Download Autonomic Nervous System

Autonomic Nervous System
Regulates the activities of:
- smooth muscle
- cardiac muscle
- some glands
- composed entirely of visceral efferent neurons (motor neurons to internal organs)
- has no sensory function MOTOR ONLY
- neurons are organized into:
- nerves
- ganglia
- plexuses
Divisions of the ANS
1. Sympathetic
2. Parasympathetic
- these two systems are often antagonistic to one another
Structure of the ANS
- composed of two efferent neurons
1. preganglionic neurons.
2. postganglionic neurons
Preganglionic Neurons
- axon is called preganglionic fiber
- runs from CNS to and autonomic ganglion
Postganglionic Neurons
- the axon is called the postganglionic fiber
- runs from the autonomic ganglion to a visceral effector
A two motor neuron pathway exists in the ANS with the preganglionic neuron
conveying efferent impulses from the CNS to the autonomic ganglion, and the
postganglionic neuron relaying impulses to the visceral effectors
Preganglionic Neurons
- the cell bodies of these neurons are in the lateral gray horns of the spinal cord
Sympathetic Division
- the preganglionic fibers arise from the spinal cord segments T1 through T12 and
L1, L2, and L3
- for this reason they are called the Thoracolumbar Division
- the fibers of this system are called the thoracolumbar outflow
Parasympathetic Division
- preganglionic fibers arise from the nuclei of cranial nerves III, VII, IX, and X
and the lateral gray horns of spinal cord segments S2, S3, and S4.
- for this reason this division is called the Craniosacral Division
- the fibers of this system are called the craniosacral outflow
Neurotransmitters
- can be classified as:
1. Cholinergic - release acetylcholine
- includes all sympathetic and parasympathetic preganglionic fibers
- all parasympathetic postganglionic fibers
- a few sympathetic postganglionic fibers
- all somatic motor neurons
2. Adrenergic - release norepinephrine
- most sympathetic postganglionic fibers
Activities of the ANS
- most effectors have dual innervation (innervation from both sympathetic and
parasympathetic divisions)
- structures that have dual innervations respond antagonistically to sympathetic and
parasympathetic stimulation
- the sympathetic division is said to be Fight or Flight
- parasympathetic activation is said to be Energy conservation-restoration
- for individual activities of the sympathetic and parasympathetic division see
Visceral Autonomic reflexes
- like a somatic reflex except for the two efferent neurons involved in the ANS reflex
- adjusts the homeostatic activity of the viscera such as:
- heart rate and force of contraction
- blood pressure
- respiration
- digestion
- defecation
- urinary bladder functions
ANS Control
- hypothalamus is the major control center