Nerves and Special Senses Anatomy Physiology Rainier JrSr

Nerves and Special Senses Anatomy & Physiology Rainier Jr/Sr High School Mr. Taylor

Structure of a Nerve • Endoneurium surrounds each fiber • Groups of fibers are bound into fascicles by perineurium • Fascicles are bound together by epineurium Figure 7. 20

Classification of Nerves • Afferent (sensory) nerves – carry impulses toward the CNS • Efferent (motor) nerves – carry impulses away from the CNS • Mixed nerves – both sensory and motor fibers

Functions of the Nervous System • Sensory input – gathering information – To monitor changes occurring inside and outside the body – Changes = stimuli • Integration – To process and interpret sensory input and decide if action is needed • Motor output – A response to integrated stimuli – The response activates muscles or glands

Structural Classification of the Nervous System • Central nervous system (CNS) – Brain – Spinal cord • Peripheral nervous system (PNS) – Nerves outside the brain and spinal cord • Afferent (sensory) division • Efferent (motor) division – 2 subdivisions » Somatic (voluntary) » Autonomic (involuntary) (2 subdivisions) » Parasympathetic » Sympathetic

Functional Classification of the Peripheral Nervous System • Sensory (afferent) division – Nerve fibers that carry information to the central nervous system Figure 7. 1

Functional Classification of the Peripheral Nervous System • Motor (efferent) division – Nerve fibers that carry impulses away from the central nervous system Figure 7. 1

Functional Classification of the Peripheral Nervous System • Motor (efferent) division – Two subdivisions • Somatic nervous system = voluntary • Autonomic nervous system = involuntary Figure 7. 1

Organization of the Nervous System Figure 7. 2

Peripheral Nervous System • Nerves and ganglia outside the central nervous system • Nerve = bundle of neuron fibers • Neuron fibers are bundled by connective tissue

Nervous Tissue: Support Cells (Neuroglia) • Astrocytes – Abundant, star-shaped cells – Brace(support) neurons – Form barrier between capillaries and neurons – Control the chemical environment of the brain Figure 7. 3 a

Nervous Tissue: Support Cells • Microglia – Spider-like phagocytes – Dispose of debris • Ependymal cells – Line cavities of the brain and spinal cord – Circulate cerebrospinal fluid Figure 7. 3 b–c

Nervous Tissue: Support Cells • Oligodendrocytes – Produce myelin sheath around nerve fibers in the central nervous system Figure 7. 3 d

Nervous Tissue: Support Cells • Satellite cells – Protect neuron cell bodies • Schwann cells – Form myelin sheath in the peripheral nervous system Figure 7. 3 e

Nervous Tissue: Neurons • Neurons = nerve cells – Cells specialized to transmit messages – Major regions of neurons • Cell body – nucleus and metabolic center of the cell • Processes – fibers that extend from the cell body – Axons carry signals away from the body – Dendrites carry signals toward the body

Neuron Anatomy • Extensions outside the cell body – Dendrites – conduct impulses toward the cell body – Axons – conduct impulses away from the cell body Figure 7. 4 a

Axons and Nerve Impulses • Axons end in axonal terminals • Axonal terminals contain vesicles with neurotransmitters • Axonal terminals are separated from the next neuron by a gap – Synaptic cleft – gap between adjacent neurons – Synapse – junction between nerves

Nerve Fiber Coverings • Schwann cells – produce myelin sheaths in jelly-roll like fashion • Nodes of Ranvier – gaps in myelin sheath along the axon Figure 7. 5

Neuron Cell Body Location • Most are found in the central nervous system – Gray matter – cell bodies and unmylenated fibers – Nuclei – clusters of cell bodies within the white matter of the central nervous system • Ganglia – collections of cell bodies outside the central nervous system

Functional Classification of Neurons • Sensory (afferent) neurons – Carry impulses from the sensory receptors • Cutaneous sense organs • Proprioceptors – detect stretch or tension • Motor (efferent) neurons – Carry impulses from the central nervous system

