brain down to the spinal cord, or from the cord to muscles or secreting glands, or directly from the brain to muscles or to a gland.

Synapse

-          The point of contact of the axon of one nerve with the Dendron/dendrites of another neuron.

Endorgan

-          A structure at the free end of an axon or Dendron designed to pass on or pick up impulses or secretions.

·         Receptor

-          An endorgan at the outer end of a Dendron of a sensory neuron that picks up sensations.

·         Effector

-          An endorgan at the distal end of an axon of a motor neuron.

Reflex Arc

-          A complete circuit consisting of a sensory neuron ending in the spinal cord, a connecting neuron within the cord, and a motor to a muscle.

Reflex Act

-          Sensory neuron carries a sensation of pain, etc., to the spinal cord.

Ganglion

-          Group of nerve cell bodies outside of the spinal cord or brain.

Nucleus

-          Group of cell bodies within the brain and spinal cord.

Center

-          Group of nerve cell bodies concerned with some specific function.

Plexus

-          A network of cell processes.

White Matter / White Substance

-          Consists of nerve fibers, axon and dendrites, and are white in color.

Gray Matter / Gray Substance

-          Made up of the nerve cell bodies and some processes and it appears darker that the white matter.

Funiculi

-          Large columns of nerve fibers that pass and down the spinal cord or brain.

Tracts

-          Bundles of nerve fibers and parts of funiculi, in close contact with each other pass up or down the spinal cord.

Irritability

-          Property of nerves that renders them responsive to stimuli such as heat, cold, etc.

Conductivity

-          The ability to convey impulses, generated by stimuli, along the neurons.

Divisions of the Nervous System

I.                    Central Nervous System

-          It includes the brain within the cerebral cranium, and the spinal cord within the vertebral column.

1.       Brain

-          Divided into the forebrain, midbrain and hindbrain.

1.1. Forebrain

-          Consists of the cerebrum and diencephalon or interbrain or between brain.

1.2. Midbrain

-          Includes two cerebral penducles, and four small bodies, the corpora quadrigemina or colliculi.

1.3. Hindbrain

-          Have pons, medulla oblongata and cerebellum.

2.       Spinal Cord

The brain / Enkephalos / Encephalon

-          Occupies the entire cerebral cranium.

1.       Forebrain

-          Large expanded mushroom-like upper part of the brain. It occupies the entire cranium except the posterior cranial fossa.

1.1. Cerebrum

-          Forms the visible part of the forebrain and is by far its larger part. It composed of gray and white matter.

·         Gyri or Convolutions

-          Rounded ridges visible on the outer curved surfaces of the cerebrum.

·         Sulci

-          Shallow grooves that dip down between adjacent gyri.

·         Cortex / Cerebral Cortex

-          The outer part of the cerebrum. Composed of several layers of nerve cell bodies that form the gray matter of the cerebrum.

·         White Matter

-          Lies inside the cortex and consists of nerve fibers arising from or ending in the cortical cell bodies.

·         Corpus Collusum

-          Composed of these nerve fibers that connect the two halves or hemispheres.

·         Cerebral Hemisphere

-          Right and left, are two lateral halves of the cerebrum separated along the median plane by the longitudinal fissure.

Fissures and Grooves

Longitudinal Fissure

-          A deep groove that extends from the front to the back of the cerebrum, along the median line, and under saggital suture.

Transverse Fissure

-          Passes transversely across the upper limit of the posterior cranial fossa.

Central Sulcus

-          Deep groove that begins at the longitudinal fissure at about its midpoint.

Lateral Sulcus / Lateral Cerebral Fissure

-          Definite groove that begins on the inferior surface of each cerebral hemisphere.

Parietooccipital Sulcus

-          Extends upward and backwards from the inferior margin of the lateral surface of the hemisphere close to its posterior margin.

Lobes of the Cerebral Hemisphere

Frontal Lobe

-          Lies adjacent to the frontal bone, and anterior to the central sulcus.

Parietal Lobe

-          Lies under the parietal bone, and between the central sulcus in front, and the lateral sulcus and parietooccipital sulcus behind.

Occipital Lobe

-          Lies in contact with the occipital bone posterior to the parietooccipital sulcus, and the parietal and temporal bone.

Temporal Lobe

-          Lies adjacent to the temporal bone and below and behind the lateral sulcus.

