Download Nervous System Lecture- Part II

BIOL 2304
Peripheral Nervous System
PNS Functions
Provides vital links to the body and outside world
Allows CNS to receive information and initiate action
Sensory inputs and motor outputs categorized as
Somatic or visceral
General or special
Basic Structure of PNS
Sensory receptors—pick up stimuli from inside or outside the body
Nerves and ganglia
Nerves—bundles of peripheral axons
Ganglia—clusters of peripheral neuronal cell bodies
Motor endings—axon terminals of motor neurons
Innervate effectors (muscle fibers and glands)
Cranial Nerves
Attach to the brain and pass through foramina of the skull
Numbered from I–XII
Cranial nerves I and II attach to the forebrain
All others attach to the brain stem
Primarily serve head and neck structures
The vagus nerve (X) is the only cranial nerve that extends into the abdomen
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Cranial Nerves
Cranial Nerve I – Olfactory
A sensory nerve involved in the sense of smell.
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Cranial Nerve II – Optic
Transmits visual information from the retina to the brain
Cranial Nerve III – Oculomotor
Controls most of the eye's movements, including constriction of the pupil and maintaining an open
eyelid by innervating the Levator palpebrae superiors muscle.
Cranial Nerve IV – Trochlear
A motor nerve (a “somatic efferent” nerve) that innervates a single
muscle: the superior oblique muscle of the eye.
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Cranial Nerve V – Trigeminal
Contains both sensory and motor fibers. It is responsible for sensation in the face and certain motor
functions such as biting, chewing, and swallowing.
Cranial Nerve VI – Abducens
A somatic efferent nerve that controls the movement of a single muscle, the lateral rectus muscle of the
eye, in humans
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Cranial Nerve VII – Facial
Controls the muscles of facial expression, and functions in the conveyance of taste sensations from the
anterior two-thirds of the tongue and oral cavity. It also supplies preganglionic parasympathetic fibers to
several head and neck ganglia.
Cranial Nerve VIII – Vestibulocochlear
Is responsible for transmitting sound and equilibrium (balance) information from the inner ear to the
brain.
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Cranial Nerve IX – Glossopharyngeal
Receives general sensory fibers (ventral
trigeminothalamic tract) from the tonsils, the
pharynx, the middle ear and the posterior 1/3 of
the tongue.
Receives special sensory fibers (taste) from the
posterior one-third of the tongue.
Receives visceral sensory fibers from the carotid
bodies, carotid sinus.
Supplies parasympathetic fibers to the parotid
gland via the otic ganglion.
Supplies motor fibers to stylopharyngeus
muscle, the only motor component of this
cranial nerve.
Contributes to the pharyngeal plexus.
Cranial Nerve X – Vagus
Parasympathetic innervation of organs
The vagus nerve contributes to the innervation
of the viscera.
Besides output to the various organs in the
body, the vagus nerve conveys sensory
information about the state of the body's organs
to the central nervous system.
80-90% of the nerve fibers in the vagus nerve
are afferent (sensory) nerves communicating
the state of the viscera to the brain.
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Cranial Nerve XI – Accessory
The Spinal Accessory nerve controls specific muscles of the shoulder and neck.
It is considered a cranial nerve because part of it was formerly believed to originate in the brain.
It provides motor innervation from the central nervous system to two muscles of the neck: the
sternocleidomastoid muscle and the trapezius muscle.
Cranial Nerve XII – Hypoglossal
It supplies motor fibers to all of the muscles of the tongue, except the palatoglossus muscle, which is
innervated by the vagus nerve
Spinal Nerves
31 pairs—contain thousands of nerve fibers
Connect to the spinal cord
Named for point of issue from the spinal cord
8 pairs of cervical nerves (C1–C8)
12 pairs of thoracic nerves (T1–T12)
5 pairs of lumbar nerves (L1–L5)
5 pairs of sacral nerves (S1–S5)
1 pair of coccygeal nerves (Co1)
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Spinal Nerves
Although there are seven cervical vertebrae (C1-C7), there are eight cervical nerves (C1-C8)
All nerves emerge above their corresponding vertebrae except for C8
The C8 nerve emerges below the C7 vertebra
Spinal Nerves
Connect to the spinal cord by the dorsal root
and ventral root
Dorsal root – contains sensory fibers
Cell bodies – located in the
dorsal root ganglion
Ventral root – contains motor fibers
arising from anterior gray column
Spinal Nerves
Branch into dorsal ramus and ventral ramus
Dorsal and ventral rami contain
sensory and motor fibers
Rami communicantes connect to the base of
the ventral ramus
Lead to the sympathetic chain ganglia
Singular: Ramus communicans
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Nerve Plexuses
Ventral rami form nerve plexuses
Cervical
Located under sternocleidomastoid
Formed by rami of C1-C4
Supplies nerves of neck (phrenic nerve)
Brachial
Located in neck and axilla
Formed by rami of C5-C8
Supplies nerves of upper limbs (radial, ulnar nerves)
Lumbar
Formed by rami of L1-L4
Supplies nerves of anterior lower limb (femoral nerve)
Sacral
Formed by rami of L4-S4
Supplies nerves of gluteal muscles, perineum, and posterior lower limb (sciatic nerve)
Autonomic Nervous System
The ANS responds to stimuli by activating smooth muscle, cardiac muscle, and glands.
