OVERVIEW OF PNS
A. Sensory receptors
B. Nerves and ganglia
C. Motor endings
A. Sensory receptors respond to stimuli. They can be classified by their location, the type of stimulus they detect or by their structural complexity.
1. Location
a. Exteroceptors - stimuli from outside the body
b. Interoceptors - A.K.A. visceroceptors, within the body
c. Proprioceptors - also inside but only in association with skeleton & muscles
2. Stimulus type
a. Mechanoreceptors - touch, pressure, stretch and vibration
b. Thermoreceptors - temperature changes
c. Photoreceptors - light energy
d. Chemoreceptors - chemicals in solution
e. Nociceptors - harm (all of the above can act as nociceptors)
3. Structural complexity
a. Complex receptors - associated with organs of special senses
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 are mechanoreceptors.
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. Nerves and associate ganglia
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 junctions.
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.
REFLEX ACTIVITY
The reflex arc - 5 components
A. Receptor
B. Sensory neuron
C. Integration center - somewhere in CNS
1. Monosynaptic reflex
2. Polysynaptic reflex
D. Motor neuron
E. Effector.
There can be autonomic or somatic reflexes.