The nervous system chapter 12, 13 and 14 Central Nervous System (CNS) Brain Spinal Cord Peripheral Nervous System (PNS) Sensory Neurons Motor Neurons Somatic Nervous System Autonomic Nervous System Sympathetic Parasympathetic The Nervous System

Our approach will be to cover functional systems: Subjects we will cover Our approach will be to cover functional systems: Somatic nervous system – sensory and motor Autonomic nervous system Special senses All system will cover both the central and peripheral nervous systems

Neurons in the nervous system link together to form circuits with specific functions. http://www.thefullwiki.org/Nervous_System

Nervous system functions Motor functions Control contraction of skeletal muscles – somatic nervous system Control contractions of smooth and cardiac muscle – autonomic nervous system Control of glands function – autonomic nervous system Sensory and integrative functions Process incoming information to ensure appropriate motor response More than 99% of the incoming information is classified as irrelevant and unimportant

3 Major levels of the CNS Spinal cord involved in walking movement, reflexes of both skeletal muscles and the autonomic nervous system effectors Lower brain/subcortical level Subconscious activities of both the autonomic and somatic nervous system. Involved in several emotional patterns Higher brain/cortical level Very large memory storage, place of higher functions like thoughts, awareness

Peripheral nervous system components Includes nerves and ganglia Most nerves are mixtures of afferent and efferent fibers and somatic and autonomic (visceral) fibers Pure sensory (afferent) or motor (efferent) nerves are rare Peripheral nerves classified as cranial or spinal nerves Ganglia contain neuron cell bodies associated with nerves Dorsal root ganglia (sensory, somatic) (Chapter 12) Autonomic ganglia (motor, visceral) (Chapter 14)

CNS protection Bony protection by the skeleton – brain by skull and spinal cord by vertebrae 3 layers of membrane collectively called meninges that are found between the bone and the CNS tissue Cerebro-spinal fluid – CSF - found in the ventricles, central canal and subarachnoid space Blood brain barrier - BBB

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Choroid Plexuses and CSF Clusters of capillaries and the ependymal cells http://www.sci.uidaho.edu/med532/choroid.htm

The choroid plexus forms tissue fluid filters CSF production The choroid plexus forms tissue fluid filters Have ion pumps that selectively pump ions from the plasma into the ventricles. The ions pumps create osmotic pressure that draws water to the CSF the choroid plexus of the lateral ventricles producing the most. The rate of formation is approximately 0.35 ml/min or 500 ml/day; a rate which replaces the total volume of CSF approximately 2-3 times over in 24 hours.

CSF – cerebrospinal fluid functions Liquid cushion for brain and spinal cord Nourishes brain Removes waste Conducts chemical signals between parts of CNS

Why are the capillaries in the BBB less permeable? Endothelial cells form tight junctions that prevent solutes movement between cells Astrocytes Selective transport properties of the endothelial cells The BBB Helps maintain a stable environment for the brain Separates neurons from some bloodborne substances

Blood-Brain Barrier: Functions Selective barrier that allows nutrients to pass freely Is ineffective against substances that can diffuse through plasma membranes (ex. Ethanol, caffeine) Absent in some areas: Ex. - hormones generally do not penetrate the brain from the blood, so in order to control the rate of hormone secretion effectively, there are specialized sites where neurons can "sample" the composition of the circulating blood. At these sites, the blood-brain barrier is 'leaky‘ (pituitary gland) Capillaries of the choroid plexus The BBB can break down under certain conditions: hypertension, radiation, infection and brain trauma

Spinal Cord Because the cord does not reach the end of the vertebral column, the lumbar and sacral spinal nerve roots angle sharply downward and travel inferiorly before reaching their intervertebral foramina This collection of nerve roots at the inferior end of the vertebral canal is called the cauda equina

Gray Matter: Organization Dorsal root (sensory) Dorsal root ganglion Dorsal horn (interneurons) Somatic sensory neuron Visceral sensory neuron Visceral motor neuron Spinal nerve Ventral horn (motor neurons) Ventral root (motor) Somatic motor neuron Interneurons receiving input from somatic sensory neurons Interneurons receiving input from visceral sensory neurons Visceral motor (autonomic) neurons Somatic motor neurons Figure 12.32

White Matter in the Spinal Cord Composed of both myelinated and unmyelinated fibers that run in three directions Ascending – to higher centers; sensory Descending – to lower levels from brain or higher places in the cord; motor Transversely – from one side of the cord to the other Divided into three funiculi (columns) – posterior, lateral, and anterior Each funiculus contains several fiber tracks with similar destination and function Fiber tract names reveal their origin and destination Fiber tracts are composed of axons with similar functions

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

Spinal Nerves: Rami Small dorsal ramus Larger ventral ramus The short spinal nerves branch into three or four mixed, distal rami Small dorsal ramus Larger ventral ramus Tiny meningeal branch – innervate the meninges and blood vessels within the vertebral canal Rami communicantes at the base of the ventral rami in the thoracic region that contain autonomic nerve fibers

Nerve Plexuses All ventral rami except T2-T12 form nerve networks called plexuses Plexuses are found in the cervical, brachial, lumbar, and sacral regions Each resulting branch of a plexus contains fibers from several spinal nerves Fibers travel to the periphery via several different routes Each muscle receives a nerve supply from more than one spinal nerve Damage to one spinal segment cannot completely paralyze a muscle

Plexus Main spinal nerves Regions innervated Major nerves Cervical C1-C5 Skin and muscles of head & neck. Superior chest and shoulder Phrenic (diaphragm) Brachial C5-C8, T1 Shoulder and upper limbs Axillary Musculocutaneous Radial Median Ulnar Lumbar L1-L4 Antero-lateral abdominal wall, external genitalia, part of lower limbs Femoral Sacral L4-L5, S1-S4 Buttocks, perineum, lower limbs Sciatic