NERVOUS SYSTEM

ORGANIZATION

Composed of neurons
Related neurons organized into nerves
Organized into CNS (central n.s.) and peripheral n.s.
CNS- brain and spinal cord
Peripheral n.s.- nerves radiate from brain (cranial) and spinal cord (spinal) to body
Ganglion-mass of neurons outside CNS
Receives messages from body and carries them to brain
Receptor- receive information (internal and external stimuli)
Conductor- transmit messages from receptors to CNS
Brain integrates and interprets message
Responses transmitted to body
Effectors- receive messages from CNS and carry them to body
Activates muscles and glands
Diagram - Nervous System
Diagram - Nervous system
Encarta - Nervous System

BRAIN

1.4 Kg (2% of body weight)
Approximately 100 billion neurons
20% of oxygen consumption
Diagram - Brain 1
Diagram - Brain 2
Diagram - Brain- 3
Encarta - Brain

Cerebrum

Largest portion of brain
2 cerebral hemispheres
Connected by nerves (corpus callosum)
4 lobes (frontal, parietial, temporal, and occipital)
Highly folded outer layer (cerebral cortex)
Maximizes surface area of brain
Outer portion contains 75% of bodies neuron cell bodies (gray matter)
Inner portion (beneath cerebral cortex) contains bundles of nerve axons that connect the cortex with other parts of nervous system (white matter)
Separate areas control motor and sensory activities
Both hemispheres control voluntary movement of skeletal muscles
Interpret sensory stimulation; impulses crossed (L -> R hemisphere; R -> L)
Right controls verbal and non verbal artistic abilities
Left controls reading, writing, and analyzing information

Cerebellum

Below occipital lobes
Coordination of muscle action
Receives impulses from muscles, tendons, joints, eyes, and ears
Sends motor impulses -> spinal cord to stimulate/inhibit skeletal muscles
Controls balance (involuntary action)
Coordinates with sense receptors (eyes and ears)
Relays information regarding body position to appropriate muscles

Brain Stem

Connects cerebrum with spinal cord
Relays information to and from cerebrum
Controls body actions
Midbrain- controls reflexes
Medulla oblongata- nerve relay station and control center
Information crossover (R -> L cerebral hemisphere; L -> R)
Controls homeostatic activities (heart rate, breathing rate, and blood flow/pressure)
Pons- relay station between cerebrum and cerebellum and mid brain and medulla oblongata
Thalamus-directs incoming sensory impulses (except odors) to cerebral cortex
Hypothalamus- connected to thalamus, cerebral cortex and brain stem
Interacts with pituitary gland (endocrine system), maintains chemical homeostasis
Limbic system- group of neurons that connect thalamus, hypothalamus, and corpus collosum regulates emotions
Detects changes in physiological state; responds by producing feelings (fears and anxiety)
Reticular formation- nerve network; runs from brain stem to thalamus
Filters incoming sensory impulses (information)

Protection

CNS surrounded by meninges (3 tissue layers)
Dura mater (outer layer)- connective tissue, blood vessels, & nerves
Arachnoid layer (middle layer)- elastic and weblike
Pia mater (inner layer)- contains many nerves and blood vessels
Cerebrospinal fluid- separates arachnoid layer and pia mater; provides fluid cushion; protects CNS from shock
Cranium encloses brain
Vertebrae surround spinal cord

SPINAL CORD

Starts in the medulla oblongata through vertebral column
Outer sheath white matter; inner core gray matter
31 pair spinal nerves- each consists of dorsal and ventral root
Dorsal root- contains sensory neurons (receptor -> spinal cord)
Ventral root- contains motor neurons (spinal cord -> effectors)
Association neurons (interneurons)- connections within spinal cord

PERIPHERAL NERVOUS SYSTEM

12 pair of cranial nerves connect brain with various parts of head and upper neck
31 pair of spinal nerves connect spinal cord with body
Reflex actions- involuntary and usually self protective
Reflex arc- involving a minimum of one
Sensory, one association, and one motor neuron (spinal cord responsible)
Sensory impulse reaches brain later
Somatic n.s.-control voluntary movement (skeletal muscles)
Autonomic n.s.- controls involuntary action (respiration/heart rate)
Sympathetic n.s.- controls stimulation of internal organs during high stress or increased activity; emergency preparation
Parasympathetic n.s.- controls internal organs during routine conditions; counteracts sympathetic n.s.
Diagram - Autonomic nervous system
Alternating action of systems help maintain homeostasis

TRANSMISSION OF NERVE IMPULSE

Neuron Structure

Impulses (electrical signals) carried by neurons
Composed of nucleated cell body, numerous dendrites, and a single axon
Dendrites- branched; receive impulses; carry impulses to cell body
Axon- elongated extension; carries impulses away from cell body
Axon surrounded by a layer of cells (Schwann cells) which form myelin sheath (insulates and protects axon)
Exposed at gaps called nodes of Ranvier
Impulses travels from dendrites -> cell body -> axon -> axon terminals -> chemical messenger crosses gap (synapse) between neurons -> organ/gland/dendrite
Diagram - Neurons- types
Encarta - Neurophysiology

Nerve Impulse

Flow of electrical charges along the cell membrane of neuron
Begins at a receptor cell and flows from neuron to neuron until it reaches effector
Resting state (not conducting impulse)
High concentration of K+ and negatively charged organic ions inside cell membrane; concentration of Na+ higher outside
Differences maintained by (1) selectively permeable membrane (impermeable to Na+ and organic ions; permeable to K+) and (2) sodium- potassium pump (Na+ pumped out; K+ pumped inside; requires active transport (ATP)
Differences in ion concentration results in negative charge inside membrane creates electrochemical potential (polarized neuron)

