NOTES: CH 50 - Sensory &

Motor Mechanisms!

Sensory Mechanisms • Sensations: action potentials that reach the

brain via sensory neurons

• Perception: interpretation of a sensation

*(this occurs in the brain!)

“If a tree falls in the woods….?”

Sensory receptors are:

1) exteroreceptors: detect stimuli outside the body (heat, light, pressure, chemicals)

2) interoreceptors: detect stimuli within the body (blood pressure, body position)

The transmission of signals to the nervous system begins with:

• SENSORY TRANSDUCTION:

-conversion of stimulus energy

a change in membrane potential

Sensory transduction is followed by:

• AMPLIFICATION: strengthening of stimulus

• TRANSMISSION: conduction of impulses to the CNS

• INTEGRATION: processing of information (usually in the brain)

5 types of sensory receptors:

1) MECHANORECEPTORS: stimulated by physical deformation (pressure, touch, stretch, motion)

2) PAIN RECEPTORS: stimulated by excess heat, pressure, specific chemicals (like those released by damaged tissues/cells)

Sensory receptors: (continued)

3) THERMORECEPTORS: respond to heat or cold; regulate body temp.

4) CHEMORECEPTORS: detect and transmit info. about solute concentration; involved w/taste & smell; osmoreceptors that regulate kidneys &rate of urine production

5) ELECTROMAGNETIC RECEPTORS: include photoreceptors (detect light); used in some animals for migration (use magnetic field of earth)

CHEMO-RECEPTORS in

an insect!

CHEMORECEPTORS:

ELECTRO-MAGNETIC

RECEPTORS:

ELECTROMAGNETIC RECEPTORS:

PHOTORECEPTORS:

• PHOTORECEPTORS:

cells that contain pigment molecules that absorb light;

transduce the light stimulus into an electrical signal (by changing the membrane potential of the cell)

Vertebrate EYE: see fig. 50.17 Structure Function

Sclera tough outer layer of connective tissue

Choroid thin inner pigmented layer

Cornea anterior, transparent area of sclera;

allows light to enter eye

Iris anterior choroid; regulates amt. of light

entering pupil; gives eye its color

Pupil hole in center of iris

Conjunctiva Mucous membrane covering the sclera; keeps the eye moist

Vertebrate EYE, continued

Retina innermost layer of eyeball;

contains photoreceptor cells

Lens transparent, protein disc that

focuses image on retina

Ciliary body produces aqueous humor

Aqueous humor fills cavity between lens and cornea; fxns as liquid lens; clear & watery

Vitreous humor fills cavity behind lens; fxns as liquid lens; jellylike

Optic nerve carries visual sensory info to brain

Vertebrate EYE, continued

Fovea Center of the visual field; near the optic nerve; densely packed with cone cells

Blind spot Where the optic nerve exits the eye – no photoreceptors here

Two types of photoreceptors on retina:

1) Rod cells: more sensitive to light (so they help us to see indim light); do not distinguishcolors; allow us to see at night; more numerous in nocturnal animals (approx. 125 million; make up 70% of all sensory receptors in the body!)

2) Cone cells: do not function at night; can distinguish colors in daylight; approx. 6 million

*Rods and cones contain visual pigments which consist of light-absorbing pigment molecules (retinal…derived from vitamin A) and a membrane protein (opsin).

Retinal Opsin

RHODOPSIN:

● the visual pigment of ROD CELLS

● has 2 alternating forms depending on the conditions:

DARK or LIGHT

IN THE DARK…

● rhodopsin is INACTIVE.

● rod cells are highly permeable to sodium and are therefore in a DEPOLARIZED state

● rod cells in this INACTIVE (dark) state are releasing neurotransmitter molecules that INHIBIT the firing of postsynaptic cells in the retina.

