Chapter 12,13,14,15

1. Chapter 12,13,14,15 Nervous Tissue, Spinal Cord and Nerves, brain and cranial nerves, and Autonomic Lecture Outline

2. Structures of the Nervous System - Overview • Twelve pairs of cranial nerves emerge from the base of the brain through foramina of the skull. • A nerve is a bundle of hundreds or thousands of axons, each of which courses along a defined path and serves a specific region of the body. • The spinal cord connects to the brain through the foramen magnum of the skull and is encircled by the bones of the vertebral column. • Thirty-one pairs of spinal nerves emerge from the spinal cord, each serving a specific region of the body. • Ganglia, located outside the brain and spinal cord, are small masses of nervous tissue, containing primarily cell bodies of neurons. • Enteric plexuses help regulate the digestive system. • Sensory receptors are either parts of neurons or specialized cells that monitor changes in the internal or external environment.

3. Functions of the Nervous Systems • The sensory function of the nervous system is to sense changes in the internal and external environment through sensory receptors. • Sensory (afferent) neurons serve this function. • The integrative function is to analyze the sensory information, store some aspects, and make decisions regarding appropriate behaviors. • Association or interneurons serve this function. • The motor function is to respond to stimuli by initiating action. • Motor(efferent) neurons serve this function.

4. Nervous System Divisions • Central nervous system (CNS) • consists of the brain and spinal cord • Peripheral nervous system (PNS) • consists of cranial and spinal nerves that contain both sensory and motor fibers • connects CNS to muscles, glands & all sensory receptors

5. Subdivisions of the PNS • Somatic (voluntary) nervous system (SNS) • neurons from cutaneous and special sensory receptors to the CNS • motor neurons to skeletal muscle tissue • Autonomic (involuntary) nervous systems • sensory neurons from visceral organs to CNS • motor neurons to smooth & cardiacmuscle and glands • sympathetic division (speeds up heart rate) • parasympathetic division (slow down heart rate) • Enteric nervous system (ENS) • involuntary sensory & motor neurons control GI tract • neurons function independently of ANS & CNS

6. Neurons • Functional unit of nervous system • Have capacity to produce action potentials • electrical excitability • Cell body • single nucleus with prominent nucleolus • Nissl bodies (chromatophilic substance) • rough ER & free ribosomes for protein synthesis • neurofilaments give cell shape and support • microtubules move material inside cell • lipofuscin pigment clumps (harmless aging) • Cell processes = dendrites & axons

7. Dendrites • Conducts impulses towards the cell body • Typically short, highly branched & unmyelinated • Surfaces specialized for contact with other neurons • Contains neurofibrils & Nissl bodies

8. Axons • Conduct impulses away from cell body • Long, thin cylindrical process of cell • Arises at axon hillock • Impulses arise from initial segment (trigger zone) • Side branches (collaterals) end in fine processes called axon terminals • Swollen tips called synaptic end bulbs contain vesicles filled with neurotransmitters Synaptic boutons

9. Axonal Transport • Cell body is location for most protein synthesis • neurotransmitters & repair proteins • Axonal transport system moves substances • slow axonal flow • movement in one direction only -- away from cell body • movement at 1-5 mm per day • fast axonal flow • moves organelles & materials along surface of microtubules • at 200-400 mm per day • transports in either direction • for use or for recycling in cell body

10. Diversity in Neurons • Both structural and functional features are used to classify the various neurons in the body. • On the basis of the number of processes extending from the cell body (structure), neurons are classified as multipolar, biopolar, and unipolar (Figure 12.4). • Most neurons in the body are interneurons and are often named for the histologist who first described them or for an aspect of their shape or appearance. Examples are Purkinje cells (Figure 12.5a) or Renshaw cells (Figure 12.5b).

