Nervous System Anatomy

Nervous System Overview

• The nervous system controls and coordinates all bodily activities
• Made of nerve cells/neurons, neuroglia, and loose connective tissue
• Three main functions: gathering sensory input, integrating data, and forming motor outputs

Classification of the Nervous System

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

Protection of the Central Nervous System

• Skull and vertebral column provide physical protection
• Meninges are protective membranes
• Cerebrospinal fluid cushions the brain and spinal cord

Meninges

• Membranes surrounding the brain and spinal cord
• Dura Mater (outer layer): strong, composed of two fibrous connective tissue layers that form dural sinuses when separated
• Arachnoid Mater (middle layer): a loose membrane separated from the dura mater by the subdural space and from the pia mater by the subarachnoid space which contains CSF and large blood vessels
• Arachnoid villi allows CSF is absorbed into venous blood
• Pia Mater (inner layer): internal, fine fibrous connective tissue with many minute blood vessels and adheres to the surface of the brain, dipping into each fissure

Meningeal Spaces

• Subdural Space: area between arachnoid mater and dura mater
• Subarachnoid Space: area between arachnoid mater and pia mater that's filled with CSF
• Epidural Space: area between the dura mater and vertebral wall that contains fat and small blood vessels

Cerebrospinal Fluid (CSF)

• A watery solution similar to blood plasma which is formed by the choroid plexus
• Forms a watery cushion to protect the brain and spinal cord
• Circulates in the arachnoid space, ventricles, and central canal of the spinal cord
• Functions: gives resistance to CNS organs, protects the CNS from blows and trauma, and nourishes the brain while carrying chemical signals

Brain Structure

• Divided into 3 parts: forebrain, midbrain, and hindbrain

Forebrain

• Consists of the cerebrum and diencephalon

Cerebrum

• Largest part of the brain, consisting of outer gray matter and inner white matter
• Composed of two cerebral hemispheres connected by the corpus callosum
• Each hemisphere has four lobes: frontal, parietal, temporal, and occipital
• Each lobe is separated by sulcus
• The surface includes sulci and gyri
• Main sulci: central, lateral, and parietooccipital

Function of Lobes

• Frontal lobe: responsible for motor, intellectual functions, and personality
• Parietal lobe: involved in sensation
• Temporal lobe: handles hearing and memory
• Occipital lobe: dedicated to vision

Diencephalon

• Sits on top of the brain stem and is enclosed by the cerebral hemispheres
• Subdivided into 4 parts: thalamus, hypothalamus, subthalamus, and epithalamus

Brain Stem

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

Brainstem: Midbrain

• Integral part of auditory pathways in CNS
• Controls involuntary functions

Brainstem: Pons

• Located below the midbrain
• Contains sleep and respiratory centers regulating rate and depth of breathing
• Involved in the control of consciousness and the level of concentration

Brainstem: Medulla Oblongata

• Cardiac center: regulates heart rate
• Vasomotor center: controls blood vessel diameter
• Respiration center: involved in coughing and sneezing
• Controls swallowing and vomiting

Cerebellum

• Lies posterior to the pons and the medulla oblongata
• Composed of two cerebellar hemispheres connected by the vermis
• Functions: coordination of movement, control of posture, and equilibrium

Brain Ventricles

• Four connecting cavities located within the cerebrum and brain stem
• Continuous with the central canal of the spinal cord
• Filled with CSF

Brain Ventricles - Specific

• Lateral ventricles: located in cerebral hemispheres and are horseshoe-shaped
• Third ventricle: lies in diencephalon and connects with lateral ventricles through the interventricular foramen
• Cerebral aqueduct: connects the 3rd and 4th ventricles
• Fourth ventricle: located in the hindbrain and connects to the central canal of the spinal cord

Spinal Cord Features

• A long, cylindrical part of the lower portion of the brain
• Located in the vertebral canal
• Extends from C1 to L1 vertebra
• Passes through the foramen magnum
• It's the continuation of the medulla oblongata
• The lower spinal cord is called conus

