Chapter 50: Nervous System and Sensory Organs
50-1 Central Nervous System
50-2 Peripheral Nervous System
50-3 Transmission of Nerve Impulses
50-4 Sensory Systems
50-1 Central Nervous System
I. Organization (neuron is the cell type)
• TWO divisions of NERVE NETWORKS regulate our body:
(1) CNS (MASTER headquarters)
(2) PNS (SATELLITE centers)
Critical Thinking
(1) What functional advantages might a neuron with SEVERAL dendrites
have over a neuron with only ONE dendrite?
(1) Axon (signal sent AWAY)
• EXTENSION of soma (cell body),  transmits NERVE IMPULSES
between neurons of CNS and PNS.
(2) Central Nervous System (CNS)
• Includes BRAIN and SPINAL CORD, nerves ENCASED in axial skeleton.
(3) Peripheral Nervous System (PNS)
• 32 PAIRS of NERVES that EXTEND spinal cord, receiving AND
transmitting IMPULSES throughout BODY.
(4) Afferent Neurons (i.e., SENSORY neurons  “What do we have here?”)
• COLLECT data from SENSES and transmit impulses TOWARDS CNS [via
dorsal roots].
(5) Efferent Neurons (i.e., MOTOR neurons  “What do we do now?”)
• RECEIVE orders from CNS and RELAY it to PNS (typically to MUSCLES
and GLANDS) [ via ventral roots].
II. Brain (conscious AND subconscious organ ~ 100 billion neurons)
• 2% of body weight, BUT DEMANDS 20% of water AND glucose supply.
(A) Cerebrum (controls MOTOR and SENSORY activities)
• Largest portion of brain, composed of TWO hemispheres.
Critical Thinking
(2) Strokes result in the DEATH of neurons in the brain. How might a
doctor tell WHICH AREAS of the brain have been AFFECTED if a person
has suffered from a stroke?
(1) Cerebral Hemispheres (L (Speech and Language), R (Reasoning)
• Connected by CORPUS CALLOSUM and contain cerebral CORTEX.
(2) Corpus Callosum
• THICK BAND of AXONS of many INTERCONNECTED neurons of 2
cerebral HEMISPHERES.
(3) Cerebral Cortex (wrinkled OUTER layer—contains FOUR lobes)
• OCCIPITAL Lobe (vision), PARIETAL Lobe (tactile senses), TEMPORAL
Lobe (hearing), and FRONTAL Lobe (problem-solving).
(4) White Matter (and Gray Matter)
• Composed of AXONS that link CORTEX with other CENTERS of brain.
(NOTE: GRAY matter consists of the cell BODIES of neurons)
(B) Upper Brain Stem (Diencephalon—hypothalamus AND thalamus)
• LINKS cerebrum WITH spinal cord, contains RELAY centers for data
ENTERING and EXITING cerebrum.
(1) Thalamus (upper RELAY center FOR SENSES)
• DIRECTS incoming SENSORY signals to proper LOBE of cerebral cortex.
(NOTE: Hypothalamus—assists w/hormone production/homeostasis)
Critical Thinking
(3) Synesthesia is a puzzling phenomenon in which one type of sensory
input is interpreted by the brain as another type. For example, a person
hearing music might associate certain notes with certain colors. What may
be happening in the central nervous system to produce this effect?
(2) Limbic System (includes diencephalon & PRIMITIVE brain)
• Plays a role in EMOTION, MEMORY, REFLEXES, and MOTIVATION.
(C) Lower Brain Stem (i.e., midbrain, pons, AND medulla oblongata)
• BELOW diencephalon, brain stem NARROWS (into midbrain, pons, and
medulla oblongata) BECOMING continuous with SPINAL CORD.
(1) Midbrain (innervates optic AND auditory nerves—SEE RED BELOW)
• Relay center for VISUAL (occipital lobe) and AUDITORY (temporal lobe)
data.
(2) Pons (critical region for GYMNASTS)
• Relay center between neurons of CEREBRUM and CEREBELLUM.
(3) Medulla Oblongata (BOTH relay AND control center—involuntary)
• Regulates HEART and BREATHING rates—homeostatic activity of body.
(4) Reticular Formation (network of neurons THROUGHOUT brain stem)
• A FILTER that separates IMPORTANT sensory signals FROM
UNIMPORTANT signals to CEREBRUM.
(D) Cerebellum (tied into PONS and CEREBRUM)
• Works with brain MOTOR CENTERS to coordinate responses (i.e., muscle
contractions, movements, BALANCE, and body posture)
(E) Protection (in addition to SKULL)
• Tissue protected by meninge CUSHION and CSF MATRIX.
