Nervous System: Peripheral Nervous System Anatomy

The peripheral nervous system (PNS) is the part of the nervous system that lies outside the central nervous system (CNS), which includes the brain and spinal cord. It consists of all the nerves that transmit information between the CNS and the rest of the body, including the muscles, organs, and skin.

The PNS is divided into two major components: the somatic nervous system and the autonomic nervous system. The somatic nervous system controls voluntary movements and sensory perception, while the autonomic nervous system controls involuntary actions such as heart rate, digestion, and breathing.

The somatic nervous system includes 12 pairs of cranial nerves and 31 pairs of spinal nerves. These nerves are responsible for carrying sensory information from the body to the CNS and for transmitting motor signals from the CNS to the muscles. The somatic nervous system also includes specialized receptors in the skin, muscles, and joints, which detect changes in temperature, pressure, and position.

The autonomic nervous system is further divided into the sympathetic and parasympathetic systems. The sympathetic system is responsible for the "fight or flight" response, which prepares the body for physical activity and stress. It increases heart rate, dilates the pupils, and diverts blood flow to the muscles. The parasympathetic system, on the other hand, is responsible for the "rest and digest" response, which promotes relaxation and digestion. It decreases heart rate, constricts the pupils, and stimulates digestion.

Both the sympathetic and parasympathetic systems work together to maintain homeostasis, or the balance of the body's internal environment. For example, during exercise, the sympathetic system increases heart rate and breathing rate to provide oxygen and nutrients to the muscles. After exercise, the parasympathetic system slows down heart rate and breathing rate to conserve energy and allow for recovery.

In addition to the somatic and autonomic nervous systems, the PNS also includes specialized sensory receptors, such as photoreceptors in the eyes, chemoreceptors in the nose and mouth, and mechanoreceptors in the skin and inner ear. These receptors convert physical stimuli into electrical signals that are transmitted to the CNS for processing and interpretation.

Branches of the PNS

While the CNS includes the brain and spinal cord, the PNS is responsible for transmitting sensory and motor signals between the CNS and the rest of the body. The sensory division sends signals to the brain, while the motor division is the response from the brain.  The motor division of the PNS is further divided into two major branches: the somatic nervous system and the autonomic nervous system.

Sensory Division

The sensory division of the PNS is responsible for gathering sensory information from both the external environment and the internal body organs. It can be further subdivided into two categories: somatic sensory and visceral sensory.

Somatic Sensory

The somatic sensory is the division collects sensory information from the skin, muscles, joints, and other external sense organs. It enables us to perceive and respond to various stimuli such as touch, temperature, pressure, and pain. Somatic sensory information is relayed to the CNS via sensory neurons.

Visceral Sensory

The visceral sensory division receives sensory information from the internal organs, including the heart, lungs, stomach, and intestines. It monitors the internal environment and provides feedback to the CNS about organ function, blood pressure, pH levels, and other internal conditions. Visceral sensory information is conveyed through sensory neurons specialized for these organs.

Motor Division

The motor division of the PNS is responsible for transmitting signals from the CNS to the muscles and glands throughout the body. Similar to the sensory division, it can be further divided into somatic motor and visceral motor divisions.

The Somatic Nervous System

The somatic nervous system is responsible for controlling voluntary muscle movement and transmitting sensory information to the CNS. It is composed of motor neurons that innervate skeletal muscles and sensory neurons that relay information from sensory receptors in the skin, muscles, and joints to the CNS. The somatic nervous system controls voluntary movements, such as walking, running, and typing, and is also responsible for reflex actions, which are rapid, involuntary responses to a stimulus.

The Autonomic Nervous System

The autonomic nervous system is responsible for regulating involuntary processes such as heart rate, digestion, and respiration. It is further divided into two branches: the sympathetic nervous system and the parasympathetic nervous system.

The Sympathetic Nervous System

The sympathetic nervous system is activated during the "fight or flight" response, which prepares the body to respond to a threat or stressor. It increases heart rate and blood pressure, dilates airways, constricts blood vessels in non-essential organs such as the digestive system, and releases glucose into the bloodstream to provide energy for the muscles. The sympathetic nervous system also releases adrenaline and noradrenaline, which stimulate the body to increase its activity levels.

