Major Effectors of Vertebrates Responsible for:

• Body movement
• Movement of most materials through the body
• Support
• Generation of heat

Muscle cells are made of

Myofibrils

Myofibrils are made of 

Myofilaments

Types of Contraction

• Isotonic
• Isometric
• Negative work
  

Isotonic Contraction

Shortening of a muscle against a constant load or force

Isometric Contraction

Little or no contraction although muscle is acting against a force

Negative Work

Muscle fibers elongate rather than contract as tension increases

Types of Muscle

• Smooth muscle
• Cardiac
• Skeletal

Smooth Muscle

• Enlongated, nonstriated, spindle shaped cells
• Provide slow, sustained contraction
• Two Types:
• Unitary smooth muscle
• Multiunit smooth muscle

Unitary Smooth Muscle

• Occurs in most viscera
• Myogenic (originating in muscle) contraction but modulated by nerve impulses

Multiunit Smooth Muscle

• Blood vessels
• Eye
• Neurogenic contraction

Cardiac Muscle

• Moderately elongated
• Striated and branching cells tightly united by intercalating discs
• Myogenic modulated by nerve impulse
• One or more nuclei per cell

Skeletal Muscle

Extremely long striated and multinucleated cells

Supports body and effects movement of the organism

Motor Units

A neuron and the skeletal muscle fibers it innervates

Motor End Plates

Points where branches of a neuron attach to muscle fiber

Tonic Fibers

• Contract slowly to provide tonus
• Each muscle fiber reveives multiple motor endplates from the neuron innervating it
• Extent and force of contraction are graded by frequency of nerve stimulation
• Muscles that move the eyeball in mammals
• A few postural muscles (affecting posture) in appendages of lower vertebrates

Phasic Fibers

• Most vertebrate skeletal muscle
• Each muscle fiber receives a single muscle motor end plate
• Contraction is "all or none"
• Two types:
• Slow phasic fibers
• Fast phasic fibers

Slow Phasic Fibers

• Contract slowly
• Adapted for slow, repetitive, isotonic contractions
• Energy comes from oxidative metabolism
• High myoglobin content
• Example: dark meat on a chicken

Fast Phasic Fibers

• Contract rapidly
• Adapted for rapid movements of short duration
• Energy comes from anaerobic glycolysis
• High glycogen content
• Example: white meat on a chicken

Strap and Fusiform Muscles

• Muscle fibers long, run parallel to each other and to their tendon
• Adapted for contractions resulting in extensive movement
• Maximum strength of contraction
• Examples: muscles of thigh region, sartorius, etc.

Pennate Muscles

• Short fibers that pull obliquely on their tendon
• Cannot cause as extensive a movement but generate move force
• Often found where space is limited
• Well adapted for forceful isotonic contractions and isometric contractions of short extent
• Examples: deltoid

Muscle Grouping

The pattern of embryonic development of muscles and thier nerve supply provides the basis for divding muscles into groups whose homologies can be recognized in different vertebrate groups.

Muscle Grouping using Embryology

Embryology is more meaningful in tracing evolution of the muscle system than is grouping based on histological or physical properties.

Somatic vs. Visceral Muscles

• Somatic muscles orient body with respect to the external environment
• Visceral muscles are involved in maintaining an internal environment

Somatic Muscles

• Typically develop from myotomes
• Axial
• Muscles of body wall and tail - epaxials, hypaxials, and epibranchials
• Hypobranchials and muscles of tongue
• Extrinsic eyeball
• Appendicular - muscles of the appendages

Visceral Muscles

• Typically develop from splanchnic mesoderm
• Branchiomeric muscles - on visceral arches, muscles of tubes, vessels, and hollow organs
• Also intrinsic eyeball (iris), heart, and erectors of feathers and hair

Muscle of Fishes

Somatic Axial

Trunk Muscles

• Form a series of muscles segments (myomeres)
• Typically composed of slow phasic (for cruising and maintaining position against a current) and fast phasic (sudden bursts of speed) fibers
• General structure of trunk musculature is closely correlated with swimming activity

Muscles of Fishes

Somatic Axial

Epaxials and Hypaxials

• Trunk muscles posterior to the gill region
• Epaxials above and hypaxials below horizontal septum

Muscles of Fishes

Somatic Axial

Epibranchials

• Lie dorsal to the gill region
• Forward continuation of epaxials and functions with them

Muscles of Fishes

Somatic Axial

Hypobranchials

• Somatic muscles ventral to gill region
• Functionally associated with the visceral skeleton
• Act to:
• Open mouth
• Expand pharynx during the intake of respiratory water current

Muscles of Fishes

Somatic Axial

Extrinsic Ocular Muscles

• 6 strap-shaped muscles attached to the outside of the eyeball
• Responsible for movement of the eye

Muscles of Fishes

Somatic Appendicular

• Muscles of paired appendages, usually quite simple
• Two types:
• Dorsal abductor
• Ventral adductor

