Peripheral Nervous System

Anatomy and physiology

1. Peripheral Nervous System & Special Senses Subject: Human anatomy and physiology-I Unit-IV Prepared by: Kajale Fulchand V . (M.Pharm Pharmacology) Assistant Professor Shivai Charitable trust’s College of Pharmacy .

2. Classification of peripheral nervous system

3. Central Nervous System The Central nervous system (CNS), consisting of the brain and the spinal cord the Peripheral Nervous System (PNS), consisting of all the nerves outside the brain and spinal cord. The PNS comprises paired cranial and sacral nerves Some of these are sensory ( afferent ) transmitting impulses to the CNS, some are motor ( efferent ) transmitting impulses from the CNS and others are mixed . It is useful to consider two functional parts within the PNS. The sensory division ( afferent ) The motor division ( efferent ) The motor division has two parts: the somatic nervous system, which controls voluntary movement of skeletal muscles The autonomic nervous system, controlling involuntary processes such as heartbeat, peristalsis and glandular activity. The autonomic nervous system has two divisions : Sympathetic and Parasympathetic.

4. Peripheral Nervous System 31 pairs of spinal nerves that originate from the spinal cord 12 pairs of cranial nerves , which originate from the brain Spinal Nerves 8 Cervical, 12 Thoracic, 5 Lumbar, 5 Sacral, 1 Coccygeal. Nerve roots The spinal nerves arise from both sides of the spinal cord and emerge through the intervertebral foramina. Each nerve is formed by the union of a motor (anterior) and a sensory (posterior) nerve root and is, therefore, a mixed nerve.

5. Plexuses In the cervical, lumbar and sacral regions the anterior rami unite near their origins to form large masses of nerves , or plexuses , where nerve fibres are regrouped and rearranged before proceeding to supply skin, bones, muscles and joints of a particular area. Cervical Plexuses Brachial Plexuses Lumbar Plexuses Sacral Plexuses Coccygeal Plexuses

6. Cranial Nerves I. Olfactory : sensory (smell) II. Optic : sensory (vision) III. Oculomotor : motor (eye movement) IV. Trochlear : motor (eye) V. Trigeminal : mixed (face & mouth) VI. Abducens : motor (rectus muscle of eye.) VII. Facial : mixed (expression) VIII. Vestibulocochlear : sensory (balance & hearing) IX. Glossopharyngeal : mixed (head & neck) X. Vagus : mixed (heart) XI. Accessory : motor (shoulder & neck) XII. Hypoglossal : motor. (tongue)

7. Autonomic Nervous System The autonomic or involuntary part of the nervous system controls involuntary body functions. Sympathetic Nervous System Parasympathetic Nervous System The two divisions work in an integrated and complementary manner to maintain involuntary functions and homeostasis. Such activities include coordination and control of breathing, blood pressure, water balance, digestion and metabolic rate. Smooth muscle , which controls the diameter of smaller airways and blood vessels Cardiac muscle , which controls the rate and force of cardiac contraction Glands that control the volumes of gastrointestinal secretions

8. 1. Sympathetic Nervous System (Fight or flight) The Preganglionic neuron. This has its cell body in the lateral column of grey matter in the spinal cord between the levels of the 1st thoracic and 2nd or 3rd lumbar vertebrae. Acetylcholine is the neurotransmitter at sympathetic ganglia. The Postganglionic neuron. This has its cell body in a ganglion and terminates in the organ or tissue supplied. Noradrenaline (norepinephrine) is usually the neurotransmitter at sympathetic effector organs. The major exception is that there is no parasympathetic supply to the sweat glands , the skin and blood vessels of skeletal muscles. These structures are supplied by only sympathetic postganglionic neurons, which are known as sympathetic cholinergic nerves and usually have acetylcholine as their neurotransmitter .

9. 2. Parasympathetic Nervous System (Rest & digest) The Preganglionic neuron . This is usually long in comparison to its counterpart in the sympathetic nervous system and has its cell body either in the brain or in the spinal cord The Postganglionic neuron . This is usually very short and has its cell body either in a ganglion or, more often, in the wall of the organ supplied. Functions of The Autonomic Nervous System Sympathetic stimulation causes the adrenal glands to secrete the hormones adrenaline (epinephrine) and noradrenaline (norepinephrine) into the bloodstream. Parasympathetic stimulation has a tendency to slow down cardiac and respiratory activity but it stimulates digestion and absorption of food and the functions of the genitourinary systems.

10. Special Senses 1. Olfaction: Sense of Smell

11. The receptors for the Sense of smell or olfaction are located in the olfactory epithelium of the nose. The olfactory epithelium consists of three kinds of cells: olfactory receptor cells, supporting cells, and basal cells. Chemicals that bind to and stimulate the olfactory receptors in the olfactory cilia are called Odorants . Supporting cells are columnar epithelial cells of the mucous membrane lining the nose. They provide physical support, nourishment , and electrical insulation for the olfactory receptor cells and help detoxify chemicals that come in contact with the olfactory epithelium. Basal cells are stem cells located between the bases of the supporting cells. They continually undergo cell division to produce new olfactory receptor cells, which live for only about two months before being replaced. Within the connective tissue that supports the olfactory epithelium are olfactory glands or Bowman’s glands , which produce mucus that is carried to the surface of the epithelium by ducts. The secretion moistens the surface of the olfactory epithelium and dissolves odorants so that transduction can occur.

