Chapter 9. Chapter 9 – Structure of the Nervous System Applied Learning Outcomes Use the...

Chapter 9

Chapter 9 – Structure of the Nervous System

Applied Learning Outcomes

Use the terminology associated with the nervous system

Learn about the following:• Nerve structure• Types of nerve pathways• Nervous system components• Central nervous system structure and

function• Peripheral nervous system structure and

functionUnderstand the aging and pathology of the

nervous system

OverviewNervous system composed of neurons and neuroglia.

Bundled into two major components:

Central Nervous System (CNS): Major division of the nervous system composed of the brain and spinal cord; works as a controlling network for the entire body

Peripheral Nervous System (PNS): The part of the nervous system made up of neurons and neuroglia outside of the brain and spinal cord; provides motor and sensory communication between the CNS and the body

Overview


Nerve Structure


A typical nerve is covered by a continuous protective sheet of connective tissue called the epineurium. Within that are neurofascicles surrounded by a covering called the perineurium.

Nerve StructureNerve: is defined as an enclosed bundle of neurons and associated neuroglia running to various structures throughout the body.

Nerves primarily constitute the PNS.Nerve tracts are found in the brain and spinal cord (CNS)Nerve tracts are neurons bunched together in pathways that are not indistinct bundles.

Afferent nerves: carry sensory information from the body to brain.Efferent nerves: carry information from the CNS to muscles and glands

Nerve Structure

Nerve is covered byepineurium-it forms a protective sheath around the nerve

and is an entryway for blood vessels that assist the nerve.

Within the epineurium are numerous Neurofascicles: a tight bundle of axons and associated neuroglia

It generally contains a mix of myelinated and unmylinated axons.Each neurofascicle is covered by the perineurium.

Each neuron and its associated neuroglia within a neurofascicle aresurrounded by an endoneurium.

Nerve StructureGanglia are collections of nerve cell bodies covered by the

epineurium.They are an accumulation of

nerve cell bodieslocated outside the CNS.

Ganglia of the PNS are:•Composed of unipolar neurons•Are usually associated with sensory function

Nervous System ComponentsCentral Nervous System (CNS)

Composed of the Brain and Spinal Cord

Three layers of tissues separate the brain and spinal cord from their bony covering.These three layers called meninges are made of the

dura materarachnoid mater pia mater

Dura mater •Outmost layer•Pressed tightly against the interior of the cranium and the vertebral column•Acts as a barrier against trauma to the CNS•Prevents CNS from rubbing against skull or vertebral column

The Meninges

Arachnoid mater•Located below the dura mater•Like a spider web•Cushions the CNS from rapid movements and blunt hits to the skull and vertebral column.•Under it is subarachnoid space-a cavity filled with cerebrospinal fluid and blood vessels.

The MeningesPia mater

•Directly makes contact with the brain and spinal cord•Adheres to brain and spinal cord•Carries blood vessels to CNS•Forms sheaths over nerves passing through the outer meninges layers•Assists with the production of cerebrospinal fluid•Associated with pia mater is the choroid plexus--it is a mass of blood vessels and glial cells (ependymal cells) that secrete cerebrospinal fluid

The Central Nervous System


The central nervous system is composed of the brain and the spinal cord. Afferent peripheral nerves act as trunks that feed sensory information to the brain through their entry into the spinal cord.

Central Nervous System

The Brain:Three Major parts:

Forebrain, Midbrain, and HindbrainForebrain: composed of the cerebrum and diencephalon

•responsible for emotions, memory, motor movement, and thought.

Cerebrum is divided into the left and right cerebral hemispheresCerebrum is separated by midsagittal crease called longitudinal cerebral fissure.Left Hemisphere: are specialized for language and speechRight Hemisphere: said to be the site of more “creative side.”Corpus callosum: band of white matter that connects the left and right brain.

