Nervous System: Structure and Function

Nervous System Overview

• This system controls and coordinates all bodily activities
• Components include: nerve cells/neurons, neuroglia, and loose connective tissue
• The nervous system's main functions are:
  • Gathering sensory input
  • Integrating data
  • Generating motor output

Divisions of the Nervous System

• The nervous system has two main divisions:
  • Central Nervous System (CNS)
  • Peripheral Nervous System (PNS)

Central Nervous System (CNS)

• Contains the brain and spinal cord

Peripheral Nervous System (PNS)

• Located outside the brain and spinal cord
• Includes cranial nerves, spinal nerves, ganglia, and parts of receptor organs

Protection of CNS

• The central nervous system is protected by:
  • Skull
  • Vertebral column
  • Meninges
  • Cerebrospinal fluid

Meninges

• The meninges are membranes that surround the brain and spinal cord.
• The meninges consist of three layers:
  • Dura mater (outer layer)
  • Arachnoid mater (middle layer)
  • Pia mater (inner layer)

Dura Mater

• The dura mater is a tough, strong outer layer composed of two fibrous connective tissue layers
• These layers separate in areas to form dural sinuses

Arachnoid Mater

• The arachnoid mater is the middle layer that forms a loose membrane
• It separates from the dura mater by the subdural space
• It separates from the pia mater by a wide subarachnoid space
• The subarachnoid space is filled with cerebrospinal fluid (CSF) and large blood vessels
• Arachnoid villi protrude superiorly to allow CSF absorption into venous blood

Pia Mater

• The pia mater is the inner layer containing fine fibrous connective tissue with numerous minute blood vessels
• It adheres to the brain's surface, dipping into each fissure

Meningeal Spaces

• Subdural space: Area between arachnoid mater and dura mater
• Subarachnoid space: Area between arachnoid mater and pia mater; filled with CSF
• Epidural space: Area between the dura mater and vertebral wall; contains fat and small blood vessels

Cerebrospinal Fluid (CSF)

• Watery solution composition is similar to blood plasma
• It's formed by the choroid plexus
• Forms a watery cushion to protect the brain and spinal cord
• It circulates in the arachnoid space, ventricles, and central canal of the spinal cord

CSF Functions

• Provides resistance to the CNS organs
• Protects the CNS from blows and other trauma
• Nourishes the brain and carries chemical signals

Brain

• The brain is divided into three main parts:
  • Forebrain
  • Midbrain
  • Hindbrain

Forebrain

• Forebrain includes the:
  • Cerebrum
  • Diencephalon

Hindbrain

• Hindbrain consists of:
  • Pons
  • Medulla oblongata
  • Cerebellum

Cerebrum

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

Brain Lobes

• Lobes are separated by sulci on the brains surface
• The surface of the brain consists of sulci (grooves) and gyri (ridges)
• Main sulci include: central, lateral, and parietooccipital

Functions of Brain Lobes

• Frontal lobe: responsible for motor function, intellectual activity, and personality
• Parietal lobe: responsible for sensation
• Temporal lobe: responsible for hearing and memory
• Occipital lobe: responsible for vision

Diencephalon

• Sits on top of the brain stem and is enclosed by the hemispheres
• It is subdivided into four parts: thalamus, hypothalamus, subthalamus, and epithalamus

Brain Stem

• Directly attaches to the brain and spinal cord
• It consists of nerve fibers and nerve cells
• The brain stem parts include: midbrain, pons, and medulla oblongata

Brain Stem Function

• Midbrain: Integral part of auditory pathways, controls involuntary functions
• Pons: Located below the midbrain, contains sleep and respiratory centers, controls consciousness and concentration levels
• Medulla oblongata: Cardiac center regulates heart rate, vasomotor center controls blood vessel diameter, respiration center controls coughing and sneezing, controls swallowing and vomiting

Cerebellum

• Lies posterior to the pons and medulla oblongata
• Consists of two cerebellar hemispheres connected by the vermis
• Coordinates movement, controls posture, and maintains equilibrium

Brain Ventricles

• These are the four connecting cavities within the cerebrum and rain stem that are continuous with the central canal of the spinal cord
• The ventricles are filled CSF

Brain Ventricles

• Right and left lateral ventricles: Located in the cerebral hemisphere
• Third ventricle: Located in the diencephalon
• Fourth ventricle: Located in the cavity of the brainstem

Brain Ventricles Connection

• Lateral ventricles connect with the third ventricle
  • Via the interventricular foramen
• The cerebral aqueduct connects the 3rd and 4th ventricles.
• Fourth ventricle is located in the hindbrain
  • Connects to the central canal of the spinal cord