Muscles of Fishes

Somatic Appendicular

Dorsal Abductor

• Runs between posterior dorsal portion of girdle and dorsal surface of fin
• Pulls fin dorsally and caudally

Muscles of Fishes

Somatic Appendicular

Ventral Adductor

• Runs between anteroventral portion of girdle and ventral surface of fin
• Pulls fin ventrally and crainally

Muscles of Fishes

Visceral Branchiomeric

• A very complex group assoicated with the elaborate visceral skeleton
• They can be fairly variable according to specialization for feeding and respiration

Evolutionary Trends in Muscles

Extrinsic Ocular Muscles

• All tetrapods retain 6 muscles
• Very little change in the process of terrestrialization
• Only real change is in the rectus complex, which moves the upper and lower eyelids of birds and mammals

Evolutionary Trends in Muscles

Hypobranchial Muscles

• Become much more differentiated, associated with the complex hyoid apparatus and development of a muscular tongue
• Involved in feeding and swallowing movements
• Increase in nubmer of muscles in this group

Evolutionary Trends in Muscles

Epibranchial and Epaxial Trunk Muscles

• Very important in mediating dorsoventral bending of the spine
• Control movements of the head
• Segmentation lost or reduced in all tetrapods except salamanders

Evolutionary Trends in Muscles

Hypaxial Muscles

• Subdivided into 3 major groups:
• Subvertebral group - lies just below vertebral column, interacts with epaxials
• Rectus abdominus - longitudinally on either side of midventral line, supports abdomen
• Lateral group - lie along flank, strengthen abdominal wall

Evolutionary Trends in Muscles

Appendicular Muscles

• Become large and numerous as appendages and girdles assume a major role in support and locomotion.
• Early splay-legged tetrapods had large, powerful ventral muscles that adducted the humerous and femur and flexed the antebranchium and crus, raising the body from the ground
• As limbs moved under the body the ventral musculature became less important for support

Evolutionary Trends in Muscles

Branchiomeric Muscles

Mandibular Muscles

• Closely follow changes in visceral skeleton
• Most mandibular muscles still act on the jaw
• Most close
• One (digastric) opens
• Tensor tympani follow part of the original mandibular arch into the middle ear, attached to the malleus (earbone) in mammals
• Masseter, pterygoids, temporalis - closely related to evolution of the temporal fenestrae

Evolutionary Trends in Muscles

Branchiomeric Muscles

Hyoid Muscles

• Divided into 3 main components:
• Part contribute to digastric
• Part contribute to middle ear (stapedius)
• Rest form superficial facial muscles

Electric Organs

• Modified muscle tissue in about 250 species of fish
• Composed of a series of disk-shaped electroplaxes
• May generate weak to strong pulses
• Functions:
• Weak - navigation, species recognition
• Strong - stun predators as protective measure, or stun prey

Electroplaxes

• Modified muscle cells or their motor endplates
• Multinucleated
• Flat on the innervated side and highly folded on the opposite side
• Myo fibrils are lost

Purpose of Sense Organs

Vertebrates must be able to avoid predators, find shelter, food and mates, therefore they must by able to detect changes within their external and internal environments.

Receptors

Transducers of mechanical, electrical, thermal, chemical, and radiant energy.

General Receptors

Widely distributed in the body

Special Receptors

Usually restricted to head region, usually paired

Somatic Receptors

Two Types:

• External receptors
• Proprioceptors

External Receptors

Somatic Receptor

Provide information about the external environment

Proprioceptors

Somatic Receptor

Provide information about skeletal muscle activity

Visceral Receptors

Monitor internal environment

Neuromast Organs

• Fishes and aquatic amphibians
• Function primarily in mechanoreception and electroreception
• May occur in shallow pits, grooves, ampullae, or canals, and forms the lateral line and cephalic canal system of fishes
• Receptor cells are hair cells, which project into a gelationous secretion called the cupula
• Bending the cupula causes changes in nerve impulses, which are detected

Kinocilium

Special type of cilium on the apex of hair cells located in the sensory epithelium of the vertebrate inner ear.

Cupula

A structure surrounding hair cell receptors

Inner Ear

• Sense organ present in all vertebrates
• Neuromast System
• Labyrinths are filled with endolymph
• Perilymphatic space is filed with perilymph
• Funtions in equilibration and audition
• Lower vertebrates: 3 semicircular ducts, 2 membranous sacs, and the lagena (which becomes the cochlear duct in some reptiles, birds, and mammals

Neuromast System

Membranous labyrinths located in skeletal labyrinth (otic capsule) surrounded by fluid-filled perilymphatic space

Sensory Neuromast Sites

Located in squamous epithelium of the sacculus, utriculus, and lagena/cochlea (patches of hair cells found in these structures are called maculae), and an enlarged neuromast called a crista occurs in a swelling (ampulla) located at one end of each semicircular duct