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The autonomic system consists of two subsystems: the sympathetic and parasympathetic system. These systems regulate the involuntary functions of the body, such as heart rate, digestion, and breathing. The sympathetic system is responsible for preparing the body for action (fight or flight response), while the parasympathetic system promotes relaxation and restoration. In this explanation, we will focus on the regulation of effectors that receive only sympathetic innervation.

The sympathetic nervous system originates in the spinal cord, specifically in the thoracolumbar segments (T1 to L2). The preganglionic sympathetic neurons leave the spinal cord through the ventral roots and synapse in the sympathetic chain ganglia (also called paravertebral ganglia). From there, postganglionic neurons extend to various effectors, including blood vessels, sweat glands, and organs.

Effectors that receive only sympathetic innervation are usually those involved in the fight or flight response and maintaining homeostasis during stress. These include blood vessels (vasoconstriction), adrenal medulla (releases adrenaline and noradrenaline into the bloodstream), sweat glands (sweating to cool the body), arrector pili muscles (hair standing on end), and fat cells (lipolysis, the breakdown of fats for energy).

Neurotransmitters are chemicals that transmit nerve signals across synapses. In the sympathetic nervous system, acetylcholine is released by the preganglionic neurons, and norepinephrine is released by the postganglionic neurons. These neurotransmitters bind to specific receptors on the effector cells, which are classified into two main types: adrenergic receptors (for norepinephrine) and cholinergic receptors (for acetylcholine). Adrenergic receptors are further divided into two main types: alpha (α) and beta (β) receptors, and cholinergic receptors are divided into nicotinic (N) and muscarinic (M) receptors.

The regulation of effectors with only sympathetic innervation is achieved through the balance of excitatory and inhibitory signals from the central nervous system (CNS). In response to stress, the brain activates the sympathetic neurons, which release neurotransmitters to stimulate their respective effector cells. Activation of adrenergic receptors usually causes an excitatory response (e.g., increased heart rate, blood vessel constriction), while activation of cholinergic receptors can be either excitatory or inhibitory, depending on the receptor subtype and effector cell type.

The sympathetic activity can be modulated by several factors, including hormones, sensory feedback, and interactions with other neurotransmitter systems. For example, adrenaline released from the adrenal medulla can also bind to adrenergic receptors, thus amplifying the sympathetic response. Additionally, sensory feedback from the effectors (e.g., blood pressure) can be sensed by the brain, which adjusts the sympathetic output accordingly. In summary, effectors that receive only sympathetic innervation are regulated by the autonomic system through the release of neurotransmitters and activation of specific receptors. The sympathetic nervous system promotes the fight or flight response and maintains homeostasis during stress, while other factors, such as hormones and sensory feedback, modulate this activity.