Functional Classification of Neurons • Interneurons (association neurons) – Found in neural pathways in the central nervous system – Connect sensory and motor neurons

Neuron Classification Figure 7. 6

Functional Properties of Neurons • Irritability – ability to respond to stimuli • Conductivity – ability to transmit an impulse

Starting a Nerve Impulse • The plasma membrane at rest is polarized – Fewer positive ions are inside the cell than outside the cell • Depolarization – a stimulus depolarizes the neuron’s membrane • A deploarized membrane allows sodium (Na+) to flow inside the membrane • The exchange of ions initiates an action potential in the neuron Figure 7. 9 a–c

The Action Potential • If the action potential (nerve impulse) starts, it is propagated over the entire axon • Potassium ions rush out of the neuron after sodium ions rush in, which repolarizes the membrane • The sodium-potassium pump restores the original configuration – This action requires ATP

Nerve Impulse Propagation • The impulse continues to move toward the cell body • Impulses travel faster when fibers have a myelin sheath Figure 7. 9 d–f

Continuation of the Nerve Impulse between Neurons • Impulses are able to cross the synapse to another nerve – Neurotransmitter is released from a nerve’s axon terminal – The dendrite of the next neuron has receptors that are stimulated by the neurotransmitter – An action potential is started in the dendrite

The Reflex Arc • Reflex – rapid, predictable, and involuntary responses to stimuli • Reflex arc – direct route from a sensory neuron, to an interneuron, to an effector Figure 7. 11 a

Central Nervous System (CNS) • CNS develops from the embryonic neural tube (ectodermal origin) – The neural tube becomes the brain and spinal cord – The opening of the neural tube becomes the ventricles • Four chambers within the brain • Filled with cerebrospinal fluid

Regions of the Brain • • Cerebral hemispheres Diencephalon Brain stem Cerebellum Figure 7. 12 b

Cerebral Hemispheres (Cerebrum) • Paired (left and right) superior parts of the brain • Include more than half of the brain mass Figure 7. 13 a

Cerebral Hemispheres (Cerebrum) • The surface is made of ridges (gyri) and grooves (sulci) Figure 7. 13 a

Specialized Areas of the Cerebrum • Somatic sensory area – receives impulses from the body’s sensory receptors • Primary motor area – sends impulses to skeletal muscles • Broca’s area – involved in our ability to speak

Specialized Areas of the Cerebrum • Cerebral areas involved in special senses – Gustatory area (taste) – Visual area – Auditory area – Olfactory area

Specialized Areas of the Cerebrum • Interpretation areas of the cerebrum – Speech/language region – Language comprehension region – General interpretation area

Sensory and Motor Areas of the Cerebral Cortex Figure 7. 14

Specialized Areas of the Cerebrum Figure 7. 13 c

Layers of the Cerebrum • Gray matter – Outer layer – Composed mostly of neuron cell bodies Figure 7. 13 a

Layers of the Cerebrum • White matter – Fiber tracts inside the gray matter – Example: corpus callosum connects hemispheres Figure 7. 13 a

Diencephalon • Sits on top of the brain stem • Enclosed by the cerebral hemispheres • Made of three parts – Thalamus – Hypothalamus – Epithalamus

Diencephalon Figure 7. 15

Thalamus • Surrounds the third ventricle • The relay station for sensory impulses • Transfers impulses to the correct part of the cortex for localization and interpretation

Hypothalamus • Under the thalamus • Important autonomic nervous system center – Helps regulate body temperature – Controls water balance – Regulates metabolism

Hypothalamus • An important part of the limbic system (emotions) • The pituitary gland is attached to the hypothalamus

Epithalamus • Forms the roof of the third ventricle • Houses the pineal body (an endocrine gland) • Includes the choroid plexus – forms cerebrospinal fluid

Brain Stem • Attaches to the spinal cord • Parts of the brain stem – Midbrain – Pons – Medulla oblongata

