THE NEURON
Cellular Structure

• Neurons consist of a cell body with the usual cellular organelles, and receiving and sending processes called dendrites and axons, respectively. One neuron activates a second, or an effector, through chemical substances called neurotransmitters that are produced in the cell body and released at knobby ends of the axon tree. Neurotransmitters that activate muscles are released at neuromuscular junctions.
• Many vertebrate neurons are wrapped in myelin sheaths, produced by Schwann cells outside the brain spinal cord and oligodendrocytes within. The sheaths or internodes regions, contain minute gaps called nodes.

• Neurons include sensory neurons (also afferent neurons), interneurons, and motor neurons (also efferent neurons). The first receive stimuli such as a light and heat and communicates with the second, which integrates the response and transmits it to the third, which produces the response. Neuralgia (or glial cells), especially astrocytes, are extremely common in the brain.

• Nerves, composed of numerous axons and dendrites, from cable like structures surrounded by connective tissue.

• Unlike an electrical impulse, the neural impulse travels comparatively slowly and is continually regenerated, so it is as strong at the end as it was at the beginning.
• The parts of a neural conduction include the resting state or polarized state, the action potential or depolarizing wave, and reestablishment of the resting state, or repolarization.

• Ions important to the neuron are negatively charged proteins within the neurons and positive sodium and potassium ions. In the resting state, the neural membrane is impermeable to Na⁺ , which is pumped out by the sodium/potassium ion exchange pumps. The membrane is permeable to K⁺, which is pumped in. K⁺ diffuses out but is also attracted in by the negative charges, so an outward concentration gradient is maintained.
• In the resting neuron, the positive charges predominate outside and the negative inside, producing a polarized state and resting potential of -60 mV.

• Stimulation of a neuron causes depolarizing, a shift in ion concentrations and electrical charges. Membrane permeability changes, admitting Na⁺, which shifts the potential to a peak of +40 mV, whereupon the membrane loses its permeability to Na⁺.
• Immediately, K⁺ exits the neuron, repolarizing the neuron, thus restoring the resting potential of -60 mV. Na⁺ is eventually pumped out by the ion exchange pumps, the K⁺ restores its resting equilibrium. As long as the membrane is in a depolarized state, it remains impermeable to Na⁺ and no new action potential can proceed.

• Studies of axonal membranes reveal the presence of specific voltage-sensitive sodium and potassium ion channels and sodium  and potassium ion gates. While potassium channels have one gate, sodium channels have an inactivation gate and an activation gate. The gate work as follows:
• Resting state: Sodium activation gate are closed, inactivation gates are open, and potassium gates are closed.
• Action potential: Sodium activation gate opens and Na+ rushes into the axon, but as the electrical peak occurs, the inactivation gates close and remain closed until the resting potential is restored. This is the refractory period when new impulses cannot occur.
• Repolarization: Potassium gates open (earlier), and K⁺ moves outward, soon repolarizing the region and restoring the resting potential. At this point the potassium gates close.
• Resting state: Upon repolarization, the sodium activation gates open, and the region is ready for another action potential.

• Myelinated neurons conduct impulses much faster than nonmyelinated neurons, since depolarization occurs only at the nodes.
• Action potentials at the node create enough current flow to activate the sodium gates in the next node, so the impulse jumps from node to node in what is called saltatory propagation.  Electrical current flow through internodes is almost instantaneous.

• Commonly, axons communicate with dendrites and effectors via synapses, where minute spaces (about 20 nm) called synaptic clefts occur.
• Neurotransmitters, such as norepinephrine and acetylcholine, are released from the presynaptic membrane, diffuse across the cleft to the postsynaptic membrane, and there activate the next neuron.  Transmission across the synapse occurs as follows:
• When an action potential reaches the synaptic knobs, voltage-sensitive calcium gates open, admitting Ca⁺² into the knob, where the ions cause vesicles to merge with the membrane, exocytotically releasing neurotransmitter into the cleft.
• The postsynaptic membrane has specific neurotransmitter receptors associated with chemically gated channels.  When activated, the gates open, admitting positive ions that start an action potential in the receiving neuron.
• Activity ceases when the neurotransmitter is cycled back to the presynaptic membrane, where it is recovered or inactivated by enzymes.  Acetylcholine is inactivated by the enzyme acetylcholinesterase and only choline is recycled.
• In inhibitory synapses, specific neurotransmitters activate either chloride gates or potassium gates, letting Cl^- in or K⁺ out, thus causing hyperpolarization and resulting in a much greater stimulus necessary to activate that neuron.

• A reflex arc may involve as few as two or three neurons.  In the "knee jerk reflex," a tap on a tendon just below the knee cap causes the quadriceps muscle above to relax.  Neurons called stretch receptors are activated, sending an impulse to an interneuron in the cord, which relays it through a motor neuron back to the muscle, which contracts slightly.

