Chapter 12

(1)SENSORY Through sense organs and simple sensory nerve endings, it receives information about changes in the body and the external environment and transmits coded messages to thespinal cord and brain.

(2)INTERNEURONS The spinal cord and brain process this information, relate it to past experience, and determine what response, if any, is appropriate to thecircumstances.

(3)MOTOR The spinal cord and brain issue commands primarily to muscle and gland cells to carry out such responses.

Excitability (irritability). All cells are excitable—that is, they respond to environmental changes called stimuli. Neurons have developed this property to the highest degree.

Conductivity. Neurons respond to stimuli by producing electrical signals that are quickly conducted to other cells at distant locations – Action Potentials.

Secretion. When the electrical signal reaches the end of a nerve fiber, the neuron secretes a chemical neurotransmitter that crosses the gap and stimulates the next cell.

• somewhat resemble an octopus; they have a bulbous body with as many as 15 armlike processes.
• Each process reaches out to a nerve fiber and spirals around it like electrical tape
• wrapped repeatedly around a wire. This wrapping, called the myelin sheath,
• insulates the nerve fiber from the extracellular fluid. For reasons explained
• later, it speeds up signal conduction in the nerve fiber. – form myelin in
• brain and spinal chord.

• resemble a cuboidal epithelium lining the internal cavities of the brain and spinal cord.
• Unlike true epithelial cells, however, they have no basement membrane and they exhibit
• rootlike processes that penetrate into the underlying tissue. Ependymal cells
• produce cerebrospinal fluid, a clear liquid that bathes the CNS and
• fills its internal cavities. They have patches of cilia on their apical
• surfaces that help to circulate the CSF. – line cavities of brain and spinal
• chord, secrete and circulate cerebrospinal fluid; ciliated.

• small macrophages that develop from white blood cells called monocytes. They wander through the
• CNS, putting out fingerlike extensions to constantly probe the tissue for
• cellular debris or other problems. They are thought to perform a complete
• checkup on the brain tissue several times a day, phagocytizing dead tissue,
• microorganisms, and other foreign matter. They become concentrated in areas
• damaged by infection, trauma, or stroke. Pathologists look for clusters of
• microglia in brain tissue as a clue to sites of injury.

• the most abundant glial cells in the CNS and constitute over 90% of the tissue in some areas of
• the brain. They cover the entire brain surface and most nonsynaptic regions of
• the neurons in the gray matter of the CNS. They are named for their
• many-branched, somewhat starlike shape. They have the most diverse functions of
• any glia. – nourish neutrons, produce growth factors, forms scar tissue.

• envelop nerve fibers of the PNS. In most cases, a Schwann cell winds repeatedly around a nerve fiber
• and produces a myelin sheath similar to the one produced by oligodendrocytes in
• the CNS. There are some important differences between the CNS and PNS in the
• way myelin is produced, which we consider shortly. In addition to myelinating
• peripheral nerve fibers, Schwann cells assist in the regeneration of damaged
• fibers.

• surround the neurosomas in ganglia of the PNS. They provide electrical insulation around
• the soma and regulate the chemical environment of the neurons.

Tracts and Nerves

• Sensory (afferent) neurons are specialized to detect stimuli such as light, heat, pressure, and chemicals, and
• transmit information about them to the CNS. Such neurons begin in almost every
• organ of the body and end in the CNS; the word afferent refers to signal
• conduction toward the CNS. Some receptors, such as those for pain and
• smell, are themselves neurons. In other cases, such as taste and hearing, the
• receptor is a separate cell that communicates directly with a sensory neuron.

• Interneurons (association neurons) lie entirely within the CNS. They receive signals from many other neurons and
• carry out the integrative function of the nervous system—that is, they process,
• store, and retrieve information and “make decisions” that determine how the
• body responds to stimuli. About 90% of our neurons are interneurons. The word interneuron
• refers to the fact that they lie between, and interconnect, the incoming
• sensory pathways and the outgoing motor pathways of the CNS.

• Motor (efferent) neurons send signals predominantly to muscle and gland cells, the effectors that carry out the
• body's responses to stimuli. They are called motor neurons because most
• of them lead to muscle cells, and efferent neurons to signify signal
• conduction away from the CNS.

• have one axon and multiple dendrites. This is the most
• common type and includes most neurons of the brain and spinal cord.

• have one axon and one dendrite. Examples include
• olfactory cells of the nasal cavity, certain neurons of the retina, and
• sensory neurons of the inner ear.

• have only a single process leading away from the soma. They are represented by the neurons
• that carry sensory signals to the spinal cord.

• Large fibers have more surface area and conduct signals more rapidly
• than small fibers. Myelin further speeds signal conduction for reasons
• discussed later. Slow unmyelinated fibers are quite sufficient for processes in
• which quick responses are not particularly important, such as secreting stomach
• acid or dilating the pupil. Fast myelinated fibers are employed where speed is
• more important, as in motor commands to the skeletal muscles and sensory
• signals for vision and balance.

• Some nerve fibers connect to the wrong muscle fibers or never find a
• muscle fiber at all, and some damaged motor neurons simply die.

It opens when the appropriate neurotransmitter binds

It opens and closes in response to changes in membrane potential.

• 1. The inside membrane face starts out being slightly negatively charged when compared to the outer membrane face.
• 2. Na+ ion channels quickly open and depolarize the membrane and produces nerve impulse. K+ channels open slowly.
• 3. K+ channels open completely and Na+ channels close, allowing repolarization followed by hyperpolarization as K+ channels slowly close.
• 4. Resting state is restored and system is ready to respond to next action potential.

• 1. An action potential must be propagated along an axon's entire length
• 2. The action potential is generated by an influx in Na+ ions which causes part of the axonal membrane to become depolarized.
• 3. This causes local current flows that are self-propagating (domino effect)
• 4. After depolarization, each segment of the axonal membrane repolarizes - results in repolarization wave chasing depolarization wave down length of axon.

Location: neuromuscular junctions, most synapses of the autonomic nervous system, retina, many parts of brain.

Action: excites skeletal muscle, inhibits cardiac muscle, has excitatory or inhibatory effects on smooth muscle and glands depending on location.

Location: sympathetic nervous system, cerebral cortex, hypothalamus, brainstem, cerebellum, and spinal chord.

Action: involved in dreaming, waking, and mood; excites cardiac muscle; can excite or inhibit smooth muscle and glands depending on location.

Location: cerebral cortex and brainstem.

Action: accounts for about 75% of excitatory synaptic transmission in the brain; involved in learning and memory.

Location: digestive tract, spinal chord, many parts of brain, also secreted by pituitary.

Action: suppresses pain; reduces perception of fatigue; may produce "runner's high" in athletes.