Nervous System (11) and Cranial Nerves (13a)

Nervous System The master controlling and communicating system of the body Functions Sensory input – monitoring stimuli occurring inside and outside the body Integration – interpretation of sensory input Motor output – response to stimuli by activating effector organs

Nervous System Figure 11.1

Organization of the Nervous System Central nervous system (CNS) Brain and spinal cord Integration and command center Peripheral nervous system (PNS) Paired spinal and cranial nerves Carries messages to and from the spinal cord and brain

Peripheral Nervous System (PNS): Two Functional Divisions Sensory (afferent) division Somatic afferent fibers – carry impulses from skin, skeletal muscles, and joints to the brain Visceral afferent fibers – transmit impulses from visceral organs to the brain Motor (efferent) division Transmits impulses from the CNS to effector organs

Motor Division: Two Main Parts Somatic motor nervous system Conscious control of skeletal muscles Autonomic motor nervous system (ANS) Regulates smooth muscle, cardiac muscle, and glands Divisions – sympathetic and parasympathetic

Histology of Nerve Tissue The two principal cell types of the nervous system are: Neurons – excitable cells that transmit electrical signals Supporting cells – cells that surround and wrap neurons

Supporting Cells: Neuroglia The supporting cells (neuroglia or glial cells): Provide a supportive scaffolding for neurons Segregate and insulate neurons Promote health and growth

Most abundant, versatile, and highly branched glial cells Astrocytes Most abundant, versatile, and highly branched glial cells They cling to neurons and their synaptic endings, and cover capillaries Functionally they: Support and brace neurons Anchor neurons to their nutrient supplies Guide migration of young neurons Control the chemical environment

Astrocytes Figure 11.3a

Microglia and Ependymal Cells Microglia – small, ovoid cells with spiny processes Phagocytes that monitor the health of neurons Ependymal cells – range in shape from squamous to columnar They line the central cavities of the brain and spinal column Help to circulate cerebrospinal fluid with cilia

Microglia and Ependymal Cells Figure 11.3b, c

Oligodendrocytes, Schwann Cells, and Satellite Cells oligodendrocytes – branched cells that wrap around nerve fibers (axons) of neurons in the CNS to form a myelin sheath Schwann cells – wrap around nerve fibers (axons) of neurons in the PNS to form a myelin sheath satellite cells - surround neuron cell bodies in the PNS

Oligodendrocytes and Schwann Cells Figure 11.3d, e

Neurons (Nerve Cells) Structural units of the nervous system Composed of a body, axon, and dendrites Long-lived, amitotic, and have a high metabolic rate Their plasma membrane functions in: Electrical signaling Cell-to-cell signaling during development

Neurons (Nerve Cells) Figure 11.4b

Nerve Cell Body (Perikaryon or Soma) Contains the nucleus and a nucleolus Is the major biosynthetic center Is the focal point for the outgrowth of neuronal processes Has well-developed Nissl bodies (rough ER) Contains an axon hillock – cone-shaped area from which axons arise

Processes Armlike extensions from the soma Called tracts in the CNS and nerves in the PNS There are two types: axons and dendrites

Dendrites of Motor Neurons Short, tapering, and diffusely branched processes They are the receptive, or input, regions of the neuron

Axons: Structure Slender processes of uniform diameter arising from the hillock Long axons are called nerve fibers Usually there is only one unbranched axon per neuron

Axons: Function Generate and transmit action potentials Secrete neurotransmitters from the axonal terminals

Myelin Sheath Whitish, fatty (protein-lipoid), segmented sheath around most long axons It functions to: Protect the axon Electrically insulate fibers from one another Increase the speed of nerve impulse transmission

Myelin Sheath and Neurilemma: Formation Formed by Schwann cells in the PNS and oligodendrocytes in the CNS A Schwann cell: Envelopes an axon in a trough Encloses the axon with its plasma membrane Has concentric layers of membrane that make up the myelin sheath Neurilemma – remaining nucleus and cytoplasm of a Schwann cell Nodes of Ranvier: Gaps in the myelin sheath between adjacent Schwann cells

Myelin Sheath and Neurilemma: Formation Figure 11.5a-c

Axons of the CNS Both myelinated and unmyelinated fibers are present Myelin sheaths are formed by oligodendrocytes Nodes of Ranvier are widely spaced

Regions of the Brain and Spinal Cord White matter – dense collections of myelinated fibers Gray matter – mostly nerve cell bodies (soma) and unmyelinated fibers

Neuron Classification Structural: Multipolar — three or more processes Bipolar — two processes (axon and dendrite) Unipolar — single, short process

Comparison of Structural Classes of Neurons Table 11.1.1

Comparison of Structural Classes of Neurons Table 11.1.2

Comparison of Structural Classes of Neurons Table 11.1.3

Neuron Classification Functional: Sensory (afferent) — transmit impulses toward the CNS Motor (efferent) — carry impulses away from the CNS Interneurons (association neurons) — shuttle signals through CNS pathways

Structure of a Nerve Nerve – cordlike organ of the PNS consisting of peripheral axons enclosed by connective tissue Connective tissue coverings include: Endoneurium – loose connective tissue that surrounds axons Perineurium – coarse connective tissue that bundles fibers into fascicles Epineurium – tough fibrous sheath around a nerve

