Lesson 1.3: Peripheral Nervous System

• Our peripheral nervous system (PNS) consists mainly of nerves and scattered ganglia found outside the central nervous system.
• Nerves are bundles of axons encased in connective tissues. They are the basic units of the nervous system.
• Ganglia, on the other hand, are scattered clusters of neuronal cell bodies found outside the spinal cord.
• Cranial nerves are those rooted in the areas of the head. They connect the brain to different parts of the head, neck, and trunk. There are 12 nerves in our skull.
• On the other hand, the 31 pairs of spinal nerves are those that originate from the spinal cord and extend to the peripheral areas of the body like the limbs.
• Sensory neurons carry nerve impulses from our senses toward the brain, whereas motor neurons carry nerve impulses from the brain toward our muscles and glands to effect a particular response or action.
• In the group of cranial nerves, only oculomotor, trochlear, abducens, accessory, and hypoglossal are motor neurons. The rest are sensory by function.
• Spinal nerves, in contrast to cranial nerves, can be both sensory and motor by function. Because of this, the nerves in the peripheral nervous system are further sorted into divisions.
• The peripheral nervous system is functionally divided into sensory and motor divisions.
• Sensory pathways carry signals to the CNS from sensory receptors in the skin, bones, muscles, and other organs.
• Motor pathways convey information from the CNS to muscles and glands.
• These motor pathways of the PNS are further subdivided into two groups: the somatic and autonomic nervous systems. The latter has two arms, namely the sympathetic and parasympathetic divisions.
• The somatic nervous system allows us to consciously, or voluntarily, control our muscles. Hence, this subdivision of the motor pathways is also referred to as the voluntary nervous system
• Voluntary Nervous System

  • It carries signals from the brain to our skeletal muscles, such as those that enable us to high five with our friends, ride a bicycle, or talk. However, not all activities controlled by the SNS are voluntary. Muscle reflexes are also governed by the nerves found in this subdivision.
• In contrast to the SNS, the autonomic nervous system regulates events that are automatic, or involuntary, such as the activities of the smooth and cardiac muscles
• Autonomic Nervous System (ANS)

  • This subdivision of the motor pathways carries impulses from the brain to involuntary muscles and glands, enabling all internal organs to function without our conscious awareness.
• Autonomic Nervous System (ANS)

  • This subdivision is also referred to as the involuntary nervous system. ANS is further grouped into the sympathetic and parasympathetic divisions. Functionally, these two divisions bring opposite effects. What one stimulates, the other inhibits.
• The sympathetic division is often referred to as the “fight-or-flight” system. It dominates when we are excited or under stress, including emergencies or threatening situations. This division is at full speed not only when you are emotionally stressed or excited but also when you are physically stressed.
• During relaxed times, when the body is not threatened in any way, the parasympathetic division is most active. It also returns the body systems to normal conditions after being stressed.
• Parasympathetic Division

  • it is referred to as the “rest-and-digest” system. It is mainly concerned with promoting normal digestion and elimination of wastes