Brain Stem Figure 7. 15 a

Midbrain • Mostly composed of tracts of nerve fibers – Reflex centers for vision and hearing

Pons • The bulging center part of the brain stem • Mostly composed of fiber tracts • Includes nuclei involved in the control of breathing

Medulla Oblongata • • The lowest part of the brain stem (“reptilian brain”) Merges into the spinal cord Includes important fiber tracts Contains important control centers – Heart rate control – Blood pressure regulation – Breathing – Swallowing – Vomiting

Reticular Formation • Diffuse mass of gray matter along the brain stem • Involved in motor control of visceral organs • Reticular activating system plays a role in awake/sleep cycles and consciousness

Cerebellum • Two hemispheres with convoluted surfaces • Provides automatic (involuntary) coordination of body movements. – This allows for the smooth movements of arms, legs, and body. – This also allows the fine motor control necessary for activities such as writing.

Cerebellum Figure 7. 15 a

Protection of the Central Nervous System • Scalp and skin • Skull and vertebral column • Meninges §Cerebrospinal fluid §Blood brain barrier Figure 7. 16 a

Meninges • Dura mater (literally “tough mother”) – Double-layered external covering • Periosteum – attached to surface of the skull • Meningeal layer – outer covering of the brain – Folds inward in areas of major fissures.

Meninges • Arachnoid layer – Middle layer – Web-like (hence, arachnoid) • Pia mater (the “tender mother”) – Internal layer – Clings to the surface of the brain

Cerebrospinal Fluid • • Similar to blood plasma composition Formed by the choroid plexus Forms a watery cushion to protect the brain Circulated in arachnoid space, ventricles, and central canal of the spinal cord

Cranial Nerves • 12 pairs of nerves that mostly serve the head and neck • Numbered in order, front to back • Numbers I, II, VIII are purely sensory. • Numbers III, IV, VI, XI are purely motor. • Numbers V, VII, IX, X, XII are mixed.

Distribution of Cranial Nerves Figure 7. 21

Cranial Nerves • I Olfactory nerve – sensory for smell • II Optic nerve – sensory for vision • III Oculomotor nerve – motor fibers to eye muscles • IV Trochlear – motor fiber to eye muscles

Cranial Nerves • V Trigeminal nerve – sensory for the face; motor fibers to chewing muscles • VI Abducens nerve – motor fibers to eye muscles • VII Facial nerve – sensory for taste; motor fibers to the face • VIII Vestibulocochlear (old name: Auditory) nerve – sensory for balance and hearing

Cranial Nerves • IX Glossopharyngeal nerve – sensory for taste; motor fibers to the pharynx • X Vagus nerves – sensory and motor fibers for pharynx, larynx, and viscera • XI (Spinal) Accessory nerve – motor fibers to neck and upper back • XII Hypoglossal nerve – motor fibers to tongue, some sensory fibers from tongue.

Blood Brain Barrier • Includes the least permeable capillaries of the body – (where are the most permeable capillaries? ) • Excludes (keeps away from the brain) many potentially harmful substances • Useless against some substances – Fats and fat soluble molecules including hormones – Respiratory gases (oxygen/carbon dioxide) – Alcohol (all kinds) – Nicotine (a paralyzing agent) – Anesthesia

Traumatic Brain Injuries • Concussion – Slight brain injury – No permanent brain damage (latest evidence does not support this … so let’s say minimal permanent brain damage) • Contusion – Nervous tissue destruction occurs (and nervous tissue does not regenerate) • Cerebral edema – Swelling from the inflammatory response from any cause (trauma and disease most common) – May compress and kill brain tissue

Cerebrovascular Accident (CVA) • Commonly called a stroke • The result of a ruptured blood vessel supplying a region of the brain or blockage from a blood clot. • Brain tissue supplied with oxygen from that blood source dies • Loss of some functions or death may result • Immediate medical attention is necessary to minimize damage • (FAST: Face; Arms; Speech; Time)

