Muscles & Sensory Perception
1. Motor Unit = consists of a single motor neuron and all the muscle fibers it
controls. When a motor unit produces an action potential, all the muscle fibers in
its motor unit contract as a group – Motor units with fewer motor neurons are
needed for fine movement
a. Categories of Muscle Fibers
i. Twitch – complete cycle of excitation/ relaxation of a muscle fiber
1. Fast Twitch
a. Respond quickly to stimulus
b. Uses oxidative and glycolytic
2. Slow Twitch
a. Respond more slowly to stimulus
b. Uses oxidative
3. Summation of two twitches
a. Tension can be summed if stimulation happens one
after another as a result of repetitive action
potentials
b. Series of action potentials at high frequency will
cause the muscle to stay constricted at very high
tension – called tetanus
ii. Fast Twitch Muscles – need lots of oxygen, since oxidative
phosphorylation is used. Often have:
1. Myoglobin – high affinity for oxygen way for muscle fibers
to get more oxygen so that oxidative phosphorylation can
happen
2. Lots of capillaries: gets you lots of oxygen
iii. Which type of muscle fiber should have an abundance of
mitochondria?
1. Slow twich oxidative muscle fibers
2. Fast twitch oxidative muscle fibers
2. Sensory Receptors
a. Terminology –
i. Sensory receptor cells/organs = respond to stimuli
ii. Perception = the brain’s integration + interpretation of the stimulus
iii. Sensory Cells
1. Neurons  neurons  brain, CNS
2. Epithelial  neuron  brain, CNS
b. Types of Receptors
i. Chemoreceptors
1. Smell – when chemicals are dissolved in air
2. Taste – when chemicals are dissolved in solution
3. Receptors can either be epithelial or neuron
ii. Electromagnetic Receptors
1. Light, magnetic forces, electricity
iii. Mechanoreceptors
1. Responding to mechanical stimuli like pressure,
temperature, stretching, motion, sound waves
iv. Theroreceptors
1. ~6 different receptors
a. Heat in the range of >42ºC (108ºF)
i. Also triggered by capaicin, so chili peppers
are hot because they also triggered heat
sensitive receptors
b. Cold in the range of 8-28ºC (48-85ºF)
i. Sensitive to methanol
v. Pain Receptors
1. Sense pain
3. Chemoreception
a. Taste
i. Receptors
1. Epithelial cells that form little clusters called taste buds
2. Taste buds are somewhat recessed in the tongue
a. Each taste receptor cell has a little pore that food
molecules bind to
b. After the signal binds to a receptor, the signal gets
sent to a neuron
ii. Types of Molecules we can taste (tastants)
Taste
Relative Sensitivity
Example
Salty (mineral salts)
1x
NaCl, KCl
Sour (acidity)
10x
Vinegars, sour fruits
Sweet (carbohydrates)
1x
Sucrose
Umami (meaty/savory)
10x
Mushrooms, meat
Bitter (toxins)
10,000x
Caffeine, hops
iii. Two types of signal transduction pathways
1. Salty/Sour
a. Tastant binds to a
ligand gated ion
channel and then
Na+ flows in and
sets off action
potential
b. Ulimately sets off
action potential at
the base; response
ends when the
tastant leaves the ligand gated channel receptor site
2. Sweet/Bitter/Umami
a. Receptor sits on the membrane of an epithelial cell
b. Tastant molecule binds to its receptor and activated
G protein coupled complexes, which activate an
enzyme
c.  which results in triggering an action potential
iv. After a G protein coupled receptor is triggered, the activated Gprotein will most likely : activate an enzyme
b. Smell = Odorant molecules bind to specific receptor proteins in the plasma
membrane of olfactory receptor cells, triggering action potential
i. In olfaction, the sensory cells are neurons
ii. Olfactory receptor cells line the upper portion of the nasal cavity
and send impulses along their axons directly to the olfactory bulb
iii. The Process
1. Odorants bind to the receptors on the plasma membrane,
which are located off of cilia
2. Membrane becomes depolarized and an action potential
gets sent from the olfactory bulb