Cerebrum

The hemispheres are divided into four lobes:• Frontal lobe- processes intellectual

information that helps with organization of

thoughts.also posterior region has

voluntary control over skeletal muscles.

• Parietal lobe- involved with emotions and sensory

interpretation.-interprets sensory info from thelips, skin, and tongue.

•Temporal lobe- organizes and stores memories of sounds and vision.•Occipital lobe- interprets vision and assists with eye function.Insula is a small region near the temporal lobe that plays And important role in processing memories.

Forebrain

Diencephalon contains the thalamus and hypothalamus.Ventricles are also found in the forebrain.

•Consists of four connected cavities that contain cerebrospinal fluid•They are associated with the choroid plexus and continue into the spinal cord•They protect the brain from trauma by acting as a cushion when head is hit or violently moved.

Hydrocephalus is a disease in children where they produce tooMuch cerebrospinal fluid.

Midbrain

Small strip of neurons that connects cerebrum to the hindbrainPossesses the auditory and visual reflex areasControls the ability of the eyes to adjust to changes in light Intensity or sound.

Hindbrain

Pons: first part of the hindbrain. It is connected to midbrain•Organizes and transmits sensory information received from the body.

Medulla oblongata lies just below the pons•Regulates involuntary body functions such as blood pressure, breathing, heart rate, and swallowing.

Cerebellum lies posterior to the pons•Plays a role in balance, posture, and coordination of body movement

Brain Stem: refers to the midbrain and hindbrain by many people

Spinal Cord

Is a cylinder of nervous tissue enclosed in the spinal canal of theVertebral column.Meninges covers its surfaceCentral canal contains cerebral spinal fluid

is continuous with ventricles of the brainSurface of spinal cord is covered with white matter.Ascending tract found along the dorsal portion of the spinal cord

They carries sensory information up to the brain.Descending tract run along the ventral and lateral portions of the

spinal cord.They carry motor information directly to the PNS and our effectors.

The Peripheral Nervous System


The peripheral nervous system is composed of somatic nerves, autonomic nerves, and ganglia. It is divided into the sympathetic and parasympathetic nervous systems.

Peripheral Nervous System

PNS:•composed of nerves that branch out from the brain and

spinal cord•not covered with meninges•do not have a cavity containing cerebrospinal fluid

•Divided into the •Somatic nervous branch•Autonomic nervous branch

Peripheral Nervous System

Two nervous branchesSomatic nerves

•Enable the voluntary control of body movements through their communication with skeletal muscles.Composed of:

afferent neurons which collect sensory informationefferent neurons which relay commands for muscle action.

Autonomic nervous system•Controls involuntary body functions•Its primary purpose is to maintain a stable internal environment for the body.

12 Cranial Nerves

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Spinal Nerves

Nerves that originate from the spinal cord and pass out of The vertebral column.

Spinal nerves carry motor and sensory information for reflex controland two way communication to the brain.

Each spinal nerve exits the spinal cord from two short, lateral Branches.

The sensory branch called the dorsal root.The motor branch is called the ventral root.

Autonomic Nervous System

Uses the cranial and spinal nerves to carry out a wide array of tasks.It performs these tasks by sending out regulatory information fromthe brain.

This information controls the glands, cardiac muscle, andsmooth muscles necessary for maintaining homeostasis.

•It does not require conscious thought•It is integrated with the endocrine system to assist body with digestion, sexual functions, andstress responses.

Autonomic Nervous System has two regions:Parasympathetic nerve divisionSympathetic nerve division

Autonomic Nervous SystemParasympathetic nerve division emerges from the cranial nerves

and the sacral spinal nerves.

Sympathetic nerve division arises from the thoracic and lumbarspinal cord.

These two components typically counteract each other’s actionsto fine tune the body’s responses to internal changes and environmental stimuli.

Autonomic Nervous System

Sometimes called the “fight or flight and rest to digest” system.

Sympathetic system prepares the body to react to stress.Parasympathetic system promotes relaxation and digestion.