Alzheimer’s Disease • Progressive degenerative brain disease • Mostly seen in the elderly, but may begin in middle age • Structural changes in the brain include abnormal protein deposits and twisted fibers within neurons • Victims experience memory loss, irritability, confusion and ultimately, hallucinations and death

Spinal Cord • Extends from the medulla oblongata to the region of T 12 • Below T 12 is the cauda equina (literal meaning is “horse tail”) a collection of spinal nerves) • Enlargements occur in the cervical and lumbar regions Figure 7. 18

Spinal Cord Anatomy • Meninges cover the spinal cord • Nerves leave at the level of each vertebrae – Dorsal root (afferent neurons) • Associated with the dorsal root ganglia – collections of cell bodies outside the central nervous system – Ventral root (efferent neurons)

Spinal Nerves • There is a pair of spinal nerves at the level of each vertebrae for a total of 31 pairs • Spinal nerves are formed by the combination of the ventral and dorsal roots of the spinal cord • Spinal nerves are named for the region from which they arise

Spinal Nerves Figure 7. 22 a

Anatomy of Spinal Nerves • Spinal nerves divide soon after leaving the spinal cord – Dorsal rami – serve the skin and muscles of the posterior trunk – Ventral rami – forms a complex of networks (plexus) for the anterior Figure 7. 22 b

Autonomic Nervous System • The involuntary branch of the nervous system • Consists of only motor nerves • Divided into two divisions – Sympathetic division (fight or flight) – Parasympathetic division (dine and doze)

Differences Between Somatic and Autonomic Nervous Systems • Nerves – Somatic – one motor neuron – Autonomic – preganglionic and postganglionic nerves • Effector organs – Somatic – skeletal muscle – Autonomic – smooth muscle, cardiac muscle, and glands

Differences Between Somatic and Autonomic Nervous Systems • Nerurotransmitters – Somatic – always use acetylcholine – Autonomic – use acetylcholine, epinephrine, or norepinephrine

Comparison of Somatic and Autonomic Nervous Systems Figure 7. 24

Anatomy of the Sympathetic Division • Originates from T 1 through L 2 • Ganglia are at the sympathetic trunk (near the spinal cord) • Norepinephrine and epinephrine are neurotransmitters to the effector organs

Anatomy of the Parasympathetic Division • Originates from the brain stem and S 1 through S 4 • Terminal ganglia are at the effector organs • Always uses acetylcholine as a neurotransmitter

Anatomy of the Autonomic Nervous System Figure 7. 25

Autonomic Functioning • Sympathetic – “fight-or-flight” – Response to unusual stimulus – Takes over to increase activities – Remember as the “E” division = exercise, excitement, emergency, and embarrassment

Autonomic Functioning • Parasympathetic – housekeeping activites – Conserves energy – Maintains daily necessary body functions – Remember as the “D” division - digestion, defecation, and diuresis – My nickname for this system is “dine and doze”.

Development Aspects of the Nervous System • The nervous system is formed during the first month of embryonic development • Any maternal infection can have extremely harmful effects • The hypothalamus is one of the last areas of the brain to develop

Development Aspects of the Nervous System • No more neurons are formed after birth, but growth and maturation continues for several years • The brain reaches maximum weight as a young adult

The Senses • General senses of touch – Temperature – Pressure – Pain • Special senses – Smell – Taste – Sight – Hearing – Equilibrium

The Eye and Vision • 70 percent of all sensory receptors are in the eyes • Each eye has over a million nerve fibers • Protection for the eye – Most of the eye is enclosed in a bony orbit – A cushion of fat surrounds most of the eye

Accessory Structures of the Eye • Eyelids • Eyelashes Figure 8. 1 b

Accessory Structures of the Eye • Meibomian glands – modified sebacious glands produce an oily secretion to lubricate the eye Figure 8. 1 b

Accessory Structures of the Eye • Ciliary glands – modified sweat glands between the eyelashes Figure 8. 1 b

