chapter 14 the somatic nervous system

Chapter 14

The Somatic Nervous

System

Neural Information

An Overview of Neural Integration.

Ch 14 Ch 15

Somatic Nervous System (SNS)

Nervous system- responsible for our conscious

perception of the environment and our

voluntary responses to our perceptions with

skeletal muscles.

Receive- Incoming( Afferent)sensory

information- process this information- and send

out information through motor neurons and

pathways that control skeletal muscles.

Nervous Information Review

Afferent Division of the Nervous System

Receptors (General and Special senses)

Sensory neurons

Sensory pathways

Efferent Division (from CNS) of the Nervous

System

Nuclei

Motor tracts

Motor neurons

Sensory Information

Sensory Receptors

Specialized cells that monitor specific

conditions in the body or external environment

When stimulated, a receptor passes

information to the CNS in the form of action

potentials along the axon of a sensory neuron

Sensory Information

Sensory (Afferent) Pathways

Deliver somatic and visceral sensory

information to their final destinations

inside the CNS using

Nerves

Nuclei (CNS)

Tracts (CNS)

Sensory Motor Information

Somatic Motor Portion of the Efferent Division

Controls peripheral effectors

Somatic Motor Commands

Travel from motor centers in the brain along

somatic motor pathways of

Motor nuclei (CNS)

Tracts (CNS)

Nerves

14-1 Sensory Receptors

and PerceptionGeneral and Special Senses

1. General Senses

Describe our sensitivity to

Temperature

Pain

Touch

Pressure

Vibration

Proprioception

Sensory Receptors

Sensation

The arriving information from these senses

Perception

Conscious awareness of a sensation through

processing in primary sensory areas of

cerebral cortex.

Sensory Receptors

2. Special Senses

Olfaction (smell)

Vision (sight)

Gustation (taste)

Equilibrium (balance)

Hearing

Sensory Receptors

Special senses are provided by special

sensory receptors

Special sensory receptors

Are located in sense organs such as the eye

or ear

Are protected by surrounding tissues

Sensory Receptors

The Detection of Stimuli

Receptor sensitivity

Each receptor has a characteristic sensitivity

Receptive field

Area is monitored by a single receptor cell

The larger the receptive field, the more difficult it is

to localize a stimulus

Sensory Receptors

Receptors and Receptive Fields

Sensory Receptors

The Interpretation of Sensory Information

Arriving stimulus

Takes many forms:

– physical force (such as pressure)

– dissolved chemical

– sound

– light

Interpretation Sensory Information

Sensations

Taste, hearing, equilibrium, and vision provided by

specialized receptor cells

Communicate with sensory neurons across

chemical synapses

Adaptation

Reduction in sensitivity of a constant stimulus (smells)

Your nervous system quickly adapts to stimuli that are

painless and constant

Classifying Sensory Receptors

General sensory receptors are divided into types

by the nature of the stimulus that excites them

Nociceptors (pain)

Thermoreceptors (temperature)

Mechanoreceptors (physical distortion)

Chemoreceptors (chemical concentration)

Osmoreceptors (concentration of body fluids)


Nociceptors

Are free nerve endings with large receptive

fields

Not protected by accessory structures

Can be stimulated by many different stimuli

Sensations reach the CNS quickly and often

trigger somatic reflexes

Relayed to the primary sensory cortex and receive

conscious attention


Thermoreceptors - Are free nerve endings

located in

The dermis, skeletal muscles, liver, hypothalamus

Conducted along the same pathways that carry

pain sensations

Sent to:

– the reticular formation

– the thalamus

– the primary sensory cortex (to a lesser extent)


Mechanoreceptors -Sensitive to stimuli that distort

their plasma membranes

Contain mechanically gated ion channels whose

gates open or close in response to

Stretching

Compression

Twisting

Other distortions of the membrane


Three Classes of Mechanoreceptors

1. Tactile receptors

provide the sensations of touch, pressure,

and vibration:

– touch sensations provide information about

shape or texture

– pressure sensations indicate degree of

mechanical distortion

– vibration sensations indicate pulsing or

oscillating pressure


Tactile Receptors in the Skin.