The Parasympathetic Nervous System

The parasympathetic nervous system is activated during times of rest and relaxation. It decreases heart rate and blood pressure, constricts airways, and increases blood flow to the digestive system. The parasympathetic nervous system is responsible for activities such as digestion, urination, and defecation, and helps the body conserve energy.

The peripheral nervous system is composed of the somatic and autonomic nervous systems. The somatic nervous system controls voluntary muscle movement and relays sensory information to the CNS, while the autonomic nervous system regulates involuntary processes such as heart rate, digestion, and respiration. The autonomic nervous system is further divided into the sympathetic and parasympathetic branches, which work in opposition to maintain a balance in the body's physiological processes.

Special Senses

The human body has five special senses which include vision, hearing, taste, smell, and equilibrium. These senses are special because they are highly evolved and allow us to interact with our environment in complex ways. In this article, we will discuss each of the special senses in detail, focusing on their anatomical structures and physiological processes.

Eye Sight

Vision is the special sense that allows us to see and interpret the visual world around us. The eyes are the organs of vision and are located in the bony sockets of the skull called the orbits. The eye is made up of several layers, including the sclera, choroid, retina, and the lens. The sclera is the white outer layer that forms the tough outer shell of the eye. The choroid is the layer of blood vessels that nourish the eye. The retina is the innermost layer that contains the photoreceptor cells responsible for detecting light and converting it into electrical signals. The lens is a flexible structure that focuses the light entering the eye onto the retina.

The physiological process of vision begins when light enters the eye and is focused onto the retina by the lens. The photoreceptor cells in the retina, known as rods and cones, detect the light and convert it into electrical signals. These signals are then transmitted to the brain via the optic nerve, where they are processed and interpreted as visual images.

Anatomy of the Eye

The eye is a complex and highly specialized organ responsible for the sense of vision. Its structure is designed to capture and focus light onto a specialized layer of cells called the retina, which contains photoreceptors that convert light energy into neural signals that can be processed by the brain.

The eye is composed of several distinct structures, including the cornea, pupil, iris, lens, ciliary body, retina, optic nerve, and various supporting structures such as the sclera and conjunctiva.

·         The conjunctiva is a clear membrane that covers the most superficial anterior layer of the eye and the interior wall of eyelid.  Protecting against debris entering the eye. 

·         The sclera is the white fibrous covering of the eye making up the external wall of the eye.

·         The choroid is a blood-filled tissue that is just under the sclera.  It anchors the ciliary body and provides nutrients to the eye.

·         The cornea is a clear, dome-shaped structure that covers the front of the eye, and its function is to help refract light and protect the eye from injury.

·         The aqueous body is a fluid body filling the space behind the cornea.

·         The pupil is a small opening in the center of the iris, which controls the amount of light that enters the eye.

·         The iris is a colored muscle that surrounds the pupil and controls its size in response to changes in light intensity.

·         The lens is a transparent structure located behind the iris that helps focus light onto the retina.

·         The ciliary body is a ring of muscles that surrounds the lens and helps adjust its shape to enable the eye to focus on objects at different distances.

·         The virtuous humor is a large jelly like fluid that fills the bulk of the eye between the lens and retina.  It holds the retina in place.

·         The retina is a thin layer of cells at the back of the eye that contains photoreceptors, which are specialized cells that convert light energy into neural signals.

·         The optic disc, also known as the blind spot, is located where the retina and optic nerve connect.

·         The fovea centralis is the point to the side of the optic disc which has the greatest visual acuity.

·         The optic nerve is a bundle of nerve fibers that carries these signals from the retina to the brain.

·         The lacrimal gland is the tear producing gland that sits on the superior-lateral edge of the eye.  The tear production lubricates and debrides the eye.  These sections drain through the lacrimal duct to the nose.

Function of the Eye

The process of vision begins when light enters the eye through the cornea and is refracted (bent) towards the lens. The lens then further refracts the light, focusing it onto the retina. The retina contains two types of photoreceptors - rods and cones. Rods are responsible for detecting light in low-light conditions, while cones are responsible for color vision and work best in bright light. When light strikes the photoreceptors, they undergo a series of chemical reactions that generate neural signals that are transmitted to the brain via the optic nerve. Once the neural signals reach the brain, they are processed and interpreted to form a visual image. This process involves a complex network of neurons in the visual cortex, which is located at the back of the brain. The brain also integrates visual information with other sensory inputs, such as sound and touch, to help us perceive our environment and navigate through it.

