[Session 8]

The Sense and The Somatic

Nervous System

Widya Indriani, M.Sc.

widya.indriani@i3l.ac.id

[FS2106] – Human and Animal Physiology

RELATED LEARNING OUTCOMES

• By the end of this session, students will be able to:

1. Explain the structural organization, input pathway of cells responsible for general and special senses and the somatic nervous systems.

COURSE OUTLINE

1. The Senses• The sense introduction

2. The Special Senses• Vision• Hearing and vestibular system• Olfaction: sense of smell• Gustation: sense of taste

2. The Somatic Nervous System• Introduction and structure• Control of voluntary movement

THE SENSES

• The sense introduction

This Photo by Unknown Author is licensed under CC BY-ND

The sense introduction>> General Principles

• Sensation – sensory information that reaches the brain whatthe stimulus is, where it is, how strong (action potentialfrequency, more neurons stimulated)

• Perception – how we interpret the sensation

• Adaptation – decrease in sensitivity, decreased actionpotential frequency with the same stimulus. Some systemsmore sensitive to adaptation.

“filter out most sensory information collection”

The sense introduction >> Sensory modalities

The general senses

• Somatic senses✓ Tactile sensations (touch, pressure, vibration, itch, and tickle)✓ Thermal sensations (warm and cold)✓ Pain sensations✓ Proprioceptive sensations

• Visceral senses✓ Provide information about conditions within internal organs

(pressure, stretch, chemicals, nausea, hunger, and temperature)

The special senses

• Smell, taste, vision, hearing, and equilibrium or balance

The sense introduction >> The process of sensation

1. Stimulation of the sensory receptors

2. Transduction of the stimulus

3. Generation of nerve impulses

4. Integration of sensory input

The sense introduction>> Types of stimulus

• Mechanoreceptors – provide sensations of touch, pressure, vibration, proprioception, and hearing and equilibrium

• Thermoreceptors – detect changes in temperature

• Nociceptors – respond to painful stimuli resulting from physical or chemical damage to tissue

• Photoreceptors – detect light that strikes the retina of the eye

• Chemoreceptors – detect chemicals in the mouth (taste), nose (smell), and body fluids

• Osmoreceptors – detect the osmotic pressure of body fluids

THE SPECIAL SENSE

• Vision

• Hearing and vestibular system

• Olfaction: sense of smell

• Gustation: sense of taste

Vision>> Visual System

The visual system

• Is used to determine the shape, color of objects, and their movement.

• Light particles (photons) have wavelengths and energies associated with different colors.

Eye

• Functions like a camera to focus light on the retina using a lens and an aperture (pupil) whose size can be adjusted to change the amount of entering light.

Vision>> Visual System

Vision

• Process by which light reflected from external objects aretranslated into a mental image. This process consists of 3steps:

1. Light enters the eye and is focused by a lens on to the retina.

2. Retinal photoreceptors transduce light energy into electrical signal.

3. Processing of the electrical signals through neural pathways.

Vision>> Anatomy of the eyeball

The pupil adjusts the amount of light entering.

Changes in the shape of the lensfocus the light onto the retina.

The retina contains the photoreceptors, rods & cones.

Vision>> Focusing Changes and Lens Shape

Contraction state of the ciliary muscles (circular like a sphincter) determinesthe amount of tension that the zonular fibers (ligaments) exert on the lens:• contracted = less tension, more rounded lens (focus on near object), • relaxed = more tension, more flattened lens (focus on distant object).

Vision>> Phototransduction

Steps 2-5 - Starts a signal transduction cascade that closes cGMP-gated cation channel.

Since the photoreceptor cell is depolarized at rest, closing of the cation channel causes a graded potential where the cell becomes hyperpolarized which reduces neurotransmitter secretion .

The steps of light detection by photoreceptor cells.

Vision>> Transduction of Signal

• Photoreceptors and bipolar cells generate graded potentials.• Ganglion cells always generate action potentials.• Ratio of photoreceptor cells to bipolar cells determines the resolution.

Vision>> Light, Cones and Rods

• Rods are used for night vision. B/c they exhibit convergence, rodsgive low resolution sight at low levels of illumination. Rods containrhodopsin and are more numerous than cones.