Pacinian

corpuscle


2. Baroreceptors

Detect pressure changes in the walls of

blood vessels and in portions of the

digestive, reproductive, and urinary tracts

3. Proprioceptors

Monitor the positions of joints and muscles

The most structurally and functionally

complex of general sensory receptors


Mechanoreceptors: Tactile Receptors

Fine touch and pressure receptors

Are extremely sensitive

Have a relatively narrow receptive field

Provide detailed information about a source of

stimulation, including:

– its exact location, shape, size, texture, movement

Tactile Receptors


Tactile Receptors


Meissner

corpuscle

Baroreceptors

Monitor change in pressure

Consist of free nerve endings that branch

within elastic tissues in wall of distensible

organ (such as a blood vessel)

Respond immediately to a change in

pressure, but adapt rapidly

Chemoreceptors

Respond only to water-soluble and lipid-soluble

substances dissolved in surrounding fluid

Receptors exhibit peripheral adaptation over

period of seconds

Located in the

Carotid bodies, Aortic bodies:

– between the major branches of the aortic arch

Receptors monitor pH, carbon dioxide, and oxygen

levels in arterial blood

Special Senses

Five Special Senses

Olfaction

Gustation

Vision

Equilibrium

Hearing

Figure 17–1a

Smell (Olfaction)

Olfactory Organs

Provide sense of smell

Located in nasal cavity on either side of

nasal septum

Made up of two layers

Olfactory epithelium

Lamina propria

Smell (Olfaction)

The Olfactory Organs.

Smell (Olfaction)

Olfactory Glands -Secretions coat surfaces of olfactory

organs

Olfactory Receptors- Highly modified neurons

Olfactory reception -Involves detecting dissolved

chemicals as they interact with odorant-binding proteins

Axons leaving olfactory epithelium

Penetrate cribriform plate of ethmoid

Reach olfactory bulbs of cerebrum where first synapse

occurs -Axons leaving olfactory bulb: travel along olfactory tract

to reach olfactory cortex, hypothalamus, and portions of limbic

system

Taste (Gustation)

Gustation provides information about the

foods and liquids consumed

Taste receptors (or gustatory receptors)

are distributed on tongue and portions of

pharynx and larynx

Clustered into taste buds

Taste (Gustation)

Gustatory

Receptors.

Taste (Gustation)

Taste buds contain

Basal (stem) cells

Gustatory cells

Extend taste hairs through taste pore

Survive only 10 days before replacement

Monitored by cranial nerves (VII-facial) that synapse

within the medulla oblongata, then on to thalamus

and primary sensory cortex

The Eye

Accessory Structures of the Eye

Provide protection, lubrication, and support

Includes

The palpebrae (eyelids)

The superficial epithelium of eye

The lacrimal apparatus

Eyelids (Palpebrae) -Continuation of skin

Blinking keeps surface of eye lubricated, free of dust

and debris

Accessory Structures of the EyeLacrimal caruncle -soft tissue with glands producing thick

secretions

Contributes to gritty deposits that appear after good night’s

sleep

Conjunctiva

Epithelium covering inner surfaces of eyelids (palpebral

conjunctiva) and outer surface of eye (ocular

conjunctiva)

Lacrimal Apparatus

Produces, distributes, and removes tears

Lacrimal gland (tear gland)


Gross and Superficial Anatomies of the Accessory Structures.


The Organization of the Lacrimal Apparatus.

The Eye

Three Layers of the Eye (know anatomy & functions)

Outer fibrous tunic- sclera, cornea, limbus

Middle vascular tunic- iris, ciliary body, choroid

Inner neural tunic- retina (rods & cones), optic disk,

associated cells

Eyeball

Is hollow

Is divided into two cavities (segments in lab):

Large posterior cavity

Smaller anterior cavity

The Eye

The Sectional Anatomy of the Eye.

The Eye

The Pupillary Muscles.

The Eye

The Optic Disc

of the Retina.

The Eye

The Cellular Organization of the Retina.

The Eye

Photograph of the Retina as Seen through the Pupil.

The Chambers of the Eye

Ciliary body and lens divide eye into

Large posterior cavity (vitreous body-supports

retina)

Smaller anterior cavity (aqueous humor- shape):

The Eye

The Circulation of Aqueous Humor.

The Eye- Lens

Lens fibers

Cells in interior of lens-No nuclei or organelles

Filled with crystallins, which provide clarity and focusing

power to lens

Cataract

Condition in which lens has lost its transparency

Light refraction

Bending of light by cornea and lens

Focal point Focal distance

The Eye

Factors Affecting Focal Distance.

C

The Eye

Light Refraction of Lens

Accommodation

Shape of lens changes to focus image on retina

Astigmatism

Condition where light passing through cornea and lens is not

refracted properly

Visual image is distorted

Visual acuity – highest at fovea

Clarity of vision

“Normal” rating is 20/20

The Eye

Accommodation.

