FORM & FUNCTION (Components of PNS) Flashcards
Functional organization of the NS:
- Somatic
- Autonomic
Somatic:
-conscious afferent and efferent (ex. pain, temperature)
-voluntary motor control (ex. muscle)
Autonomic:
-unconscious efferent
-involuntary motor control of internal organs to maintain homeostasis
*entirely EFFERENT
Autonomic divisions:
-sympathetic
-parasympathetic
Somatic afferents:
-sensory fibers from skin, muscle, joints, tendons
Visceral afferents:
-sensory fibers from visceral organs
-some result in conscious sensations, but others do not
-NOT considered part of autonomic NS
Components of PNS:
- Nerves (bundles of nerve fibers)
- Nerve endings
- Ganglia (cell bodies): sensory vs. autonomic ganglia
Myelination:
-speeds up the conduction of electrical impulses along axons
Axons in CNS are myelinated by:
-oligodendrocytes
Axons in PNS are myelinated by:
-Schwann cells
*each cell myelinates a single internode
How is myelination formed?
- Neurolemmocyte starts to wrap around a portion of an axon
- N. cytoplasm and N. PM begin to form consecutive layers around axon
- Overlapping inner layers of N. PM form the myeline sheath
4.Eventually N. cytoplasm and N. nucleus are pushed to periphery of the cell as the sheath is formed
How is an unmyelinated nerve fiber formed?
- Neurolemmocyte starts to envelope multiple axons
- Unmyelinated axons are enveloped by the neurolemmocyte, but there are no myelin sheath wraps around each axon
Nodes of Ranvier:
-areas of myelinated axon that are not covered by myelin sheath
Internode:
-area between two nodes
Paranode:
-transitional zone of myeline sheath (ex. on either side of node)
Connective tissue layers of PNS:
-endoneurium
-perineurium
-epineurium
*PNS nerve fibers are grouped into bundles to form nerves
Endoneurium:
-surrounds axons
Perineurium:
-surrounds axon fascicles
-blood-nerve barrier using tight junctions
Epineurium:
-surrounds the entire nerve
Mixture of myelinated and unmyelinated axons:
-most nerves in PNS are a mixture
-different functions and have different conductions speeds
Rapid signal transmission axons:
-usually myelinated
Ex. sensory and motor pathways, somatic NS
Unmyelinated axons commonly found in:
-autonomic nervous system
-slow signal conduction
Nerve endings of PNS:
-afferent nerves
-efferent nerves
Afferent nerves:
Contain sensory receptors
-free nerve endings
-encapsulated nerve endings
Free nerve endings:
-most common
Ex. somatic and visceral sense of pain (nociceptors) and temperature (thermoreceptors)
Encapsulated nerve endings
-ensheathed by a connective tissue capsule
Ex. mechanoreceptors in the inner dermis, proprioceptors in skeletal muscles
Efferent nerves:
-innervate target organs
Ex. neuromuscular junction
Neuromuscular junction:
-motor endplate
-synapse between motor neuron and muscle fiber
Ganglia:
-part of PNS
-ovoid structures containing neuronal cell bodies and glial cells supported by connective tissues
-relay station to transmit nerve impulses
-sensory or autonomic
Glial cells:
-satellite cells
Sensory ganglion:
-the impulse travels toward the CNS
-located in cranial and spinal nerves
Autonomic ganglion:
-the impulse travels away from the CNS
-controls involuntary movement
-2 neuron circuits of the sympathetic and parasympathetic NS
Histological differences between autonomic and sensory ganglion:
1.Location of nucleus
-on side (autonomic) vs. center (sensory)
2. Size of neuron
-smaller (autonomic) vs. bigger (sensory)
3. distribution of Nissi substances:
-edge of cell (autonomic) vs. evenly distributed (sensory)