• Sensory receptors are highly specialized for the stimuli they receive, but all act as transducers, converting external stimuli first into generator potentials and then into action potentials.  Generator potentials are graded, increasing from threshold level to an intense level, thus providing greater information about the stimulus.
• While all action potentials are the same, the number and speed of impulses can act as a code in the central nervous system.  Interpretation of sensory information depends on neural organization in the brain, along with experience, memory, and learning.

• Tactile (touch) or mechanoreceptors respond to deforming force.  In invertebrates, they are often associated with sensory hairs.  They detect moving solid objects and air and water currents.
• In vertebrates mechanoreceptors include the distance receptors (such as the lateral line organ of fishes) and contact receptors that specialize in touch.  In humans, Pacinian corpuscles are activated by pressure, while Meissner's corpuscles respond to light touch.  Baroreceptos in the arteries respond to changes in blood pressure.  Body hairs and the outer dead skin layer act as piezoelectric crystals, discharging current when deformed.

• Thermoreception is the detection of changes in temperature.  Heat detection in invertebrates is especially important to ectoparasites of the warm-bodied mammals and birds.
• In vertebrates, heat sensors include the pit of pit vipers, and Ruffini corpuscles and Krause end bulbs in mammals.  Thermoreception in humans is poorly understood and, up to a point, the perception of heat and cold may depend on the last experience of the receptors involved.

• Chemoreception is widespread in animals.  Insects have chemoreceptors on many body parts, but the sensillum of certain moths is the most acute known and can be activated by a single molecule of bombkol, the sex attractant.
• Vertebrate chemoreceptors include general receptors and gustatory and olfactory receptors (taste and smell).
• In rodents, a keen olfactory sense aids in social interactions.  Reptiles sample chemicals with their forked tongue, which is inserted into the paired Jacobson's organ in the mouth where receptors are located.  In mammals, an olfactory epithelium contains sensory neurons that respond to chemicals, synapsing with neurons in the olfactory bulb above.
• In fish, taste receptors may be scattered over the body surface, while in terrestrial vertebrates they are in the mouth.  In humans, taste receptors are located in taste buds on the tongue, where there is some specialization for sweet, sour, salty, and bitter.

• Proprioception -sensing the position of the body or its parts-in invertebrates occurs through sensors in the body hairs and muscles.  In vertebrates, proprioceptors include the stretch receptors involved in the reflex arc.

• Audition in insects includes the specialized tympanal organ, a drum like device.
• Some fishes have an inner ear containing tiny bones, Weberian ossicles that move in response to sound, stimulating action potentials in nearby sensors.  In land vertebrates, the hearing organs include an auditory canal, and one to three middle ear bones that transmit sound from the tympanic membrane to the inner ear.
• The human hearing organ includes the external ear (pinna, auditory canal and tympanum).  The middle ear bones include the malleus, incus, and stapes, which connect the tympanum with the cochlea.  The inner ear includes the snail shaped cochlea and the vestibular apparatus.
• The cochlea is a fluid filled, U-shaped tube with the oval window (stapes attachment) on one end and the round window on the other.  Sound sets up motion in the fluids, which activates the organ of Corti-a basilar membrane containing hair cells embedded in the tectorial membrane.  Bending of the hair cells creates generator potentials, which in turn start action potentials in neurons from the cochlear nerve.  Differences in intensity and pitch are produced by activity at various parts of the basilar membrane.

• Some invertebrates have statocysts, chambers containing grains that move against sensory hairs when changes in body position occur.
• In humans and other mammals, movement, position, and balance are detected by the vestibular apparatus.  Three fluid filled semicircular canals, lying in three planes, detect acceleration and deceleration as the fluids move against sensory hairs.  Fine granules in the saccule and utricle shift with movement that activates sensory hairs, which inform the brain about position.

• Visual receptors in arthropods include simple eyes and compound eyes, the first of which lack lenses.  Compound eyes contain immovable units called ommatidia that register the field of vision as a mosaic.
• The vertebrate eyeball consists of tough, outer sclera, with a forward, transparent cornea and more internal choroid.  The choroid supports the iris, which surrounds the pupil.  The lens separates two fluid filled chambers.  The light sensitive retina within the eyeball communicates with the optic nerve.
• Light entering the eye passes through the anterior chamber (aqueous humor), lens, and posterior  chamber (vitreous humor), to the retina.  Focusing is done through changes in lens shape brought about by the ciliary muscles.  The amount of light entering is altered by the iris, which determines the size of the pupil.
• The retina consists of four cell layers, including an innermost pigmented layer, a layer of rods and cones, the sensory receptors, a layer of bipolar cells, and a final layer of ganglion cells.  The bipolar cells and ganglion cells are neurons.
• Rods respond to all wavelengths of the visible spectrum and function well at night, while each cone responds to either red, green, or blue light wavelengths and has little night function.  Cones are most concentrated in the fovea.
• Upon exposure to light, rhodopsin in rods bleaches to opsin and retinal, and the chemical change activates the synapses with the neurons above, where action potentials begin.  The two products recycle, using vitamin A to produce rhodopsin.  Since much of the rhodopsin is in the opsin/retinal form of bright light, temporary night blindness occurs when the surroundings are suddenly darkened.