Human Senses


• Gustation—Chemoreceptors on the tongue sense taste

• Olfaction—Chemoreceptors in the nose sense smell

• Vision—Photoreceptors in the retina of the eye sense light

• Hearing—Neurons in the cochlea sense sound vibrations

• Equilibrium—Neurons in the semicircular canals and vestibule of the ear sense position

• Taction or Tactility—Sensory receptors in the skin perceive touch

Human Senses



The human head contains a concentration of sensory structures.

Human Senses

Sense of smell -- olfaction Sense of taste-- gustationSense of vision-- photoreceptorsSense of hearing-- audition Sense of balance--equilibrium

Both smell and taste rely on chemoreceptors to detect chemicals dissolved in the air or water.

Vision uses photoreceptors to convert light into a neural signal.

Hearing uses a series of receptors that are sensitive to different types of sounds and vibrations.

Equilibrium uses a combination of receptors to detect the body’s position as it Is moving and standing still.

Taste



Taste buds are distributed throughout the upper surface of the tongue. Saliva is to ensure wet environment for taste to occur.Taste is detected by chemoreceptors.Five taste sensations: bitter, salt, sour, sweet, and umaniUmani the taste that occurs when food ingredient monosodiumGlutamate (MSG) dissolves in mouth. Umani receptors are distributed throughout the tongue.Thought to stimulate appetite.

Smell

Olfaction serves two purposes:detect potentially harmful or valuable chemicals in the airan important supplement to taste.

Chemoreceptors for smell are within the olfactory bulbThese are located in upper region of nasal cavityOlfactory bulb is an extension of olfactory nerve.

Vision

Eyes are specialized organs used to see objects and perceive movement.Eyelids and eyelashes are used to protect eyes.Conjunctiva-thin transparent layer that tightly covers anterior

surface of the eyeLacrimal gland- produces tears that lubricate and protect eye

from infectionLacrimal duct-tube that connects with nasal cavity, tears usually

drain off surface of the eye Retina-inside layer located at back of eye, contains photoreceptorsSclera-tough outermost layer of the eye, white in colorCornea-clear covering at the front surface of the eye

Extrinsic muscles- the six muscles that move the eyeChoroid- layer of blood vessels lining the inner surface of the scleraCiliary body-a ring of muscles and connective tissue attached to the lens, adjusts lens of the eyeLens-transparent structure inside the eye that focuses the light

rays for clear visionIris-colored part of the eye, ring of muscles that adjusts the

pupilPupil-opening in the eye that lets the light enter

Parts of the Eye

Aqueous humor-a clear watery fluid in the anterior chamber

of the eyeball, helps maintain shape of cornea for properly focused visionVitreous humor-gel-like fluid that fills the cavity behind the

eye lens, maintains the spherical shape of the

eyeCones-a photoreceptor sensitive to bright light and color. Rods-a photoreceptor sensitive to dim light. Fovea-central point at back of the eye that contains only the

densest concentration of cones.

Parts of the Eye

Diagram of the eye



The Ear



The Ear

Human Ear converts sounds and body orientations into neuron Signals for the brain

The ear is composed of three regions:external earmiddle earinner ear

The external ear is composed of the: auricle or pinna-fleshy part of the

ear that catches sound waves and funnels them into

the ear canalexternal auditory meatus or auditory

canal- ear canal

The Ear

The Middle ear:tympanic membrane or eardrum-vibrates in response to

sounds transmitted by the auditory canal.ossicles are the three ear bones that the eardrum vibrates

against.These vibrations are transmitted to the inner ear.Three ossicles are the

malleus -hammerincus- anvil stapes-stirrup

Eustachian tube connects the middle ear to the throat.It prevents damage to the middle ear by adapting to changes in air pressure.

The Ear

The Inner ear:is housed in a cavity in the temporal

boneit is composed of the

cochlea-is a coiled, fluid filled organ that converts

vibrations into neuron impulses that

are sent to the brain.semicircular canals-are fluid

filled chambers that respond to body position.