Sympathetic division – fight/flight
Parasympathetic division – rest/digest
ANS Regulates visceral functions
Heart rate
Blood pressure
Digestion
Urination
Contrast with the somatic nervous system which responds to stimuli by activating skeletal muscle
Comparing SNS and ANS
Somatic Nervous System
1 motor neuron from spinal cord to skeletal muscle
Axon is myelinated; impulse is rapid
Skeletal muscle movement
Autonomic Nervous System
2 motor neuron chain from brain stem or spinal cord
Axon is unmyelinated
Smooth or cardiac muscle movement or gland secretion
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Comparison of Somatic and Autonomic Systems
Autonomic Nervous System (ANS)
Sympathetic NS
Pre-ganglionic neuron arises in thoracic or lumbar spinal cord segment and is short
Post-ganglionic neuron long and synapses onto target organ/gland.
Parasympathetic NS
Pre-ganglionic neuron arises in brainstem or sacral spinal cord segment and is long
Post-ganglionic neuron short and close to and synapses onto target organ/gland
Most organs are dually innvervated by both branches of ANS
Allows for fine tuning of body for homeostasis
Autonomic Pathways
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Special Senses
General Senses
General senses:
Respond to
Temperature
Pain
Touch
Pressure
Vibration
Proprioception
Receptors
Located throughout body
Structurally simple
General Senses
Sensation – transduction of stimulus energy
Perception – conscious awareness of stimulus energy
Receptive field – area on body monitored by a single receptor cell
Adaptation – reduction in sensitivity in the presence of a constant stimulus
Peripheral (receptor activity change)
Central (inhibition of sensory pathway)
Sensory Receptors
Classified by Location:
Exteroceptors – sensitive to stimuli arising from outside the body
Located at or near body surfaces
Include receptors for touch, pressure, pain, and temperature
Interoceptors – receive stimuli from internal viscera
Located in digestive tube, bladder, and lungs
Monitor a variety of stimuli
Changes in chemical concentration
Taste stimuli
Stretching of tissues
Temperature
Proprioceptors
Located in skeletal muscles, tendons, joints, and ligaments
Monitor degree of stretch
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Send inputs on body movement to the CNS
Sensory Receptors
Classified by Stimulus Detected
Mechanoreceptors respond to mechanical forces (touch, pressure, stretch, vibration, and itch)
Baroreceptors monitor blood pressure
Thermoreceptors respond to temperature changes
Chemoreceptors respond to chemicals in solution
Photoreceptors respond to light
Nociceptors respond to harmful stimuli that result in pain
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Sensory Receptors
Classified by Structure
Free Nerve Endings – naked dendrites
Merkel’s – stratum basale (basal layer) of epidermis, light touch
Hair follicle receptors – reticular layer of dermis
Encapsulated Nerve Endings – dendrites enclosed by connective tissue
Meissner’s – dermal papilla (papillary layer of dermis), discriminative touch
Pacinian – hypodermis, deep touch
Ruffini – dermis and hypodermis, deep touch
Proprioceptors – in muscles/tendons, monitor stretch
Major Somatosensory Pathaways
Spinothalamic Pathways
Anterior spinothalamic tract carries crude touch and pressure sensations.
Crude touch to right side of body activates receptor that synapses in spinal cord
2nd neuron decussates anteriorly and ascends medially to thalamus, where it synapses.
Thalamic neuron ascends to primary somatosensory cortex (post-central gyrus)
where it synapses, and perception occurs.
Lateral spinothalamic tract carries pain and temperature sensations.
Painful stimulus to right side of body activates receptor that synapses in spinal cord.
2nd neuron decussates anteriorly and ascends laterally to thalamus, where it synapses.
Thalamic neuron ascends to primary somatosensory cortex (post-central gyrus)
where it synapses, and perception occurs.
Posterior Column Pathway
Posterior column tract carries fine touch sensations.
