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Chapter 50:

Sensory / Motor

Mechanisms

Pathway of a stimulus:

Campbell et al. – Figure 50.2

Ability to distinguish stimuli depends on the brain:

• Sensations: Electrical impulses that reach the brain via sensory neurons

• Perceptions: Interpretation of electrical impulses by the brain

Chapter 50: Sensory / Motor Mechanisms

(1) Reception

Receptor detects

stimuli

Exteroreceptors (external)

Interoreceptors (internal)

(2) Transduction

Stimuli converted to

electrical impulse

(3) Transmission

Signal conducted

to CNS

Amplification

Strengthening of

stimulus signal

Intensity = AP frequency

Low

intensity

High

intensity

(4) Summation

Integration of

signal by CNS

Sensory Adaptation

Decrease in

responsiveness

over time

Sensory receptors are categorized by the type of energy they transduce:

1) Mechanoreceptors: Stimulated by physical deformation (e.g., touch)

• Muscle spindles = stretch receptors monitoring skeletal muscle length

• Hair cells = receptors detecting motion (e.g., hearing)

2) Nociceptors (pain receptors): Stimulated by inflamed / damaged tissue

3) Thermoreceptors: Stimulated by heat / cold

4) Chemoreceptors: Stimulated by specific molecule types

• Osmoreceptors = detect changes in [solute]

• Gustatory (taste) / Olfactory (smell) receptors

5) Electromagnetic receptors: Stimulated by electromagnetic energy

Heat

Magnetic Field


Light

Photoreceptors and Vision:

Invertebrates:

Allows for detection of light

intensity / directionality

B) Image-Forming Eyes:

• Accurate at detecting movement

2) Single-lens Eye (Spiders / mollusks)

• Consists of multiple ommatidia (facets)

1) Compound Eye (insects / crustaceans)


Chapter 50: Sensory / Motor MechanismsCampbell e t al. – Figure 50.16

A) Light Detection Eyes:

Planaria – Ocellus

Photoreceptors and Vision:

Vertebrates:

A) Single-lens Eye:

Cornea: Transparent; allows

light into eye


Pupil: Regulates amount of

light entering eye

Retina: Inner surface of eye;

contains photoreceptors• Fish = move lens forward / back

• Mammals = change shape of lens

Lens: Focuses image onto

back of eye

Rods (125 million) = light sensitive

Cones (6 million) = color sensitive

Color vision common in lower

vertebrates but rare in mammals…

WHY?

Chapter 50: Sensory / Motor MechanismsCampbell et al. – Figure 50.20

Physiology of Vision (example = rods):

A) Rhodopsin (light absorbing structure) triggers signal-transduction pathway:

Opsin = membrane protein

Retinal = light-absorbing pigment

• When rhodopsin absorbs light, the retinal changes shape and separates from opsin

“Bleaching”

Separation of retinal

Why does it take

several minutes for

eyes to adjust to dark?


2


A) Rhodopsin (light absorbing pigment) triggers signal-transduction pathway:

• Altered opsin (minus retinal) triggers enzymatic pathway (similar to Figure 50.21):

IMPORTANT FACT:

Light does not depolarize rod cell,

but hyperpolarizes it




B) Retina assists cerebral cortex in processing visual information

Vertical Pathway:

Rod Bipolar Cell Ganglion Cell

Lateral Pathway:

Horizontal / Amacrine cells link

neighboring cells

Lateral Inhibition:

Horizontal cells inhibit nearby rods from firing

(sharpens edges / enhances contrast)Campbell et al. – Figure 50.23


Skeletal System:

• Supporting framework for the body

Types of Animal Skeletons:

1) Hydrostatic Skeleton

• Fluid-filled compartments provide

support (via pressure)


• Movement = contraction of:

1) Circular muscles

2) Longitudinal muscles

Peristalsis

• Ideal for aquatic life

• Limited vertical support

Who:

Cnidarians

Flatworms

Nematodes

Annelids


Skeletal System:

