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NERVOUS SYSTEM
- One of the two control and communication systems of the body
- Nervous system rapid, complex network
- Endocrine system slow/via blood
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The nervous system is divided into two subdivisions:
- CENTRAL NERVOUS SYSTEM (CNS) includes the brain and spinal cord
- PERIPHERAL NERVOUS SYSTEM (PNS)
- Afferent division-brings sensory information from receptors to the CNS
- Efferent division-carries motor commands from CNS to muscles and glands
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SENSORY NERVOUS SYSTEM (AFFERENT)
- Subdivided into:
- Somatic sensory-caries information from skeletal muscles, joints and skin to CNS
- Visceral sensory-caries information from smooth muscle, cardiac muscle and gland activity
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MOTOR NERVOUS SYSTEM (EFFERENT)
- Subdivided into:
- Somatic motor-which controls the contraction of skeletal muscles
- Autonomic motor- which regulates smooth, cardiac muscle and gland activity
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The CNS and PNS perform three general functions:
- Collecting information-receptors detect changes in the internal and external environment (sensory input) and send it to the CNS
- Processing and evaluating information-CNS determines what response is required
- Responding to information-CNS initiates nerve impulses (motor output) to effectors (muscles or glands)
- TWO TYPES OF CELLS IN THE NERVOUS SYSTEM:
- 1. Neurons (nerve cells)-electrically excitable cells that initiate, transmit and receive nerve impulses
- 2. Neuroglial cells-non-excitable cells that support and protect the neurons
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NEURONS
- Made up of cell body and processes:
- Cell body (perikaryon/soma)
- Contains nucleus
- (Amitotic, large nucleolus, chromosomes)
- Cytoplasm, mitochondria, golgi, nissl granules (ER), microtubules, and microfilaments
- High metabolic rate
- Most cells bodies in CNS
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2 TYPES OF PROCESSES:
- Dendrites
- Short, highly branched, unmyelinated receptive surfaces of neuron
- Carry information to cell body
- Axon
- Long slender process arising from hillock of cell body
- Most are myelinated
- Carry information away from cell body
- Axon may have collateral branches
- Axon ending have synaptic knobs
- Most axons surrounded by phospholipid . . . . myelin sheath
- 1. Neurolemmocytes-from myelin sheath on axons outside CNS
- 2. Oligodendrocytes-from myelin sheaths on axons inside CNS
- Myelinated axons transmit impulses faster
- Interruptions in myelin sheath called . . . . nodes of Ranvier
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WITH THE DISEASE MULTIPLE SCLEROSIS
Myelin sheaths are lacking on axons because it either doesn’t form or degenerates which interferes with transmission
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STRUCTUREAL TYPES OF NEURONS
- Neurons are classified according to the number of processes coming off the cell body:
- Unipolar-single short process that branches like a T (sensory neurons
- Bipolar-two processes, one dendrite and one axon (eye, ear, and embryo)
- Multipolar- many dendrites and a single axon, most common of all neurons ( most motor neurons and interneuron)
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FUNCTIONAL TYPES OF NEURONS
- Neurons are classified according to the direction the nerve impulse is traveling:
- Sensory(afferent)-transmit impulses to the CNS (from sensory receptors)
- Motor (efferent)-transmit impulses from the CNS (to muscle or glands)
- Interneurons- transmit impulses between sensory and motor neurons
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NEUROGLIAL CELLS
- Aka glia
- In both CNS and PNS
- Smaller than nerurons
- Physically protect and nourish neurons
- More numerous than neurons
- Capable of mitosis
- Brain tumors are more likely to be derived from glia than neurons
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NEUROGLIAL CELLS OF THE CNS
- Four types:
- 1. Astrocytes-blood brain barrier, support
- 2. Ependymal cell- lines ventricles, central canal, choroid plexuses
- 3. Microglial cell-Phagocytes
- 4. Oligodendrocyte-myelin sheaths in CNS
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NEUROGLIAL CELLS OF THE PNS
- Two types:
- 1. Satellite cells –support/protection in ganglia
- 2. Neurolemmocytes-myelin sheaths in PNS
- (schwann cells)
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CONNECTIVE TISSUE IN NERVOUS SYSTEM
