The Divisions of the Peripheral Nervous System
The nervous system consists of two major branches: the central nervous system (CNS) and the peripheral nervous system (PNS). The CNS consists of the brain and spinal cord. The peripheral nervous system consists of all the nerves on the periphery(outside) of the CNS. The PNS consists of two primary divisions: the somatic nervous system and the autonomic nervous system.
The somatic nervous system (SNS) controls your skeletal muscles. The SNS is the only division of the PNS that is under voluntary control. For example, I am in conscious control of my fingers while writing this chapter. However, skeletal muscle reflexes are involuntary. The skeletal muscle that is the diaphragm is under conscious (speech) and unconscious (respiration)control. We will explore the SNS in more depth in the movement unit.
The autonomic nervous system (ANS) consists of two subdivisions: the sympathetic nervous system (SyNS) and the parasympathetic nervous system (PyNS).
The SyNS handles the fight-or-flight response and energy expenditure. Activation of our SyNS increases heart rate, respiratory rate, blood flow to the limbs, and reflexes. Also, the pupils dilate, and the senses are more responsive to a person’s surroundings.
The PyNS tells the body to conserve energy. Parasympathetic responses are in opposition to sympathetic responses— i.e., slow heart rate and respiration. Also, the extra blood in the limbs diverts to the digestive system.
Both branches of the ANS are involuntary. You have no direct control over your heart rate, pupil diameter, digestion, blood flow, and fight-or-flight response.
The Spinal Nerves
A spinal nerve is a cluster of sensory neuron axons or motor neuron axons wrapped in a fibrous sheet. The motor neuron’s cell body is in the spinal cord, and its axon extends the distance to its target organ. For example, the longest motor axon in the PNS spans the distance between the vertebral column base to the big toe.
A sensory neuron’s cell body is in a swelling near the spinal cord called the dorsal root ganglion. A ganglion is a cluster of cell bodies. The longest axon in the body is a sensory neuron that originates in the foot and travels to the brainstem.
The Cranial Nerves
The cranial nerves (CN) originate in the cranium, hence their name. The twelve cranial nerves form sensory pathways and motor pathways. The location on the brainstem indicates a cranial nerve number. For example, the olfactory nerve (CN #1) is the superior cranial nerve, and the hypoglossal nerve (CN #12) is the inferior cranial nerve.
Cranial Nerves Mnemonic
To learn the order of the cranial nerves, remember this phrase:
- Oh, oh, oh, to touch and feel very good velvet, ah heaven
Let’s explain this mnemonic:
- Oh = olfactory nerve
- oh = optic nerve
- oh = oculomotor nerve
- to = trochlear nerve
- touch = trigeminal nerve
- and = abducens nerve
- feel = facial nerve
- very = vestibulocochlear nerve
- good = glossopharyngeal nerve
- velvet = vagus nerve
- ah = accessory nerve
- heaven = hypoglossal nerve
There are other mnemonics out there, so you can search for those or create your own.
Here’s one I just thought of:
- Orange origami octopus’ tip toe always furry velociraptor great victory, alligator Harold
It’s a work in progress.
Cranial Nerve #1
The olfactory nerve is the first cranial nerve (CN #1) and consists of sensory neurons that send olfactory information (scents) to the brain.
The olfactory nerve is the only sensory nerve with a direct, unfiltered link to our emotions, which is why the smell of a dead rat invokes a more nauseating emotional response than the sight of the rat.
Cranial Nerve #2
The optic nerve is the second cranial nerve (CN #2) and extends from the eye’s retina to the posterior region of the brain. The retina comprises specialized ling-sensitive neurons that transmit light information via sensory neurons to the visual cortex (the area of the brain that processes visual sensory information). Therefore, the optic nerve contains only sensory neurons.
Cranial Nerves #3, #4, and #6
The oculomotor nerve (CN #3), trochlear nerve (CN #4), and abducens nerve (CN #6) comprise motor neurons that control the movements of your eyes. Vision is complex, and slight changes in pupil diameter and eye movement affect our sight. CN #3 controls the constriction of the pupils and the upward movement of the eyes. CN #4 stimulates the eye muscles for downward and medial movements of the eyes. CN #6 stimulates muscle for the lateral movement of the eyes.
Cranial Nerve #5
The trigeminal nerve (CN #5) has the largest diameter of the cranial nerves. CN #5 consists of three branches. The first two are sensory branches that send tactile sensations from the face, forehead, eyes, nose, and upper jaw to the brain. The third branch consists of sensory and motor neurons. The sensory neurons are in the mandible and the temples of the skull. The motor neurons control the mandibular muscles involved in mastication (chewing).
Cranial Nerve #7
The facial nerve(CN #7) sends taste sensations from the anterior part of the tongue to the brain. The motor pathway stimulates the muscles of the face and scalp. Bell’s palsy is a condition that affects CN #7 by causing muscle paralysis on one side of the face. The cause of Bell’s palsy is unknown, but it usually follows infections such as Herpes, Measles, and Lyme Disease.
Cranial Nerve #8
The vestibulocochlear nerve (CN #8) consists of sensory neurons. The vestibular apparatus(a sense organ in your inner ear) only contains sensory neurons. The vestibular sense orients the position of our head to the body, which helps us maintain balance. The cochlear branch sends auditory information from the cochlea to the brain.
Cranial Nerve #9
The muscles involved in swallowing and saliva secretion receive stimulation via the glossopharyngeal nerve (CN #9) motor neurons. Tongue and throat sensations get to the brain via CN #9 sensory neurons. Also, taste sensations from the posterior part of the tongue travel to the brain via CN #9.
Cranial Nerve #10
The vagus nerve (CN #10) communicates with many major organs in the thoracic and abdominal cavities. The primary sensations are visceral, which are unconscious sensations of the internal organs. Visceral sensations from the eardrum, voice box (larynx), trachea, lungs, heart, esophagus, and digestive tract travel to the brain via the vagus nerve. Vagus motor neurons stimulate skeletal muscles in the throat and larynx, cardiac muscle, and the smooth muscle surrounding the GI tract. CN #10 is the central nerve of the PyNS. Spinal nerves innervate the organs of the SyNS.
Cranial Nerve #11
Motor neurons in the accessory nerve (CN #11) stimulate the sternocleidomastoid and trapezius neck muscles. Patients with accessory nerve damage find it challenging to move their necks and raise their shoulders.
Cranial Nerve #12
The hypoglossal nerve (CN#12) comprises motor neurons that control tongue movements. Patients with hypoglossal nerve damage will display a tilted tongue. The misshaped tongue affects speech, swallowing, and breathing.
Summary
- The peripheral nervous system (PNS) comprises all neurons outside the brain and spinal cord.
- The somatic nervous system is the only voluntary division of the PNS, and it controls skeletal muscle movements. Reflexes are a part of the somatic NS and are involuntary movements of skeletal muscles.
- The autonomic nervous system is involuntary and comprises the sympathetic NS and the parasympathetic NS.
- The sympathetic NS controls the expenditure of energy – i.e., fight or flight.
- The parasympathetic NS controls the conservation of energy – i.e., digestion.
- The spinal nerves begin at the spinal cord and extend to all body regions.
- There are 12 cranial nerves.
- Cranial nerves 1, 2, and 8 comprise sensory neurons.
- Cranial nerves 3, 4, 6, 11, and 12 comprise motor neurons.
- Cranial nerves 5, 7, 9, and 10 comprise motor and sensory neurons.