• Supporting framework for the body

Types of Animal Skeletons:

2) Exoskeleton (e.g., insects, crustaceans)

3) Endoskeleton (e.g., mammals)

• Rigid, encasement deposited on surface of organism

• Chitin (modified polysaccharide) / Calcium carbonate

• Muscles attach from outer “shell” to interior of body

• Must be periodically shed

• Rigid, internal skeleton supporting body (e.g., plates / bones)

• Benefits: Can grow with organism; relatively lightweight

Chitin


Human Skeleton = 206 bones

• Axial skeleton

• Skull, vertebral column, rib cage

• Appendicular Skeleton

• Upper / Lower limbs

Tissue Types:

1) Cartilage (flexible support / connections)

2) Bone (rigid support - matrix)

Bones connected at joints:

1) Hinge joint:

• 2 - dimensional movement (e.g., elbow)

2) Ball-and-socket joint:

• 3 - dimensional movement (e.g., hip)


3

Function of Muscle:

4) Guard entrance / exit (e.g., lips / anus)

5) Maintain body temperature (endotherms)

• Locomotion / manipulation (skeletal)

1) Produce movement

2) Maintain posture

3) Support soft tissue (e.g., abdominal wall)

• Blood pressure

(cardiac)

• Propulsion (smooth)

Prune Belly

Syndrome



Gross Anatomy of Muscle: Connective Tissue Layers:

• Outside muscle covering

• Form tendons

Epimysium:

• Divides muscle into fascicles

Compartments

• Contains blood vessels/nerves

Perimysium:

• Surrounds individual muscle

fibers & ties them together

Endomysium:


Microanatomy of Muscle:

Transverse Tubules: Network of passageways through fiber

• Continuous with outside of cell

Sacroplasmic Reticulum: Specialized endoplasmic reticulum

• Contain calcium ions (Ca++)

Myofibrils: Cylindrical structures containing contractile fibers

Muscle Fiber (cell):

Functional Unit of Muscle

Triad


Multi-nucleated

Sarcolemma: Cell membrane

Sarcoplasma: Cytoplasm


Myofibrils contain myofilaments (protein) :

1) Actin (Thin filament)

2) Myosin (Thick filament)

Sarcomere: Repeating units of

myofilaments (~ 10,000 / cell)

MyosinActin

Sarcomere

Z lineM lineI Band

A Band


http://www.annerpino.com/2003/heart.jpg

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Interactions between the thick and thin filaments of sarcomeres are

responsible for muscle contraction


Sliding Filament

Theory

Thick filaments:

• Composed of many myosin molecules

Tails: Attach molecules together

Heads: Bind with thin filament

Hinge





Sliding Filament

Theory

Thick filaments:

• Composed of many myosin molecules

Tails: Attach molecules together

Heads: Bind with thin filament

Thin filaments:

• Composed of interwoven actin molecules

Tropomyosin: Cover active sites

Troponin: Bind tropomyosin to actin





Sliding Filament

Theory


Sarcoplasmic Reticulum

Ca++

Ca++ Ca++

Ca++

Ca++ Ca++ Ca++Ca++

Ca++Ca++Ca++Ca++

Neuromuscular Junction:

• Neuron Muscle fiber

• 1 connection / muscle fiber

Sarcolemma

Sarcoplasm

T-tubule

Synaptic

Knob

Neuron

Motor End Plate

Muscle Contraction Events:

1) Acetylcholine (ACh - neurotransmitter)

released from synaptic knob

ACh ACh ACh

2) ACh binds to receptors on motor end

plate; generates AP

3) Action potential (AP - electrical impulse)

conducted along sarcolemma

Sarcomere of

myofibril


Sarcoplasmic Reticulum

Ca++

Ca++ Ca++

Ca++

Ca++ Ca++ Ca++Ca++

Ca++Ca++Ca++Ca++

Neuromuscular Junction:

