The Nervous System

1. The Nervous System Chapter 8 Anatomy & Physiology

2. Functions of the Nervous System • Detect changes & feel sensations • Initiate appropriate response to changes • Organize information for immediate use • Store information for future use

3. Divisions of the Nervous System • Central Nervous System (CNS) • Brain • Spinal cord • Peripheral Nervous System (PNS) • Peripheral nerves • Cranial nerves • Autonomic nervous system (ANS) • Sympathetic nerves • Parasympathetic nerves

4. Neurons • Nerve cell or nerve fiber • Cells that transmit electrical impulses • Anatomy of a neuron: • Cell body • Cytoplasm and nucleus • Dendrites • Branches that transmit impulses toward cell body • Axon • Branch that transmits impulses away from the cell body

5. Neuronal Anatomy (PNS) • Schwann cells • Specialized cells whose cell membranes provide myelin insulation by wrapping around PNS axons & dendrites • Myelin sheath • A type of insulating phospholipid which helps to control impulse transmission • Nodes of Ranvier • Spaces between adjacent Schwann cells that expose the part of the neuron cell membrane that depolarizes with impulse transmission

6. Neuronal Anatomy (PNS) • Neurolemma • Nuclei & cytoplasm of the Schwann cells that are wrapped around the outside of the myelin sheath • Have tunnels that allow regeneration of damaged axons & dendrites • Schwann cells also produce a chemical growth factor to help regeneration of damaged branches

7. CNS Microanatomy • No Schwann cells, no neurolemma, and no neuron regeneration is possible • Neuroglial cells (supporting cells) • Oligodendrocytes • Form myelin sheaths in CNS • Microglia • Phagocytes in CNS • Destroy debris, damaged cells, pathogens

8. CNS Microanatomy • More neuroglial cells: • Astrocytes • Form framework for embryonic growth of brain • Contribute to blood-brain barrier(BBB) • Maintain K+ level in brain • Ependyma • Mostly ciliated cells that line brain ventricles & help circulate the cerebrospinal fluid (CSF)

9. CNS Microanatomy • Blood-Brain Barrier (BBB) • Brain capillaries that are less permeable than other capillaries in the body and some astrocyte extensions • Keep various chemicals from diffusing into the brain tissue (more sensitive cells than other tissues) • Disadvantages: also keeps out some medicines and lymphocyte antibodies

10. Synapses • Space between the axon of one neuron and the dendrites (or cell body) of another • Pre-synaptic neuron • Also called synaptic knob or terminal end • Releases the neurotransmitter • Post-synaptic neuron • Contains an inactivator chemical to make the neurotransmitter inactive • Prevents continuous, unwanted impulse transmission

11. Synapses • Types of synapses • Excitatory • NT makes postsynaptic neuron depolarize & transmit an impulse to another cell • Inhibitory • NT makes postsynaptic neuron hyperpolarize so that it will NOT transmit an impulse • Example: slowing HR, preventing excessive muscle contractions

12. Synapses and Neurotransmitters • Some aspects of the synapse • Insure one-way transmission • NT only released by pre-synaptic neuron

13. Neurotransmitters • Acetylcholine • At NM jct., CNS, and PNS • CNS neurotransmitters • Dopamine • GABA • Norepinephrine • Glutamate • serotonin

14. Neurotransmitters • Some are inactivated after in the post-synaptic neuron • Example: cholinesterase • Some are reabsorbed by pre-synaptic neuron • Called reuptake

15. Types of Neurons • Sensory (afferent) neurons • Motor (efferent) neurons • Interneurons

16. Sensory /Afferent Neurons • Carry impulses from receptors to CNS • Receptors detect internal or external changes and relay this info to CNS • Types of sensory neurons • Somatic • From skin, skeletal muscle, joints • Visceral • From internal organs

