Chapter 49Sensory and Motor Mechanisms

Brian NguyenSophie Worthington

Yvonne Hsieh

Structure of an eyeball• Sclera: A tough, white outer layer of connective

tissue that forms the globe of the vertebrate eye. • Ciliary body: Divide the eye into two cavities, an

anterior cavity and the cornea.• Suspensory ligament: Change shapes of lens as

the eye focus on image.• Cornea: Lets light into the eye in acts to fixed

lens.• Iris: Regulates how much light is led in.• Pupil: The hole in iris that lets light in.• Aqueous humor: Clear water substance the fill

the interior.

Structure of an eyeball• Vitreous humor: Jelly-like substance that gives the

eye its volume.• Choroid: Thin pigmented inner layer• Retina: Inner most layer of eye: contains photo

receptors• Fovea: The eyes center of focus• Optic nerve: Transmits image to brain• Central artery/vein of the retina: Supplies blood to

the eye• Optic disc (blind spot): The optic disc is also called

the blind spot because there are no receptors in this part of the retina. This is where all of the axons of the ganglion cells exit the retina to form the optic nerve.

Structures of an ear• Outer ear: Consists of external pinna and the

auditory canal• Middle Ear: Consist of malleus, incus, and stapes• Inner ear: Consist of labyrinth of fluid filled

chambers• Pinna: Outer layer of skin• Auditory canal: collect sound waves and channel

them to the eardrum separating the outer ear from the middle ear.

• Eustachian: connects with the pharynx and equalizes pressure between the middle ear and the atmosphere

How the ear functions as a hearing organ

• Percussion waves cause the tympanic membrane to vibrate with the same frequency as the sound.

• The three bones of the middle ear amplify and transmit the movements to the oval window.

• The oval window produces pressure waves in the fluid with in the cochlea.

Maintaining Body Balance

• Hearing and the perception of balance, are related in most animals. Both depend on the formation of sensations by mechanoreceptors containing hair cells. When the hairs are bent they produce receptor potentials which allow the brain and body to respond to the noise or imbalance.

Functions of a Skeleton• The three main functions of a

skeleton are support, protection, and movement.

• Support : A framework to maintain its shape.

• Protection : A hard skeleton provides protection for soft tissues.

• Movement : Skeletons give muscles something firm to work against.

Exoskeleton vs. Endoskeleton

• Exoskeleton: A hard encasement deposited on the surface of an animal.

• Endoskeleton: Consists of hard supporting elements, such as bones, buried within the soft tissue of an animal.

Muscle & Bone movement

• Antagonistic muscle structures come in pairs so that the muscles can contract in two directions. Without the antagonistic muscle, the skeleton could only move one direction.

Contraction of Whole Muscles

• Nervous system produces graded contractions of muscles by varying the number of muscle fibers that contract and by varying the rate at which muscle fibers are stimulated.

Distinguishing the Three Types of Muscles • Skeletal Muscle: Muscle generally

responsible for the voluntary movements of the body.

• Cardiac Muscle: A type of muscle that forms the contractile wall of the heart. Its cells are jointed by intercalated disks that relay each heartbeat.

• Smooth Muscle: A type of muscle lacking the striations of skeletal and cardiac muscle because of the uniform distribution of myosin filaments in the cell; responsible for involuntary body activities.

Process of Muscle Contraction

1. Acetycholine(ACh) released by synaptic terminal diffuses across synaptic cleft and binds to receptor proteins on muscle fiber’s plasma membrane triggering an action potential in muscle fiber.

2. Action potential is propagated along plasma membrane and down T tubules

Process of Muscle Contraction

3. Action potential triggers Ca²+ release from sarco-plasmic(SR) reticulum.

4. Calcium ions bind to troponin; troponin changes shape removing blocking action of tropomyosin; myosin-binding sites exposed.

Process of Muscle Contraction

5. Myosin cross-bridges alternately attach to actin and detach, pulling actin filaments toward center of sarcomere; ATP powers sliding of filaments.

6. Cytosolic Ca²+ is removed by active transport into SR after action potential ends.

7. Tropomyosin blockage of myosin-binding sites is restored; contraction ends, and muscle fiber relaxes.