12. Both supporting cells of the nasal epithelium and olfactory glands are innervated by Parasympathetic neurons within branches of the facial (VII) nerve , which can be stimulated by certain chemicals. Physiology of Smell (Olfaction) Olfactory receptors react to odorant molecules in the same way that most sensory receptors react to their specific stimuli A receptor potential (depolarization) develops and triggers one or more nerve impulses. This process, called olfactory transduction , occurs in the following way: Binding of an odorant to an olfactory receptor protein in an olfactory cilium stimulates a membrane protein called a G protein . The G protein, in turn, activates the enzyme adenylyl cyclase to produce a substance called cyclic adenosine monophosphate (cAMP), a type of second messenger. The cAMP opens a cation channel that allows Na + and Ca2 + to enter the cytosol, which causes a depolarizing receptor potential to form in the membrane of the olfactory receptor cell. If the depolarization reaches threshold, an action potential is generated along the axon of the olfactory receptor cell.

13. 2. Gustation: Sense of Taste Like olfaction, gustation, or taste, is a chemical sense. However, gustation is much simpler than olfaction in that only five primary tastes can be distinguished: salty, sour, sweet, bitter, and umami . Salty taste is caused by the presence of sodium ions (Na+) in food. A common dietary source of Na+ is NaCl. Sour taste is produced by hydrogen ions (H+) released from acids. Lemons have a sour taste because they contain citric acid.

14. Sweet taste is elicited by sugars such as glucose, fructose, and sucrose and by artificial sweeteners such as saccharin , aspartame , and sucralose . Bitter taste is caused by a wide variety of substances, including caffeine, morphine , and quinine. many poisonous substances like strychnine have a bitter taste. When something tastes bitter, a natural response is to spit it out, a reaction that serves to protect you from ingesting potentially harmful substances. The umami taste, first reported by Japanese scientists, is described as “ meaty ” or “ savory .” Chemicals that stimulate gustatory receptor cells are known as Tastants . Once a tastant is dissolved in saliva, it can make contact with the plasma membranes of the gustatory microvilli, which are the sites of taste transduction. The result is a depolarizing receptor potential that stimulates exocytosis of synaptic vesicles from the gustatory receptor cell. In turn, the liberated neurotransmitter molecules trigger graded potentials that produce nerve impulses in the first-order sensory neurons that synapse with gustatory receptor cells.

15. 3. Vision: Eye

16. Anatomically, the wall of the eyeball consists of three layers : Fibrous Tunic, (2) Vascular Tunic (3) Retina (Inner Tunic) The Fibrous Tunic is the superficial layer of the eyeball and consists of the anterior Cornea and Posterior Sclera . The cornea is a transparent coat that covers the colored iris. The sclera is the “ White ” portion of the eye, is a layer of dense connective tissue made up mostly of collagen fibers and fibroblasts. The sclera covers the entire eyeball except the cornea; it gives shape to the eyeball, makes it more rigid , protects its inner parts, and serves as a site of attachment for the extrinsic eye muscles. The Vascular Tunic or uvea is the middle layer of the eyeball. It is composed of three parts: choroid , ciliary body , and iris . The highly vascularized choroid , which is the posterior portion of the vascular tunic, lines most of the internal surface of the sclera.

17. The third and inner layer of the eyeball, the Retina (Inner Tunic) , lines the posterior three-quarters of the eyeball and is the beginning of the visual pathway. The adult eyeball measures about 2.5 cm (1 in.) in diameter. Photoreceptors are specialized cells in the photoreceptor layer that begin the process by which light rays are ultimately converted to nerve impulses. There are two types of photoreceptors: Rods and Cones . Each retina has about 6 million cones and 120 million rods . Rods allow us to see in dim light , such as moonlight. Because rods do not provide color vision, in dim light we can see only black, white , and all shades of gray in between. Brighter lights stimulate cones , which produce color vision . Three types of cones are present in the retina: (1) Blue Cones , which are sensitive to blue light, (2) Green Cones , which are sensitive to green light, and (3) Red Cones , which are sensitive to red light. Color vision results from the stimulation of various combinations of these three types of cones.

18. Microscopic Structure of The Retina

19. Physiology of Vision The first step in visual transduction is absorption of light by a photopigment (visual pigment), a colored protein that undergoes structural changes when it absorbs light, in the outer segment of a photoreceptor. 1. Isomerization . In darkness, retinal has a bent shape, called cis-retinal, which fits snugly into the opsin portion of the Photopigment. When cis-retinal absorbs a photon of light, this cis-to-trans conversion is called isomerization and is the first step In visual Transduction . 2. Bleaching . In about a minute, trans-retinal completely separates from opsin . Retinal is responsible for the color of the photopigment, so the separation of trans-retinal from opsin causes Opsin to look colorless. Because of the color change, this part of The cycle is termed bleaching of photopigment . 3. Conversion . An enzyme called retinal isomerase converts trans-retinal back to cis-retinal. 4. Regeneration . The cis-retinal then can bind to opsin , reforming a functional photopigment. This part of the cycle resynthesis of a photopigment is called regeneration .

