Difference Between Autonomic And Somatic Nervous System

The human nervous system, a complex network responsible for controlling and coordinating bodily functions, is broadly divided into two main parts: the central nervous system (CNS) and the peripheral nervous system (PNS). Delving deeper, the PNS is further classified into the autonomic nervous system (ANS) and the somatic nervous system (SNS). Understanding the difference between autonomic and somatic nervous system is crucial to grasping how our bodies respond to the world around us and maintain internal equilibrium.

Unveiling the Somatic Nervous System (SNS)

The somatic nervous system is the part of the peripheral nervous system responsible for conscious control of skeletal muscles and transmitting sensory information from the body to the central nervous system. It’s the system that allows you to voluntarily move your limbs, feel the texture of a fabric, and react to changes in temperature.

Components of the SNS

The SNS is composed of two major components:

• Sensory Neurons (Afferent): These neurons carry sensory information from sensory receptors in the skin, muscles, and joints to the CNS. This information includes touch, pain, temperature, pressure, and proprioception (awareness of body position).
• Motor Neurons (Efferent): These neurons transmit motor commands from the CNS to skeletal muscles, causing them to contract and produce movement.

Function of the SNS

The primary function of the SNS is to control voluntary movements. When you decide to walk, write, or play a musical instrument, your brain sends signals through motor neurons of the SNS to the appropriate skeletal muscles, initiating the desired action. The SNS also plays a role in reflex actions. While many reflexes are processed at the spinal cord level, bypassing conscious thought, the SNS is still responsible for carrying out the motor response. For example, if you accidentally touch a hot stove, sensory neurons in your skin send a signal to your spinal cord. The spinal cord, in turn, sends a signal via motor neurons to the muscles in your arm, causing you to quickly withdraw your hand.

How the SNS Works: A Step-by-Step Example

Let's break down how the SNS works with a simple example: picking up a pen.

1. Decision: You consciously decide to pick up a pen. This decision originates in the motor cortex of your brain.
2. Signal Transmission: The motor cortex sends a signal down the spinal cord. This signal travels along motor neurons of the SNS.
3. Muscle Activation: The motor neurons reach the muscles in your hand and arm. They release a neurotransmitter called acetylcholine at the neuromuscular junction.
4. Muscle Contraction: Acetylcholine binds to receptors on the muscle fibers, causing them to contract. The contraction of the specific muscles needed to grasp the pen allows you to pick it up.
5. Sensory Feedback: Throughout this process, sensory neurons in your hand and fingers provide feedback to the brain about the position of your hand, the pressure you're applying to the pen, and the texture of the pen. This feedback allows you to adjust your grip and ensure you pick up the pen successfully.

Exploring the Autonomic Nervous System (ANS)

In contrast to the SNS, the autonomic nervous system operates largely unconsciously, controlling visceral functions such as heart rate, digestion, respiration, salivation, perspiration, pupillary dilation, urination, and sexual arousal. The autonomic nervous system is crucial for maintaining homeostasis, the stable internal environment necessary for survival.

Divisions of the ANS

The ANS is divided into three main branches:

• Sympathetic Nervous System: Often referred to as the "fight-or-flight" system, the sympathetic nervous system prepares the body for stressful or emergency situations. It increases heart rate, blood pressure, and respiration, dilates pupils, and diverts blood flow away from digestion to the muscles.
• Parasympathetic Nervous System: Known as the "rest-and-digest" system, the parasympathetic nervous system conserves energy and promotes relaxation. It slows heart rate, lowers blood pressure, stimulates digestion, and constricts pupils.
• Enteric Nervous System: Sometimes considered an independent branch of the ANS, the enteric nervous system is a complex network of neurons located in the walls of the gastrointestinal tract. It controls digestion independently of the brain and spinal cord, although it can be influenced by the sympathetic and parasympathetic systems.