- Endoneurium- areolar connective tissue sheath around fiber
- Perineurium- dense irregular connective tissue around fasiculi
- Epineurium-dense irregular connective tissue with collagen fibers around whole nerve
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NERVES AND TRACTS
- Nerves
- Contain faciculi wrapped in connective tissue
- In the PNS
- Most nerves contain both sensory and motor fibers (mixed)
- Tracts
- Contains fasiculi (no connective tissue)
- In the CNS
- Tract are either sensory (ascending) or motor (descending)
- Types: association, commissural, projection
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GANGLIA AND NUCLEI
- Ganglia
- Clusters of cells bodies in PNS
- Nuclei
- Clusters of neuron cell bodies in CNS
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NERVE IMPULSE TRANSMISSION
- Mechanism by which neurons carry signals
- Nearly all body cells have and electrical difference between the outside and the inside of their cell membranes
- Due to this electrical potential difference nerve cells (and muscle cells) are excitable
- Neurons can be excited by stimulation of sensory receptors and by other neurons
- When a neuron is stimulated the electrical difference across the cell membrane is altered
- This alteration is propagated along the length of the neuron as a . . . nerve impulse
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NEUROTRANSMITTERS
- Neurotransmitters (chemicals) transmit impulse across synapse (the space between two neurons) from a presynaptic neuron to postsynaptic neuron
- Neurotransmitters are stored in synaptic vesicles in axon endings of presynaptic neuron
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SOME NEUROTRANSMITTERS IN THE BODY
- Acetylcholine
- Epinephrine
- Norepinepherine
- Dopamine
- Endorphins/enkephalins
- Serotonin
- Gamma aminobutyric acid (gaba)
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EMBRYONIC DEVELOPMENT OF THE BRAIN
- 4th week 5th week
- Prosencephalon
- (Forebrain) Telencephalon
- Diencephalon
- Mesencephalon
- (midbrain) Mesencephalon
- Rhombencephalon
- (hindbrain) Metencephalon
- Myelencephalon
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EMBRYONIC AREAS
- 1. Telencephalon-forms the cerebrum
- 2. Diencephalon-forms thalamus, hypothalamus, epithalamus
- 3. Mesencephalon-forms the midbrain
- 4. Metencephalon-forms the pons and cerebellum
- 5. Myelencephalon-forms the medulla oblongata
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BRAIN
- Gray matter:
- Interneuron and motor neuron cell bodies
- Forms the cortex that covers the surface of the adult brain
- Also makes up neuron clusters called cerebral nuclei in CNS
- White matter:
- Made up of myelinated axons
- Lies deep to the gray matter of the cortex
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CRAINIAL MENINGES
- Connective tissue layers that:
- Separates the brain from the cranium
- Enclose and protects blood vessels brain
- Contain and circulate cerebrospinal fluid (CSF)
- Form some of the veins (sinuses) that drain blood from the brain
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Three meninges are:
- Dura mater
- Arachnoid
- Pia mater
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DURA MATER
- Tough mother
- Dense irregular connective tissue
- Outermost meninx, double layered
- 1. Outer . . . periosteal dura
- 2. Inner . . . meningeal dura
- Space between the two layers of dura creates veins filled with blood called . . . dural sinuses
- Dura terminates at level of 2nd sacral vertebra
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ARACHNOID MATER
- Delicate avasular connective tissue with collagen/elastic fibers
- Between pia/dura maters
- Subarachnoid space (space under arachnoid) is filled with CSF
- Folds in arachnoid mater called villi function as 1 way passageways to allow CSF to diffuse from subarachnoid space into the blood in the dural sinuses
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PIA MATER
- Soft mother
- Thin, transparent, areolar connective tissue
- Innermost, delicate, vascular
- Follows contour of brain
- Pia mater, capillaries and ependymal cells together make up the choroid plexuses (which produce CSF)
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VENTRICLES
- Four cavities within the brain
- Continuous with one another and with the central canal of the cord
- Ventricles contain cerebrospinal fluid
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VENTRICLES IN THE BRAIN
- Lateral ventricles (2)
- One in each hemisphere of the cerebrum
- Separated by a thin septum pellucidum