Insula / Central Lobe

-          Lies deep within each hemisphere, and deep to the lower part of the lateral sulcus.

Special Areas and Centers of the Cortex

Sensory Area

-          Receives and interprets impulses from sensory nerves, is located posterior to the central sulcus in the parietal lobe.

Center of Hearing and Smell

-          Located in the temporal lobe.

Center for Taste

-          Located in the parietal lobe.

Visual Center

-          In the posterior part of the occipital lobe.

1.2. Diencephalon

-          Between brain of after brain is located between the cerebrum and the midbrain.

Thalamus

-          Forms the lateral walls of the third ventricle.

Hypothalamus

-          Forms the floor of the third ventricle

2.       Midbrain

-          Forms a small part of the brain and is less one inch in length.

2.1. Cerebral Peducles

-          Visible on the ventral surface of the brain and are covered by the cerebrum.

2.2. Corpora Quadrigemina / Quadrageminal Bodies

-          Located in dorsal part of the midbrain and are covered by the cerebrum.

3.       Hindbrain

-          Lies below the midbrain and extends down to the foramen magnum where it becomes the spinal cord.

3.1. Pons

-          Lies immediately below the midbrain and anteriorly forms a definite prominence with a median groove.

3.2. Medulla Oblangata

-          Lies below the pons, between it and the spinal cord.

3.3. Cerebellum

-          Lies posterior to the pons and upper part of the medulla oblongata.

-          Act as a coordinator of muscular movement, rendering them smooth and graceful rather than jerky.

-          Concerned with the maintenance of posture and equilibrium.

Brain Stem

-          Includes midbrain, pons and medulla.

Ventricles of the Brain

1.       Lateral Ventricles

-          Located in the corresponding cerebral hemisphere.

Interventricular Foramen / Foramen of Monro

-          An opening between the medial margin of a lateral ventricle and the adjacent third ventricle.

2.       Third Ventricle

-          Lies in the midline of the diencephalon or afterbrain.

Cerebral Aqueduct / Aqueduct of Sylvius

-          A canal that passes between the third and fourth ventricles.

3.       Fourth Ventricle

-          Lies in the hindbrain.

Median Aperture / Foramen of Magendie

-          An opening between the fourth ventricle and subarachnoid space of the brain in the midline of the roof of fourth ventricle.

Lateral Apertures / Foramina of Luschka

-          Two opening between the fourth ventricle and subarachnoid space of the brain in the lateral parts of the roof of the fourth ventricle.

Choroid Plexuses

-          Networks of vein in the wall of each ventricle.

SPINAL CORD

-          About 40-45 cm (16-18 inches) in length lies within the vertebral canal, which is formed by the vertebral foramina of the vertebrae and the ligaments between their arches.

·         Gray Matter / Substance

-          Composed largely of nerve cell bodies forms the inner part of the cord, an arrangement opposite to that in the cerebrum.

Lateral Horn / Lateral Column

-          A bulge on each limb of gray substance in the thoracic and upper lumbar segments.

Posterior Column

-          Contains sensory cell bodies, the anterior column contains motor cell bodies.

·         White Substance

-          Composed of nerve fibers and surrounds the gray substance.

Motor Tracts / Descending Tracts

-          Located in the anterior and lateral funiculi of the cord.

Sensory Tracts / Ascending Tracts

-          Located for the most part in the posterior and lateral funiculi of white matter.

Meninges

-          The covering of the brain and spinal cord.

1.       Pia Mater

-          Delicate inner membrane.

2.       Arachnoid

-          The middle layer and overlies the pia mater.

Subarachnoid Space

-          The space between the arachnoid and pia mater.

3.       Dura Mater

-          Tough outer membrane covering the brain and cord.

Falx Cerebri

-          The fold of dura mater that dips down into the longitudinal fissure between the right and left cerebral hemisphere.

Tentorium Cerebelli

-          Fold of dura mater that passes transversely across the upper end of the posterior cranial fossa in the transverse fissure.

Epidural Space

-          The interval between the dura mater and cranial bones.

-          Space between the dura mater and the vertebral canal in spine.

Subdural Space

-          Interval between the arachnoid and dura mater of the brain or the spinal cord.

Archnoid Cisterns

-          Small cavities where the pia mater and arachnoid are more widely separated from each other that usual.