The eye is a complex organ that plays a critical role in our sense of vision. Its structure is designed to capture and focus light onto the retina, where photoreceptors convert it into neural signals that are transmitted to the brain. This process allows us to perceive the world around us and navigate through it.

Hearing

Hearing is the special sense that allows us to detect and interpret sound waves. The ears are the organs of hearing and are divided into three parts: the outer ear, middle ear, and inner ear. The outer ear consists of the pinna and the ear canal. The pinna is the external part of the ear that helps to collect sound waves and direct them into the ear canal. The ear canal is a narrow tube that extends from the pinna to the eardrum. The middle ear consists of three tiny bones called the ossicles, which amplify and transmit sound vibrations from the eardrum to the inner ear. The inner ear is the sensory organ that contains the cochlea, a spiral-shaped structure that contains hair cells responsible for detecting sound waves.

The physiological process of hearing begins when sound waves enter the ear canal and strike the eardrum, causing it to vibrate. The vibration is then transmitted to the ossicles, which amplify and transmit the sound vibrations to the cochlea. The hair cells in the cochlea detect the sound vibrations and convert them into electrical signals, which are then transmitted to the brain via the auditory nerve, where they are processed and interpreted as sound.

Anatomy of the Ear

The ear is the sensory organ responsible for hearing and balance in the human body. It is composed of three main parts: the outer ear, the middle ear, and the inner ear.

·         The outer ear is the visible part of the ear, including the pinna, or the external ear flap, and the ear canal.

o    The pinna, also called the auricle, serves to collect sound waves from the environment and funnel them into the ear canal.

o    The external acoustic meatus (ear canal), in turn, is lined with hairs and glands that produce earwax, which helps to protect the ear from foreign particles and infection.

·         The middle ear is located between the eardrum and the inner ear. It contains three small bones, known as the ossicles, which transmit vibrations from the eardrum to the inner ear.

o    The ossicles consist of the malleus (hammer), incus (anvil), and stapes (stirrup). These bones work together to amplify and transmit sound waves through the middle ear.

o    The Eustachian tubes regulate the pressure of the middle ear and drain excess fluid into the nasopharynx.

·         The inner ear is the most complex part of the ear and is responsible for converting sound waves into electrical impulses that the brain can interpret as sound. It is also involved in the sense of balance. The inner ear is composed of two main structures: the cochlea and the vestibular system.

o    The cochlea is a spiral-shaped structure filled with fluid and lined with tiny hair cells that are sensitive to different frequencies of sound. As sound waves enter the cochlea, they cause the fluid to move, which stimulates the hair cells to send electrical signals to the brain. The brain then interprets these signals as sound.

o    The vestibule is located next to the cochlea and is responsible for detecting changes in the position and movement of the head. It consists of three semicircular canals and two otolith organs.

§  The semicircular canals detect rotational movements of the head (dynamic equilibrium).

§  The otolith organs detect linear movements and changes in head position (static equilibrium).

The ear is a complex and highly specialized organ that plays a critical role in our ability to hear and maintain our sense of balance.

Function of the Ear

The physiology of the ear involves a complex system that enables us to hear and perceive sound. It can be divided into three main sections: the outer ear, middle ear, and inner ear.

The outer ear consists of the visible part called the pinna (or auricle) and the ear canal. The pinna helps in collecting sound waves and funneling them into the ear canal. As sound waves enter the ear canal, they travel towards the middle ear.

The middle ear is an air-filled chamber located between the eardrum (tympanic membrane) and the inner ear. It contains three small bones known as the ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). When sound waves reach the eardrum, they cause it to vibrate. These vibrations are transmitted through the ossicles, which act as a lever system. The malleus is connected to the eardrum and transfers the vibrations to the incus, which, in turn, transfers them to the stapes. The stapes then amplifies the vibrations and transmits them to the inner ear.