The Eye

Visual

abnormalities

Nearsightedness

Farsightedness

Visual Physiology

Rods - Respond to almost any photon, regardless of energy

content Cones - Have characteristic ranges of sensitivity

Visual Physiology

Visual pigments

Is where light absorption occurs

Derivatives of rhodopsin (opsin plus retinal)

Retinal: synthesized from vitamin A

Photoreception- Photon strikes retinal portion of rhodopsin molecule

embedded in membrane of disc

Opsin is activated

Bound retinal molecule with two possible

configurations.(isomers)

Visual Physiology

Photoreception pigment molecule structural change leads to changes in membrane potential of rods /cones.

Visual Physiology

Cone Types and Sensitivity to Color.

Figure 17–16

Color Vison

Integration of information from red,

green, and blue cones

Color blindness

Inability to detect certain colors

Visual Physiology

The Visual Pathways

Begin at photoreceptors

End at visual cortex of cerebral hemispheres

Message crosses two synapses before it

heads toward brain

Photoreceptor to bipolar cell

Bipolar cell to ganglion cell

Processing Visual Information

Axons from ganglion cells converge on optic disc

Proceed toward diencephalon as optic nerve (II)

Two optic nerves (one for each eye) reach

diencephalon at optic chiasm

From combined field of vision arrive at visual

cortex of opposite occipital lobe

Left half arrive at right occipital lobe

Right half arrive at left occipital lobe

The Visual

Pathways.

The Ear

External Ear –auricle, acoustic meatus, and tympanic

membrane (Separates external ear from middle ear), ceruminous

glands

Middle Ear

Communicates with nasopharynx via auditory tube

(Permits equalization of pressures on either side of

tympanic membrane)

Encloses and protects three auditory ossicles

Malleus (hammer), Incus (anvil), Stapes (stirrup)

The Ear

The Tympanic Membrane and Auditory Ossicles of the Middle Ear.

Tympanic Membrane

Converts arriving sound waves into mechanical

movements

Auditory ossicles conduct vibrations to inner ear

Tensor tympani muscle

Stiffens tympanic membrane

Stapedius muscle

Reduces movement of stapes at oval window

The Inner Ear

Contains fluid called endolymph

Bony labyrinth surrounds and protects membranous

labyrinth- subdivided into vestibule, semicircular

canals, and cochlea.

The Inner Ear

Vestibule (Encloses saccule and utricle)

Receptors provide sensations of gravity

and linear acceleration

Semicircular canals

Receptors stimulated by rotation of

head- rotational/angular movements

Cochlea -Contains cochlear duct (elongated

portion of membranous labyrinth)

Receptors provide sense of hearing -

Round window and oval window

Equilibrium

Sensations provided by receptors of vestibular complex

Hair cells

Basic receptors of inner ear

Provide information about direction and strength of

mechanical stimuli

The Semicircular Ducts

Are continuous with utricle- each duct contains- Ampulla

with gelatinous cupula, associated sensory receptors- hair cells with

cilia

The Ear

The Semicircular Ducts.

The Utricle and Saccule

Provide equilibrium sensations- linear acceleration

and gravity

Are connected with the endolymphatic duct, which

ends in endolymphatic sac

Maculae- oval structures where hair cells cluster

Statoconia -densely packed crystals

on surface of gelatinous mass

Otolith (ear stone)

= gel and statoconia

The Ear

Macular Function.

Neural Pathways for Equilibrium

SensationsVestibular receptors- Activate sensory neurons of

vestibular ganglia

Axons form vestibular branch of vestibulocochlear nerve

(VIII) – Synapse within vestibular nuclei

Integrate sensory information about balance and

equilibrium from both sides of head

Relay information from vestibular complex to cerebellum

Relay information from vestibular complex to cerebral

cortex

Provide conscious sense of head position and

movement

The Ear

Pathways for Equilibrium Sensations.

Hearing

Cochlear duct receptors

Provide sense of hearing

HearingAuditory ossicles -Convert pressure fluctuation in air into

much greater pressure fluctuations in perilymph of cochlea

Frequency of sound:

– determined by which part of cochlear duct is stimulated

Intensity (volume):

– determined by number of hair cells stimulated

Cochlea – three fluid-filled chambers-

Upper chamber – oval window membrane vestibule –

vestibular duct

Middle chamber- contains organ of Corti with sensory hair

cells – in cochlear duct

Lower chamber- tympani – tympanic duct -round window

membrane to middle ear chamber.

