Chapter 7 – Nervous system Ms. Harborth Anatomy and physiology

Structural classification Central nervous system (CNS) – consists of the brain and spinal cord. Command center Peripheral nervous system (PNS) – part of body outside of CNS. Spinal nerves – carry impulses to and from spinal cord Cranial nerves – carry impulses to and from the brain

PNS Sensory (afferent) – carries impulses FROM sensory organs TO the brain Motor (efferent) – carries impulses FROM the brain TO effector organs, muscles, and glands. Somatic nervous system – control skeletal muscles Autonomic nervous system – regulates automatic events (smooth and cardiac muscle) Sympathetic Parasympathetic

Nervous tissue Supporting cells of CNS “neuroglia” Neurons Astrocytes – barrier between neurons and capillaries Microglia – phagocytes that “clean” Ependymal cells – beat cilia to circulate cerebrospinal fluid Oligodendrocytes – form myelin sheaths Supporting cells of PNS: Schwann cells – form myelin sheaths Satellite cells – protective, cushioning cells Neurons Cell body (Nissl substance & neurofibrils) Processes (fibers) Myelin sheath, Schwann cells (PNS), nodes of Ranvier Dendrites (toward cell body) Axons (away from cell body) Axonal terminals, neurotransmitters, synaptic cleft, synapse

Astrocyte Ependymal cells Oligodendrocyte Microglial cell

Multiple sclerosis Immune system gradually destroys myelin sheaths, converting to scleroses (hardened processes)

Vocab Nuclei – clusters of cell bodies in the CNS Ganglia – small collections of cell bodies found in the PNS. Tracts – bundles of nerve fibers in the CNS Nerves – bundles of nerve fibers in the PNS White matter – myelinated fibers in the CNS Gray matter – unmyelinated fibers and cell bodies in the CNS

Classification of neurons Functional (direction of nerve impulse) Sensory (afferent) neurons – neurons carrying impulses from sensory receptors to the CNS. Cell bodies are found in ganglion outside of CNS. Cutaneous sense organs, proprioceptors Motor (efferent) neurons – neurons carrying impulses to viscera and/or muscles and glands. Cell bodies are in the CNS. Association neurons (interneurons) – connect motor and sensory neurons in neural pathways.

Classification of neurons Structural Multipolar neuron – several processes extending from cell body. All motor and association neurons Bipolar neuron – axon and dendrite extend from cell body Rare. Found in some special sense organs. Unipolar neuron – short, single process that usually divides into proximal and distal fibers. Axon conducts nerve impulses both toward AND away from the cell body. Sensory neurons found in PNS ganglia

Neuron physiology Nerve impulse – Irritability Conductivity animation 1. Inactive neuron is polarized (more negative inside) 2. Stimulus excites neuron, permeability increases 3. Depolarization Action Potential 4. Repolarization Saltatory conduction – occurs on myelinated nerves FASTER Cold and continuous pressure hinder conduction Conductivity When action potential reaches axonal endings, tiny vesicles fuse with axonal membranes, rupture, and releases neurotransmitters which start impulse in next neuron Electrochemical event animation

Neuron physiology Reflex Arc Autonomic reflexes Somatic reflexes Regulate activity of smooth muscles, heart, and glands. Ex: salivary reflex and pupillary reflex. Somatic reflexes Reflexes that stimulate skeletal muscles Minimum of five elements: Sensory receptor Afferent neuron Integration center Efferent neuron Effector organ

Central Nervous System Embryonic development: Neural tube develops 4th week – anterior end of neural tube expands and brain formation begins. Central canal of neural tube becomes 4 ventricles

Functional Anatomy of the Brain Cerebral Hemispheres Diencephalon Cerebellum Brain Stem

Cerebral Hemispheres Gyri: elevated ridges of tissue Sulci: shallow grooves Fissures: deeper grooves which separate large regions of the brain Hemispheres separated by longitudinal fissure Lobes are named for cranial bones that surrounding them