(1) Meninges (e.g., dura mater, arachnoid layer, and pia mater)
• THREE layers SURROUNDING neurons of CNS.
(2) Dura Mater (OUTER layer, SEE BLUE BELOW)
• Consists of connective tissue, blood vessels, and neurons.
(3) Arachnoid Layer (MIDDLE layer, SEE GOLD BELOW)
• Elastic and WEB-LIKE, provides FLEXIBILITY to spinal cord.
(4) Pia Mater (INNER layer, SEE LIGHT PINK BELOW)
• Thin layer ADHERING to CNS, rich in blood vessels and neurons.
(5) Cerebrospinal Fluid (CSF, fills inner and middle meninges)
• Provides protective CUSHION and transport MEDIUM for neurotropins.
(6) Ventricles (FOUR interconnected CAVITIES in brain)
• Filled with CSF, acting as CHAMBERS for transport medium and cushion.
(F) Spinal Cord (neural tissue—medulla THROUGH vertebral column)
• OUTER sheath of WHITE matter (AXONS of neurons) surrounds an
INNER core of GRAY matter (SOMA of neurons).
Critical Thinking
(4) Why might an injury to the LOWER spinal cord cause a loss of
sensation in the LEGS?
(1) Nerve (a group of BUNDLED axons)
• Each SPINAL NERVE consists of a DORSAL root (carries sensory signals
to CNS) and a VENTRAL root (carries signals to MUSCLES and GLANDS).
(2) Sensory Receptor (data FROM receptor to spinal cord—DORSAL roots)
• AFFERENT neuron to detect a STIMULUS, (pressure, heat, or pain).
(3) Motor Neurons (axons in VENTRAL roots of spinal nerve)
• EFFERENT neuron carry data from spinal cord to MUSCLES and glands.
(4) Interneurons (located in spinal cord AND in body)
• CONNECT neurons (often of different types) to EACH OTHER.
50-2 Peripheral Nervous System (S & M Divisions)
I. Sensory Division (PART of PNS, sensory neurons and interneurons)
• ACQUIRES DATA from external AND internal sources of body (e.g.,
spinal and cranial nerves connect PNS to CNS)
II. Motor Division (PART of PNS, motor neurons and interneurons)
• Allows body to REACT to sensory information
(NOTE: MD is divided into TWO subdivisions):
(1) Somatic Nervous System (fed from EXTERNAL ENVIRONMENT)
(2) Autonomic Nervous System (fed from INTERNAL ENVIRONMENT)
(A) Somatic Nervous System (i.e., VOLUNTARY behaviors and reflexes)
• Motor neurons that REGULATE movement of SKELETAL muscles.
(1) Reflexes (e.g., match to finger)
• INVOLUNTARY relay of signals, ~ for SELF-PROTECTIVE movements.
(2) Spinal Reflex (e.g., patellar reflex, BRAIN is LEFT in the DARK)
• Involves ONLY neurons in body AND spinal cord (i.e., ONLY sensory
receptors, sensory neurons, and interneurons are involved).
(B) Autonomic Nervous System (i.e., Involuntary Behaviors—2 Divisions)
• Regulates INTERNAL conditions by affecting SMOOTH muscles 
MOST IMPORTANT to homeostasis. (e.g., respiration rate, heartbeat)
Critical Thinking
(5) Most organs in the body are stimulated by BOTH the sympathetic
division and the parasympathetic division of the autonomic nervous system.
Explain HOW this may help maintain homeostasis in the body.
(1) Sympathetic Division (i.e., “fight-OR-flight” response)
• Activated by physical or emotional STRESS, shunting blood AWAY from
digestive organs and TOWARDS heart and skeletal muscles.
(2) Parasympathetic Division (i.e., “rest-AND-digest” response)
• Signals organs to revert to NORMAL levels of activity; body is induced to
CONSERVE energy (NOTE: Both systems work together in BALANCE).
50-3 Transmission of Nerve Impulses
I. Neuron Structure (transmits “action potentials,” i.e., a nerve impulse)
• Electrochemical NRG is made as CHARGED IONS move into AND out of a
neuron’s membrane.
Critical Thinking
(6) An imbalance of electrolytes, the ion-containing fluids of the body, can
impair transmission of nerve signals. Why might this be so?
(1) Dendrites (NUMEROUS extensions of CELL BODY)
• RECEIVE chemicals from OTHER neurons AND either CONTINUE impulse
(excitatory) or STOP signal (inhibitory) down AXON.