Fine touch to right side of body activates receptor that ascends dorsally and synapses
ipsilaterally in the medulla.
2nd neuron decussates and ascends medially to thalamus, where it synapses.
Thalamic neuron ascends to primary somatosensory cortex (post-central gyrus)
where it synapses, and perception occurs.
Spinocerebellar Pathway
Spinocerebellar tract carries proprioceptive sensations.
Proprioceptive sensations from the right and left sides of the body activate receptors
that synapse in the spinal cord.
One of the 2nd order neurons decussates and ascends to the cerebellum.
The other 2nd order neuron ascends ipsilaterally to the cerebellum.
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Sensory Homunculus
Each region of the sensory cortex corresponds to a body regions.
The amount of cortical space dedicated to that body region is proportional to the number of sensory
receptors located in that body region.
This is the sensory homunculus
Motor Homunculus
Corticospinal pathway provides voluntary control over skeletal muscles.
Neurons in primary motor cortex (pre-central gyrus) descend, decussate in the medulla and
synapse onto anterior spinal cord neurons.
The 2nd order neuron is a motor neuron that synapses onto a skeletal muscle
There is a corresponding motor homunculus
Sensory and Motor Maps
Mapping is plastic
Changes with experiences
Changes with use/disuse
Changes with injury
Olfactory Anatomy
Olfactory receptors are part of the olfactory epithelium
Olfactory epithelium is pseudostratified columnar and contains three main cell types:
Olfactory sensory neurons
Supporting epithelial cells
Basal epithelial cells
Cell bodies of olfactory sensory neurons
Located in olfactory epithelium
Have an apical dendrite that projects to the epithelial surface
Ends in a knob from which olfactory cilia radiate
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Olfactory cilia act as receptive structures for smell
Mucus captures and dissolves odor molecules
Pathway for Olfactory Perception
Axons of olfactory epithelium
Gather into bundles—filaments of the olfactory nerve
Pass through the cribriform plate of the ethmoid bone
Attach to the olfactory bulbs and synapse with mitral cells
Mitral cells transmit impulses along the olfactory tract to
Limbic system
Piriform lobe of the cerebral cortex
Anatomy of Gustation
Taste receptors
Occur in taste buds
Most are found on the surface of the tongue
Located within tongue papillae
Two types of papillae (with taste buds)
Fungiform papillae
Vallate papillae
Anatomy of Gustation
Collection of 50 –100 epithelial cells
Contain two major cell types
Gustatory epithelial cells supporting cells
Basal epithelial cells gustatory cells
Contain long microvilli – extend through a taste pore to the surface of the epithelium
Cells in taste buds replaced every 7–10 days
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Anatomy of Gustation
Pathway for Perception of Taste
Taste information reaches the cerebral cortex
Primarily through the facial (VII) and glossopharyngeal (IX) nerves
Some taste information through the vagus nerve (X)
Sensory neurons synapse in the medulla
Located in the solitary nucleus
Impulses are transmitted to the thalamus and ultimately to the gustatory area of the
cerebral cortex in the insular lobe
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Accessory Structures of the Eye
Eyebrows – coarse hairs on the superciliary arches
Eyelids (palpebrae) – separated by the palpebral
fissure
Meet at the medial and lateral angles (canthi)
Lacrimal caruncle – reddish elevation at the
medial canthus
Tarsal plates – connective tissue within the
eyelids
Tarsal glands – modified sebaceous glands
Conjunctiva –
transparent mucous membrane
Palpebral conjunctiva
Bulbar conjunctiva
Conjunctival sac
Accessory Structures of the Eye
Lacrimal apparatus – keeps the surface of the eye
moist
Lacrimal gland – produces lacrimal fluid
Lacrimal sac –
fluid empties into nasal cavity
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Extrinsic Eye Muscles
Six muscles that control movement of the eye
Originate in the walls of the orbit
Insert on outer surface of the eyeball
Annular ring – origin of the four rectus muscles
The six extrinsic eye muscles are:
Lateral rectus and medial rectus
Superior rectus and inferior rectus
Superior oblique and inferior oblique
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Extrinsic Eye Muscles
Anatomy of the Eyeball
The Fibrous Layer – most external layer of the eyeball
Composed of two regions of connective tissue
Sclera – posterior five-sixths of the tunic
White, opaque region
Provides shape and an anchor for eye muscles
Cornea – anterior one-sixth of the fibrous tunic
Limbus – junction between sclera and cornea
Scleral venous sinus – allows aqueous humor to drain
The Vascular layer – the middle coat of the eyeball
Composed of choroid, ciliary body, and iris
Choroid – vascular, darkly pigmented membrane