Cerebral Hemisphere Parietal lobe: posterior to central sulcus Somatic sensory area – localizes and interprets impulses from sensory receptors. Sensory pathways cross (left -> right, right -> left) Occipital lobe: visual area Temporal lobe: auditory and olfactory area Frontal lobe: Higher intellectual reasoning in anterior part Primary motor area - Axons of motor neurons form pyramidal (corticospinal) tract which descends to spinal cord. Pathways are crossed. Broca’s area – speech. Only found in one hemisphere Speech area – junction of temporal, parietal, and occipital lobes. Sound out words. One hemisphere. Gray matter (cerebral cortex): cell bodies of neurons White matter: fiber tracts carrying impulses to or from the cortex Corpus callosum: large fiber tract that connects cerebral hemispheres. Allows halves to communicate with one another. Basal nuclei: “islands” of gray matter buried in white matter. Modify instructions sent to muscles.

Diencephalon Thalamus Hypothalamus Epithalamus Encloses third ventricle Relay station for sensory impulses Hypothalamus Regulation of temperature, water balance and metabolism. Regulates pituitary Limbic system – “emotional-visceral brain”. Thirst, appetite, sex, pain, pleasure centers. Mammillary bodies – reflex involved in olfaction Epithalamus Pineal body Choroid plexus of third ventricle

Diencephalon

Brain stem Midbrain Pons Medulla oblongata Reticular formation Cerebral aqueduct – canal that connects third ventricle of diencphalon to fourth ventricle below Cerebral peduncles – convey ascending and descending impulses Corpora quadrigemina – reflex centers for vision and hearing Pons Mostly fiber tracts, but does have some nuclei involved in controlling breathing Medulla oblongata Merges into spinal cord. Regulate vital visceral activities. Controls heart rate, blood pressure, breathing, swallowing, and vomiting Reticular formation Neurons involved in motor control of the visceral organs. RAS (reticular activating system) plays a role in consciousness and awake/sleep cycles. Damage to this area results in coma.

Brain Stem

Cerebellum Two hemispheres Timing for skeletal muscle activity and controls balance/equilibrium Takes info from eyes, ears and muscles to coordinate movements in a smooth fashion

meninges Three connective tissue membranes covering and protecting brain and spinal cord Dura mater – outermost layer. Double-layered, forms periosteum and meningeal layer. Arachnoid mater – span subarachnoid space to attach to innermost membrane Subarachnoid space filled with cerebrospinal fluid. Arachnoid villi protrude through dura mater. Dural sinuses absorb fluid into blood through arachnoid villi. Pia mater – delicate layer clings tightly to surface of brain and spinal cord. Meningitis – inflammation of meninges

Cerebrospinal fluid (CSF) similar to blood plasma. continually formed from blood by choroid plexuses continually moving. Blood-brain barrier – any changes in fluid around brain could be detrimental. Barrier composed of the LEAST permeable capillaries in whole body. Water, glucose and essential amino acids can pass through.

Spinal Cord 17 inches long, thickness of a thumb Extends from foramen magnum to 1st or 2nd lumbar vertebrae, right below ribs Cauda equina

Peripheral Nervous System Endoneurium Perineurium Fascicles Epineurium Nerves are classified by which direction they transmit Mixed nerves Afferent (sensory) Efferent (motor)

Cranial nerves 12 pairs (table 7.1) Serve head and neck (except vagus nerves – extend to thoracic and abdominal cavities) I. Olfactory II. Optic III. Oculomotor IV. Trochlear V. Trigeminal VI. Abducens VII. Facial VIII. Vestibulochochlear IX. Glossopharyngeal X. Vagus XI. Accessory XII. Hypoglossal

Spinal Nerves/ Nerve Plexuses 31 pairs of spinal nerves formed by combo of ventral and dorsal roots of spinal cord. Named for region of spinal cord from which they arise. Each spinal nerve divides into dorsal and ventral rami Dorsal rami serve skin and muscles of posterior body trunk Ventral rami of T1 – T12 form intercostal nerves Ventral rami of other spinal nerves form 4 plexuses (Table 7.2)

Autonomic nervous system Motor subdivisions that control automatic activities (cardiac and smooth muscle, and glands) Sympathetic and parasympathetic Sympathetic – mobilizes body during extreme situation (fear, exercise, rage, etc.) Parasympathetic – allows us to “unwind” and conserve energy