Cerebrospinal Fluid (CSF)

-          Clear colorless that is formed by cells in the choroid plexuses of all of the ventricles, and is secreted into their cavities.

Arachnoid Granulations

-          Small knoblike protrusions of the arachnoid into the walls of the superior sagittal sinus.

Lumbar Puncture

-          A procedure whereby some CSF is obtained from the spinal cord.

CRANIAL NERVES

I Cranial / Olfactory Nerve

-          The sensory nerve of smell.

II Cranial / Optic Nerve

-          The sensory nerve of sight.

III Cranial / Oculomotor Nerve

-          A motor nerve to muscles of the eye.

IV Cranial / Trochlear Nerve

-          A motor nerve to superior oblique muscle of the eyeball.

V Cranial / Trigeminal Nerve

-          A motor and sensory nerve from the pons, with three dimensions:

·         Opthalmic Nerve

-          Sensory through the superior orbital fissure to the eyeball, lacrimal gland, and upper face.

·         Maxillary Nerve

-          Sensory through the foramen rotundum to the face and upper teeth.

·         Mandibular Nerve

-          Sensory and motor, sensory to the lower teeth, mouth and lower part of the face, motor to the muscles of masticaton, through the foramen ovale.

VI Cranial / Abdocent Nerve

-          A motor nerve from the pons through the superior orbital fissure to the lateral rectus muscle of the eye.

VII Cranial / Facial Nerve

-          A motor nerve to muscle of the face, from the pons through stylomastoid formen and sensory nerve to taste buds on the tongue.

VIII Cranial / Vestibulocochlear Nerve

-          Acoustic or auditory nerve, through the internal acoustic porus and internal acoustic meatus to the internal ear.

·         Vestibular Nerve

-          Sensory from semicircular canals of internal ear to medulla, pons and the cerebellum, controlling equilibrium and coordination.

·         Cochlear Nerve

-          Nerve of hearing, sensory from the organ of corti in the internal ear to the temporal lobe of the cerebrum.

IX Cranial / Glossopharyngeal Nerve

-          Sensory and motor from medulla through jugular foramen to muscles of pharynx and to taste buds on posterior part of tongue.

X Cranial / Vagus Nerve

-          Sensory and motor, from medulla through jugular foramen and parasympathetic nerve to the thoracic and abdominal organs, visceral sensory and motor to the larynx and pharynx.

XI Cranial / Accessory Nerve

-          Two parts; the cranial part from the medulla through the jugular foramen with the vagus nerve to the thoracic and abdominal organs, and the cervical part from the upper cervical segments of the spinal cord, motor and sensory about the neck, shoulder and upper back.

XII Cranial / Hypoglossal Nerve

-          Motor nerve to the tongue and muscles under this organ, from the medulla through the hypoglossal canal.

SPINAL NERVES

Sensory Fibers

-          Begin as receptors of sensory impulses in the skin, muscles, joints, etc.

Motor Fibers

-          Axons originated in cell bodies in the anterior column of gray substance.

Ventral Root

-          Consists of the bundle of axons that leaves the cord at its anterior surface.

Dorsal Root

-          Consists of a bundle of dendrites that enters each intervertebral foramen and contains sensory nerve fibers.

DISTRIBUTION OF SPINAL NERVES

Cervical Plexus

-          Made up of the ventral divisions’ f the 1^st, 2^nd, 3^rd and 4^th cervical nerves.

Brachial Plexus

-          Composed of ventral divisions of the upper four cervical and first thoracic nerves.

Thoracic Nerves

-          Ventral divisions pass around the thoracic wall as intercostals nerves between adjacent nerves.

Lumbar Plexus

-          Composed of the ventral divisions of the 1^st, 2^nd, 3^rd and part of the 4^th lumbar spinal nerve.

Sacral Plexus

-          Formed from the ventral divisions of the 4^th and 5^th lumbar, and the 1^st, 2^nd and 3^rd sacral spinal nerves.

Autonomic Nervous System / Involuntary Nervous System

-          The motor subdivision of the PNS that controls body activities automatically.

Sympathetic Nervous System

-          Have nerve centers in the spinal cord, sympathetic trunks and trunk ganglia, communicating rami, autonomic plexuses and ganglia, nerve fibers to the organ supplied.

Parasympathetic Nervous System

-          Provides nerve fibers to many but not all of the same organs supplied by the sympathetic part.