Nervous System: Structure, Function, Classification

Nervous System Overview

• A system controls and coordinates all bodily activities
• Composed of nerve cells/neurons, neuroglia, and loose connective tissue

Functions of the Nervous System

• Gathers sensory input
• Integrates data
• Forms motor output

Classification of the Nervous System

• Central Nervous System (CNS) consists of the brain and spinal cord
• Peripheral Nervous System (PNS) contains nerves outside the brain and spinal cord
• PNS includes cranial nerves, spinal nerves, ganglia, and parts of receptor organs

Central Nervous System (CNS) Protection

• Skull and vertebral column provide physical protection
• Meninges (membranes) surround the brain and spinal cord
• Cerebrospinal fluid offers a watery cushion
• Blood-brain barrier protects against harmful substances

Meninges

• Membranes surround the brain and spinal cord
• Dura Mater is the tough, outer layer with two fibrous connective tissue layers that form dural sinuses
• Arachnoid Mater is the middle, loose membrane separated from the dura mater by the subdural space
• Arachnoid Mater is also separted from the pia mater by the subarachnoid space, which contains CSF and large blood vessels
• Arachnoid villi allow CSF to be absorbed into venous blood
• Pia Mater is the inner layer of fine fibrous connective tissue with many minute blood vessels, adhering to the brain's surface

Meningeal Spaces

• Subdural space is the area between the arachnoid mater and dura mater
• Subarachnoid space lies between the arachnoid mater and pia mater that contains CSF
• Epidural space is located between the dura mater and vertebral wall, containing fat and small blood vessels

Cerebrospinal Fluid (CSF)

• Watery solution similar to blood plasma and is formed by the choroid plexus
• Provides a watery cushion to protect the brain and spinal cord
• Circulates in the arachnoid space, ventricles, and central canal of the spinal cord
• Gives resistance to the CNS organs
• Protects the CNS from blows and other trauma
• Nourishes the brain and carries chemical signals

Brain Divisions

• Forebrain includes the cerebrum and diencephalon
• Midbrain is a distinct region
• Hindbrain consists of the pons, medulla oblongata, and cerebellum

Cerebrum

• Largest part of the brain
• Consists of outer gray matter and inner white matter
• Two cerebral hemispheres connected by the corpus callosum
• Each hemisphere is composed of frontal, parietal, temporal, and occipital lobes

Cerebral Lobes and Sulci

• Each lobe is separated from others by sulci
• The surface includes sulci (grooves) and gyri (ridges)
• Main sulci include the central, lateral, and parietooccipital sulci

Functions of Cerebral Lobes

• Frontal lobe is responsible for motor function, intellect, and personality
• Parietal lobe is responsible for sensation
• Temporal lobe is responsible for hearing and memory
• Occipital lobe is responsible for vision

Diencephalon

• Sits on top of the brain stem and is enclosed by the cerebral hemispheres
• Subdivided into the thalamus, hypothalamus, subthalamus, and epithalamus

Brain Stem

• Directly attaches to the brain and spinal cord
• Consists of nerve fibers and nerve cells
• Includes the midbrain, pons, and medulla oblongata

Brainstem Function

• Midbrain integrates auditory pathways and controls involuntary functions
• Pons is below the midbrain where sleep and respiratory centers, consciousness, and concentration are controlled
• Medulla oblongata regulates heart rate (cardiac center), blood vessel diameter (vasomotor center), respiration via coughing and sneezing, and swallowing/vomiting

Cerebellum

• Lies posterior to the pons and medulla oblongata
• Composed of two cerebellar hemispheres connected by the vermis

Cerebellum functions

• Coordinates movement
• Controls posture
• Maintains equilibrium

Brain Ventricles

• Four connecting cavities within the cerebrum and brainstem
• They are continuous with the central canal of the spinal cord
• Filled with the cerebrospinal fluid (CSF)
• Right and left lateral ventricles are in the cerebral hemisphere
• Third ventricle is the cavity of the diencephalon
• Fourth ventricle is the cavity of the brainstem

Types of Brain Ventricles

• Lateral ventricles are located in cerebral hemispheres and demonstrate a horseshoe shape
• Third ventricle lies in the diencephalon, connected to the lateral ventricles via the interventricular foramen
• Cerebral aqueduct connects the 3rd and 4th ventricles
• Fourth ventricle lies in the hindbrain, connected to the central canal of the spinal cord

Spinal Cord

• It is a long, cylindrical part of lower brain located in the vertebral canal that extends from vertebrae C1-L1
• Continues as the medulla oblongata, passing through foramen magnum
• Lower part called conus

Spinal Cord Anatomy

• The spinal cord has a length between 42-45cm and enlargements in the cervical and lumbar regions
• Meninges cover this area
• 31 pairs of spinal nerves present in the intervertebral foramina
• Spinal nerves contain both dorsal and ventral roots

