The autonomic nervous system (ANS)

The autonomic nervous system (ANS)
The nervous system is divided into two anatomical divisions: the central nervous system (CNS), which is composed of the brain and spinal cord, and the peripheral nervous system, which includes neurons located outside the brain and spinal cord—that is, any nerves that enter or leave the CNS. The peripheral nervous system is subdivided into the efferent and afferent divisions. The efferent neurons carry signals away from the brain and spinal cord to the peripheral tissues, and the afferent neurons bring information from the periphery to the CNS. The efferent portion of the peripheral nervous system is further divided into two major functional subdivisions: the somatic and the ANS (Figure 1).
The somatic efferent neurons are involved in the voluntary control of functions such as contraction of the skeletal muscles. The ANS, conversely, regulates the everyday requirements of vital bodily functions without the conscious participation of the mind (involuntary action).



Anatomy of the ANS

Efferent neurons: The ANS carries nerve impulses from the CNS to the effector organs by way of two types of efferent neurons: the preganglionic neurons and the postganglionic neurons.
1-Preganglionic neuron: The cell body of the preganglionic neuron, is located within the CNS. The preganglionic neurons emerge from the brainstem or spinal cord and make a synaptic connection in ganglia.
2-Ganglia : an aggregation of nerve cell bodies located in the peripheral nervous system. The ganglia function as relay stations.
3-Postganglionic neuron: The cell body of the postganglionic neuron originates in the ganglion. It is generally nonmyelinated and terminates on effector organs, such as smooth muscles of the viscera, cardiac muscle, and the exocrine glands.

The efferent ANS is divided into the sympathetic , the parasympathetic nervous systems and enteric nervous system .



Anatomical differences between sympathetic and the parasympathetic nervous systems explained by the figure 3.6



A .Functions of the sympathetic nervous system
Effects of stimulation of the sympathetic division: The effect of sympathetic output is to increase heart rate and blood pressure, to mobilize energy stores of the body, and to increase blood flow to skeletal muscles and the heart while diverting flow from the skin and internal organs. Sympathetic stimulation results in dilation of the pupils and the bronchioles (Figure 3.3). It also affects GI motility and the function of the bladder and sexual organs. Sympathetic stimulation effect also called :Fight-or-flight response which mean, The changes experienced by the body during emergencies .

B .Functions of the parasympathetic nervous system
The parasympathetic division is involved with maintaining homeostasis within the body. It is required for life, since it maintains essential bodily functions, such as digestion and elimination of wastes. The parasympathetic division usually acts to oppose or balance the actions of the sympathetic division and generally predominates the sympathetic system in “rest-and-digest” situations.






Type of tissue Innervation by the ANS
1. Dual innervation: Most organs in the body are innervated by both divisions of the ANS. Thus, vagal parasympathetic innervation slows the heart rate, and sympathetic innervation increases the heart rate. Despite this dual innervation, one system usually predominates in controlling the activity of a given organ. For example, in the heart, the vagus nerve is the predominant factor for controlling rate.
2. Organs receiving only sympathetic innervation: Although most tissues receive dual innervation, some effector organs, such as the adrenal medulla, kidney, pilomotor muscles, and sweat glands, receive innervation only from the sympathetic system.
Somatic nervous system
The efferent somatic nervous system differs from the ANS in that a single myelinated motor neuron, originating in the CNS, travels directly to skeletal muscle without the mediation of ganglia. As noted earlier, the somatic nervous system is under voluntary control, whereas the ANS is involuntary.

Chemical signaling between cells
A. Hormones
Specialized endocrine cells secrete hormones into the bloodstream, where they travel throughout the body, exerting effects on broadly distributed target cells.

B. Local mediators
Most cells in the body secrete chemicals that act locally on cells in the immediate environment. Because these chemical signals are rapidly destroyed or removed, they do not enter the blood and are not distributed throughout the body. Histamine and the prostaglandins are examples of local mediators.
C. Neurotransmitters
Communication between nerve cells, and between nerve cells and effector organs.

The primary chemical signals in the ANS are Acetylcholine and norepinephrine.

Types of neurons in ANS according to neurotransmitters:

A -Cholinergic neurons: the neurons called cholinergic if Acetylcholine mediates the transmission of nerve impulses . Acetylcholine mediates the transmission of nerve impulses across:
1- Autonomic ganglia in both the sympathetic and parasympathetic nervous systems.
2- It is the neurotransmitter at the adrenal medulla.
3-Transmission from the autonomic postganglionic nerves to the effector organs in the parasympathetic system.
Types of receptors for acetylcholine:
1-nicotinic receptors (Nm ,Nn)
2-muscarinic receptors

B -Adrenergic neurons: the neurons called adrenergic When, norepinephrine (noradrenalin) and epinephrine (adrenalin)are the neurotransmitters. Norepinephrine mediates the transmission of nerve impulses from autonomic postganglionic nerves to effector organs in the sympathetic system.
Types of receptors for norepinephrine (noradrenalin) and epinephrine (adrenalin)are:
1-? receptors
2-? receptors


Figure 3.8 :Summary of the neurotransmitters released, types of receptors, and types of neurons within the autonomic and somatic nervous systems.