20. 1. In darkness , cis-retinal is the form of retinal associated with the photopigment of the photoreceptor. 2. During darkness is that there is a high concentration of cyclic GMP (cGMP) in the cytosol of the photoreceptor outer segment. This is due to the continuous production of cGMP by the enzyme guanylyl cyclase in the disc membrane. 3. After it is produced, cGMP binds to and opens nonselective cation channels in the outer segment membrane. These cGMP-gated channels mainly allow Na+ ions to enter the cell. 4. The inflow of Na+, called the dark current , depolarizes the photoreceptor. As a result, in darkness, the membrane potential of a photoreceptor is about −40 mV . This is much closer to zero than a typical neuron’s resting membrane potential of −70 mV . 5. The depolarization during darkness spreads from the outer segment to the synaptic terminal, which contains voltage-gated Ca2+ channels in its membrane. The depolarization keeps these channels open, allowing Ca2+ to enter the cell. The entry of Ca2+in turn triggers exocytosis of synaptic vesicles, resulting in tonic release of large amounts of neurotransmitter from the synaptic terminal.( glutamate )is an inhibitory neurotransmitter

21. 1. When light strikes the retina, cis-retinal undergoes isomerization to trans-retinal. 2. Isomerization of retinal causes activation of a G-protein known as Transducin that is located in the disc membrane. 3. Transducin in turn activates an enzyme called c-GMP phosphodiesterase, which is also present in the Disc membrane . 4. Once activated, c-GMP Phosphodiesterase breaks down cGMP. The breakdown of cGMP lowers the concentration of cGMP in the Cytosol of the outer segment. 5. As a result, the number of open cGMP-gated channels in the outer segment membrane is reduced and Na+ inflow decreases . 6. The decreased Na+ inflow causes the membrane potential to drop to about − 65 mV, thereby producing a hyperpolarizing receptor potential. 7. The hyperpolarization spreads from the outer segment to the synaptic terminal, causing a decrease in the number of open voltage-gated Ca2+ channels. Ca2+ entry into the cell is reduced, which decreases the release of neurotransmitter from the synaptic terminal. ( Inhibitory )

22. 4. Hearing

23. Anatomy of the Ear The ear is divided into three main regions: (1) External ear , which collects sound waves and channels them inward; (2) Middle ear , which conveys sound vibrations to the oval window and (3) Internal ear , which houses the receptors for hearing and equilibrium. 1. External (Outer) Ear The external (outer) ear consists of the auricle, external auditory canal, and eardrum. The auricle or pinna is a flap of elastic cartilage shaped like the flared end of a trumpet and covered by skin. The external auditory canal is a curved tube about 2.5 cm (1 in.) long that lies in the temporal bone and leads to the eardrum. The Tympanic membrane or eardrum is a thin, semitransparent partition between the external auditory canal and middle ear. 2. Middle Ear The middle ear is a small, air-filled cavity in the petrous portion of the temporal bone that is lined by epithelium. ( Auditory ossicles ) 3. Internal (Inner) Ear The internal (inner) ear is also called the labyrinth because of its complicated series of canals . Structurally, it consists of two main divisions: an outer bony labyrinth that encloses an inner membranous labyrinth. It is like long balloons put inside a rigid tube.

24. Physiology of Hearing 1. The auricle directs sound waves into the external auditory canal . 2. When sound waves strike the tympanic membrane , the alternating waves of high and low pressure in the air cause the tympanic membrane to vibrate back and forth .

25. 3. The central area of the tympanic membrane connects to the malleus , which vibrates along with the tympanic membrane. 4. As the stapes moves back and forth, its oval-shaped footplate , which is attached via a ligament to the circumference of the oval window, vibrates in the oval window. 5. The movement of the stapes at the oval window sets up fluid pressure waves in the perilymph of the cochlea. 6. Pressure waves are transmitted from the scala vestibuli to the scala tympani and eventually to the round window, causing it to bulge outward into the middle ear. 7. As the pressure waves deform the walls of the scalea vestibuli and scala tympani, they also push the vestibular membrane back and forth, creating pressure waves in the endolymph inside the cochlear duct. 8. The pressure waves in the endolymph cause the basilar membrane to vibrate , which moves the hair cells of the spiral organ against the tectorial membrane. This leads to bending of the stereocilia and ultimately to the generation of nerve impulses in first-order neurons in cochlear nerve fibers .

26. Reference Anatomy and Physiology in Health and Illness by Kathleen J.W. Wilson, Churchill Livingstone, New York Principles of Anatomy and Physiology by Tortora Grabowski. Palmetto, GA, U.S.A. Essentials of Medical Physiology by K. Sembulingam and P. Sembulingam. Jaypee brothers medical publishers, New Delhi. 25-12-2023 33