17. Motor / Efferent Neurons • Carry impulses from CNS to effectors • Effectors are muscles and glands • Types of Motor neurons • Somatic • Go to skeletal muscle • Visceral • Go to smooth muscle, cardiac muscle & glands • These make up the autonomic nervous system

18. Interneurons • Found only in CNS • Carry only sensory or only motor impulses • Some integrate functions

19. Nerves • Nerve • Group of many axons +/or dendrites from from many neurons, blood vessels & CT • Types of nerves • Motor nerves • Sensory nerves • Mixed nerves • Majority of peripheral nerves

20. Nerve Tracts • Groups of neurons in CNS • All neurons in a tract are either sensory or motor • Also called “white matter” due to the color of their myelin sheath

21. The Nerve Impulse • Resting state of Polarization • Neuron more positive on outside of its membrane due to Na+ abundance there • Depolarization • Stimulus arrives at neuron making membrane very permeable to Na+ • Na+ rushes into cell & charges reverse • Repolarization • Membrane now very permeable to K+ • K+ leaves cell, restoring positive charge outside

22. Nerve Impulse • After repolarization • K+ & Na+ pumps work • Action potential = depolarization, followed by repolarization • Takes milliseconds only • Saltatory conduction • Increased transmission velocity due to the myelin sheath’s insulation of neuron

23. Spinal Cord • Functions • Transmits impulses to & from the brain • Integrating center for spinal cord reflexes • Protected by layers of • Meninges (connective tissue) • Vertebral canal (formed by bones)

24. Parts of the Spinal Cord Gray Matter interior part (H-shaped), cell bodies of motor and interneurons White Matter external part made of myelinated axons and dentrites of interneurons Ascending tracts go from periphery to brain (sensory) dorsal column, spinothalamic tract

25. Parts of Spinal Cord • Descending tracts • Go from the brain to the periphery (motor) • Corticospinal & rubrospinal tracts • Central canal • Continuous with brain ventricles • Contains cerebrospinal fluid (CSF) • Insulating, protective fluid that surrounds CNS

26. Spinal Nerves • Nerves that emerge from the spinal cord • Named according to their vertebrae • 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, one coccygeal pair • Cauda equina • Lumbar & sacral nerves that hang below the end of the spinal cord

27. Spinal Nerves • Each spinal nerve has two roots and is a mixed nerve: • Dorsal root • Sensory neurons • Has dorsal root ganglion • Group of cell bodies outside of CNS • Ventral root • Motor neurons

28. Spinal Cord Reflexes • Reflex • Involuntary response to a stimulus • An automatic action stimulated by a specific change • Spinal Cord reflexes • Reflexes that do not depend on the brain

29. Reflex Arc • The pathway traveled by nerve impulses when a reflex is elicited. • Parts of the arc • Receptor • Sensory neuron • CNS • Motor neuron • Effector

30. Spinal Cord Reflexes • Stretch reflex • A stretched muscle will automatically contract • Examples: knee-jerk • Flexor reflexes • Withdrawal reflexes • Stimulus is painful or harmful and person pulls away

31. Brain Anatomy • Major components • Brainstem • Medulla, pons, midbrain • Cerebellum • Hypothalamus • Thalamus • Cerebrum

32. Brain Anatomy • Ventricles • Four cavities within the brain • Two lateral ventricles • One third (III) ventricle • One fourth (IV) ventricle • Each has a choroid plexus • A capillary network that makes CSF

33. The Brainstem • Medulla • Between spinal cord and pons • Contains cardiac, vasomotor, and respiratory centers • Contains centers for cough, swallowing, vomiting also • “vital sign” center

34. The Brainstem • Pons • Its neurons connect the medulla to the brain • Two additional respiratory centers • Midbrain • Extends from pons to hypothalamus • Contains cerebral aqueduct (connects ventricles) • Integrates visual, auditory, & other reflexes • Coordinates eye movements & righting relexes

35. Cerebellum • Coordination & regulation of muscle tone • Maintenance of posture & equilibrium • Trajectory & endpoint of movements • Sensory perception (tactile) • stereognosis

