3.1 Peripheral Nervous System Flashcards
where do the somatic efferent signals originate from?
Efferent signals originate at the cerebral cortex as a conscious decision and activate neurons in the brainstem or spinal cord.
where do the autonomic efferent nervous signals originate from?
unconscious signals originate in hypothalamus, brain stem and spinal cord and activate target neurons that lie in the peripheral nervous system.
somatic vs autonomic nervous system: myelination?
- somatic nervous system: efferent axons are myelinated (fast conduction)
- autonomic nervous system: postsynaptic axons are non myelinated (slow conduction), presynaptic axons are myelinated
somatic vs autonomic nervous system: NMJ?
- somatic nervous system: efferent axons are specific and localized
- autonomic nervous system: synapses at the target organs for the axons of the ANS may be diffuse (varicosities)
Dorsal root ganglia
- All the _____________ are located in the ganglia and one of the projections extend to synapse with peripheral organs such as the skin.
- The other axonal projection from the dorsal root ganglia form synapses with ______________.
- There is a connective tissue surrounding each ganglia called the capsule.
- These axon bundles will form your ____________–.
- Gliocytes surrounding the neuronal soma control the neuronal microenvironment.
soma or cell bodies;
neurons in the Posterior horn of the spinal cord;
spinal nerves
Peripheral nerves consists of axonal bundles projected from neurons
- Each nerve contains axons, connective tissue, glial cells, and blood vessels
- Each nerve bundle contains axons, but also other cellular components such as Schwann cells, connective tissue to hold the axons together as well as blood vessels to supply oxygen and nutrients to the nerves.
- Nerves are arranged in a particular order and ensheathed in a protective layer of connective tissue
- Endoneurium – surrounds each ___________
- Perineurium – surrounds _______________
- Epineurium – surrounds ______________
There are two types of PNS nerves which correspond to the ganglia from which they project: Sensory (afferent) nerves and motor (efferent) nerves
- Motor neuron nerves can further be classified as somatic and autonomic
- Spinal nerves are mixed nerves which carry motor, sensory and autonomic signals from the spinal cord to the target organs. However, many of these nerve fibers will diverge and synapse with different populations of cells in the spinal cord.
- Nerves can also be myelinated and unmyelinated
neuron;
bundles of axons (fascicles);
entire nerve
Myelinated and unmyelinated axons
- Myelination by Schwann cells results in formation of ______________ which accelerates electrical transmission along axons.
–> Along myelinated nerve fibres, conduction of impulses is fast, since the myelin sheath forms an insulating layer, and the impulse “jumps” from one intercellular region free of myelin to the next myelin-free region.
–> In nerve fibres lacking a myelin sheath, the nerve impulses move continuously and less rapidly.
- The majority of ____________ in the PNS are myelinated. These are neurons that require rapid conductance to allow fast propagation of information between neurons in the CNS-PNS or between neurons and effector organs –> This is critical for initiating muscle contraction during voluntary movement. Hence the relay of signals must be fast.
- In contrast, unmyelinated axons are found mainly in _________________. The nerve impulses are less rapid compared to the somatic motor neurons.
- Typically, unmyelinated axons are smaller in diameter.
Many other factors can influence conductance and these include the diameter of myelination, temperature of the body, compression and also disease (multiple sclerosis which damages the myelin).
- Note that both CNS and PNS contain myelinated and unmyelinated neurons. Unlike ____________ that myelinate PNS axons, ______________ myelinate CNS axons
Nodes of Ranvier;
somatic sensory and motor neurons;
autonomic nervous system motor neurons, particularly in axonal projections from postganglionic neurons;
Schwann cells;
Oligodendrocytes
There are two types of motor neurons: the somatic and autonomic neurons.
- The somatic motor neuron cell bodies are located in the ________. They project and form synaptic connections with different target organs include the skeletal muscle. The signal itself is being relayed from the soma to the target organs.
- The other type of motor neuron is the autonomic neuron. Information flows from the preganglionic neurons to the postganglionic neuron. The preganglionic neurons is located in the ______ while the post-ganglionic neurons are found in __________________
The majority of sensory neurons such as dorsal root ganglion neurons do not have elaborate dendritic branches. These sensory neurons are known as ______________ with axons coming out of the cell bodies projecting to two different target sites. In the case of DRG neurons, one synaptic terminal innervates the target organs, the other terminal forms a connection with ___________. Information received from the target organs travel toward the axons until they arrive at the synaptic junction with CNS neurons.