• Cones are used for daylight vision. Cones are in a 1:1 ratio tobipolar and ganglion cells. Cones give high resolution sight at highlevels of illumination.

Vision>> Cones and Color Vision

Hearing and vestibular system>> Auditory System

Hearing and vestibular system>> Anatomy of the Ear

Pinna and external auditory canal (air filled) focus sound waves on thetympanic membrane, which rocks the malleus, incus, and stapes (air filled),causing ripples in the cochlear fluid (fluid filled) and auditory transduction

Hearing and vestibular system>> Sound Transmission

• Middle ear amplifies sound waves & couple vibrations of tympanicmembrane with oval window.

• Vibrations at oval window sets off the ripples in the cochlear fluidwithin the scala vestibuli and scala tympani. These ripples cause thebasilar membrane to vibrate.

Hearing and vestibular system>> Organ of Corti

• Stereocilia of hair cells are embedded in the tectorial membrane.

• As the basilar membrane bounces up & down, stereocilia bend.

• Bending in one direction depolarizes the cell; in the other direction hyperpolarizes the cell.

Hearing and vestibular system>> Sound Transduction

• After a hair cell (= receptor) activates the afferent neuron, axons from these neurons join to form the cochlear nerve.

• The region of the basilar membrane that vibrates the most correlates with the frequency of the sound.

• The louder the sound, the more vibration and the greater frequency of action potentials produced in the afferent neurons.

Hearing and vestibular system>> Sense of Balance: Vestibular System

Two receptor organs in the inner ear sense movement of the head.

They detect:

• angular acceleration (shake or nod your head);

• linear acceleration (elevator drops, or body leans to one side).

Semicircular canals: respond to changes in head rotation.

Otolith organs:

saccule detects vertical movement.

utricle detects horizontal movement.

Hearing and vestibular system>> Sense of Balance: Vestibular System

Semicircular canals detect angular acceleration during rotation of the head along three perpendicular axes:• Nodding (yes)• Shaking (no)• Tipping ear to shoulder

Hearing and vestibular system>> Semicircular Canals

Hearing and vestibular system>> Otolith Organs

OTOLITH ORGANS: sheets of hair cells that detect changes in linear acceleration or position of the head. Since the otoliths are heavier than the fluid around the hair cells, a change in position causes them to move and pull on the stereocilia

OTOLITHS – calcium carbonate crystals embedded in gel at the tips of stereocilia

UTRICLE detects movement in the horizontal plane (acceleration during take off in a plane or car).

SACCULE detects vertical movement (jumping). Cells are at a 90-degree angle compared to utricle.

Olfaction: sense of smell>> Anatomy of olfactory receptors

Olfaction: sense of smell>> Olfactory transduction

• Binding of an odorant molecule to an olfactory receptor protein.

• Activation of a G protein and adenylate cyclase, resulting in the production of cAMP.

• Cyclic AMP opens sodium ion (Na+) channels, Na+ ions enter the olfactory receptor cell.

• Activation triggers graded potentials which lead to action potentials.

Gustation: sense of taste>> Anatomy of tastebud

Gustation: sense of taste>> Chemoreception and taste

Chemoreceptors include: • taste buds (tongue) • receptors in carotids (O2, H+,

CO2)• neurons in brain (osmolarity) • odor receptors (nose)

Gustation : sense of taste>> Taste modalities

• Taste ligands dissolved in saliva bind to chemoreceptors (proteins) on taste buds.

• Receptor binding raises intracellular Ca++ causing the release of neurotransmitters (graded potentials).

• This leads to the initiation of action potentials in the postsynaptic neuron (primary neuron).

Five Sensations: sweet (sugars) - GPCRs sour (H+) – ion channels salt (Na+) – ion channels bitter (plant alkaloids) - GPCRs umami (glutamate) - GPCRs

THE SOMATIC NERVOUS SYSTEM

• Introduction and structure

• Control of voluntary movement

Introduction and structure>> Nervous system organization

Introduction and structure>> Somatic nervous system organization

• The somatic nervous system:

1. Sensory neurons - convey input from receptors for somatic senses and from receptors for the special senses

2. Motor neurons - innervate skeletal muscles and produce both reflexive and voluntary movements

✓ Stimulate → the muscle contracts

✓ Cease to stimulate → the muscle paralyzes, limp muscle that has no muscle tone

Example: breathing

✓ Muscles that generate respiratory movements → If the respiratory motor neurons become inactive, breathing stops

Introduction and structure>> Motor unit

One neuron can control from 3-1000 fibers, all will be in the same muscle, usually spread out.

