The nervous system plays a central role in controlling body activities and maintaining coordination between organs. In this unit, we focus on the peripheral nervous system (PNS), which includes the nerves that connect the central nervous system to the rest of the body, and the special senses, which allow the body to perceive and respond to external stimuli. Understanding these systems is vital in pharmacology, especially in the context of nerve-targeting drugs, sensory disorders, and autonomic regulation.
what is Peripheral Nervous System?
The Peripheral Nervous System (PNS) is a crucial part of the nervous system. While the Central Nervous System (CNS) consists of the brain and spinal cord, the PNS comprises all the nerves and ganglia that lie outside the brain and spinal cord.
Think of it this way: if the CNS is the main highway, the PNS is all the connecting roads and pathways that branch out to every corner of your body.
Here’s a breakdown of its key aspects:
What it does:
- Connects CNS to the rest of the body: The primary function of the PNS is to serve as a communication network, relaying messages to and from the brain and spinal cord to the limbs, organs, glands, and skin.
- Sensory information: It carries information from sensory receptors (in your eyes, ears, nose, tongue, and skin) to the brain, allowing you to experience the world around you (sight, sound, smell, taste, touch, pain, temperature).
- Motor control: It transmits signals from the brain and spinal cord to muscles, enabling voluntary movements like walking, talking, or picking something up. It also controls involuntary actions like reflexes.
- Autonomic functions: It manages vital, unconscious processes that keep your body running, such as heart rate, breathing, digestion, blood pressure, and sweating.
Divisions of the PNS:
Peripheral Nervous System and Special Senses (PNS) is generally divided into two main functional components:
- Somatic Nervous System (SNS): This system is responsible for voluntary actions and conscious control.
- Sensory (afferent) nerves: Carry information from your senses (skin, muscles, joints) to the CNS.
- Motor (efferent) nerves: Carry commands from the CNS to your skeletal muscles, allowing for movement.
- Autonomic Nervous System (ANS): This system controls the involuntary body functions. It operates largely without conscious thought. The ANS is further divided into two parts that often have opposing effects:
- Sympathetic Nervous System: Prepares the body for “fight or flight” responses in stressful or dangerous situations. It increases heart rate, dilates pupils, and diverts blood flow to muscles.
- Parasympathetic Nervous System: Promotes “rest and digest” activities, conserving energy and maintaining normal body functions when you are calm. It slows heart rate, constricts pupils, and stimulates digestion.
Nerves in the PNS:
The PNS includes:
- Cranial Nerves: 12 pairs of nerves that emerge directly from the brain (with some exceptions) and primarily control functions in the head and neck, such as facial expressions, vision, hearing, and taste.
- Spinal Nerves: 31 pairs of nerves that branch out from the spinal cord to the rest of the body, including the limbs, trunk, and internal organs.
what is Special Senses?
In the realm of human physiology, “Special Senses” refer to a distinct group of senses that possess highly specialized organs devoted to their detection and processing. This is in contrast to the “General Senses” (like touch, pain, temperature, pressure, and proprioception), which are detected by simpler, more widely distributed receptors throughout the body, particularly in the skin and internal organs.
The key distinguishing feature of special senses is the presence of dedicated, complex sensory organs located in specific regions of the head.
The five traditional special senses are:
- Vision (Sight):
- Mechanism: Specialized photoreceptor cells (rods and cones) in the retina detect light, which is then converted into electrical signals and sent to the brain for interpretation as images.
- Function: Allows us to perceive light, color, shape, depth, and movement, enabling us to navigate and understand our visual environment.
- Hearing (Audition):
- Mechanism: Sound waves cause vibrations in the eardrum and tiny bones (ossicles) in the middle ear, which are then transmitted to fluid in the inner ear. Hair cells in the cochlea convert these fluid movements into electrical signals, sent to the brain.
- Function: Enables us to detect sound, including speech, music, and environmental noises, crucial for communication and awareness of surroundings.
- Equilibrium (Balance):
- Mechanism: Hair cells in the vestibular system detect changes in head position, acceleration, and rotation, providing information about our body’s orientation in space.
- Function: Crucial for maintaining balance, posture, and coordinating eye movements during head movements. It helps prevent dizziness and falls.
- Smell (Olfaction):
- Mechanism: Olfactory receptor neurons have specialized receptors that bind to airborne chemical molecules (odorants), triggering electrical signals that are sent to the olfactory bulb in the brain.
- Function: Allows us to detect and differentiate a vast array of smells, which plays a role in identifying food, detecting danger, and influencing emotions and memories.
- Taste (Gustation):
- Mechanism: Taste receptor cells within taste buds chemically react with dissolved molecules (tastants) from food and beverages, leading to the perception of five basic tastes: sweet, sour, salty, bitter, and umami.
- Function: Enables us to assess the palatability and nutritional value of food, and also to detect potentially harmful substances.
Why the Distinction Matters:
The classification of senses into “special” and “general” is important in anatomy and neurophysiology because:
- Dedicated Organs: Special senses have unique, complex anatomical structures built specifically for their function.
- Cranial Nerve Innervation: Information from special senses is typically transmitted to the brain via specific cranial nerves (e.g., optic nerve for vision, vestibulocochlear nerve for hearing and balance, olfactory nerve for smell, facial/glossopharyngeal/vagus nerves for taste). General senses, in contrast, primarily use spinal nerves.
- Distinct Processing Pathways: The neural pathways in the brain for processing special senses are highly specific and often involve specialized cortical areas.
Frequently Asked Questions (FAQs)
1. What is the function of the peripheral nervous system?
The PNS connects the CNS to the body, transmitting sensory input and motor output to regulate bodily functions.
2. What is the difference between sympathetic and parasympathetic systems?
The sympathetic system triggers the body’s stress response, while the parasympathetic system maintains rest and recovery processes.
3. How many cranial and spinal nerves are there?
There are 12 cranial nerves and 31 spinal nerves in the human body.
4. What are special senses?
Special senses include vision, hearing, smell, taste, and balance, each associated with specialized organs.
5. Why are special senses important in pharmacy?
Many drugs affect sensory functions, and understanding these systems helps in treating sensory disorders and managing side effects.