● so in the dark, no message is sent from the rod cells to the visual centers of the brain

IN THE LIGHT…● rhodopsin absorbs light, and breaks apart, as

its retinal component changes shape; opsin is now ACTIVE;

 ● this triggers a chain of metabolic events

(signal-transduction pathway!) that makes the rod cell membrane less permeable to sodium and therefore hyperpolarizes the rod cell membrane;

● the rod cell synaptic terminals stop releasing neurotransmitter (which was inhibiting the postsynaptic cell);

IN THE LIGHT…

● thus, a decrease in chemical signal to the cells with which the photoreceptors synapse serves as the message that rods have been stimulated by light…these postsynaptic neurons, now freed from inhibition, can develop action potentials which are transmitted to the brain for processing

Photoisomerization of rhodopsin:

● over time, very bright light keeps the rhodopsin “bleached” (most of the rhodopsin decomposes into retinal and opsin) and rod cells eventually become unresponsive cone cells take over;

● in the dark, enzymes convert the retinal back to its original form (rhodopsin) and the rod cells can once again respond to faint light

(e.g. walking from a bright environment into a dark room or movie theatre…)

CONES and COLOR VISION

• there are 3 subclasses of cone cells, each with a different opsin protein

• each photopsin is best at absorbing a specific wavelength (color) of light

• the 3 subclasses are: red green blue

Integration of visual information:

• begins in the retina

• rod and cone cells synapse with:

BIPOLAR CELLS, which synapse with: GANGLION CELLS

• HORIZONTAL CELLS and AMACRINE CELLS also involved

 

Vertical pathway: info. passes directly from receptor (i.e. the rod or cone cell) to bipolar to ganglion cells

Lateral pathway: info. passes from rod/cone to horizontal/amacrine cells and spreads out over several bipolar or ganglion cells

*(nearby cells are stimulated; distant receptor & bipolar cells are inhibited “lateral inhibition”…sharpens the edges of objects)

Integration of visual info (cont.):

• Finally, the info. is transmitted along the optic nerves formed by axons of ganglion cells;

• optic nerves from the 2 eyes meet at the OPTIC CHIASM;

• they pass through the thalamus;

• they continue back to the primary visual cortex in the occipital lobe of the cerebrum (integration occurs here and in other cortex areas)

HEARING AND EQULIBRIUM:

• vibrating objects create percussion waves in air;

 

• these waves cause the tympanic membrane (EARDRUM) to vibrate w/same freq.;

• the 3 bones of the middle ear (malleus, incus, stapes) amplify the sound and transmit the mechanical movements to the oval window (membrane covering cochlea in the inner ear);

 

• vibrations of oval window produce pressure waves in the fluid w/i the cochlea;

 

• mechanoreceptors in the cochlea convert the energy of the vibrating fluid into action potentials (which travel on the auditory pathway to the cerebral cortex)

The receptor cells – hair cells – are part of the organ of Corti; as hair cells are physically moved by the sound waves (transmittedthrough the cochlearfluid), they brush againstthe tectorial membrane;this causes them to depololarize and releaseneurotransmitter, thereby triggering an action potential in a sensory neuron!

*(the middle ear also contains the Eustachian tube which connects with the pharynx)

*Equilibrium and balance are affected by the semicircular canals in the inner ear

CHEMORECEPTION: Taste and Smell

• chemoreceptors in taste buds or nasal cavity are triggered when specific molecules bind and trigger action potentials

 

**4 basic taste perceptions:

sweet, sour, salty, bitter

 

• taste and smell interact; if the olfactory system is blocked (e.g. you have a cold) the perception of taste is sharply reduced

**Example of chemoreceptors…solutes in air dissolve in mucus; bind to chemoreceptors; result in the opening of ion channels;sensory neuron is depolarized; action potential goes to thebrain via the olfactory nerve…you perceive a SMELL!

(He thought it was funny)

CH 50, continued…Muscles & Movement

● Movement is necessary to:-catch food

-escape predators

-locate mates

● Skeletons are essential to movement:

-they provide a firm attachment against which muscles can work during movement

The muscular system consists of three types of muscle tissue:

● Skeletal

● Smooth

● Cardiac

STRUCTURE OF A SKELETAL MUSCLE:

*Individual muscles are the organs of the muscular system. They contain skeletal muscle tissue, nervous tissue, blood, and connective tissues.