A muscle fiber is usually innervated by a single neuron.

A MOTOR UNIT consists of a motor neuron and all of the muscle fibers it controls.

Acetylcholine released by somatic motor neuron.

Muscle motor end plate contains nicotinic AChR.

Sarcolemma (muscle PM) contains voltage-gated channels.

Introduction and structure>> Neuromuscular junction

Control of movement>> Types of Lower Motor Neuron

ALPHA MOTOR NEURONS innervate extrafusal muscle fibers to generate force for posture & movement.

GAMMA MOTOR NEURONS innervate intrafusal muscle fibers (muscle spindle). Shortening of the muscle spindle concurrent with shortening of the extrafusal muscle fibers enables continuous feedback via afferent sensory neurons to the spinal cord. The muscle spindle is critical for monitoring the length of the muscle as it shortens.

Alpha and gamma motor neurons are efferent fibers which are coactivated to finely regulate the intensity of muscle contraction.

Control of movement>> Muscle Sensory Reception

The afferent neuron coiled around the

muscle spindle senses stretch (muscle

length & speed of stretch). It acts in

reciprocal innervation causing

contraction of stretched muscle and

relaxation of antagonistic muscle.

The gamma motor neurons, muscle

spindle, and sensory neuron act as part of

a negative feedback loop to maintain muscle length at a desired value.

Control of movement>> Muscle Sensory Reception

Afferent neuron associated with Golgi

tendon organ senses tension generated

by muscle.

Golgi tendon organs are encapsulated

afferent nerve endings located at junction of

muscle and tendon in series with the extrafusal

fibers. Output from each Golgi tendon organ is

sent via an afferent axon.

Golgi tendon organs are activated by

contraction of the muscle, NOT passive stretch.

They act in a negative feedback to regulatetension.

Control of movement>> Muscle Stretch Reflex - knee jerk reflex

The afferent neuron coiled around the muscle spindle senses stretch (muscle length & speed of stretch). It acts in reciprocal innervation causing contraction of stretched muscle and relaxation of antagonistic muscle.

Control of movement>> Golgi Tendon Reflex

Contraction of the flexor muscle of the arm pulls the Golgi tendon organ activating afferent neurons.

It acts in reciprocal innervation causing relaxation of the contracted muscle and contraction of the antagonistic muscle.

Limits tension within the muscle to protect muscle and muscle-tendon junction.

Control of movement>> Withdrawal Reflexes

Reflex circuitry mediates

withdrawal of limb from a painful

sensory stimulus (flexor reflex).

Crossed extensor reflex provides

postural support during withdrawal

of affected limb.

Pain input to spinal cord

excites/inhibits reciprocal muscles on

pained side of the body (causing

withdrawal reflex) and on opposite

side of the body to support body weight.

Control of movement>> Central Pattern Generators

CENTRAL PATTERN GENERATORS (CPG):Neural circuits produce timing and coordination of complexpatterns of movement independent of sensory input andadjust them in response to sensory feedback.

CPG Characterized by:• Oscillatory• Flexible due to control by brain (change from walking to running by

reducing stance phase and sequence of limb movements in quadrupeds).

LocomotionForward flexion (swing phase) of a limb and then backward extension (stance phase) that is repeated.Reciprocal innervation of limbs

REFERENCES

• Duke University Introduction of Human Physiology: The Sense and the Somatic Nervous

• Tortora, G. J., & Derrickson, B. H. (2008). Principles of anatomy and physiology. John Wiley & Sons.

Chapter 16. Sensory, motor and integrative systemChapter 17. The Special Senses

• Randall, D., Burggren, W., French, K., & Eckert, R. (2002). Eckert animal physiology. Macmillan

Chapter 7. Sensing the EnvironmentChapter 15. The Autonomic Nervous System