Stapes hits oval window and creates fluid (perilymph) pressure in vestibule

through tympani chambers- distorting basilar membrane of middle chamber

(cochlear doctor scala media).

The Cochlea- Unwound

Cochlear Duct Receptors

Basilar membrane:

– separates cochlear duct from tympanic duct

– hair cells with stereocilia in contact with overlying

tectorial membrane

The Ear

The Organ of Corti.

The Ear

Hearing.

The Ear

Pressure Waves

Wavelength- Distance between two adjacent wave troughs

Frequency -Number of waves that pass fixed reference point at

given time -Use term cycles instead of waves:

– Hertz (Hz): cycles/second (cps)

Pitch -Our sensory response to frequency

Amplitude -Intensity of sound wave

Sound energy is reported in decibels

Auditory Pathways

Cochlear branch

Formed by afferent fibers of ganglion neurons:

– enters medulla oblongata

– synapses at dorsal and ventral cochlear nuclei

– information crosses to opposite side of brain:

» ascends to inferior colliculus of mesencephalon

Ascending auditory sensations

Synapse in thalamus

Projection fibers deliver information to auditory cortex of

temporal lobe

Figure 17–31

Pathways for Auditory Sensations

The Ear

With age, damage accumulates

Tympanic membrane gets less flexible

Articulations between ossicles stiffen

Round window may begin to ossify

14.2 Somatic Sensory (Ascending)

Pathways-Carry sensory information from the skin and

musculature of the body wall, head, neck, and limbs

Three major somatic sensory (Ascending) pathways

–lateral and posterior spinal tracts

The spinothalamic, spinocerebellar, and posterior column

pathway-

Involve three neurons – Neurotransmitter = (Ach)

First order-from receptors to medulla;

Second- from medulla to thalamus;

Third -from thalamus to sensory homunuculus.

Sensory Pathways

Sensory Pathways and Ascending Tracts in the Spinal Cord.

The Posterior

Column Pathway.

Posterior Column Pathway

Sensory homunculus

Functional map of the primary sensory cortex

Distortions occur because area of sensory cortex

devoted to particular body region is not

proportional to region’s size, but to number of

sensory receptors it contains

Spinothalamic Tract

Feeling Pain (Lateral Spinothalamic Tract)

An individual can feel pain in an uninjured part of the

body when pain actually originates at another location

Strong visceral pain

Sensations arriving at segment of spinal cord can stimulate

interneurons that are part of spinothalamic pathway

Activity in interneurons leads to stimulation of primary

sensory cortex, so an individual feels pain in specific part of

body surface:

– also called referred pain

Sensory Pathwa

The Spinothalamic Tracts of the Spinothalamic Pathway.

14.3 Somatic Motor (Descending)

Pathways

SNS controls contractions of skeletal

muscles

Always involve at least two motor neurons

1. Upper motor neuron

Cell body lies in a CNS processing center

Synapses on the lower motor neuron

Innervates a single motor unit in a skeletal muscle:

– activity in upper motor neuron may facilitate or inhibit

lower motor neuron

Somatic Motor Pathways

Always involve at least 2 motor neurons

2. Lower motor neuron

Cell body lies in a nucleus of the brain stem or

spinal cord

Triggers a contraction in innervated muscle:

– only axon of lower motor neuron extends outside CNS

– destruction of or damage to lower motor neuron

eliminates voluntary and reflex control over innervated

motor unit

Somatic Motor (Descending)Pathways

1. The Corticospinal tract

Sometimes called the pyramidal system

Provides voluntary control over skeletal muscles

System begins at pyramidal cells of primary motor cortex

Axons of these upper motor neurons descend into brain stem

and spinal cord to synapse on lower motor neurons that

control skeletal muscles

Contains three pairs of descending tracts

Corticobulbar, Lateral corticospinal and Anterior corticospinal

tracts

The Corticospinal tract

Motor homunculus

Primary motor cortex corresponds point by point with specific

regions of the body

Cortical areas have been mapped out in diagrammatic form

Homunculus provides indication of degree of fine motor

control available:

– hands, face, and tongue, which are capable of varied and

complex movements, appear very large, while trunk is relatively

small

– these proportions are similar to the sensory homunculus

The Corticospinal

Pathway.

Upper

motor neuron

Lower

motor neuron

chapter 14 the somatic nervous system

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