Sensory and Motor Mechanisms

AP Chapter 50

Notice• You do not need to know the specific

neuroanatomy of the sensory organs, rather the mechanisms of how they work, ie: type of receptors, generally how signal is carried (by vibrations, photopigments, etc), signaling mechanisms or opening of ions channels. Even though more information is included in the power point, just read for information purposes.

The brain’s processing of sensory input and motor output is cyclical rather than linear

• The way it ISN’T: sensing brain analysis action.

• The way it is: sensing, analysis, and action are ongoing and overlapping processes.

• Sensations begin as different forms of energy that are detected by sensory receptors.– This energy is converted to action potentials

that travel to appropriate regions of the brain.• The limbic region plays a major role in determining

the importance of a particular sensory input.

Sensory receptors transduce stimulus energy and transmit signals to the nervous system

4 Functions common to all sensory pathways

1. Sensory Reception

2. Sensory transduction

3. Transmission

4. Perception

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

Categories of sensory receptors

1. Mechanoreceptors – pressure, touch, motion, sound, hair cells

2. Chemoreceptors general – solute conc specific – molecules; gustatory (taste), olfactory (smell)3. Electromagnetic4. Thermoreceptors5. Photoreceptors6. Pain receptors – in humans, nociceptors in

epidermis, located in skin and other areas, aspirin/ibuprofen blocks prostaglandins

Mechanoreceptors for hearing and equilibrium

• Utilize moving fluid and settling particles• Mammals – pressure waves picked up by

ears and converted into nerve impulses• Fish – lateral line systems• Invertebrates – statocysts with ciliated

receptor cells with sand granules• Insects – body hairs that vibrate, some have

ears

Our Hearing and Balance

• Energy of fluid into energy of action potentials

• Uses sensitive hair cells

• True organ of hearing – the organ of Corti located in the cochlea

• Balance – semicircular canals

Hearing animation

Hearing

http://msjensen.cehd.umn.edu/1135/Links/Animations/Flash/0019-swf_effect_of_soun.swf

The three small bones transmit vibrationsTo the inner ear which contains fluid-filled canals.

Air pressure vibrates fluid in canals which vibrate the basilar membrane, bending the hairs of its receptor cells against the tectorial membrane which opens ion channels and allows K+ to enter the cells and cause a depolarization and releases neurotransmitters to continue to the auditory nerve to the brain.

Fig. 50-9

“Hairs” ofhair cell

Neuro-trans-mitter atsynapse

Sensoryneuron

Moreneuro-trans-mitter

(a) No bending of hairs (b) Bending of hairs in one direction (c) Bending of hairs in other direction

Lessneuro-trans-mitter

Action potentials

Mem

bra

ne

po

ten

tial

(m

V)

0

–70

0 1 2 3 4 5 6 7

Time (sec)

Sig

nal

Sig

nal

–70

–50 Receptor potential

Mem

bra

ne

po

ten

tial

(m

V)

0

–70

0 1 2 3 4 5 6 7Time (sec)

–70

–50

Mem

bra

ne

po

ten

tial

(m

V)

0

–70

0 1 2 3 4 5 6 7

Time (sec)

–70

–50

Sig

nal

Frequency (pitch) determined by areas of basilar membrane that vibrate at different frequencies; areas are thick and thin

Volume is controlled by amplitude of wave – stronger bends hair cells more and more action potentials

• Balance in the semicircular canals is also a response to hair cells; different head angles stimulate different.

• Hair cells; lateral line systems in fish and some amphibians work like this too.

The inner ear also contains the organs of equilibrium

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

• Statocysts are mechanoreceptors that function in an invertebrates sense of equilibrium.– Statocyst function

is similar to that of human semi-

circular canals– Use ciliated (hair-

like cells)

Many invertebrates have gravity sensors and are sound-sensitive

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 49.21

A diversity of photoreceptors has evolved among invertebrates.

Planaria – eyecup for light and direction Insects/crustaceans- compound eyes (ommatidia) Jellyfish, spider,

mollusks – single lens eye

Taste and Smell

• Odor/taste molecules bind to ciliated receptor cells and trigger a signal-transduction pathway that involves a G-protein and, often, adenylyl cyclase and cyclic AMP’s.

• cAMP to open Na+ channels, depolarizing the membrane and sending action potentials to the brain.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 49.24

Fig. 50-13

G proteinSugar molecule

Phospholipase C

Tongue

Sodiumchannel

PIP2

Na+

IP3

(secondmessenger)

Sweetreceptor

ER

Nucleus

Taste pore

SENSORYRECEPTORCELL

Ca2+

(secondmessenger)

IP3-gatedcalciumchannel

Sensoryreceptorcells

Tastebud

Sugarmolecule

Sensoryneuron

Vertebrate eyes

• Rods and cones are photoreceptors located in the retina of the eye.