36. Hypothalamus • Superior to pituitary gland & inferior to thalamus • Relatively small area of the brain • Functions: • ADH & oxytocin hormone production • Releasing factors (hormones) production • Body temperature regulation • Food intake regulation

37. Hypothalamus • More functions • Integration of ANS functions • Stimulation of various reactions to emotion • Regulation of rhythmic body cycles • Sleep, alertness, mood

38. Thalamus • Superior to hypothalamus and inferior to the cerebrum, works closely with cerebrum • Functions (mostly related to sensation) • Relays all sensory info (except olfaction) to cerebrum • Integrates impulses from skin receptors & cerebellum • Suppression of unimportant sensory info • Involved in alertness & awareness

39. Cerebrum • Biggest part of the human brain • Two hemispheres • Each has a lateral ventricle within it • Separated by the longitudinal fissure • Corpus callosum • At base of longitudinal fissure • Band of neurons that connect right & left hemispheres

40. Cerebrum • Cerebral cortex • Surface of the cerebrum • Made up of gray matter (nerve cell bodies) • Convolutions or gyri • Folds of cortex tissue • Fissures or sulci • Grooves in between convolutions • Divided into various lobes • Named same as overlying skull bones

41. Cerebral Lobes • Frontal lobes • Motor area: contralateral voluntary motor • Premotor area: learned movement sequenes • Prefrontal area: emotional responses • Broca’s motor speech area

42. Lobes of the Brain • Parietal • Contralateral sensation, conscious muscle sense, (hands & face have most receptors), taste receptors • Temporal • Olfactory input (smell), auditory input & sound interpretation • Occipital: visual areas

43. Other Areas of the Brain • Association areas • Not directly concerned with movement or sensation • Learning, memory, etc • Hippocampus: information collection, spatial cognition, sense of 3 dimensions, etc.

44. Other Areas of the Brain • Basal Ganglia • Paired masses of gray matter deep in cerebral hemispheres • Work with cerebellum to coordinates accessory movements • Regulate muscle tone and subconscious aspects of voluntary movements • Corpus Callosum • Allows one hemisphere to know of the other hemisphere’s activities

45. Meninges & CSF • Meninges • Connective tissue layers around the brain and spinal cord • Layers of meninges • Dura mater: outermost, fibrous & tough • Arachnoid membrane: spider’s web • Subarachnoid space, contains CSF • Pia mater: thin membrane on brain & cord surface

46. Cerebrospinal Fluid • Formation • In choroid plexus (capillary network) of each ventricle • Circulation • From lateral & III, to IV ventricle, to central canal to subarachnoid spaces • Reabsorption • Through arachnoid villi in cranial area into blood in the cranial venous sinuses into plasma to choroid plexus again

47. Cranial Nerves • 12 pairs of peripheral nerves that emergy from the brain or brain stem • Most have functions associated with the head and neck area • Examples of CN: • Taste, smell, sight, motor nerves to muscles of face & eyes • Vagus nerves (to chest and abdomen)

48. Autonomic Nervous System • Part of peripheral nervous system (PNS) • Carries visceral motor neurons • Smooth & cardiac muscle & glands • Receives information from visceral effectors (internal organs, etc) • Has two divisions • Sympathetic division • Parasympathetic division

49. ANS • Hypothalamus • Integrates activity of both divisions • Autonomic pathways • Preganglionic neuron • From CNS to ganglion • Postganglionic neuron • From ganglion to visceral effector • (ganglia = cell bodies of this neuron)

50. Sympathetic Division of ANS • Also called thoracolumbar division • “Fight or flight” responses • Increases HR (heart rate) • vasodilation in skeletal muscle, vasoconstriction in nonessential areas (GI, skin, kidneys) • Dilation of bronchioles • Liver breaks down glycogen into glucose • Decreases digestive enzyme action & peristalsis