Structure of a Nerve Figure 13.3b

Classification of Nerves Sensory and motor divisions Sensory (afferent) only – carry impulse to the CNS Motor (efferent) only – carry impulses from CNS Mixed – sensory and motor fibers carry impulses to and from CNS; most common type of nerve

Peripheral Nerves Mixed nerves – carry somatic and autonomic (visceral) impulses The four types of mixed nerves are: Somatic afferent and somatic efferent Visceral afferent and visceral efferent Peripheral nerves originate from the brain or spinal column

The 12 Cranial Nerves Twelve pairs of cranial nerves arise from the brain They may have sensory, motor, or both sensory and motor functions Each nerve is identified by a number (I through XII) and a name Four cranial nerves carry parasympathetic fibers that serve muscles and glands

Cranial Nerves Figure 13.5a

Summary of Function of Cranial Nerves Figure 13.5b

Make up your own memory device! I. O______________ VII. F_______________ II. O______________ VIII. V______________ III. O_____________ IX. G_______________ IV. T_____________ X. V________________ V. T______________ XI. A________________ VI. A_____________ XII. H_______________

Cranial Nerve I: Olfactory Functions solely by carrying afferent impulses for the sense of smell Arises from the olfactory epithelium Passes through the cribriform plate of the ethmoid bone Fibers run through the olfactory bulb and terminate in the primary olfactory cortex

Cranial Nerve I: Olfactory Figure I from Table 13.2

Cranial Nerve II: Optic Functions solely by carrying afferent impulses for vision Arises from the retina of the eye Optic nerves pass through the optic canals and converge at the optic chiasm They continue to the thalamus where they synapse From there, the optic radiation fibers run to the visual cortex

Cranial Nerve II: Optic Figure II Table 13.2

Cranial Nerve III: Oculomotor Functions in raising the eyelid, directing the eyeball, constricting the iris, and controlling lens shape Fibers extend from the ventral midbrain, pass through the superior orbital fissure, and go to the extrinsic eye muscles Parasympathetic cell bodies are in the ciliary ganglia

Cranial Nerve III: Oculomotor Figure III from Table 13.2

Cranial Nerve IV: Trochlear Functions primarily as a motor nerve that directs the eyeball Fibers emerge from the dorsal midbrain and enter the orbits via the superior orbital fissures; innervate the superior oblique muscle

Cranial Nerve IV: Trochlear Figure IV from Table 13.2

Cranial Nerve V: Trigeminal Conveys sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication Composed of three divisions: ophthalmic (V1), maxillary (V2), and mandibular (V3) Fibers run from the face to the pons via the superior orbital fissure (V1), the foramen rotundum (V2), and the foramen ovale (V3)

Cranial Nerve V: Trigeminal Figure V from Table 13.2

Cranial Nerve VI: Abducens Functions primarily as a motor nerve innervating the lateral rectus muscle Fibers leave the inferior pons and enter the orbit via the superior orbital fissure Figure VI from Table 13.2

Cranial Nerve VII: Facial Motor functions include facial expression, and the transmittal of autonomic impulses to lacrimal and salivary glands; Sensory function is taste from the anterior two-thirds of the tongue Fibers leave the pons, travel through the internal acoustic meatus, and emerge through the stylomastoid foramen to the lateral aspect of the face Mixed nerve with five major branches

Cranial Nerve VII: Facial Figure VII from Table 13.2

Cranial Nerve VIII: Vestibulocochlear Functions are solely sensory – equilibrium and hearing Fibers arise from the hearing and equilibrium apparatus of the inner ear, pass through the internal acoustic meatus, and enter the brainstem at the pons-medulla border Two divisions – cochlear (hearing) and vestibular (balance)

Cranial Nerve VIII: Vestibulocochlear Figure VIII from Table 13.2

Cranial Nerve IX: Glossopharyngeal Motor – innervates part of the tongue and pharynx, and provides motor fibers to the parotid salivary gland; Sensory – fibers conduct taste and general sensory impulses from the tongue and pharynx Fibers emerge from the medulla, leave the skull via the jugular foramen, and run to the throat Nerve IX is a mixed nerve with motor and sensory functions

Cranial Nerve IX: Glossopharyngeal Figure IX from Table 13.2

Cranial Nerve X: Vagus Motor - Most motor fibers are parasympathetic fibers to the heart, lungs, and visceral organs; Sensory function is in taste The only cranial nerve that extends beyond the head and neck Fibers emerge from the medulla via the jugular foramen The vagus is a mixed nerve

Cranial Nerve X: Vagus Figure X from Table 13.2

Cranial Nerve XI: Accessory Primarily a motor nerve: Supplies fibers to the larynx, pharynx, and soft palate Innervates the trapezius and sternocleidomastoid, which move the head and neck Formed from a cranial root emerging from the medulla and a spinal root arising from the superior region of the spinal cord The spinal root passes upward into the cranium via the foramen magnum The accessory nerve leaves the cranium via the jugular foramen

Cranial Nerve XI: Accessory Figure XI from Table 13.2

Cranial Nerve XII: Hypoglossal Innervates both extrinsic and intrinsic muscles of the tongue, which contribute to swallowing and speech Fibers arise from the medulla and exit the skull via the hypoglossal canal

Cranial Nerve XII: Hypoglossal Figure XII from Table 13.2