11. Functional Classification of Neurons • Sensory (afferent) neurons • transport sensory information from skin, muscles, joints, sense organs & viscera to CNS • Motor (efferent) neurons • send motor nerve impulses to muscles & glands • Interneurons (association) neurons • connect sensory to motor neurons • 90% of neurons in the body

12. Neuroglial Cells • Half of the volume of the CNS • Smaller cells than neurons • 50X more numerous • Cells can divide • rapid mitosis in tumor formation (gliomas) • 4 cell types in CNS • astrocytes, oligodendrocytes, microglia & ependymal • 2 cell types in PNS • schwann and satellite cells

13. Myelination • A multilayered lipid and protein covering called the myelin sheath and produced by Schwann cells and oligodendrocytes surrounds the axons of most neurons (Figure 12.8a). • The sheath electrically insulates the axon and increases the speed of nerve impulse conduction.

14. Axon Coverings in PNS • All axons surrounded by a lipid & protein covering (myelin sheath) produced by Schwann cells • Neurilemma is cytoplasm & nucleusof Schwann cell • gaps called nodes of Ranvier • Myelinated fibers appear white • jelly-roll like wrappings made of lipoprotein = myelin • acts as electrical insulator • speeds conduction of nerve impulses • Unmyelinated fibers • slow, small diameter fibers • only surrounded by neurilemma but no myelin sheath wrapping

15. Myelination in PNS • Schwann cells myelinate (wrap around) axons in the PNS during fetal development • Schwann cell cytoplasm & nucleus forms outermost layer of neurolemma with inner portion being the myelin sheath • Tube guides growing axons that are repairing themselves

16. Myelination in the CNS • Oligodendrocytes myelinate axons in the CNS • Broad, flat cell processes wrap about CNS axons, but the cell bodies do not surround the axons • No neurilemma is formed • Little regrowth after injury is possible due to the lack of a distinct tube or neurilemma

17. Gray and White Matter • White matter = myelinated processes (white in color) • Gray matter = nerve cell bodies, dendrites, axon terminals, bundles of unmyelinated axons and neuroglia (gray color) • In the spinal cord = gray matter forms an H-shaped inner core surrounded by white matter • In the brain = a thin outer shell of gray matter covers the surface & is found in clusters called nuclei inside the CNS • A nucleus is a mass of nerve cell bodies and dendrites inside the CNS.

18. INTRODUCTION • The spinal cord and spinal nerves mediate reactions to environmental changes. • The spinal cord has several functions. • It processes reflexes. • It is a conduction pathway for sensory and motor nerve impulses. • The size of the vertebral canal varies in different regions of the vertebral column and affects spinal cord injuries.

19. Meninges • The meninges are three coverings that run continuously around the spinal cord and brain (Figures 13.1a, 14.4a). • The outermost layer is the dura mater. • The middle layer is the arachnoid. • The innermost meninx is the pia mater, a thin, transparent connective tissue layer that adheres to the surface of the spinal cord and brain • Denticulate ligaments are thickenings of the pia mater that suspend the spinal cord in the middle of its dural sheath.

20. Structures Covering the Spinal Cord • Vertebrae • Epidural space filled with fat • Dura mater • dense irregular CT tube • Subdural space filled with interstitial fluid • Arachnoid = spider web of collagen fibers • Subarachnoid space = CSF • Pia mater • thin layer covers BV • denticulate ligs hold in place

21. Applications • The subarachnoid space is between the arachnoid mater and pia mater and contains cerebrospinal fluid (CSF). • Inflammation of the meninges is known as meningitis. • Removal of cerebrospinal fluid from the subarachnoid space is called a spinal tap (lumbar puncture). This procedure is used to diagnose pathologies and to introduce antibiotics, contrast media, anesthetics, and chemotherapeutic drugs.

22. External Anatomy of the Spinal Cord • The spinal cord begins as a continuation of the medulla oblongata and terminates at about the second lumbar vertebra in an adult (Figure 13.2). • It contains cervical and lumbar enlargements that serve as points of origin for nerves to the extremities. • The tapered portion of the spinal cord is the conus medullaris, from which arise the filum terminale and cauda equina.