Spinal Cord Anatomy

• 42-45 cm in length
• Meninges covers the spinal cord
• Has 31 pairs of spinal nerves present in the intervertebral foramina
• Each spinal nerve has two roots: dorsal and ventral

Spinal Cord Cross Section

• Exterior contains white matter for conduction tracts
• Internal gray matter contains mostly cell bodies
• Gray matter has dorsal (posterior) and anterior (ventral) horns
• Central canal is filled with cerebrospinal fluid

Peripheral Nervous System: Spinal Nerves

• Spinal nerves arise from the spinal cord
• Attach to it by an anterior (motor) root and posterior (sensory) root
• These mix together and then divide into two rami (anterior and posterior)
• 31 pairs of spinal nerves: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal

Cranial Nerves

• There are twelve cranial nerves in total
• Olfactory (CN I) and optic nerve (CN II) originate from the cerebrum
• CN III – XII arise from the brain stem (midbrain, pons or medulla)

Cranial Nerves origins

• 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) are attached 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 sensation (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 sensory/motor neurons that affect sweating, peristalsis, heart rate, opening the larynx for speech and breathing. The ear also has branches in the ear canal.
• Accessory Nerve (XI): neck and upper back muscles (motor)
• Hypoglossal (XII): tongue (motor)

Sympathetic Nervous System

• "Fight or flight" response
• Increases heart rate, blood pressure, and respiratory rate
• Causes bronchodilation and decreases intestinal motility and urination

Parasympathetic Nervous System

• Operates during rest
• Lowers heart rate and breathing rate
• Stimulates intestinal motility and increases gland secretion
• Causes bronchoconstriction and vasodilation

Special Senses

• Taste
• Smell
• Sight
• Hearing
• Touch

General Senses of Touch

• Temperature
• Pressure
• Pain

Taste Buds

• Tiny sensory organs on the tongue that send taste messages to the brain
• Located in papillae of the tongue mucosa
• Taste buds are found in fungiform and circumvallate papillae
• Adult human tongue contains between 2,000 and 8,000 taste buds

Sense of Smell

• The organ of smell is the olfactory epithelium, which covers the superior nasal concha
• Olfactory receptor cells are bipolar neurons with radiating olfactory cilia

Vision

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

Eye: Fibrous Tunic

• Outermost coat of the eye, which includes opaque sclera (posteriorly) and clear cornea (anteriorly)
• The sclera protects the eye and anchors extrinsic muscles
• The cornea lets light enter the eye

Eye: Vascular Tunic (Uvea)

• Choroid region: supplies blood to all eye tunics
• Ciliary body: composed of smooth muscle (ciliary muscles) that anchors the lens
• Iris: the colored part of the eye
• Pupil: the central opening of the iris that regulates the amount of light entering the eye

Eye: Sensory Tunic (Retina)

• Delicate two-layered membrane, with a neural layer containing photoreceptors that transduce light energy
• Rods: respond to dim light
• Cones: respond to bright light and have high-acuity color vision

Eye Chambers and Fluids

• Eyeball composed of two chambers: anterior (between the cornea and the iris) and posterior (between the iris and the lens)

Ear: General Structure

• Three parts: inner, outer, and middle ear
• Outer and middle ear: involved with hearing
• Inner ear functions: hearing and equilibrium

Outer Ear

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

Middle Ear (Tympanic Cavity)

• A small, air-filled, mucosa-lined cavity
• The Eustachian tube connects the middle ear to the nasopharynx, which equalizes pressure in the middle ear cavity with the external air pressure

Middle Ear: Ear Ossicles

• Tympanic cavity contains three small bones: malleus, incus, and stapes
• Transmit vibratory motion of the eardrum to the oval window

Inner Ear

• Bony labyrinth: contains the vestibule, the cochlea, and the semicircular canals; all filled with perilymph
• Membranous labyrinth: series of membranous sacs within the bony labyrinth which are filled with a potassium-rich fluid