The inner ear is a fluid-filled structure consisting of the cochlea and the vestibular system. The cochlea is responsible for hearing, while the vestibular system is involved in balance and spatial orientation. Within the cochlea, the vibrations from the ossicles are transformed into electrical signals that can be interpreted by the brain. The cochlea contains a spiral-shaped structure called the basilar membrane, which is lined with tiny hair cells. These hair cells are responsible for converting the mechanical vibrations into electrical signals. As the vibrations travel through the cochlear fluid, they cause the basilar membrane to move. This movement stimulates the hair cells, which in turn generate electrical signals. The hair cells are tuned to specific frequencies, allowing them to respond to different pitches of sound. The electrical signals generated by the hair cells are then transmitted through the auditory nerve to the brain, specifically the auditory cortex. In the auditory cortex, the signals are processed and interpreted, allowing us to perceive and understand sound.

Equilibrium 

Equilibrium is the sense that allows us to maintain our body position and orientation in space. The sensory organ responsible for balance is the vestibular apparatus, which is located in the inner ear. The vestibular apparatus contains three semicircular canals that detect rotational movements of the head and two otolith organs that detect linear movements and gravity. Information from the vestibular apparatus is transmitted to the brain via the vestibular nerve.

The sense of equilibrium is the physiological system responsible for maintaining balance and spatial orientation. It includes two types of sensory organs: the vestibular system, which is located in the inner ear, and the proprioceptive system, which is located in the muscles and joints.

·         The vestibular system is composed of the semicircular canals and the otolith organs. The semicircular canals are three fluid-filled tubes that are oriented in three planes of space, and they are responsible for detecting rotational movements of the head. The otolith organs, which include the utricle and the saccule, are responsible for detecting linear accelerations and changes in head position.

o    The semicircular canals contain specialized hair cells that are embedded in a gelatinous structure called the cupula. When the head rotates, the fluid in the canals also moves, causing the cupula to bend and activate the hair cells. The hair cells then send signals to the brainstem and cerebellum, which process the information and generate motor commands to maintain balance. Semicircular canals are responsible for processing dynamic equilibrium movements such as rotation.

o    The otolith organs, inside the vestibule, contain hair cells that are embedded in a gelatinous structure called the otolithic membrane, which is covered with tiny calcium carbonate crystals called otoliths or otoconia. When the head moves linearly or changes position, the otolithic membrane shifts, causing the hair cells to bend and activate, these movements are called static equilibrium. This sends signals to the brainstem and cerebellum, which also generate motor commands to maintain balance.

·         The proprioceptive system is responsible for providing information about the position and movement of the body in space. This system includes specialized sensory receptors called proprioceptors, which are located in the muscles, tendons, and joints. When the body moves, these receptors send signals to the brain, which processes the information and generates motor commands to maintain balance.

The sense of equilibrium is an intricate and complex system that involves multiple sensory organs and motor pathways. It is essential for maintaining balance and spatial orientation during various activities, such as walking, running, and even standing still. Dysfunction of this system can result in various balance disorders, such as vertigo, dizziness, and falls, which can significantly impair an individual's quality of life.

Taste

Taste is the special sense that allows us to detect and interpret different flavors of food. The tongue is the organ of taste and is covered in small bumps called papillae, which contain taste buds. Taste buds are clusters of specialized cells that detect different tastes such as sweet, sour, bitter, and salty. The physiological process of taste begins when food enters the mouth and comes into contact with the taste buds on the tongue. The taste buds detect the different flavors and send signals to the brain via the gustatory nerve, where they are processed and interpreted as taste.

Anatomy of the Tongue

The tongue is a muscular organ located in the oral cavity that plays a crucial role in various functions, including speech, mastication, and taste perception. In this response, we will focus on the structure and function of the tongue, with a particular emphasis on gustatory cells.

The tongue is a highly specialized muscular organ that is divided into different regions, each with specific functions. It is covered by a mucous membrane that contains numerous papillae, small bumps on the tongue's surface that house the taste buds. There are three types of papillae: filiform, fungiform, and circumvallate.

Figure 157: Tongue anatomy, OpenStax

·         Filiform Papillae are the most abundant papillae on the tongue's surface and are responsible for detecting texture and temperature. They are also responsible for providing friction to move food particles around the mouth.

·         Foliate papillae are leaf-like ridges found on the sides of the tongue. They appear as a series of vertical folds or grooves. They are located on the lateral edges of the tongue, towards the back. These papillae contain taste buds, which are responsible for detecting different tastes.

