Chapter 15: The Autonomic Nervous System

Autonomic Nervous System

• The autonomic nervous system (ANS) controls involuntary responses in the body.

• Often referred to as the "automatic" nervous system.

• Main divisions: Sympathetic and Parasympathetic.

Sympathetic Division

• Known as the fight or flight response.

• Activated during perceived threats or stressful situations (e.g., encountering a snake, public speaking).

• Emergency response mechanism.

• Key Effects of the Sympathetic Division:

• Increased heart rate: Prepares body for action.

• Increased respiratory rate: Enhances oxygen supply.

• Increased blood flow to skeletal muscles: Prepares muscles for quick response.

• Decreased blood flow to digestive system: Redirects energy toward essential systems for survival.

• Increased sweat gland secretion: Helps cool the body during stress.

• Pupil dilation: Improves vision and focus on threats.

• Anatomical Considerations:

• Travels from the central nervous system (CNS) through spinal cord to target organs.

• Preganglionic fibers: Shorter in sympathetic; exit via ventral roots.

• Postganglionic fibers: Longer; connect to target organs.

Parasympathetic Division

• Known as the rest and digest response.

• Promotes relaxation and restoration of body functions after stress.

• Key Effects of the Parasympathetic Division:

• Decreased heart rate: Helps the heart regain normal function.

• Decreased respiratory rate: Dials down the body's heightened state.

• Pupil constriction: Protects eyes and reduces light intake.

• Stimulated digestive activities: Encourages digestion and nutrient absorption.

• Anatomical Differences:

• Also connects through the CNS but has distinctively longer preganglionic fibers.

• Preganglionic fibers: Longer in parasympathetic; close to target organs.

• Postganglionic fibers: Shorter, usually found within target organs.

Reflexes

• Defined as rapid, predictable, involuntary responses to stimuli.

• Two main types of reflexes:

• Autonomic reflexes: Mediated through the autonomic nervous system.

  • Typically affect smooth and cardiac muscles and glands (not consciously perceived).

• Somatic reflexes: Involve stimulation of skeletal muscles, such as quickly withdrawing a hand from a hot surface.

Long Reflexes vs. Short Reflexes

• Long Reflexes:

• Involve the CNS (spinal cord or brain).

• Example: Reactions to food or danger that require CNS processing.

• Short Reflexes:

• Bypass CNS; directly stimulate postganglionic fibers by sensory neurons.

• Primarily mediated by the enteric nervous system (ENS) in digestion,

  • Example: Reflexive responses to digestive movement and chemical changes.

This concludes the overview of the autonomic nervous system, focusing on its divisions, reflex mechanisms, and physiological impacts on the body.