Accessory Structures of the Eye • Conjunctiva – Membrane that lines the eyelids – Connects to the surface of the eye – Secretes mucus to lubricate the eye

Accessory Structures of the Eye • Lacrimal apparatus – Lacrimal gland – produces lacrimal fluid – Lacrimal canals – drains lacrimal fluid from eyes into lacrimal sac. Figure 8. 1 a

Accessory Structures of the Eye • Lacrimal sac – provides passage of lacrimal fluid towards nasal cavity • Nasolacrimal duct – empties lacrimal fluid into the nasal cavity Figure 8. 1 a

Function of the Lacrimal Apparatus • Properties of lacrimal fluid – Dilute salt solution (tears) – Contains antibodies and lysozyme • Protects, moistens, and lubricates the eye

Extrinsic Eye Muscles • Muscles attach to the outer surface of the eye • Produce eye movements Figure 8. 2

Structure of the Eye • The wall is composed of three tunics – Fibrous tunic – outside layer – Choroid – middle layer – Sensory tunic – inside layer Figure 8. 3 a

The Fibrous Tunic • Sclera – White connective tissue layer – Seen anteriorly as the “white of the eye” • Cornea – Transparent, central anterior portion – Allows for light to pass through – Repairs itself easily and quickly – The only human tissue that can be transplanted without fear of rejection

Choroid Layer • Blood-rich nutritive tunic • Pigment prevents light from scattering • Middle part is modified into two structures – Ciliary body – smooth muscle – Iris • Pigmented layer that gives eye color • Pupil – rounded opening in the iris

Sensory Tunic (Retina) • Contains receptor cells (photoreceptors) – Rods • Most are found towards the edges of the retina • Allow dim light vision and peripheral vision • Perception is all in gray tones – Cones • Allow for detailed color vision • Densest in the center of the retina • Fovea centralis – area of the retina with only cones – Signals pass from photoreceptors via a two-neuron chain • Signals leave the retina toward the brain through the optic nerve

Cone Sensitivity • There are three types of cones • Different cones are sensitive to different wavelengths • Color blindness is the result of lack of one cone type Figure 8. 6

Neurons of the Retina Figure 8. 4

Lens • Biconvex crystallike structure • Held in place by a suspensory ligament attached to the ciliary body Figure 8. 3 a

Internal Eye Chamber Fluids • Aqueous humor – Watery fluid found in chamber between the lens and cornea – Similar to blood plasma – Helps maintain intraocular pressure – Provides nutrients for the lens and cornea – Reabsorbed into venous blood through the canal of Schlemm

Internal Eye Chamber Fluids • Vitreous humor – Gel-like substance behind the lens – Keeps the eye from collapsing – Lasts a lifetime and is not replaced

Lens Accommodation • Light must be focused to a point on the retina for optimal vision • The eye is set for distance vision (over 20 ft away) • The lens must change shape to focus for closer objects Figure 8. 9

Visual Pathway • Photoreceptors of the retina • Optic nerve crosses at the optic chiasma Figure 8. 11

Visual Pathway • Optic tracts • Thalamus (axons form optic radiation) • Visual cortex of the occipital lobe Figure 8. 11

Eye Reflexes • Internal muscles are controlled by the autonomic nervous system – Bright light causes pupils to constrict through action of radial and ciliary muscles – Viewing close objects causes accommodation • External muscles control eye movement to follow objects • Viewing close objects causes convergence (eyes moving medially)

The Ear • Houses two senses – Hearing – Equilibrium (balance) • Receptors are mechanoreceptors • Different organs house receptors for each sense

Anatomy of the Ear • The ear is divided into three areas – Outer (external) ear – Middle ear – Inner ear Figure 8. 12

The External Ear • Involved in hearing only • Structures of the external ear – Pinna (auricle) – External auditory canal Figure 8. 12

The External Auditory Canal • • Narrow chamber in the temporal bone Lined with skin Ceruminous (wax) glands are present Ends at the tympanic membrane

