OVERVIEW OF PNS
A. Sensory receptors
A. Sensory receptors respond to stimuli. They can be classified by their
location, the type of stimulus they detect or by their structural complexity.
B. Nerves and ganglia
C. Motor endings
B. Nerves and associate ganglia
a. Exteroceptors - stimuli from outside the body
2. Stimulus type
b. Interoceptors - A.K.A. visceroceptors, within the body
c. Proprioceptors - also inside but only in association with skeleton
a. Mechanoreceptors - touch, pressure, stretch and vibration
3. Structural complexity
b. Thermoreceptors - temperature changes
c. Photoreceptors - light energy
d. Chemoreceptors - chemicals in solution
e. Nociceptors - harm (all of the above can act as nociceptors)
a. Complex receptors - associated with organs of special senses
4. Sensory receptor potentials - the stimulus is in some form of energy
that is transduced to electrical energy. The stimulus causes changes in
membrane permeability which allows ions to move and sets up a graded potential
(called a receptor potential in this case). Just proximal to the sensor
are voltage gated ion channels. Similar to post-synaptic potentials, receptor
potentials can summate to generate an action potential. If a receptor potential
is high enough to trigger an A.P. it's called a generator potential. A
strong stimulus will result in more rapidly reaching a generator potential.
The faster that happens the more frequent the A.P.s can be, indicating
a strong stimulus to the CNS.
b. Simple receptors - associated with the general senses
1. Free dendritic endings - abundant in epithelial tissue.
Pain and temp. ex: Merkel discs and root hair plexuses.
2. Encapsulated dendritic endings - connective tissue capsule. Most
1. Nerve structure - A "nerve" consists of bundles of axons
wrapped in connective tissue. Remember that the PNS has sensory and motor
divisions. Nerves can have bundles of axons that travel toward the CNS
only. They are sensory nerves. Nerves that only carry impulses away from
the CNS are called motor nerves. The most common are mixed nerves, with
both sensory and motor fibers.
C. Motor endings - Activate effectors by releasing neurotransmitters. There
are 2 types of effectors, somatic and autonomic.
1. Axonal terminals of somatic fibers - these are the neuromuscular
2. Varicosities of autonomic fibers - the axonal endings on smooth
muscles or glands have a series of knob-like swellings that contain vesicles
of neurotransmitters. Synaptic clefts are often wider, leading to slower
responses than on somatic effectors.
The reflex arc - 5 components
There can be autonomic or somatic reflexes.
B. Sensory neuron
C. Integration center - somewhere in CNS
1. Monosynaptic reflex
D. Motor neuron
2. Polysynaptic reflex