·         Fungiform Papillae are mushroom-shaped papillae located on the upper surface of the tongue, mainly towards the tip. Each of these papillae contains several taste buds containing gustatory cells which are responsible for detecting sweet, sour, salty, bitter, umami, and potentially carbohydrate and lipid tastes.

o    Each taste bud contains 50-100 gustatory cells, which are classified into three types: type I, type II, and type III. Type I cells are supporting cells that do not respond to tastes. Type II cells respond to sweet, bitter, and umami tastes. Type III cells respond to sour and salty tastes.

o    When a gustatory cell is stimulated, it triggers a series of events that result in the release of neurotransmitters. These neurotransmitters stimulate the gustatory nerve fibers, which send signals to the brain, where they are interpreted as specific tastes.

·         Circumvallate Papillae are the largest papillae on the tongue and are arranged in a V-shape at the back of the tongue. Each of these papillae contains up to 100 taste buds and is responsible for detecting bitter taste.

Function of the Tongue

The tongue is composed of various muscles that are responsible for its movement and function. These muscles are divided into intrinsic and extrinsic muscles. The intrinsic muscles are responsible for shaping the tongue, while the extrinsic muscles move the tongue in different directions.

The tongue plays a crucial role in various functions, including speech, mastication, and taste perception.

·         Speech: The tongue plays a vital role in forming sounds and words by altering its shape and position. The tongue's movements help to produce different sounds and make it possible to speak different languages.

·         Mastication: The tongue helps in moving food around the mouth during chewing, which breaks down the food into smaller particles that can be easily swallowed.

·         Taste Perception: The tongue houses the taste buds, which are responsible for detecting different tastes. There are five primary tastes: sweet, sour, salty, bitter, and umami (savory). There are two additional taste buds that are in dispute, carbohydrate and lipid taste buds, further evidence is still pending.  Each taste bud contains specialized gustatory cells that respond to specific tastes.

The tongue is a highly specialized muscular organ that plays a crucial role in various functions, including speech, mastication, and taste perception. The tongue's taste perception is achieved through the specialized gustatory cells located in the taste buds, which respond to different tastes and send signals to the brain for interpretation.

Olfactory

Olfactory (smell) is the special sense that allows us to detect and interpret different odors in the environment. The nose is the organ of smell and is divided into two nasal cavities separated by the septum. The nasal cavity is lined with a mucous membrane that contains specialized cells called olfactory receptors, which detect different odors. The physiological process of smell begins when odor molecules enter the nasal cavity and come into contact with the olfactory receptors. The receptors detect the different odors and send signals to the brain via the olfactory nerve, where they are processed and interpreted as smell.

Anatomy of the Nose

The nose is an essential organ of the respiratory system, responsible for breathing air into the body and filtering it before it reaches the lungs. It is also the primary organ of the sense of smell, or olfaction. The structure and function of the nose are complex and interrelated, and involve multiple layers and regions.

The nose is made up of several structures, including the nasal cavity, the septum, the sinuses, and the olfactory epithelium.

·         The naris, also known as the nostrils, are the external openings of the nose through which air enters and exits.

·         The nasal concha, also called turbinates, are bony structures covered with nasal mucosa that extend from the lateral walls of the nasal cavity. There are three pairs of nasal conchae: superior, middle, and inferior.

·         The nasal meatus are the spaces between the nasal conchae. They help to warm, humidify, and filter the inhaled air.

·         The nasal cavity is the main airway into the body, consisting of two openings called nostrils, or nares, that are separated by the septum, a thin layer of bone and cartilage that runs down the middle of the nose.

·         The sinuses are air-filled cavities within the skull that are lined with mucous membranes. Each type of sinus is located in a specific area of the skull and has unique features and functions. The sinuses are lined with ciliated epithelium and produce mucus, which drains into the nasal cavity.  The function of the sinuses is not entirely clear, but they may play a role in warming and humidifying inspired air and providing resonance for the voice.

o    The frontal sinuses are located within the frontal bone of the skull, just above the eyebrows. They are typically present at birth but do not fully develop until around the age of 7. 

o    The maxillary sinuses are the largest of the paranasal sinuses and are located within the maxillary bones of the skull, above the upper teeth. They are present at birth but do not fully develop until the teenage years.

o    The ethmoid sinuses are located within the ethmoid bone of the skull, between the eyes. There are two groups of ethmoid sinuses: anterior and posterior. The anterior ethmoid sinuses are present at birth, while the posterior ethmoid sinuses do not fully develop until around the age of 12.

o    The sphenoid sinuses are located within the sphenoid bone of the skull, behind the ethmoid sinuses. They are typically present at birth but do not fully develop until adolescence.