Function of the ANS

The ANS regulates a wide range of bodily functions essential for survival, including:

• Cardiovascular Function: Controls heart rate, blood pressure, and blood vessel constriction.
• Respiratory Function: Regulates breathing rate and depth, and controls the dilation and constriction of airways.
• Digestive Function: Controls the motility of the gastrointestinal tract, the secretion of digestive enzymes, and the absorption of nutrients.
• Excretory Function: Regulates urination and defecation.
• Endocrine Function: Influences the release of hormones from various glands.
• Thermoregulation: Controls sweating, shivering, and blood vessel constriction in the skin to maintain body temperature.

How the ANS Works: An Example of Stress Response

Let's examine how the sympathetic nervous system responds to a stressful situation: encountering a barking dog while walking.

1. Sensory Input: Your eyes and ears detect the barking dog, and this information is relayed to the brain.
2. Sympathetic Activation: The brain perceives this as a potential threat and activates the sympathetic nervous system.
3. Hormone Release: The sympathetic nervous system stimulates the adrenal glands to release adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream.
4. Physiological Changes: Adrenaline and noradrenaline cause a cascade of physiological changes:
   • Increased Heart Rate: Your heart beats faster to pump more blood to your muscles.
   • Increased Blood Pressure: Your blood vessels constrict to increase blood pressure.
   • Increased Respiration Rate: You breathe faster to take in more oxygen.
   • Pupil Dilation: Your pupils dilate to allow more light into your eyes, enhancing your vision.
   • Blood Diversion: Blood is diverted away from digestion and towards your muscles, providing them with more energy.
5. Response: These changes prepare you to either fight the dog or run away.
6. Parasympathetic Activation (After the Threat): Once the threat has passed, the parasympathetic nervous system kicks in to restore your body to a relaxed state. Your heart rate slows down, your blood pressure lowers, your breathing becomes slower and deeper, and digestion resumes.

Key Differences Between Autonomic and Somatic Nervous System

To summarize, here's a table highlighting the key differences between the autonomic and somatic nervous systems:

Detailed Comparison: Delving Deeper

While the table above provides a concise overview, let's explore some of these differences in more detail:

Voluntary vs. Involuntary Control

This is perhaps the most fundamental difference. The SNS is under conscious, voluntary control. You decide to move your arm, and your SNS executes that command. The ANS, on the other hand, operates largely without conscious awareness. You don't consciously control your heart rate, digestion, or sweating. These processes are regulated automatically by the ANS. While it's generally accepted that the ANS functions involuntarily, practices like meditation and biofeedback suggest a degree of conscious influence can be exerted over autonomic functions.

Target Tissues

The SNS exclusively targets skeletal muscles. Its primary function is to control their contraction, allowing for voluntary movement. The ANS, in contrast, targets a wider range of tissues, including smooth muscle (found in the walls of blood vessels, the digestive tract, and other internal organs), cardiac muscle (found in the heart), and various glands throughout the body. This broader range of target tissues reflects the ANS's role in regulating a wide variety of internal functions.

Efferent Pathways

The efferent pathways (motor pathways) of the SNS and ANS differ in their structure. The SNS uses a single-neuron pathway. A motor neuron originates in the CNS (brain or spinal cord) and extends directly to the skeletal muscle it innervates. The ANS, however, uses a two-neuron pathway. The first neuron, called the preganglionic neuron, originates in the CNS and extends to a ganglion (a cluster of nerve cell bodies) located outside the CNS. The preganglionic neuron synapses with a second neuron, called the postganglionic neuron, which extends from the ganglion to the target tissue (smooth muscle, cardiac muscle, or gland). This two-neuron pathway allows for more complex regulation of autonomic functions.

Neurotransmitters

While both the SNS and ANS use acetylcholine as a neurotransmitter, the ANS also uses norepinephrine (noradrenaline). In the SNS, motor neurons release acetylcholine at the neuromuscular junction, causing muscle contraction. In the ANS, preganglionic neurons in both the sympathetic and parasympathetic branches release acetylcholine. However, postganglionic neurons in the parasympathetic branch also release acetylcholine, while postganglionic neurons in the sympathetic branch typically release norepinephrine. This difference in neurotransmitters contributes to the different effects of the sympathetic and parasympathetic nervous systems on their target tissues. Acetylcholine generally has a calming or inhibitory effect, while norepinephrine generally has a stimulating or excitatory effect.