Impulse Transmission

All-or-none stimulation
Generated impulse results in sudden change in membrane permeability
Neuron becomes permeable to Na+ (diffuse in)
Inner surface becomes positively charged
Reverses electrical potential (depolarized)
Immediately disrupts the adjacent part of membrane
Creates wave of depolarization (action potential)
Myelin sheath and nodes of Ranvier speed up transmission
Refractory period- time that follows the passage of nerve impulse (return to its resting potential)
Diagram - Nerve impulse

Transmission Across Synapse

Synapse-gap between axon terminal of one neuron and dendrite of another neuron
Carried across synapse by neurotransmitters (chemical messengers)
Chemicals stored in synaptic vesicles embedded in bouton at tip of axon terminal; enclosed by presynaptic membrane
(1) synaptic vesicles fuse with presynaptic membrane; (2) release neurotransmitters into synapsis; (3) neurotransmitters bind to receptor molecules in postsynaptic membrane of adjacent dendrite; (4) crates new action potential; (5) continues down dendrite
Neurotransmitters destroyed by enzymes (stimulation discontinued)
Impulse transmitted along neuron until reaches muscle or gland
Diagram - Synapses- anatomy and types
Diagram - Synapse

Effects of Neurotransmitters

Either stimulates action potential or inhibits it
30 different neurotransmitters identified
Some used to treat or cause various nervous disorders

THE SENSES

Different stimuli -> highly developed system of nerves -> specialized cells and organs
Environmental stimuli -> receptor cells -> conductors -> central nervous system -> brain
Receptors
Mechanoreceptors (movement, pressure, or tension)
Photoreceptors (variation in light)
Chemoreceptors (chemicals)
Pain receptors (tissue damage)
Encarta - Sense Organs

Sense Organs (eyes, ears, nose, mouth and skin)

Specific stimuli -> sense organs -> receptors -> electrical impulses -> specific region of brain
Impulses are electrically similar; region of brain varies
Brain has sense specific regions

Vision (eyes)

Eyes receive light -> transmit impulse to visual cortex of brain
Structures function together
Retina (light-sensitive inner layer of eye)
Pupil (opening in iris)
Iris (controls amount of light entering eye; pigmented)
Lens (convex crystalline structure; focuses light onto retina)
Rods (photoreceptor, stimulated by weak light)
Contains rhodopsin (detects light as shades of gray)
Cones (photoreceptor; stimulated by bright light)
Sharp images and colors
3 kinds of cones; different pigments respond to different wave lengths
Integration of impulses -> visible color spectrum
Color blindness -> inability to distinguish certain colors
Light -> cones and rods -> optic nerve -> occipital lobe of brain -> interpreted as vision
Diagram - Eye 1
Diagram - Eye 2
Diagram - Eye 3
Encarta - Eyes
Encarta - Vision

Hearing and Balance (ears)

Sound = air vibrations
Air vibrations -> auditory canal -> vibrate tympanic membrane (ear drum) -> 3 ear ossicles (bones; hammer, anvil and stirrup) -> oval window -> cochlea (filled with fluid and hair cells) -> vibrations bend hair cells -> auditory nerve -> auditory region of brain -> interpreted as sound
Different pitches stimulate different hair cells
Intensity proportional to number of stimulated hair cells
Air pressure in middle and inner ear regulated by Eustachian tube
Balance maintained by mechanoreceptors in 3 semicircular canals (inner ear)
Filled with fluid and lined with hairs
Movement of fluid stimulates hair cells
Other hair cells (between cochlea and semicircular canal) respond to the movement of calcium carbonate particles on their surface and detect gravitational changes
Neurons -> signal -> cerebrum where motion and position are interpreted
Diagram -  Ear 1
Diagram -  Ear 2
Diagram -  Ear 3
Encarta - Ear
Encarta - Hearing

Taste and Smell (tongue and nose)

Special chemoreceptors perceive variations in taste and smell
Taste
Clustered in taste buds (>10,000) between bumps (papillae) on the tongue, roof of mouth and throat
Chemicals and saliva -> different tastes -> opening of taste buds -> stimulates chemoreceptors -> impulse -> cerebrum -> interpreted as taste (sweet, sour, salty and bitter)
Other flavors are perceived by receptors in nasal passage
Diagram - Coordination- taste
Encarta - Taste
Smell
Olfactory receptors located in olfactory epithelium of nasal passage
Different receptors respond to different chemicals
Chemicals mix with mucus before stimulating the olfactory receptors -> impulse -> olfactory nerve -> olfactory region of cerebral cortex -> interpreted as odor
Diagram - Coordination - smell
Encarta - Smell

Other Senses

Mechanoreceptors (skin) -> sense of touch, pressure, and tension
Touch receptors concentrated on face and finger tips
Receptors also at base of hairs
Encarta - Touch
Thermoreceptors
Cold receptors (sensitive to temperatures < 20o)
Heat receptors (sensitive to temperatures > 25o)
Pain receptors
Base of epidermis
Stimulated by mechanical, thermal, electrical or chemical energy
Type and number vary throughout body

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Body Quest
The Virtual Body
Innerbody
St. Mary's Biology - Human Body Service Manual
The Mississippi School for Mathemataics and Science - Nervous System
Wadsworth - Nervous System
On-Line Biology Book - The Nervous System (Excellent)
Human Body Atlas- Brain
Human Body Atlas- Eyes
Human Body Atlas- Ears
Human Body Atlas- Nose
Mayo Clinic - Alzheimer's Resource Center
Wayne State - Nerves
Synaptic transmission
Neuroscience for kids:
Brain Facts
Brain Briefings (Society for Neuroscience)