CNS;
CNS; different gangli;
pseudounipolar neurons;
other CNS neurons in the spinal cord
Spinal nerves are nerve bundles that project from the spinal cord and contains a mixture of sensory, motor and autonomic axons.
- There are 31 pairs of spinal nerves along the spinal cord with the axons branching out and innervating both sides of the body.
- These 31 pairs are identified primarily by where their ganglia are located on the spinal cord. These ganglia are located at __ cervical, __ thoracic, __ lumbar, __ sacral and __ coccyx.
- Each spinal nerve is connected to 2 roots: dorsal and ventral roots that enter the spinal cord. Dorsal roots carry _______________ while ventral roots convey _______________
Location of ganglia and axonal projections of somatic and autonomic neurons
- The somatic nerve bundles enter and exit and spinal cord via the ventral and dorsal roots with the dorsal roots mainly carrying sensory inputs and the ventral root carrying motor outputs. Note that the sensory dorsal root ganglia is situated along the dorsal root between the ___________________.
In addition, autonomic neurons also sends axonal projections to the _______________ via the ramus communicantes.
- If the projection from the spinal cord is _______________, then the axons run along the white ramus.
- If the axons _______________, then it utilizes the grey ramus.
8, 12, 5, 5, 1;
sensory afferents; motor efferent;
spinal nerves and spinal cord;
ramus communicantes;
paravertebral and prevertebral ganglia;
ascending or descending within the sympathetic ganglion;
exit the sympathetic ganglion
Dermatomes
- The territory innervated by each spinal nerve is called a dermatome
- Each ganglion is located in a specific segment of the spinal cord. Each sensory ganglion and associated spinal nerve sends axonal projections to a specific region of the body.
- Due to the fixed location of the spinal nerve projection, the area of innervation is closely related to ___________________
- Segmental arrangement of spinal nerves reveals close relationship between location of skin and nerve fibres which it innervates
- Dermatomes vary among individuals
- There is overlap between dermatomes which prevents loss of sensation if any single dermatome is damaged
- Note that are 7 cervical dermatomes starting from ______________________
- -> 12 thoracic dermatomes (T1-T12) - “T” refers to the thoracic, or to the upper torso area of the body or chest (frontal and dorsal).
- -> 5 lumbar dermatomes (L1-L5) that supply sensation from these spinal nerves in the lower limb (leg, foot, hip, etc.)
- -> 5 sacral dermatomes (S1-5) - “S” refers to the sacral or the sacrum, which are located below L5.
- -> 1 coccygeal nerve that originates in the spinal cord and emerges at the level of the tail bone (coccyx).
One of the best ways to study the presence of dermatome is during shingles reactivation.
- The Varicella zoster virus that is latent in the ganglia will reactivate and infect the epithelium in the dermatome.
- As the virus spreads in the dermatome, causing painful blisters, more virus are shed and it is possible that the reactivated virus on the skin further infects and then reactivates the latent virus in an adjacent ganglion. This causes the blisters to spread in a _______________ pattern.
- They also found that overlap existed between nearby nerve territories and that body shape was a factor that influenced the differences in the shape of the affected skin (Downs & Laporte, 2011).
- Head attributed areas of tenderness after zoster infection with corresponding spinal cord regions and constructed dermatome maps based on the lesions that appeared on the skin.
where the ganglion resides;
C2-C8. There is no dermatome for C1;
specific “snake-like”;
Myotomes
- Myotome is the motor neuron equivalent of a dermatome. It is defined as the __________________
- Just like the dermatome, there is some overlap in the myotome such that different spinal nerves can innervate different muscle groups and loss of one myotome does not result in a complete loss of motor neuron innervation to a muscle.
- Unlike dermatomes, myotomes originate from _________________, as such nomenclature is focused on target muscle group
- The nomenclature for myotomes tend to be more focused on the muscle groups that receive spinal nerve innervation. However, like dermatome, because each ganglion is identified with the segments of the spinal cord, the myotome is also closely related to the location of each ganglion.
muscle or muscles that are innervated by a single spinal nerve;
group of muscles that are innervated by a motor neuron
Myotome: C1
upper cervical flexion
Myotome: C2
upper cervical extension
Myotome: C3
Cervical lateral flexion
Myotome: C4
shoulder girdle elevation
Myotome: C5
Shoulder abduction (deltoid)
Myotome: C6
Elbow flexion (biceps, brachioradialis)
