The Peripheral Nervous System and Spinal Cord

The Nervous System

master regulatory systems of the body: endocrine system and nervous system

endocrine system: works via circulatory system, produces slow responses (secs/mins/hrs)
nervous system: works via snyapses, produces fast responses (millisec)

2 main classes of cells: neurons (process info) and glia (supporting cells)

Cellular Organization of the PNS

3 main components of PNS

(1) sensory neurons (afferent)

receive input from periphery and enter spinal cord via dorsal roots
cell bodies (ganglia) are localized in dorsal root ganglia outside the spinal cord

(2) somatic motoneurons (efferent)

exit through the ventral roots to synapse on skeletal muscle fibers
cell bodies are localized in the spinal cord and receive information along with the dendrites

(3) autonomic nervous system

two-neuron chain

preganglionic – originate in spinal cord and synapse in autonomic ganglia

postganglionic – innervate their visceral targets (sm mus, heart, glands)

glial cells in PNS

Schwann cells: myelinate axons
Nodes of Ranvier: gaps between myelin (Saltatory conduction)

glial cells in CNS

astrocytes: supporting cells
oligodendrocytes: myelinate axons

The Spinal Cord and Reflex Arc

synapses to motoneurons

descending inputs from brain
monosynaptic inputs from sensory neurons
polysnaptic inputs from interneurons (short axons, entirely contained in spinal cord)
can be excitatory or inhibitory (synaptic integration = summation of inputs)

monosynaptic reflex (knee-jerk reflex)

gray matter = cell bodies; white matter = axons

ascending sensory pathways from spinal cord to brain:

large sensory neurons of fine touch, vibration, joint position are ipsalateral (same side)
small sensory neurons of pain and temp synapse in dorsal horn and are contralateral (opposite side) via the lateral spinothalamic tract

Neuromuscular Transmission

many motoneurons innervate a muscle; each motoneuron innervates serveral muscle fibers (forming a motor unit) giving rise to a synchronous contraction

at the neuromuscular junction:

AP (all or none; electrical signal) causes Ca to enter presynaptic membrane
Ca causes synaptic vessicles to fuse with presynaptic membrane
acetylcholine is released (chemical signal) and fuse with acetylcholine receptors (nicotinic) on postsynaptic membrane
Causes voltage gated channels to open creating an EPP which triggers an AP (electrical signal)
Occurs in 1 ms