- Interventricular foramina (foramina of Monro) connect to 3rd
- Third ventricle
- In the diecephalon
- Cerebral aqueduct (mesencephalon/aqueduct of Sylvius)
- Fourth ventricle
- Between pons and cerebellum
- Median and lateral (2) apertures (foramina of Magendie and Luschka) connect to subarachnoid space
- Also connects to central canal of cord
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CEREBROSPINAL FLUID (CSF)
- Formed by an ultrafiltration process from choroid plexuses in the roof of ventricles
- Circulates inside the brain (in ventricles) and outside the brain in subarachnoid space (between arachnoid and pia mater)
- Reabsorbed into arachnoid villi then into dural sinuses (returned to venous circulation
- CNS contains 150ml of CSF
- 500ml CSF produced/reabsorbed each day
- Clear, colorless fluid similar to plasma (except more Na+, Cl-, no protein)
- CSF functions to cushion and protect the brain
- CSF carries nutrients and metabolic waste to/from nervous tissue
- CNS “floats” in CSF
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CEREBRUM
- 2 cerebral hemispheres
- Tracts (bundles of axons) allow communication between hemispheres
- Corpus callosum
- Convolutions (gyri/sulci) central and lateral
- Deep groves called fissures
- Cerebral cortex (gray matter/ white matter)
- Longitudinal fissure between hemispheres
- Falx cerebri
- Lateral ventricles
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Basal nuclei are clusters of neurons found deep within white matter of cerebrum
- Basal nuclei:
- Provide subconscious control and integration of skeletal muscles tone
- Coordination of learned movement patterns
- Process, integrate and relay information from cerebral cortex to thalamus
- Caudate nucleus, amygdaloid body, lentiform nucleus (putamen and globus pallidus)
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The two hemispheres appear as anatomic mirror images but they display some functional differences, termed hemisphere lateralization.
- Left hemisphere. . . . categorical hemisphere (language, analytical, math, etc.)
- Right hemisphere. . . Representational hemisphere (imagination, music, art, etc)
- However, both hemispheres are in constant communication via corpus callosum
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CEREBRAL HEMISPHERE
- Divided into five lobes:
- 1. Frontal lobe . . . . . .voluntary motor function, decision making
- verbal communication and personality
- 2. Parietal lobe . . . . . .general sensory functions
- 3. Temporal lobe . . . .hearing and smelling
- 4. Occipital lobe . . . . processes incoming visual information
- 5. Insula . . . . . . . . . . memory and taste
- Motor areas . . . . . . . . .frontal lobe
- (control skeletal muscles)
- Sensory areas . . . . . . . parietal, occipital, temporal lobes and insula
- (sensations)
- Association areas . . . . frontal, occipital, parietal and temporal lobes
- (integrate information)
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DIENCEPHALON
- Components of the diencephalon include:
- Epithalamus
- Thalamus
- Hypothalamus
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EPITHALAMUS
- Roof of diencephalon
- Pineal gland which secretes the hormone melatonin to regulate day-night cycles
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THALAMUS
- Pair of oval masses on each side of 3rd ventricle connect via intermediate mass
- Between cerebral hemispheres
- Gray matter
- Relay station for sensory information projecting to somatosensory cortex
- All sensory impulses except olfactory converge on thalamus and synapse
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HYPOTHALAMUS
- Anterior inferior to thalamus
- Lower sidewalls and floor of 3rd ventricle
- Infundibulum (stalk) extends inferiorly to attach to the pituitary gland
- Control of the ANS and endocrine system
- Produces, releasing, and inhibitory factors
- Produces oxytocin and antidiuretic hormone
- Controls behavior, food, and water intake
- Controls body temperature
- Controls sleep-wakefulness
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LIMBIC SYSTEM
- Structures that form a ring (limbus) around the diencephalon
- The limbic system is composed of nuclei and tracts that collectively process and experience emotions
- Cingulated gyrus, hippocampus, amygdaloid body, fornix, nuclei, etc
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BRAINSTEM
- Three regions form the brainstem:
- 1. Midbrain (mesencephalon)
- 2. Pons
- 3. Medulla oblongata
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MIDBRAIN
- Dorsal midbrain consists of 4 rounded elevations:
- Corpora quadrigemina. . . .