(2) Axon Terminal
• END POINTS of an axon that may touch a muscle, gland, OR a dendrite of
another neuron.
Critical Thinking
(7) Epilepsy affects one of every 200 Americans. Brain neurons normally
produce small bursts of action potentials in varying patterns. During an
epileptic seizure, large numbers of brain neurons send rapid bursts of
action potentials simultaneously. The body of an individual having a seizure
may grow rigid and convulse. From what you know about the brain’s control
of muscles and posture, how might one explain these symptoms?
(3) Myelin Sheath (FATTY covering of an AXON)
• SPEEDS UP transmission of action potentials ALONG axon of a neuron.
(4) Schwann Cells (i.e., “glial cells” surround AXONS)
• Builds myelin sheath around AXON, BUT leaves special GAPS
(known as “Nodes of Ranvier”).
(5) Nodes of Ranvier (electrical impulse JUMPS from node to node)
• GAPS in myelin sheath  forces impulse to JUMP along length of axon.
(6) Synaptic Cleft (i.e., neurons do NOT touch each other directly)
• A gap between AXON TERMINALS of one neuron and DENDRITE of
another neuron—site of NEUROTRANSMITTER activity.
(7) Neurotransmitters (released by PRE-synaptic terminal into CLEFT)
• Chemicals that ELICIT electrical activity  will either CONTINUE or
INHIBIT signal.
(i.e., ELECTRICAL activity WITHIN neurons and CHEMICAL activity
BETWEEN neurons).
II. Neuron Function (dependent on ELECTRICAL activity)
• Neurons have an e- charge DIFFERENT from ECM that surrounds them
(e.g., this produces a “POTENTIAL”).
(1) Potential (i.e., DIFFERENCE in electrical charge, due to ION movement)
• Ion movement  (1) PERMEABILITY of Membrane,
(2) CONCENTRATION gradient, and (3) Ion CHARGE.
(A) Resting Potential (~ -70 millivolts in a human neuron, Na-K pump)
• At REST, [K+] > INSIDE CELL with (-) charged proteins AND
[Na+] > OUTSIDE CELL;
•K+ diffuses OUT, BUT larger proteins are trapped—results with the
INTERIOR becoming slightly (-) CHARGED.
(B) Action Potential (involves voltage gated channels for Na/K)
• Neurotransmitter  Permeability to Na + Ions  A wave of (+) charge
passes DOWN AXON (1-way)  AP reaches axon terminals, resulting in a
release of neurotransmitters.
(NOW, neuron enters the “refractory period”).
(1) Refractory Period (neuron CANNOT fire until resting returns)
• Voltage-gated channels for Na + CLOSE, and channels for K+ OPEN 
OUTER surface again becomes “+” charged, and inner returns to “–” charge.
NOTE: Neurons use a HUGE amount of ATP to drive Na-K PUMPS, which
help to recharge a neuron BACK to its RESTING POTENTIAL.
(C) Neurotransmitter Function (can either EXCITE or INHIBIT p.s.n.)
• NT binds to POST-synaptic membrane, and if capable of OPENING (+) ion
channels INTO cell, AP will CONTINUE.
50-4 Sensory Systems
I. Receptors and Sense Organs (5 classes of receptors in PNS)
• Data COLLECTED and TRANSLATED into electrical signals to CNS for
INTERPRETATION.
(1) Mechanoreceptors (located in DERMIS of skin)
• Respond to stimuli of movement, PRESSURE, and tension.
(2) Photoreceptors (located deep within RETINA of eye)
• Respond to variations of LIGHT and DARK, including COLOR.
(3) Chemoreceptors (found in TASTE BUDS, nasal passage, mouth)
• Respond to types of SUBSTANCES found in AIR or in FOOD.
(4) Thermoreceptors (found in DERMIS of the skin)
• Respond to CHANGES in TEMPERATURE.
(5) Pain Receptors (concentrated HIGHLY in hands and mouth)
• Respond to stimuli resulting in TISSUE DAMAGE.
II. Hearing and Balance (human ears are specialized for BOTH)
• Sound is VIBRATIONS turned electrical (our medium is air)—balance
achieved by mechanoreceptors in INNER EAR.
(1) Auditory Canal
• Connects EXTERNAL ear into TYMPANIC MEMBRANE.
(2) Tympanic Membrane (i.e., the eardrum)
• SOUND WAVES in air  eardrums VIBRATE and transfer NRG to 3
SMALL BONES.