Spinal Cord

• The spinal cord is the long, cylindrical part of the lower brain
• It is in the vertebral canal
• Extends from C1-L1 vertebra
• Passes through the foramen magnum
• Continuation of the medulla oblongata
• The lower part of the spinal cord is called conus

Dimensions of Spinal Cord

• The spinal cord measures 42-45 cm in length
• Enlargements occur in the cervical and lumbar regions

Spinal Cord Anatomy

• Meninges cover the spinal cord
• There are 31 pairs of spinal nerves present in the intervertebral foramina
• Spinal nerves have two roots: dorsal and ventral

Spinal Cord Composition

• Consists of exterior white matter and interior grey matter
• Anterior (ventral) and posterior (dorsal) exist

Spinal Cord Canal

• The central canal is filled with cerebrospinal fluid

Peripheral Nervous System: Spinal Nerves

• Spinal nerves originate from the spinal cord and attach to it by an anterior (motor) root and a posterior (sensory) root
• These roots connect to form a mixed spinal nerve, which then divides into anterior and posterior rami

Spinal Nerve Count

• There are 31 pairs of spinal nerves:
  • 8 cervical
  • 12 thoracic
  • 5 lumbar
  • 5 sacral
  • 1 coccygeal

Cranial Nerves

• There are twelve cranial nerves in total
• The olfactory (CN I) and optic (CN II) nerves originate from the cerebrum
• Cranial nerves III – XII arise from the brain stem
  • This includes from from the midbrain, pons and medulla

Cranial Nerves Origins

• Oculomotor (III) and trochlear (IV) nerves stem from the midbrain
• Trigeminal (V), abducent (VI), and facial (VII) and vestibulocochlear (VIII) nerves arise in the pons
• Glossopharyngeal (IX), vagus (X), accessory (XI), and hypoglossal (XII) nerves attach to the medulla oblongata

Cranial Nerves Functions

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

Sympathetic Division

• Triggers "fight or flight" response
• Increases heart rate, blood pressure, and respiratory rate
• Causes is responsible for Bronchodilation, decreased intestinal mobility, urination and sweating

Parasympathetic Division

• Functions primarily at rest
• Lowers heart rate and breathing rate
• Stimulates intestinal motility and secretion of glands
• Responsible for Bronchoconstriction and vasodilation

Special Senses

• Special senses include: taste, smell, sight, hearing, and touch

General Senses

• General senses of touch include: temperature, pressure and pain

Taste Buds

• Taste buds are tiny sensory organs on the tongue that send taste messages to the brain
• These buds are found in the papillae of the tongue mucosa
• Fungiform and circumvallate papillae contain taste buds
• The adult human tongue contains between 2,000 and 8,000 taste buds

Smell

• The olfactory epithelium covers the superior nasal concha and serves as this systems smell organ
• Olfactory receptor cells are bipolar neurons with radiating olfactory cilia

Vision

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

Fibrous Tunic

• Forms the outermost coat of the eye and includes:
  • Opaque sclera (posteriorly)
  • Clear cornea (anteriorly)
• The sclera protects the eye and anchors extrinsic muscles
• The cornea lets light enter the eye

Vascular Tunic (Uvea)

• Choroid region: Supplies blood to all eye tunics
• Ciliary body: Composed of smooth muscles that anchor the lens in place
• Iris: The colored part of the eye
• Pupil: Central opening of the iris, regulates amount of light entering the eye

Sensory Tunic: Retina

• The system has a delicate two-layered membrane, a neural layer containing photoreceptors that transduce light energy
  • Rods respond to dim light
  • Cones respond to bright light, and have increased visuals depending on colors .

Eye Chambers and Fluids

• The eyeball contains two chambers:
  • Anterior: Located between the cornea and the iris
  • Posterior: Located between the iris and the lens

The Ear: Hearing and Balance

• The ear's three parts are the inner, outer, and middle ear
• The outer and middle ear are involved with hearing
• The inner ear functions in both hearing and equilibrium

Outer Ear

• The auricle (pinna) is part of this system.
• External auditory canal: Short, curved tube filled with ceruminous glands
• Tympanic membrane (eardrum): Is connective tissue that vibrates to sound, this also:
  • Transfers sound to the middle ear ossicles
  • Is the Boundary between outer and middle ears

Middle Ear (Tympanic Cavity)

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

Ear Ossicles

• Housed in the is located in the Tympanic cavity it helps with the function of of 3 small bones: malleus, incus, and stapes, which are used to transmit the vibratory motion of the eardrum to the oval window.

Inner Ear

• Bony labyrinth: Contains the vestibule, cochlea, and semicircular canals; filled with perilymph
• Contains membranous labyrinth: Series of membranous sacs; filled with a potassium-rich fluid