The Middle Ear or Tympanic Cavity • Air-filled cavity within the temporal bone • Only involved in the sense of hearing

The Middle Ear or Tympanic Cavity • Two tubes are associated with the middle ear – The opening from the auditory canal is covered by the tympanic membrane – The auditory tube connecting the middle ear with the throat • Allows for equalizing pressure during yawning or swallowing • This tube is otherwise collapsed

Bones of the Tympanic Cavity • Three bones span the cavity – Malleus (hammer) – Incus (anvil) – Stapes (stirrup) Figure 8. 12

Bones of the Tympanic Cavity • Vibrations from eardrum move the malleus • These bones transfer sound to the inner ear Figure 8. 12

Inner Ear or Bony Labyrinth • Includes sense organs for hearing and balance • Filled with perilymph Figure 8. 12

Inner Ear or Bony Labrynth • A maze of bony chambers within the temporal bone – Cochlea – Vestibule – Semicircular canals Figure 8. 12

Organs of Hearing • Organ of Corti – Located within the cochlea – Receptors = hair cells on the basilar membrane – Gel-like tectorial membrane is capable of bending hair cells – Cochlear nerve attached to hair cells transmits nerve impulses to auditory cortex on temporal lobe

Organs of Hearing Figure 8. 15

Mechanisms of Hearing • Vibrations from sound waves move tectorial membrane • Hair cells are bent by the membrane • An action potential starts in the cochlear nerve • Continued stimulation can lead to adaptation

Organs of Equilibrium • Receptor cells are in two structures – Vestibule – Semicircular canals Figure 8. 14 a–b

Organs of Equilibrium • Equilibrium has two functional parts – Static equilibrium – Dynamic equilibrium Figure 8. 14 a–b

Static Equilibrium • Maculae – receptors in the vestibule – Report on the position of the head – Send information via the vestibular nerve • Anatomy of the maculae – Hair cells are embedded in the otolithic membrane – Otoliths (tiny stones) float in a gel around the hair cells – Movements cause otoliths to bend the hair cells

Function of Maculae Figure 8. 13 a–b

Dynamic Equilibrium • Crista ampullaris – receptors in the semicircular canals – Tuft of hair cells – Cupula (gelatinous cap) covers the hair cells Figure 8. 14 c

Dynamic Equilibrium • Action of angular head movements – The cupula stimulates the hair cells – An impulse is sent via the vestibular nerve to the cerebellum Figure 8. 14 c

Chemical Senses – Taste and Smell • Both senses use chemoreceptors – Stimulated by chemicals in solution – Taste has four types of receptors – Smell can differentiate a large range of chemicals • Both senses complement each other and respond to many of the same stimuli

Olfaction – The Sense of Smell • Olfactory receptors are in the roof of the nasal cavity – Neurons with long cilia – Chemicals must be dissolved in mucus for detection • Impulses are transmitted via the olfactory nerve • Interpretation of smells is made in the cortex

The Sense of Taste • Taste buds house the receptor organs • Location of taste buds – Most are on the tongue – Soft palate – Cheeks Figure 8. 18 a–b

The Tongue and Taste • The tongue is covered with projections called papillae – Filiform papillae – sharp with no taste buds – Fungifiorm papillae – rounded with taste buds – Circumvallate papillae – large papillae with taste buds • Taste buds are found on the sides of papillae

Structure of Taste Buds • Gustatory cells are the receptors – Have gustatory hairs (long microvilli) – Hairs are stimulated by chemicals dissolved in saliva

Anatomy of Taste Buds Figure 8. 18

Taste Sensations • Sweet receptors – Sugars – Saccharine – Some amino acids • Sour receptors – Acids • Bitter receptors – Alkaloids • Salty receptors – Metal ions

Developmental Aspects of the Special Senses • Formed early in embryonic development • Eyes are outgrowths of the brain • All special senses are functional at birth; vision takes time to become fully functional