A. Resting Membrane Potential.
        1. Neurons, like most cells, have a negative charge inside them.
 

Concentration of Ions (millimoles) in the Giant Axon of a Squid
Ion	Seawater	Squid Blood	Squid Axon
K+	10	20	400
Na+	450	450	50
C-	550	550	100
Ca2+	10	10	0.5
Mg2+	55	55	10
Organic ions	-	-	400

        2. This asymmetric distribution of ions across their membranes is maintained by the sodium-potassium pump.
            a. ATP is used to expel Na⁺ and import K⁺.
            b. The pump must work constantly against the diffusion of these ions down the concentration gradient.
        3. K⁺ diffuses out more freely than Na⁺ diffuses in, so a net positive charge is set up outside the cell.

    B. Local Changes in Membrane Potential.
        1. Ions cross the membrane by passing through certain membrane proteins.
        2. These proteins form permeability channels with “gates” which can open or close.
        3. Opening a gate allows the ion to rush across the membrane, down the concentration gradient.
        4. The Na⁺ gates are opened by a change in membrane potential.
        5. A stimulus impinging on a dendrite causes a change in the local membrane potential.
        6. Then the gates open and Na⁺ ions rush through, changing the potential further.
        7. The polarity of the cell is reversed, with the inside becoming positive in relation to the outside.
        8. This opens the other Na⁺ gates nearby.
        9. Thus the change in membrane potential passes rapidly along the membrane.
        10. This change in membrane permeability lasts for less than 1 millisecond.
        11. Then the Na⁺ gates immediately close and the K⁺ gates open.
        12. K⁺ flows out of the cell, restoring the membrane potential.
        13. The cytoplasm resists this ionic flow, and the impulse may be lost.
        14. This prevents us from being distracted by every little input from our environment.
        15. The impulse will continue if certain conditions are met.
            a. It is strong enough.
            b. It is sustained.
            c. It is joined by impulses from many dendrites.
        16. When the impulse exceeds the axon’s threshold, it will move down the axon unchanged.

    C. Transmission Across a Synapse.
        1. Most signals are carried across a synapse by chemicals called neurotransmitters.
        2. These are stored in synaptic vesicles.
        3. When a signal reaches the end of an axon, it triggers an influx of Ca²⁺ ions.
        4. These Ca²⁺ ions cause the synaptic vesicles to burst and release their contents into the synaptic cleft.
        5. They cross the space (only 20 nm wide) and bind to receptor sites on the dendrite opposite the axon.
        6. This changes the membrane potential and sets the signal on its way again.
        7. Since neurotransmitters are found only in axons, signals can be transmitted in only one direction.
        8. Some synapses are “electrical” synapses.
        9. The synaptic cleft is only 2 nm wide and the electrical charge jumps across the space.
        10. These synapses are called “gap junctions”.
        11. In contrast to “chemical” synapses, these can transmit a signal in both directions.

IV. Central Nervous System.

    A. Brain.
        1. Coverings.
            a. Dura mater.
            b. Arachnoid.
            c. Pia mater.
        2. Cerebrospinal fluid.
            a. Bathes the cells of brain and spinal cord.
            b. Protects.
        3. Cerebrum.
            a. The large part.
            b. Two hemispheres.
            c. Cerebral cortex (outer portion) contains 12-15 billion neurons.
            d. Controls conscious activities.
        4. Thalamus.
            a. Two oval gray masses near the center of the brain.
            b. Where environmental sensations go first.
        5. Hypothalamus.
            a. Controls many involuntary activities.
            b. Controls hormone release by the pituitary gland and makes two hormones itself.
        6. Midbrain: controls body movement, vision, and hearing.
        7. Pons.
            a. Carries information from one side of the brain to the other.
            b. Controls some reflexes.
        8. Medulla oblongata.
            a. Connects the spinal cord and the brain.
            b. Controls respiration, blood vessel diameter, and heart rate.
        9. Cerebellum.
            a. Second largest part.
            b. Behind and below the cerebrum.
            c. Two hemispheres.
            d. Subconscious functions only.

    B. Spinal Cord.
        1. A cylindrical mass of nervous tissue composed of 31 segments.
        2. Transverse section reveals two masses of tissue joined at the center.
            a. H-shaped gray matter region in the center.
            b. White matter around the gray matter.
        3. Gray matter contains interneurons and the soma of sensory and motor neurons.
        4. White matter is vertical myelinated axons.
        5. It is a high-speed insulated communication cable.

V. Peripheral Nervous System.

    A. Somatic nervous system.
        1. The part of the peripheral nervous system we consciously control.
        2. Contains both motor and sensory neurons.
        3. Controls skeletal muscles.

    B. Autonomic nervous system.
        1. The involuntary part of the peripheral nervous system.
        2. Contains only motor neurons.
        3. Sympathetic nervous system.
            a. Enables the body to adjust to stressful situations.
            b. Causes the adrenal glands to release adrenalin.
        4. Parasympathetic nervous system: maintains normal body functions.