23. Inferior End of Spinal Cord • Conus medullaris • cone-shaped end of spinal cord • Filum terminale • thread-like extension of pia mater • stabilizes spinal cord in canal • Caudae equinae (horse’s tail) • dorsal & ventral roots of lowest spinal nerves • Spinal segment • area of cord from which each pair of spinal nerves arises

24. Spinal nerves • The 31 pairs of spinal nerves are named and numbered according to the region and level of the spinal cord from which they emerge (Figure 13.2). • 8 pairs of cervical nerves, • 12 pairs of thoracic nerves, • 5 pairs of lumbar nerves, • 5 pairs of sacral nerves, and • 1 pair of coccygeal nerves. • Spinal nerves are the paths of communication between the spinal cord and most of the body. • Roots are the two points of attachment that connect each spinal nerve to a segment of the spinal cord (Figure 13.3a).

25. Internal Anatomy of the Spinal Cord • The anterior median fissure and the posterior median sulcus penetrate the white matter of the spinal cord and divide it into right and left sides (Figure 13.3b). • The gray matter of the spinal cord is shaped like the letter H or a butterfly and is surround by white matter. • The gray matter consists primarily of cell bodies of neurons and neuroglia and unmyelinated axons and dendrites of association and motor neurons. • The gray commissure forms the cross bar of the H-shaped gray matter. • The white matter consists of bundles of myelinated axons of motor and sensory neurons.

26. Sensory and Motor Tracts • Figure 13.12 shows the principal sensory and motor tracts in the spinal cord. These tracts are discussed in detail in Chapter 16 summarized in tables 16.3 and 16.4. • Sensory (ascending) tracts conduct nerve impulses toward the brain. • the lateral and anterior spinothalamic tracts and the posterior column tract. • Motor (descending) tracts conduct impulses down the cord. • Direct pathways include lateral and anterior corticospinal and corticobulbar tracts. • Indirect pathways include rubrospinal, tectospinal, and vestibulospinal tracts.

27. Function of Spinal Tracts • Spinothalamic tract • pain, temperature, deep pressure & crude touch • Posterior columns • proprioception, discriminative touch, two-point discrimination, pressure and vibration • Direct pathways (corticospinal & corticobulbar) • precise, voluntary movements • Indirect pathways (rubrospinal, vestibulospinal) • programming automatic movements, posture & muscle tone, equilibrium & coordination of visual reflexes

28. SPINAL NERVES • Spinal nerves connect the CNS to sensory receptors, muscles, and glands and are part of the peripheral nervous system. • The 31 pairs of spinal nerves are named and numbered according to the region and level of the spinal cord from which they emerge (Figure 13.2). • Roots of the lower lumbar, sacral, and coccygeal nerves are not in line with their corresponding vertebrae and thus form the cauda equina (Figure 13.2). • Spinal nerves connect to the cord via an anterior and a posterior root (Figure 13.3a). Since the posterior root contains sensory axons and the anterior root contains motor axons, a spinal nerve is a mixed nerve.

29. A Nerve Plexus • Joining of ventral rami of spinal nerves to form nerve networks or plexuses • Found in neck, arm, low back & sacral regions • No plexus in thoracic region • intercostal nn. innervate intercostal spaces • T7 to T12 supply abdominal wall as well

30. INTRODUCTION • The brain is the center for registering sensations, correlating them with one another and with stored information, making decisions, and taking action. • It is also the center for intellect, emotions, behavior, and memory. • It also directs our behavior towards others. • In this chapter we will consider the principal parts of the brain, how the brain is protected and nourished, and how it is related to the spinal cord and to the 12 pairs of cranial nerves.

