o Spinal cord is enclosed and protected by the bones of the vertebral column in
the vertebral canal and contains ~ 100 million neurons.
o CNS processes many different kinds of incoming sensory information and is the
source of thoughts, emotions, and memories.
o Most nerve impulses that stimulate muscles to contract and glands to secrete,
originate in the CNS.
2. Peripheral Nervous System (PNS): “cranial and spinal nerves containing sensory/motor
o All nervous structures that are outside of the CNS (i.e. cranial nerves + branches;
spinal nerves + all of their branches; sensory receptors).
o These structures link all of the parts of the body to the CNS (muscles, glands,
and all sensory receptors).
• Divisions of the PNS:
1. Somatic (voluntary):
- Consists of sensory neurons (somatic sensory neurons) that convey information to the
CNS from the sensory receptors in the skin, skeletal muscles, and joints, and from the
receptors for the special senses (i.e. vision, hearing, equilibrium, taste, and smell.)
- Also consists of somatic motor neurons, that convey information from the CNS to the
skeletal muscles only. “somatic motor pathways” – involved in the CNS’s output of
information that results in muscular contraction.
- The motor responses can be CONSCIOUSLY CONTROLLED – thus, the actions of these
parts of the SNS are “voluntary”.
2. Autonomic (involuntary):
- Also has sensory and motor components.
- Sensory neurons (autonomic (visceral) sensory neurons) convey information to the
CNS from autonomic sensory receptors, primarily in the visceral organs (i.e. smooth
muscle organs in the thorax, abdomen, and pelvis).
- Autonomic motor neurons will convey information from the CNS to the smooth
muscle, cardiac muscle, and glands and cause the muscles to contract and the glands
- Motor responses are NOT normally under conscious control – the action of ANS is
- 3 subdivisions of the ANS (2 main + 1 small subdivision)
a. Sympathetic division: “fight or flight”
b. Parasympathetic division: “rest and digest”
c. Enteric nervous system: “local circuit neurons in the gut” – sensory neurons of the
ENS will monitor chemical changes within the GI tract, as well as the stretching of
its walls; motor neurons of the ENS govern the contraction of the GI tract smooth
muscle to propel food through the GI tract + control secretions of the GI tract
organs (i.e. stomach acid) and endocrine cells (secrete hormones).