    C. Reflex Arc.
        1. Sensory neurons are stimulated.
        2. The impulse is transmitted to the spinal cord.
        3. It is passed to one or several interneurons.
        4. The impulse is transmitted to one or more motor neurons.
        5. The signal is then carried to the muscle, which moves the body.
        6. A signal is also sent to the brain, informing it of the stimulus.
        7. The motor neurons transmit the signal before the brain receives it, however.

VI. Minor Senses.

    A. Introduction.
        1. All sense organs contain receptors, specialized dendrites of sensory neurons.
        2. Receptors are usually stimulated by only one sort of stimulus (e.g. light, or heat, etc.).

    B. Touch.
        1. 5 cutaneous sensations.
        2. Touch.
            a. Near exterior of skin.
            b. Most numerous in finger tips, palms, soles, roots of hair.
        3. Heat: deep in the dermis.
        4. Pain: ubiquitous.
        5. Cold: dermis and subcutaneous regions of skin; cornea of eye; tip of tongue.
        6. Pressure: below the skin; membranes of abdominal cavity; around joints and tendons.

    C. Taste.
        1. 10,000 taste buds in adults.
        2. Mainly in tongue, but also in mouth, tonsils, epiglottis.
        3. Taste bud cells are not neurons, but are attached to nerve endings.
        4. Short life span – replaced constantly.
        5. Replacement decreases with age.

    D. Smell.
        1. Olfactory receptors are present in the mucous membranes of the nasal cavities.
        2. Decrease with age.
        3. Several 1,000 odors distinguishable.

VII. Ear.

    A. Structure.
        1. Outer ear.
            a. Auricle: collects sound waves.
            b. External auditory canal: a tube in the head connecting the auricle and the tympanic membrane.
            c. Tympanic membrane (eardrum).
        2. Middle ear.
            a. A moist, air-filled chamber containing 3 ossicles.
                i. Malleus (hammer).
                ii. Incus (anvil).
                iii. Stapes (stirrup).
            b. These 3 bones transfer vibrations from the tympanic membrane to the oval window of the inner ear.
            c. Two small muscles amplify soft noises and dampen loud noises.
            d. The Eustachian tube connects to the pharynx, allowing air pressure in the middle ear to remain the same as atmospheric pressure.
        3. Inner ear.
            a. Bony labyrinth containing a membranous labyrinth.
            b. The cochlea is divided into the vestibular canal, the  middle canal, and the tympanic canal.
            c. These are divided by the vestibular membrane and the basilar membrane.
            d. The middle canal contains the organ of Corti along its full length.
            e. The organ of Corti contains the sound receptors.
            f. Semicircular canals, the saccule, and the utricle maintain balance.

    B. Operation.
        1. Atmospheric sound waves travel down the external auditory canal and vibrate the tympanic membrane.
        2. The tympanic membrane moves the bones of the middle ear which conduct the vibrations to the oval window.
        3. Ripples are set up in the fluid of the cochlea which disturb the vestibular and basilar membranes.
        4. These membranes move the hair cells of the organ of Conti.
        5. The membranes of these cells send nerve impulses to the brain.

    C. Balance.
        1. Static equilibrium.
            a. The sense of body position when not moving.
            b. Governed by the fluid-filled saccule and utricle in the inner ear.
            c. Hair cells in these compartments are embedded in a jellylike substance containing mineral crystals.
            d. When the head moves, the crystals slide, pulling the jelly.
            e. This bends the hairs, stimulating nerve impulses to the brain.
        2. Dynamic equilibrium.
            a. Enables the body to respond automatically to positional changes when it is moving.
            b. The semicircular canals are filled with a jellylike fluid.
            c. Movement causes this fluid to bend hairs of the sensory cells, which send a nerve impulse to the brain.
            d. The 3 canals are at right angles to each other, so all possible spatial movements can be sensed.

VIII. Eye.

    A. Structure.
        1. The eyeball has 3 tissue layers.
        2. Sclera: outer layer.
            a. “White of the eye”.
            b. Fibrous tissue which maintains the shape of the eye.
            c. The anterior portion is transparent: the cornea.
            d. The cornea allows light to enter the eye.
        3. Choroid: middle layer.
            a. Thin, many blood vessels for nourishing the retina.
            b. The anterior portion contains muscles and is colored: the iris.
            c. The iris has an opening: the pupil.
            d. The muscles change the diameter of the opening, regulating the amount of light that enters.
        4. Retina: inner layer.
            a. Lined with over 100 million photoreceptors in each eye.
            b. Most are rods.
                i. Sensitive to low-intensity light.
                ii. Unable to distinguish color.
            c. Color-sensitive receptors are cones.
        5. There are 2 cavities.
            a. One anterior to the lens is filled with aqueous humor.
            b. This diffuses from the blood and nourishes the cornea.
            c. One posterior to the lens is filled with clear, permanent jellylike vitreous humor.

    B. Operation.
        1. Light passes through the cornea and the opening of the iris.
        2. The lens focuses the light onto the retina.
            a. Ciliary muscles change the shape of the lens.
            b. When viewing a close object, they contract, making the lens thicker.
        3. The photoreceptors are stimulated by light and pass a signal to connecting neurons.
        4. Connecting neurons pass the signal to the optic nerve which transmits it to the brain.

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