Functional Roles

The SNS is primarily responsible for voluntary movement and sensory perception. It allows us to interact with the external environment in a conscious and deliberate way. The ANS, on the other hand, is primarily responsible for regulating the internal environment, maintaining homeostasis. It ensures that our heart rate, blood pressure, digestion, and other vital functions are operating within the optimal range, even in the face of changing external conditions.

Clinical Significance: When Things Go Wrong

Understanding the difference between autonomic and somatic nervous system is also crucial for diagnosing and treating various medical conditions. Dysfunction in either system can lead to a wide range of symptoms and disorders.

Somatic Nervous System Disorders

Disorders affecting the SNS can result in:

• Muscle Weakness or Paralysis: Damage to motor neurons in the brain, spinal cord, or peripheral nerves can lead to muscle weakness (paresis) or paralysis. Examples include stroke, spinal cord injury, and peripheral neuropathy.
• Sensory Loss: Damage to sensory neurons can result in loss of sensation, such as numbness, tingling, or pain. Examples include diabetic neuropathy and carpal tunnel syndrome.
• Movement Disorders: Conditions like Parkinson's disease and amyotrophic lateral sclerosis (ALS) affect motor neurons and can cause tremors, rigidity, and difficulty with movement.

Autonomic Nervous System Disorders

Disorders affecting the ANS can manifest in a variety of ways, depending on which branch of the ANS is affected and which organs are involved. Some common autonomic disorders include:

• Postural Orthostatic Tachycardia Syndrome (POTS): Characterized by an excessive increase in heart rate upon standing, leading to dizziness, lightheadedness, and fatigue.
• Neurocardiogenic Syncope (Vasovagal Syncope): A sudden drop in heart rate and blood pressure that can cause fainting.
• Multiple System Atrophy (MSA): A progressive neurodegenerative disorder that affects the ANS, leading to problems with blood pressure regulation, bladder control, and motor function.
• Diabetic Neuropathy: Diabetes can damage nerves throughout the body, including those of the ANS, leading to problems with heart rate, blood pressure, digestion, and bladder control.
• Hyperhidrosis: Excessive sweating, often caused by overactivity of the sympathetic nervous system.
• Gastrointestinal Disorders: The enteric nervous system, a branch of the ANS, plays a crucial role in digestion. Dysfunction of the enteric nervous system can contribute to conditions like irritable bowel syndrome (IBS) and gastroparesis.

The Interplay Between SNS and ANS

While it's helpful to understand the differences between autonomic and somatic nervous system, it's important to remember that these two systems are not entirely independent. They often work together to coordinate bodily functions. For example, when you exercise, the SNS increases your heart rate and blood pressure to deliver more oxygen to your muscles, while the SNS also controls the voluntary movements of your limbs. Similarly, sensory feedback from the muscles and joints, transmitted via the SNS, can influence autonomic functions like blood pressure and breathing rate. The brain integrates information from both the SNS and ANS to create a coordinated response to the environment.

Conclusion

In conclusion, the autonomic and somatic nervous systems are two distinct but interconnected parts of the peripheral nervous system, each playing a vital role in controlling bodily functions. The somatic nervous system governs voluntary movements and sensory perception, allowing us to interact with the external world consciously. The autonomic nervous system, on the other hand, regulates involuntary functions such as heart rate, digestion, and respiration, maintaining internal homeostasis. Understanding the differences between these two systems is crucial for appreciating the complexity of the human nervous system and how it allows us to adapt to and thrive in a constantly changing environment. Disruptions to either system can have significant impacts on health and well-being, highlighting the importance of continued research and improved treatments for nervous system disorders.