Overview of the Central and Peripheral Nervous Systems

CNS

The brain and spinal cord.

PNS

To carry sensory information to the CNS and motor commands away from the CNS.

Divisions of the PNS

Sensory (afferent)—brings in input; Motor (efferent)—carries out responses.

Somatic motor division

Controls voluntary skeletal muscle.

Meningeal layers

Dura mater → Arachnoid mater → Pia mater.

Functions of the spinal cord

Conduit for ascending/descending tracts and a reflex center.

Arachnoid mater

Contains the CSF-filled subarachnoid space.

Ventral horn

Contains motor neuron cell bodies.

Ascending tracts

Carries sensory data up the spinal cord.

Commissural fibers

Connect left and right sides of the cord within the white matter.

Spinal nerve formation

The dorsal (sensory) root and ventral (motor) root merge.

Dermatome

The skin area innervated by a single spinal nerve.

Varicella-zoster virus

Lies dormant in the dorsal root ganglia before causing shingles.

Divergence in a neuronal pool

One input neuron synapsing onto many output neurons.

Convergence

Many inputs converging on one output.

Parallel processing

Simultaneous pain withdrawal reflex plus increased heart rate via separate pathways.

Basic reflex arc steps

Receptor → Sensory neuron → Integration center → Motor neuron → Effector.

Monosynaptic reflex

Has one synapse; polysynaptic has two or more via interneurons.

Visceral reflex example

Pupillary light reflex.

Tendon reflex

Uses the Golgi tendon organ to prevent muscle damage.

Crossed-extensor reflex

The injured side withdraws while the opposite side extends to support weight.

Stretch (myotatic) reflex

Maintains muscle length when stretched.

Cortex facilitation of spinal reflex

By increasing motor neuron excitability via descending pathways.

Inhibition of a reflex

By activating inhibitory interneurons that dampen motor neuron output.

Voluntary override of a spinal reflex

Suppressing the patellar response when you don't want your leg to kick.