(3) Eustachian Tube (e.g., opening of MIDDLE ear to throat)
• Allows air BEHIND eardrum to be BALANCED to fine-tune hearing
(higher altitude, ears “POP” volume drops).
(4) Oval Window (membrane separating middle ear FROM inner ear)
• Transfers vibrations RECEIVED by middle ear bones (incus, malleus, and
stapes) to COCHLEA.
(5) Cochlea (coiled tube ~ semicircular canals AND organ of Corti)
• Cochlear fluid = MEDIUM for vibrations and works with HAIR CELLS to
send NERVE IMPULSES (to auditory nerve  thalamus  temporal lobe).
(6) Organ of Corti (i.e., the organ of HEARING)
• Contains HAIR CELLS  send AP from VIBRATIONS into cochlear fluid.
(7) Semicircular Canals (three continuous CANALS of cochlear fluid)
• Lined with hair cells STUDDED with CaCO3 on top  detects GRAVITY
and body’s ORIENTATION in space.
III. Vision
• Light BECOMES nerve impulses at OPTIC nerve, to THALAMUS, and to
OCCIPITAL lobe of cerebrum.
(1) Retina (sends signal to optic nerve)
• LIGHT-SENSITIVE inner layer of eye (photoreceptors).
(2) Cornea (FIRST structure light MUST pass through)
• Clear, PROTECTIVE layer of eyeball COVERING pupil and inner eye.
(3) Pupil
• Opening to LIGHT  regulated by INVOLUNTARY contractions of IRIS.
(4) Iris (PIGMENTED part of the eye)
• Regulates pupil OPENING to extreme brightness (tiny pupils) or darkness
(wide pupils).
(5) Lens (convex structure, directly BEHIND pupil)
• Muscles ATTACH to lens, bending light rays to FOCUS image onto retina.
(6) Rods (~125 million PHOTORECEPTORS, send data to retina)
• Contain rhodopsin, a LIGHT-sensitive pigment  allows vision in dim light
(i.e, DETECTS LIGHT/DARKNESS).
(7) Cones (~7 million PHOTORECEPTORS (3 types), send data to retina)
• Contains a PIGMENT that absorbs DIFFERENT wavelengths of light
(i.e., DETECTS COLORS of visible spectrum).
(8) Optic Nerve (results AT a CONVERGENCE of retinal photoreceptors)
• Carries VISUAL information (i.e., AP) from RETINA to THALAMUS, and
to OCCIPITAL lobe.
In the diagram above, notice the exit of the optic nerve
to the brain. Any light falling directly on this point does
not stimulate the retina, making this point blind. You can
use the figure below to demonstrate the blind spot.
Cover your right eye. Look at the "+" with the left
eye and move your head toward or away from the
screen. Although you are looking at the plus mark,
concentrate on the spot. At some point in your head
movement, the spot will disappear
... It is being projected onto the area of the blind spot
where the optic nerve exits the retina. Since the blind
spot in each eye is NOT aimed at the same spot, their
images do not overlap. If you uncover your right eye, the
spot is projected onto the retina of that eye and you "see"
it.
IV. Taste and Smell (tied closely into our LIMBIC system)
• Chemoreceptors allow for differences in TASTES and AROMAS.
(1) Taste Buds (~10,000 clustered chemoreceptors)
• Embedded between PAPILLAE on tongue, PALATES, and THROAT.
(2) Papillae
• Bumps of TONGUE epithelium that contain (in valleys) our TASTE BUDS.
(3) Olfactory Receptors (in nasal passageway)
• Bind to ODOR molecules, triggering AP towards olfactory bulb (part of
LIMBIC system).
(4) Amygdala (a SECOND part of our limbic system)
• Receives signals from our OLFACTORY senses and is tied INTO our
EMOTIONS and long-term MEMORY.
V. Other Senses
• Tactile senses (mechanoreceptors—pressure, thermoreceptors—hot and
cold, pain receptors—damaged cells)
NOTE: Sensory input from UPPER regions of our body enters though
UPPER dorsal sections of spinal cord, while sensory input from LOWER
body enters dorsal roots of LOWER spinal cord.
Cover your right eye. Look at the "+" with the left
eye and move your head toward or away from the
screen. Although you are looking at the plus mark,
concentrate on the spot. At some point in your head
movement, the spot will disappear
... It is being projected onto the area of the blind spot
where the optic nerve exits the retina. Since the blind
spot in each eye is NOT aimed at the same spot, their
images do not overlap. If you uncover your right eye, the
spot is projected onto the retina of that eye and you "see"
it.