·         The nasal septum is a wall composed of bone and cartilage that divides the nasal cavity into left and right sides. The nasal septum is lined with a mucous membrane.

·         The olfactory nerves, also called olfactory epithelium, is a specialized tissue located at the top of the nasal cavity that contains the receptors for smell. This tissue contains several types of cells, including olfactory sensory neurons, supporting cells, and basal cells. Olfactory sensory neurons are responsible for detecting odor molecules in the air and sending signals to the brain, while supporting cells provide structural support and help to maintain the health of the olfactory epithelium. Basal cells are stem cells that can differentiate into olfactory sensory neurons or supporting cells as needed.

Function of the Nose

The nose performs several functions, including breathing, filtering, warming, and humidifying the air, and olfaction (sense of smell). When air enters the nose, it first passes through the nostrils and into the nasal cavity. As it does, tiny hairs called cilia lining the inside of the nose filter out dust, pollen, and other particles, while mucus traps bacteria and other pathogens. As air moves through the nasal cavity, it also comes into contact with the sinuses, which help to warm and humidify the air before it reaches the lungs. The sinuses contain a layer of mucus that traps moisture and heat, which is then transferred to the air as it passes through.

The olfactory epithelium is responsible for detecting odor molecules in the air. When odor molecules bind to receptors on olfactory sensory neurons, they trigger an electrical signal that is transmitted to the brain via the olfactory nerve. The brain then processes this signal and interprets it as a particular odor. In addition to its role in detecting odors, the nose also plays a key role in the sense of taste. The tongue and other taste receptors in the mouth can only detect five basic tastes (sweet, sour, bitter, salty, and umami), but the sense of smell can enhance and enrich these flavors by detecting the complex aromas that are released as food is chewed and swallowed.

Overview

The nervous system is a complex network that includes the central nervous system (CNS) and the peripheral nervous system (PNS). The peripheral nervous system lies outside the CNS and consists of nerves that transmit information between the CNS and the rest of the body, including muscles, organs, and skin. The PNS is divided into the somatic nervous system and the autonomic nervous system.

The somatic nervous system controls voluntary movements and sensory perception, while the autonomic nervous system controls involuntary actions like heart rate, digestion, and breathing. The somatic nervous system includes cranial nerves and spinal nerves responsible for carrying sensory information to the CNS and transmitting motor signals from the CNS to muscles. It also includes specialized receptors in the skin, muscles, and joints that detect changes in temperature, pressure, and position.

The autonomic nervous system further divides into the sympathetic and parasympathetic systems. The sympathetic system triggers the "fight or flight" response, preparing the body for physical activity and stress. It increases heart rate, dilates pupils, and diverts blood flow to the muscles. The parasympathetic system triggers the "rest and digest" response, promoting relaxation and digestion. It decreases heart rate, constricts pupils, and stimulates digestion. Both systems work together to maintain homeostasis, balancing the body's internal environment.

The PNS also includes specialized sensory receptors, such as photoreceptors in the eyes, chemoreceptors in the nose and mouth, and mechanoreceptors in the skin and inner ear. These receptors convert physical stimuli into electrical signals transmitted to the CNS for processing.

Understanding the structure and function of the PNS is crucial for health professionals in various fields, including medicine, physical therapy, and sports science. Additionally, the PNS includes 12 pairs of cranial nerves responsible for transmitting sensory, motor, and autonomic signals to different parts of the head, neck, and trunk. Each cranial nerve has specific functions and plays a crucial role in various sensory and motor processes.

The special senses, including vision, hearing, taste, smell, and equilibrium, allow humans to interact with their environment in complex ways. The eye, as the organ of vision, captures and focuses light onto the retina, which contains photoreceptor cells responsible for detecting light and converting it into electrical signals. The ear, as the organ of hearing, consists of the outer ear, middle ear, and inner ear, working together to detect and interpret sound waves.

A detailed understanding of the anatomy and physiological processes of the special senses is essential for diagnosing and treating neurological disorders and maintaining overall health.