- Superior colliculi-visual reflex centers
- Inferior colliculi-auditory reflex centers
- Trochlear nerve (IV) emerges dorsally, just bellow corpora
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Ventral midbrain consists of . . .
- Cerebral peduncles
- . . . which are descending motor axons
- Ocularmotor nerve(III) emerges ventrally
- Cerebral aqueducts run thru midbrain
- Nuclei of cranial nerves (III) and (IV) in midbrain
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PONS
- Rounded bridge on underside of brainstem
- Sensory and motor tracts in pons connect brain to cord
- Contains the pneumotaxic and apneustic centers . . .
- . . . regulates rate/depth of breathing and influence respiratory center of medulla
- Nuclei for cranial nerves V, VI, VII, some of VIII
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MEDULLA OBLONGATA
- Most inferior part of the brainstem which continues with the spinal cord inferior
- Fourth ventricle to central canal of cord
- All tracts ascend/descend thru medulla
- Pyramids on ventral surface are motor tracts. Some tracts in medulla decussate. (so motor areas on one side of cortex control muscles on other side of body)
- Nuclei of cranial nerves VII, IX, X, XI, XII
- Medulla oblongata . . .contains several autonomic nuclei which group to form three important centers
- Cardiac center-regulates heart rate and strength of contraction
- Vasomotor center-controls blood pressure by regulating the contraction and relaxation of smooth muscle in the walls of arterioles
- Respiratory center-reglates respiratory rate. Influenced by the apneustic and pneumotaxic centers of the pons
- Other nuclei-coughing, sneezing, salivation, swallowing, gaging, and vomiting
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CEREBELLUM
- 2nd largest part of the brain
- Behind cerebrum
- Highly convoluted surface
- 2 cerebellar hemispheres
- Cortex/gray matter
- Arbor vitae/white matter
- Three pairs of cerebellar peduncle connect cerebellum to midbrain, pons and medulla
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CEREBELLAR FUNCTION
- Coordinates skeletal muscle movements for smooth, coordinated movements
- Stores learned movement patterns
- Maintains equilibrium and posture
- Receives proprioceptive (sensory) information from the muscle
- and joints to regulate body position
- Monitors the position of joints and muscle tone
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SPINAL CORD
- 18 inches in length
- CNS below the medulla
- Passes thru vertebral canal of vertebrae
- Extends from foramen magnum to level of 1st or 2nd lumbar vertebra
- Serves as link between brain and the rest of body
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FUNCTIONS
- Pathway (sensory/motor impulses
- Spinal reflexes
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GROSS ANATOMY OF THE SPINAL CORD
- Spinal cord is shorter than the vertebral canal
- Tapering inferior end of the spinal cord is called the conus medullaris (usally at the level of the first/second lumbar vertebra)
- Inferior to the conus medullaris group of axons called cauda equine project inferiorly from the spinal cord
- Within the cauda equine is the filum terminale which is a thin strand of pia mater (anchors conus medullaris to coccyx)
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Spinal cord…is separated into 2 regions
- Inner…gray matter
- Outer…white matter
- Gray matter—dendrites and cell bodies of neurons, unmyelinated axons, and glial cells
- White matter—myelinated axons
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GRAY MATTER
- Anterior horn: contains cell bodies of somatic neurons which innervate skeletal muscle
- Lateral horn: found only in the T1-L2 of the spinal cord. Contains cells bodies of autonomic motor neurons which innervate cardiac muscle, smooth muscle and glands
- Posterior horn: contains axons of sensory neurons and cell bodies of interneurons
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WHITE MATTER
- The white matter of the spinal cord is external to the gray matter and is partitioned into three areas called funiculi:
- Posterior
- Lateral
- Anterior
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Axons within each funiculus = tracts
A tract + its nucleus is called a …pathway
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PATHWAY
- Pathways are ascending and descending
- Ascending pathways……carry sensory information from the body to the brain
- Descending pathways……transmit motor information from the brain to muscles or glands
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