• Neuron Muscle fiber

• 1 connection / muscle fiber

Sarcolemma

Sarcoplasm

T-tubule

Synaptic

Knob

Neuron

Motor End Plate

ACh ACh ACh

4) AP descends into muscle fiber via

T-tubules

Ca++ Ca++Ca++Ca++Ca++

5) AP triggers release of Ca++ from

sarcoplasmic reticulum

6) Ca++ initiates cross-bridging (actin / myosin)Sarcomere of

myofibril

Muscle Contraction Events:


5

Cross-bridging Events:

1) Ca++ binds with troponin; exposes

active sites on actin

Cross-Bridging in Action:


Troponin

Actin

Tropomyosin

Myosin

Head

Ca++




2) Myosin head (cocked) binds with

active site

3) Myosin head pivots – pulls actin

forward







active site


forward

4) ATP binds to myosin head; head

detaches and re-cocks



ATP

ATPATP

ADP

P

Re-cock

6





active site


forward



5) Myosin head binds to active site;

Process repeated


• Energy = Creatine phosphate


ATP

ATP

ADP

P

Re-cock

ATP

ADP

P

Re-cockATP





active site


forward



5) Myosin head binds to active site;

Process repeated

6) Process ends when APs cease

• Ca++ returned to sarcoplasmic

reticulum (active transport)

• ACh broken down by

acetylcholinesterase (AChE)


• Energy = Creatine phosphate


• Rigor Mortis • Tetanus


• Lou Gerig’s Disease (ALS) • Botulism


What Regulates Muscle Tension Production?

Muscle Tension: Force exerted on an object by a contacting muscle

Muscle Mechanics:

1) Number of muscle fibers activated:

Motor Unit: A single motor neuron and all the muscle fibers innervated by it

All-or-none

response

Recruitment:

Addition of motor units to produce

smooth, steady muscle tension(small large motor units)

Motor unit size dictates control:

• Fine Control = 1-5 fibers / MU (e.g., eye)

• Gross Control = 1000’s fibers / MU (e.g., leg)

Diverse body movements

require variation in muscle activity




Muscle Mechanics:

2) Frequency of stimulation:

Twitch = Single stimulus-contraction-relaxation sequence

Tensio

n

Time

Stim

ulu

s

Latent Period:

Time between stimulus

and tension development

LP

Ca++ release;

cross-bridge formation

Ca++ uptake;

cross-bridge detachment

CP

Contraction Phase:

Period where tension

rises to peak level

RP

Relaxation Phase:

Period where tension

falls to resting level

Twitches alone are not a

useful contraction


7



Muscle Mechanics:

Incomplete Tetanus: Rapid cycles of contraction & relaxationTensio

n

Time

Stim

ulu

sMaximum

Tension

Summation:

Addition of twitches to

produce a more powerful

contraction





Muscle Mechanics:

Tensio

n

Time

Complete Tetanus: Rapid stimulation erases relaxation phase

Stim

ulu

s

Maximum

Tension

SR can not reclaim Ca++

(stimulation too rapid)

Most normal muscle contraction

involves complete tetanus



Too contracted = no room for movement; poor

cross-bridge formation

Too stretched = no cross-bridge

formation

Resting length = # of cross-bridges;

distance to slide

(maximal muscle force is

near / at normal resting length)

Degree of Muscle Stretch can also Affect Muscle Performance:


Cardiac Muscle:Skeletal Muscle: Smooth Muscle:Property

Filament

Organization

Control

Mechanism

Calcium

Source

Contraction

Energy Source

Sarcomeres along

myofibrils

Sarcomeres along

myofibrilsScattered in

sarcoplasm

NeuralAutomaticity

(pacemaker cells)

Automaticity,

neural, hormonal

Sarcoplasmic

reticulum

SR / across

sarcolemma

Across

sarcolemma

Rapid onset;

tetanus can occur;

rapid fatigue

Slower onset;

no tetanus;

fatigue-resistant

Slow onset;

tetanus can occur;

fatigue-resistant

Aerobic / Anaerobic

metabolismAerobic

metabolism

Aerobic

metabolism