Skeletal Muscle Fibers: each muscle fiber is a single muscle cell,

which is the unit of contraction just beneath the cell membrane the

cytoplasm (SARCOPLASM) contains: * many small, oval nuclei* mitochondria* SARCOPLASMIC RETICULUM (a modified

endoplasmic reticulum)* MYOFIBRILS (fibers of the proteins ACTIN

and MYOSIN)

Skeletal Muscle Fibers:**the organization of actin and myosin

filaments produces STRIATIONS (bands)

**the thick (myosin) and thin (actin) filaments are organized into structural units called SARCOMERES

SARCOMERE: unit of organization of skeletal muscle

● Z lines: borders of sarcomere

● I bands: area near the edge; contains only thin filaments

● A bands: regions where thick and thin filaments overlap

● H zones: areas in center of A bands containing only thick filaments

Also part of a muscle fiber…

**TRANSVERSE TUBULES (T tubules) extend inward from the cell membrane and associate with the SARCOPLASMIC RETICULUM (whose membranes surround each myofibril)

Neuromuscular Junction:MOTOR NEURONS

stimulate muscle fibers to contract

in response to a nerve impulse, the end of a motor neuron axon secretes a NEUROTRANSMITTER, which stimulates the muscle fiber to contract

Neuromuscular Junction:

one MOTOR NEURON and the MUSCLE FIBERS associated with it constitute a MOTOR UNIT

all muscle fibers of a motor unit contract together!

SKELETAL MUSCLE CONTRACTION

*Muscle fiber contraction results from a sliding movement of actin and myosin filaments.

(known as the SLIDING FILAMENT MODEL in which individual sarcomeres shorten)

Role of MYOSIN and ACTIN:

cross-bridges of myosin filaments form linkages with actin filaments

the reaction between actin and myosin filaments generates the force of contraction

                                                                                                                                                                                                                                                                        

OTHER (REGULATORY) PROTEINS INVOLVED:

● TROPONIN and TROPOMYOSIN: together form a complex that covers the myosin-binding sites on actin; by covering these binding sites, myosin cannot bind to actin and a contraction cannot occur.

Stimulus for and Steps of…a CONTRACTION:

> ACETYLCHOLINE (a neurotransmitter) is released from the distal end of a motor neuron axon and stimulates a skeletal muscle fiber

> acetylcholine causes the muscle fiber to conduct an impulse (membrane potential) over the surface of the fiber that reaches deep within the fiber through the TRANSVERSE TUBULES

> a muscle impulse signals the sarcoplasmic reticulum to release CALCIUM IONS

Steps of a Muscle Contraction…

> calcium ions bind to troponin protein & tropomyosin is pulled aside, uncovering the myosin-binding sites on actin

> linkages form between actin and myosin

> the myosin cross-bridges pull on actin filaments, shortening the fiber

****energy for the sliding filament model comes from ATP!!!

                                                                                                                                                                                                                                                                        

The end of a contraction…> the muscle fiber relaxes (and the contraction

ends) when cross-bridges release from actin and when calcium ions are actively transported back into the sarcoplasmic reticulum (without calcium present, the troponin-tropomyosin complex re-covers the myosin-binding sites on actin)

> acetylcholine is broken down by the enzyme ACETYLCHOLINESTERASE

Energy Sources for Muscles:

● ATP supplies the energy for muscle fiber contraction

● for sustained muscle contractions, a molecule called creatine phosphate is used to make more ATP

Oxygen Supply and Cellular Respiration● aerobic respiration

requires oxygen

● red blood cells carry oxygen to body cells (oxygen binds to HEMOGLOBIN in the RBCs)

● MYOGLOBIN in muscle cells temporarily stores oxygen

Oxygen Debt● during rest or moderate exercise, muscles receive

enough oxygen to respire aerobically

● during strenuous exercise, oxygen deficiency may cause LACTIC ACID to accumulate

● OXYGEN DEBT is the amount of oxygen required to convert accumulated lactic acid to glucose and to restore supplies of ATP

**the metabolic capacity of a muscle may change with training!

Muscle Fatigue:

● a fatigued muscle loses its ability to contract

● muscle fatigue is usually due to accumulated lactic acid

Heat Production

**muscle action is an important source of body heat!