General visceral fibres do not supply
**Core Concept**
General visceral fibres are part of the autonomic nervous system, which plays a crucial role in controlling involuntary functions of the body, such as heart rate, digestion, and blood pressure. These fibres are responsible for regulating the activity of smooth muscles, cardiac muscles, and glands. The autonomic nervous system is divided into two main subdivisions: the sympathetic nervous system and the parasympathetic nervous system.
**Why the Correct Answer is Right**
General visceral fibres specifically supply smooth muscles, cardiac muscles, and glands, which are all components of the involuntary nervous system. These fibres release neurotransmitters such as acetylcholine and noradrenaline, which act on specific receptors to regulate the activity of these muscles and glands. Skeletal muscles, on the other hand, are under voluntary control and are supplied by somatic motor fibres. The distinction between general and somatic visceral fibres lies in their target organs and the type of control they exert.
**Why Each Wrong Option is Incorrect**
**Option A:** While it is true that general visceral fibres do supply smooth muscles, this option is not the correct answer because the question asks for what they do not supply. Smooth muscles are indeed a target of general visceral fibres.
**Option C:** General visceral fibres do supply cardiac muscles, which are a type of involuntary muscle. This makes option C incorrect.
**Option D:** General visceral fibres also supply glands, such as the adrenal glands, pancreas, and salivary glands. This option is therefore incorrect.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the autonomic nervous system controls involuntary functions, which are often referred to as "automatic" functions. This includes heart rate, digestion, and blood pressure, among others. A key distinction to make is between the sympathetic and parasympathetic nervous systems, which have opposite effects on the body's involuntary functions.
**✓ Correct Answer: B. Skeletal muscles**