31. Principal Parts of the Brain • Cerebrum • Diencephalon • thalamus & hypothalamus • Cerebellum • Brainstem • medulla, pons & midbrain

32. Blood Supply to Brain • Arterial blood supply is branches from circle of Willis on base of brain • Vessels on surface of brain----penetrate tissue • Uses 20% of our bodies oxygen & glucose needs • blood flow to an area increases with activity in that area • deprivation of O2 for 4 min does permanent injury • at that time, lysosome release enzymes • Blood-brain barrier (BBB) • protects cells from some toxins and pathogens • proteins & antibiotics can not pass but alcohol & anesthetics do • tight junctions seal together epithelial cells, continuous basement membrane, astrocyte processes covering capillaries

33. BBB • A blood-brain barrier (BBB) protects brain cells from harmful substances and pathogens by serving as a selective barrier to prevent passage of many substances from the blood to the brain. • An injury to the brain due to trauma, inflammation, or toxins causes a breakdown of the BBB, permitting the passage of normally restricted substances into brain tissue. • The BBB may also prevent entry of drugs that could be used as therapy for brain cancer or other CNS disorders, so research is exploring ways to transport drugs past the BBB.

34. Protective Covering of the Brain • The brain is protected by the cranial bones (Figure 7.4) and the cranial meninges (Figure 14.2). • The cranial meninges are continuous with the spinal meninges and are named dura mater, arachnoid, and pia mater. • Three extensions of the dura mater separate parts of the brain: the falx cerebri, falx cerebelli, and the tentorium cerebelli.

35. Cerebrospinal Fluid (CSF) • 80-150 ml (3-5oz) • Clear liquid containing glucose, proteins, & ions • Functions • mechanical protection • floats brain & softens impact with bony walls • chemical protection • optimal ionic concentrations for action potentials • circulation • nutrients and waste products to and from bloodstream

36. Ventricles • There are four CSF filled cavities within the brain called ventricles (Figure 14.3). • A lateral ventricle is located in each hemisphere of the cerebrum. The lateral ventricles are separated by the septum pellucidum. • The third ventricle is a narrow cavity along the midline superior to the hypothalamus and between the right and left halves of the thalamus. • The fourth ventricle is between the brain stem and the cerebellum.

37. Medulla Oblongata • Continuation of spinal cord • Ascending sensory tracts • Descending motor tracts • Nuclei of 5 cranial nerves • Cardiovascular center • force & rate of heart beat • diameter of blood vessels • Respiratory center • medullary rhythmicity area sets basic rhythm of breathing • Information in & out of cerebellum • Reflex centers for coughing, sneezing, swallowing etc.

38. Pons • The pons is located superior to the medulla. It connects the spinal cord with the brain and links parts of the brain with one another by way of tracts (Figures 14.1, 14.5). • relays nerve impulses related to voluntary skeletal movements from the cerebral cortex to the cerebellum. • contains the pneumotaxic and apneustic areas, which help control respiration along with the respiratory center in the medulla (Figure 23.24). • contains nuclei for cranial nerves V trigeminal, VI abducens, VII facial, and VIII vestibulocochlear (vestibular branch only).(Figure 14.5).

39. Midbrain • One inch in length • Extends from pons to diencephalon • Cerebral aqueduct connects 3rd ventricle above to 4th ventricle below

40. Cerebellum • 2 cerebellar hemispheres and vermis (central area) • Function • correct voluntary muscle contraction and posture based on sensory data from body about actual movements • sense of equilibrium

41. Thalamus • The thalamus is located superior to the midbrain and contains nuclei that serve as relay stations for all sensory impulses, except smell, to the cerebral cortex (Figure 14.9). • seven major groups of thalamic nuclei on each side (Figure 14.9 c and d). • They are the Anterior nucleus, medial nuclei, lateral group, ventral group, intralaminar nuclei, midline nucleus, and the reticular nucleus. • It also registers conscious recognition of pain and temperature and some awareness of light touch and pressure. • It plays an essential role in awareness and the acquisition of knowledge (cognition.)