• Rods are more light sensitive and are concentrated toward the edge of the retina.

• Cones are more color sensitive and are concentrated in the center of the visual field called the

Vertebrates have single-lens eyes• Is structurally analogous to the invertebrate

single-lens eye.

How does this work?

• Rods and cones synapse with bipolar cells in the retina, which synapse with ganglion cells, whose axons form the optic nerve.

• R/C BP Ganglion Cells

Light hits the retinaand then comes backthrough the cells to theoptic nerve.

Photoreceptors

Rods and cones have visual pigments embedded in a stack of folded membranes or

disks in each cell.

Retinal is the light-absorbing pigment and is bonded to a membraneprotein – opsin. Combo – rhodopsin.

When retinal absorbs light, it changes shape and separates from opsin. In the dark, the retinal is

converted back to its original shape.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Opsin activates a G protein and opens/closes Na channels to continue/discontinue the nerve impulse.

Fig. 49.13

Notice in thelight, the Na+

channels areclosed.

remember…

The ultimate perception of the stimulus depends on the area of the brain that is stimulated!

In summary:Touch – mechanoreceptors, dendrites of

neurons pick up ions

Smell, taste – chemoreceptors, gen (solute conc), specific (individual molecules), G protein activates a second messenger that controls a Na or K ion channel

Sight (light) – electroreceptors, trans retinal activates a G protein cascade that opens/closes Na channels

Hearing, balance – mechanoreceptors, moving fluid, settling particles, bending of hair cells open ion channels

Locomotion and muscle action

The muscle cell’s structure is conducive to its purpose which is to contract upon receiving a stimulus.

How the muscle cell is organized

Animations

This is ONEmuscle cell,called a muscle fiber.

for energy

The muscle fiber (cell) is made up of many myofibrilswhich in turn are made up of sarcomeres, the units ofcontraction.

• The sarcoplasmic reticulum (SR) is a special type of smooth ER found in smooth and striated muscle.

• The SR contains large stores of calcium ions, which it releases when the cell is depolarized.

Action potentialIs spread in theT tubules

Fig. 50-25b

TEM

Thickfilaments(myosin)

M line

Z line Z line

Thinfilaments(actin)

Sarcomere

0.5 µm

The contracting unit is the sarcomere.

This is what gives skeletal muscles and heart muscles their striated appearance.

When thesarcomerecontracts, the filaments slide over each other.

The sliding filament model

Animation: Sarcomere Contraction

How does this happen? a closer look: Myosin

Myosin is made of polypeptides twisted to form a fiber helix with a globular end, which has ATPase activity & an affinity to bind to actin.

a closer look: Actin

Actin is a globular protein;each globular actin unit contains a myosin binding site. 

Remember – Actin – Ac”thin”

Mechanism of action

1. The Neuromuscular Junction – neuron to muscle

• Signal travels from motor neuron by acetycholine (excitatory) to the skeletal muscle cell and depolarizes it.

• An action potential is spread in the T tubules and changes the permeability of the sarcoplasmic reticulum which releases Ca+.

2. Actin involvement

Myosin-binding sites

are blocked by a strand

of tropomyosin whose

position is controlled by

Troponin complex molecules.

Ca+ ions bind to the complex

and move the tropomyosin and

expose the binding sites for

myosin.

3. Myosin Involvement- The globular heads of the myosin are

energized by ATP and bind to actin forming a cross-bridge

- When relaxing to its low-energy state, the myosin head bends and pulls the attached actin toward the center of the sarcomere

4. Completion

When its’s over, Ca+ returns to the sarcoplasmic reticulum and tropomyosin recovers binding sites on actin.

Acetylcholine is degraded at the synapse.

Mechanism of Filament Slidingat the Neuromuscular Junction

Animation: Action Potentials and Muscle Contraction

• The sliding-filament model of muscle contraction.

Interactions between myosin and actin generate force during uscle contractions

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 49.33

Protein models of muscle action

Contraction

• Of single muscle fiber – all or none• Twitch: slow – less SR, Ca in longer,

fibers must have many mitochondria, a good blood supply (myoglobin better which picks up O2 better and stores it)

• Fast – rapid and powerful contraction• Tetanus – smooth, sustained

contraction; action potentials arrive rapidly

Muscle Fatigue

• Depletion of ATP, loss of ion gradient, accumulation of lactic acid

• Hydrostatic skeleton: consists of fluid held under pressure in a closed body compartment.– Form and movement is controlled by

changing the shape of this compartment.– Advantageous in aquatic environments and

can support crawling and burrowing.– Does not allow for running or

walking.

Skeletons support and protect the animal body and are essential to movement

• Exoskeletons – supportive, protective but do not grow (molted)

• Endoskeletons – supportive, grow with the organism, less protective