Cross Sectional Anatomy

• Exterior white matter provides conduction tracts
• The spinal cord has internal gray matter which contains posterior/dorsal and anterior/ventral horns

Spinal/Peripheral Nerves

• Exterior white matter of spinal cord provides conduction tracts
• Internal gray matter of spinal cord contains posterior/dorsal and anterior/ventral horns
• Central canal is filled with cerebrospinal fluid

Peripheral Nervous System: Spinal Nerves

• Spinal nerves arise from the spinal cord, attaching via a motor anterior root
• Spinal nerves attach via a sensory posterior root to form trunk of spinal nerve
• 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, 1 coccygeal

Cranial Nerves

• Twelve cranial nerves in total originate from cerebrum or brain stem: midbrain, pons or medulla
• Olfactory (CN I) and optic nerve (CN II) originate from the cerebrum.
• Oculomotor (III) and trochlear (IV) cranial nerves stem from the midbrain
• Trigeminal (V), abducent (VI), and facial nerves (VII) vestibulocochlear (VIII) arise in the pons
• Glossopharyngeal (IX), vagus (X), accessory (XI) and hypoglossal nerves (XII) attach to the medulla oblongata

Cranial Nerve Functions

• Olfactory (I) = Smell (sensory)
• Optic (II) = Vision (sensory)
• Oculomotor (III) = Eye muscles (motor)
• Trochlear (IV) = Eye muscles (motor)
• Trigeminal (V) = Facial (sensory), chewing muscles (motor)
• Abducens (VI) = Eye muscles (motor)
• Facial (VII) = Taste (sensory), facial muscles (motor)
• Vestibulocochlear (VIII) = Balance and hearing (sensory)
• Glossopharyngeal (IX) = Taste (sensory), swallowing (motor)
• Vagus (X) affects sweating, peristalsis, heart rate, and opening the larynx for speech and breathing and has branches in the ear canal resulting with a cotton swab cough
• Accessory Nerve (XI) = Neck and upper back muscles (motor)
• Hypoglossal (XII) = Tongue (motor)

Sympathetic Nervous System

• Fight or flight response
• Increases heart rate and blood pressure, as well as respiratory rate
• Bronchodilation and decreased intestinal motility; urination; sweating

Parasympathetic Nervous System

• Occurs at rest
• Lowers heart and breathing rate
• Stimulates intestinal motility and increases the secretion of glands
• Bronchoconstriction and vasodilation

Special Senses

• Taste
• Smell
• Sight
• Hearing
• Touch
• General touch = Temperature, pressure and pain

Taste Buds

• Tiny sensory organs on your tongue that send taste messages
• Organs have nerve endings that have chemical reactions to the food
• Located in papillae of tongue mucosa and are found in fungiform and circumvallate papillae
• Adult human tongue contains between 2,000 and 8,000 taste buds

Sense of Smell

• The organ is olfactory epithelium covers the superior nasal concha
• Receptor cells are bipolar neurons with radiating olfactory cilia

Vision

• 70% of all sensory receptors are in the eye
• The eye has accessory structures = eyebrows, eyelids, conjunctiva, lacrimal apparatus, and extrinsic eye muscles
• Most of the eye is protected by a cushion of fat and the bony orbit

Eyeball structure

• Fibrous tunic: outermost coat that has an opaque sclera (posteriorly) and clear cornea (anteriorly)
• Sclera protects and anchors , while the cornea lets light enter
• Vascular tunic (Uvea): choroid region where blood is supplied the ciliary body where muscles anchor the lens
• Iris: colored part of eye
• Pupil: central opening of iris that regulates light
• Sensory Tunic: Retina is a delicate two-layered membrane and contains neural layer with photoreceptors that transduce light energy
• Rods respond to dim light
• Cones respond to bright light and have acuity color vision

Eye Chambers and Fluids

• The eyeball is composed of two chambers
• There is an anterior chamber (between the cornea and the iris)
• There is a posterior chamber between the iris and the lens

The Ear

• 3 parts = inner, outer and middle
• The outer and middle ear = hearing
• The inner ear = hearing and equilibrium

Outer Ear

• Includes the auricle (pinna)
• Contains the external auditory canal (short, curved tube filled with ceruminous glands)
• Composed of tympanic membrane/eardrum made of thin connective tissue that vibrates in response to sound which transfers sound to middle ear ossicles as a boundary between outer and middle ears

Middle Ear

• Small, air-filled, mucosa-lined cavity
• Contains the Eustachian tube connects the middle ear to the nasopharynx to equalize pressure with external air

Ear Ossicles

• The tympanic cavity contains three small bones
• The malleus (hammer), incus (anvil), and stapes (stirrup) that transmit vibratory motion to oval window

Inner Ear

• The bony labyrinth contains the vestibule, cochlea, and semicircular canals filled with perilymph
• The membranous labyrinth are seriers of sacs that has potassium rich fluid