Download Sensory and Motor Mechanisms

NOTES – CH 50: Nervous System – Sensory & Motor Mechanisms
• Sensations: action potentials that reach the brain via sensory neurons
• Perception:
*(this occurs in the brain!)
Sensory receptors are:
1) exteroreceptors: detect stimuli outside the body (
)
2) interoreceptors: detect stimuli within the body (
)
The transmission of signals to the nervous system begins with:
• SENSORY TRANSDUCTION:
-conversion of stimulus energy 
Sensory transduction is followed by:
● AMPLIFICATION:
● TRANSMISSION: conduction of impulses to the CNS
● INTEGRATION: processing of information (usually in the brain)
5 types of sensory receptors:
1) MECHANORECEPTORS: stimulated by
(pressure, touch, stretch,
motion)
2) PAIN RECEPTORS: stimulated by excess heat, pressure, specific chemicals (like those released by damaged
tissues/cells)
3) THERMORECEPTORS: respond to heat or cold;
4) CHEMORECEPTORS: detect and transmit info. about solute concentration;
;
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)
● PHOTORECEPTORS:
 cells that contain pigment molecules that
 transduce the
membrane potential of the cell)
HUMAN EYE: see fig. 50.17
Structure
Function
Sclera
tough outer layer of connective tissue
;
(by changing the
Choroid
Cornea
thin inner pigmented layer
anterior, transparent area of sclera; allows light to enter eye
Iris
anterior choroid; regulates amt. of light entering pupil;
______________________________
Pupil
Conjunctiva
Mucous membrane covering the sclera; keeps the eye moist
Retina
Innermost layer of eyeball; _________________________________.
Lens
Transparent; protein disc that_____________________________.
Ciliary body
produces aqueous humor
Aqueous humor
fills cavity between lens and cornea; fxns as liquid lens;
______________________
Vitreous humor
fills cavity behind lens; fxns as liquid lens;______________________
Optic nerve
Fovea
carries visual sensory info to brain
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:
(so they help us to see in dim light); do not
distinguish colors; 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:
; 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).
RHODOPSIN:
●
● has 2 alternating forms depending on the conditions:
 IN THE DARK…
●
● 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);
● 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
● 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
● the 3 subclasses are:



Integration of visual information:
●
● rod and cone cells synapse with:
● horizontal cells and amacrine cells also involved
, which synapse with:
Vertical pathway: info. passes directly from receptor (i.e. the rod or cone cell) to
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)
● Finally, the info. is transmitted along the
cells;
● optic nerves from the 2 eyes meet at the
formed by axons of ganglion
;
● they pass through the thalamus;
● they continue back to the
in the occipital lobe of the cerebrum.
HEARING AND EQULIBRIUM:
• vibrating objects create
;
• these waves cause the tympanic membrane (
) 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 middle ear also contains the Eustachian tube which connects with the pharynx)
*Equilibrium and balance are affected by the
CHEMORECEPTION: Taste and Smell
• chemoreceptors in taste buds or nasal cavity are triggered when
**4 basic taste perceptions:
• taste and smell interact; if the olfactory system is blocked (cold) the perception of taste is sharply reduced
NOTES – CH 50, continued: Muscles & Movement
● Movement is necessary to:
-
-
-
● 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:
●
●
●
STRUCTURE OF A SKELETAL MUSCLE:
*Individual muscles are the organs of the muscular system. They contain
.
Skeletal Muscle Fibers:
 each muscle fiber is a single muscle cell
 just beneath the cell membrane, the cytoplasm (
*
) contains:
*
*
(a modified endoplasmic reticulum)
*
(fibers of the proteins ACTIN and MYOSIN)
**the organization of actin and myosin filaments produces STRIATIONS (bands)
**the thick (
SARCOMERES
) and thin (
) filaments are organized into structural units called
SARCOMERE: unit of organization of skeletal muscle
♦ Z lines:
♦ I bands: area near the edge; contains only thin filaments
♦ A bands:
♦ H zones: areas in center of A bands containing only thick filaments
Also part of a muscle fiber…
**TRANSVERSE TUBULES extend inward from the cell membrane and associate with the SARCOPLASMIC
RETICULUM (whose membranes surround each myofibril)
Neuromuscular Junction:
 MOTOR NEURONS
 in response to a nerve impulse, the end of a motor neuron axon secretes a NEUROTRANSMITTER, which
stimulates the muscle fiber to contract
 one MOTOR NEURON and the MUSCLE FIBERS associated with it constitute a
 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
in which individual sarcomeres shorten)
Role of MYOSIN and ACTIN:
 cross-bridges of myosin 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
by covering these binding sites, myosin cannot bind to actin and a contraction cannot occur.
Stimulus for and Steps of…a CONTRACTION:
> ACETYLCHOLINE (a
stimulates a skeletal muscle fiber
;
) released from the distal end of a motor neuron axon
> acetylcholine causes the muscle fiber to conduct an impulse (
of the fiber that reaches deep within the fiber through the TRANSVERSE TUBULES
) over the surface
> a muscle impulse signals the sarcoplasmic reticulum to release CALCIUM IONS
> calcium ions bind to troponin protein &
binding sites on actin
, uncovering the myosin-
> linkages form between
> the myosin cross-bridges
****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 troponintropomyosin complex re-covers the myosin-binding sites on actin)
>
Energy Sources for Contraction
by the enzyme ACETYLCHOLINESTERASE!!! 
● 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
●
● red blood cells carry oxygen to body cells (oxygen binds to
●
in the RBCs)
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
to accumulate
●OXYGEN DEBT is the
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
Heat Production
**
!