Divisions of the Nervous System and Brain Structure

Brain and cranial nerves

Brain and spinal cord

Spinal cord and peripheral nerves

Somatic and autonomic pathways

C1–C5

C1–C8

C1–C12

C1–C7

Afferent carry signals away from the CNS; efferent arrive at the CNS

Afferent arrive at the CNS; efferent carry signals to skeletal muscles

Afferent control autonomic functions; efferent mediate sensory input

Afferent regulate digestion; efferent regulate hearing

Sensory (afferent) fibers

Autonomic (sympathetic/parasympathetic) fibers

Motor efferent fibers of the somatic nervous system

Enteric division of the autonomic nervous system

Acetylcholine

Norepinephrine (noradrenaline)

Dopamine

Serotonin

Parasympathetic division supporting relaxation

Sympathetic division preparing for fight-or-flight

Enteric division modulating digestion independently

Somatic division controlling voluntary skeletal movement

Efferent neuron → dorsal root → interneuron → afferent neuron → muscle

Afferent neuron → spinal ganglion → interneuron in spinal cord → efferent neuron → muscle

Interneuron → afferent neuron → ventral root → spinal ganglion → muscle

Skin receptor → cerebrum → afferent neuron → efferent neuron → muscle

It is the smallest branch of the ANS and depends entirely on the CNS for control

It spans the digestive tract with two large nerve plexuses and can act independently of the CNS

It primarily mediates fight-or-flight responses through norepinephrine

It only regulates salivation and has no role in intestinal motility

Neurons are impulse-conducting cells and make up about 10% of nerve cells, while neuroglial cells primarily support and insulate neurons and comprise about 90% of nerve cells.

Neurons form the blood-brain barrier and are the most abundant cells, whereas neuroglial cells conduct impulses and are least common.

Neurons act as brain macrophages that monitor the environment, while neuroglial cells generate action potentials only during injury.

Neurons convert glucose to lactate for nourishment, while neuroglial cells absorb neurotransmitters from synaptic fluid.