A P P R O P R I A T E D F R O M M R S . H A R B O R T HA N A T O M Y A N D P H Y S I O L O G Y

CHAPTER 7 – NERVOUS SYSTEM

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”• Astrocytes• Microglia• Ependymal cells• Oligodendrocytes• Schwann cells• Satellite 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

Microglial cell

Ependymal cells

Oligodendrocyte

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• 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• 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• Effector organ• Afferent neuron• Efferent neuron• Integration center

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• Fissues: 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• 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• 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