The vibrations are transferred from the stapes to part of the cochleacalled the oval window.

Organ of Corti: is the part of the inner ear that

contains the neurons that respond to

different sounds.Sound waves then vibrate again the round window of the cochlea.This permits pressure from sound waves to be released so they don’techo.

The Ear

The Inner Ear continued:Semicircular canals- respond to body position

Changes in the direction of flow stimulates neurons, whichthe brain interprets as movement. Three canals superior canal detects forward and backward motion

lateral canal detects forward and side to side motion posterior canal also detects side to side motion

Vestibule-is the base of the semicircular canals detects body position.

Wellness and Illness over the Life Span


• Most brain aging is due to loss of myelinization and decreased blood flow.

• Most people show a decrease in complex brain functions as they age.

• Aging is accompanied by some neuron loss in the brain.

Pathology of the Nervous System

Common categories are traumacerebrovascular and neurovascular diseasesnervous system tumorsdevelopmental disordersmetabolic and toxic diseasesCNS infections neurogenerative diseases

Trauma categorized by its location

Peripheral neuropathy It causes numbness, pain, tingling, or weakness in the PNS.

Traumatic neuropathies of the CNSWhiplash-nerve damage in the neck

caused by an abrupt, forced movement of the head.

Shaken baby syndrome is severe injuries that result when a young child is violently shaken.

Cerebrovascular and neurovascular diseases are blood vessel disorders that impair the nervous system function

Aneurysms-bulges in blood vessels caused by stretchingand thinning of the vessels.

Ateriovenous malformations is an unusual tangling of blood vessels in the CNS or PNS that disrupt blood flowoccur after birth but may not cause problems until adulthood.

Ischemic attacks a condition caused by insufficient flood flow toa body part. (Stroke)

Transient ischemic attack (mini stroke)caused by temporary loss of blood flow

Nervous system tumors

Abnormal growths that develop from neuroglia (glioma) cells of the meninges (meningioma)Immune cells in the nervous system (lymphoma)Neurons (neuroblastoma and neuroma)

Developmental disorders of the nervous system

Caused by some factor that interferes with the DNA’s ability to formOr carry out normal functions of a body component.

Can produce four characteristic problems with nerve function:athetosis causes slow, involuntary movements of the

hands and feet.repetitive, involuntary twisting movements

chorea causes muscular twitching of the arms, legs, and face.short, irregular, nonrepetitive muscle contractions

palsy causes paralysis of a muscle or group of muscles. muscle paralysis caused by nerve losstremor causes uncontrollable, rhythmic shaking movements

Metabolic or toxic nervous system diseases

Caused by poisons that impede the functions of neuroglia orneurons.

Large amounts of calcium sodium, and potassium affect the action potential of the neurons.Many types of metals interfere with the metabolism of nervous system cells.Bacterial, fungal, and protistan infections of the nervoussystem damage cells due to the toxins they produce.

Aging of the Nervous System

Nerve cell mass of the brain and spinal cord decreases after theAge of 40.The average person loses 5 to 10% of his or her brain’s weightbetween ages 20 and 90 years. Decreased blood flow may account for brain shrinkage and weightloss.People in older age groups have diminished and slower memorycapabilities.PNS degeneration also occurs. This causes a reduction or loss of reflexes. This produces a problem in balance and mobility.Sensory loss is also seen with age. It is most likely due to degradationOf the sensory structure.Hearing loss affects men more than women in the age range of 70-79.Vision loss occurs equally in each gender.

Summary

• The human nervous system is formed of two components that work together to coordinate body functions: the central and peripheral nervous systems.

• Information from the environment is transmitted to the CNS by nerves of the PNS.

• Sensory information is used by the brain to formulate a response. Responses of the brain are channeled to the body via the somatic or autonomic nervous system.

• Nervous system structure is subject to damage resulting from a variety of diseases.


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