Anatomy Chapter 13 - PNS

first-order neurons

•Conduct impulses from cutaneous receptors and proprioceptors
•Cell body located in the dorsal root ganglion of the spinal cord
•Branch diffusely as it inters the spinal cord
•Synapse with second-order Neurons

second-order neurons

•Interneurons
•Cell body in the dorsal horn of the spinal cord
•Axons extend to the Thalamus or the Cerebellum (depending on the pathway)
•Synapse with third-order neurons

third-order neurons

•Interneurons
•Cell body located in the thalamus
•Axon extends to the somatosensory cortex
•Synapses with a somatosensory neurons located in the appropriate region (homunculus)

Dorsal Column-Medial Lemniscal Pathway

•How we determine things based on touch
-Ex. penny vs. dime vs. quarter vs. nickel
•Mostly originates from skin (soft tissue)
•First order neuron goes all the way to the medulla
•The second order goes from the medulla, crosses hyperlaterally, then to the thalamus
•The third order goes from the thalamus to a specific cortex

Spinothalamic Pathway

•Pain pathway
•First order neuron synapses at the level of injury
•The second order immediately contralaterally crosses and goes to the thalamus
•Third-order neuron goes from thalamus to specific cortex

Spinocerebellar Pathway

•Does not reach cerebral cortex
•Muscular and joint stuff, reflexes
•First order neuron goes from muscle spindle to dorsal root ganglion
•Second order neuron goes from dorsal root ganglion to cerebellum

Descending Pathway

•Deliver efferent impulses from brain to spinal cord
•Upper motor neurons
-Primary motor cortex
•Lower motor neurons
-Ventral horn
-Innervate skeletal muscles

Lateral Corticospinal Pathway

•Athlete’s pathway
•Skilled movements
-writing/typing
•Begins a cerebral cortex to the medulla then decisates to where the second order neuron exits the spinal cord
•Synapse with an interneuron
•Interneuron synapses with many neurons
•Travels to effector

dermatome

•the area of skin innervated by the
cutaneous branches of a single spinal
nerve
•Most dermatomes overlap, so
destruction of a single spinal nerve
will not cause complete numbness

flaccid paralysis

Injury to the Lower Motor Neuron
•No voluntary control of muscles
•No reflex activity
•Muscles atrophy

spastic paralysis

Injury to Upper Motor Neurons
•No voluntary control of muscles
•Muscles can be stimulated by reflex
activity
•Muscle atrophy delayed

quadriplegia

loss of function in arms and legs (cervical)

paraplegia

loss of function in the legs and lower body (thoracic)

hemiplegia

loss of function on one side of the body (typically brain)

proprioception

aware of external environment

interoception

detection of internal environment

exteroception

touch, pain, temperature and vibration

mechanoreceptors

Skin tactile receptors (flutter, vibration, stretch,...)
Deep tissue proprioceptors
Hair cells (Ear)
Vestibular Receptors (Equilibrium)

chemoreceptors

Taste buds
Olfactory Receptors

photoreceptors

Rods & Cones

thermoreceptors

Hot & Cold

nociceptors

Sharp Pain, Dull Throbbing Pain

Action Potential generation

1. Stimulus energy activates receptors
causing ion channels to open
2. Activation of each receptor leads to
numerous local depolarizations—These
depolarization are summed at the Axon
Hillock (Receptor potentials are
proportional to stimulus intensity)
3. A receptor potential > threshold results
in action potential (AP) generation
4. The frequency of AP firing is
proportional to the magnitude
(intensity/strength) of the stimulu