42. Hypothalamus • The hypothalamus • inferior to the thalamus, has four major regions (mammillary, tuberal, supraoptic, and preoptic) • controls many body activities, and is one of the major regulators of homeostasis (Figure 14.10). • The hypothalamus has a great number of functions. • It controls the ANS. • It produces hormones. • It functions in regulation of emotional and behavioral patterns. • It regulates eating and drinking through the feeding center, satiety center, and thirst center. • It aids in controlling body temperature. • It regulates circadian rhythms and states of consciousness.

43. THE CEREBRUM • The cerebrum is the largest part of the brain . • The surface layer, the cerebral cortex, is 2-4 mm thick and is composed of gray matter. The cortex contains billions of neurons. • The cortex contains gyri (convolutions), deep grooves called fissures, and shallower sulci. (Figure 14.11a) • Beneath the cortex lies the cerebral white matter, tracts that connect parts of the brain with itself and other parts of the nervous system. • The cerebrum is nearly separated into right and left halves, called hemispheres, by the longitudinal fissure. • Internally it remains connected by the corpuscallosum, a bundle of transverse white fibers. Figure 14.12)

44. Cerebrum (Cerebral Hemispheres) • Cerebral cortex is gray matteroverlying white matter • 2-4 mm thick containing billions of cells • grew quickly; formed folds(gyri) and grooves (sulci or fissures) • Longitudinal fissure separates left & right cerebral hemispheres • Corpus callosum is a commisure (band of white matter) connecting left and right cerebral hemispheres • Each hemisphere is subdivided into 4 lobes

45. Lobes • Each cerebral hemisphere is further subdivided into four lobes by sulci or fissures (Figure 14.11 a,b) • frontal, parietal, temporal, and occipital. • A fifth part of the cerebrum, the insula, lies deep to the parietal, frontal, and temporal lobes and cannot be seen in an external view of the brain.

46. White Matter • The white matter is under the cortex and consists of myelinated axons running in three principal directions (Figure 14.12). • Association fibers connect and transmit nerve impulses between gyri in the same hemisphere. • Commissural fibers connect gyri in one cerebral hemisphere to the corresponding gyri in the opposite hemisphere. • Projection fibers form ascending and descending tracts that transmit impulses from the cerebrum to other parts of the brain and spinal cord.

47. Sensory Areas • The sensory areas of the cerebral cortex are concerned with the reception and interpretation of sensory impulses. • Some important sensory areas include • primary somatosensory area, • primary visual area, • primary auditory area, and • primary gustatory area

48. INTRODUCTION • The autonomic nervous system (ANS) operates via reflex arcs. • Operation of the ANS to maintain homeostasis, however, depends on a continual flow of sensory afferent input, from receptors in organs, and efferent motor output to the same effector organs. • Structurally, the ANS includes autonomic sensory neurons, integrating centers in the CNS, and autonomic motor neurons. • Functionally, the ANS usually operates without conscious control. • The ANS is regulated by the hypothalamus and brain stem.

49. SOMATIC AND AUTONOMIC NERVOUS SYSTEMS • The autonomic nervous system contains both autonomic sensory and motor neurons. • Autonomic sensory neurons are associated with interoceptors. • Autonomic sensory input is not consciously perceived. • The ANS also receives sensory input from somatic senses and special sensory neurons. • The autonomic motor neurons regulate visceral activities by either increasing (exciting) or decreasing (inhibiting) ongoing activities of cardiac muscle, smooth muscle, and glands. • Most autonomic responses can not be consciously altered or suppressed.

50. SOMATIC vs AUTONOMIC NERVOUS SYSTEMS • All somatic motor pathways consist of a single motor neuron • Autonomic motor pathways consists of two motor neurons in series • The first autonomic neuron motor has its cell body in the CNS and its myelinated axon extends to an autonomic ganglion. • It may extend to the adrenal medullae rather than an autonomic ganglion • The second autonomic motor neuron has its cell body in an autonomic ganglion; its nonmyelinated axon extends to an effector.