Forms posterior five-sixths of the vascular tunic
Brown color – from melanocytes
Prevents scattering of light rays within the eye
Choroid corresponds to the arachnoid and pia maters
Ciliary body – thickened ring of tissue, which encircles the lens
Composed of ciliary muscle
Ciliary processes – posterior surface of the ciliary body
Ciliary zonule (suspensory ligament)
Attached around entire circumference of the lens
Iris
Visible colored part of the eye
Attached to the ciliary body
Composed of smooth muscle
Pupil – the round, central opening
Sphincter pupillae muscle
Dilator pupillae muscle
Act to vary the size of the pupil
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Anatomy of the Eyeball
Anatomy of the Eyeball
The retina - the deepest tunic, the inner layer
Composed of two layers
Pigmented layer – single layer of melanocytes
Neural layer – sheet of nervous tissue
Contains three main types of neurons
Photoreceptor cells
Bipolar cells
Ganglion cells
Microscopic Anatomy of the Retina
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Humors of the Eye
Posterior segment (cavity)
Filled with vitreous humor
Clear, jelly-like substance
Transmits light
Supports the posterior surface of the lens
Helps maintain intraocular pressure
Anterior segment
Divided into anterior and posterior chambers
Anterior chamber – between the cornea and iris
Posterior chamber – between the iris and lens
Filled with aqueous humor
Renewed continuously
Formed as a blood filtrate
Supplies nutrients to the lens and cornea
Humors of the Eye
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Pathway for the Perception of Light
Pathway begins at the retina.
Light activates photoreceptors
Photoreceptors signal bipolar cells
Bipolar cells signal ganglion cells
Axons of ganglion cells exit eye as the CN II (optic
nerve)
Optic nerve signals lateral geniculate nucleus of the
thalamus
Thalamus signals primary visual cortex
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Anatomy of the Ear
The ear – receptor organ for hearing and equilibrium
Composed of three main regions:
Outer ear – functions in hearing
Middle ear – functions in hearing
Internal ear – functions in both hearing and equilibrium
Anatomy of the Outer Ear
The auricle (pinna)
Helps direct sounds
External acoustic meatus
Lined with skin
Contains hairs, sebaceous glands, and ceruminous glands
Tympanic membrane
Forms the boundary between the external and middle ear
Anatomy of the Ear
Anatomy of the Middle Ear
Composed of
The tympanic cavity
A small, air-filled space
Located within the petrous portion of the temporal bone
Medial wall is penetrated by
Oval window
Round window
Pharyngotympanic tube (auditory or eustachian tube)
Links the middle ear and pharynx
Anatomy of the Middle Ear
Ear ossicles – smallest bones in the body
Malleus – attaches to the eardrum
Incus – between the malleus and stapes
Stapes – vibrates against the oval window
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Anatomy of the Middle Ear
Anatomy of the Inner Ear
Membranous labyrinth
Series of membrane-walled sacs and ducts
Fit within the bony labyrinth
Consists of three main parts
Organs of equilibrium
Semicircular ducts
Utricle and saccule
Organ of hearing
Cochlear duct
Anatomy of the Inner Ear
Hair cells in the cochlea
Inner hair cells are the receptors that transmit vibrations of the basilar membrane
Outer hair cells actively tune the cochlea and amplify the signal
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Anatomy of the Inner Ear
Pathway for Perception of Sound
Vibrations of basilar membrane signal hair cells
Hair cells signal Cranial nerve VIII (vestibulocochlear nerve)
Cranial nerve VIII signals a nucleus (superior olivary) in the medulla/pons
SO nucleus signals the inferior colliculus
Inferior colliculus signals medial geniculate nucleus of thalamus
Thalamus signals primary auditory cortex (temporal lobe)
Anatomy of Equilibrium
Utricle and saccule – suspended in perilymph
Two egg-shaped parts of the membranous labyrinth
House the macula – a spot of sensory epithelium
Macula – contains receptor cells
Monitor the position of the head when the head is still
Contains columnar supporting cells
Hair cells synapse onto vestibular nerve.
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Anatomy of Equilibrium
Anatomy of Equilibrium
Semicircular duct – snakes through each semicircular canal
Membranous ampulla – located within bony ampulla
Houses a structure called a crista ampullaris
Cristae contain receptor cells of rotational acceleration
Epithelium contains supporting cells and receptor hair cells
Pathway for Equilibrium
Vibrations of semicircular ducts, utricle, and saccule signal hair cells
Hair cells signal Cranial nerve VIII (vestibulocochlear nerve)
Cranial nerve VIII signals the vestibular nuclear complex of the pons
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The pons signals the cerebellum
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