SECTION A: NEURAL TISSUE

1. NERVOUS SYSTEM
   1. The nervous system is divided into two parts. The central nervous system comprises the brain and spinal cord, and the peripheral nervous system consists of nerves extending to and from the CNS.

2. STRUCTURE AND MAINTENANCE OF NEURONS
   1. The basic unit of the nervous system is the nerve cell, or neuron.
   2. The cell body and dendrites receive information from other neurons.
   3. The axon (nerve fiber), which may be covered with sections of myelin separated by nodes of Ranvier, transmits information to other neurons or effector cells.

3. FUNCTIONAL CLASSES OF NEURONS
   1. Neurons are classified in three ways.
      1. Afferent neurons transmit information into the CNS from receptors at their peripheral endings.
      2. Efferent neurons transmit information out of the CNS to effector cells.
      3. Interneurons lie entirely within the CNS and form circuits with other interneurons or connect afferent and efferent neurons.

   2. Information is transmitted across a synapse by a neurotransmitter, which is released by a presynaptic neuron and combines with receptors on a postsynaptic neuron.

4. GLIAL CELLS
   1. The CNS also contains glial cells, which help regulate the extracellular fluid composition, sustain the neurons metabolically, form myelin, serve as guides for developing neurons, and provide immune functions.

5. NEURAL GROWTH AND REGENERATION
   1. Neurons develop from precursor cells, migrate to their final location, and send out processes to their target cells.
   2. Cell division to form new neurons is virtually complete by birth.
   3. After degeneration of a severed axon, damaged peripheral neurons can regrow the axon to their target organ. Damaged neurons of the CNS do not regenerate or restore function.

SECTION B: MEMBRANE POTENTIALS

1. THE RESTING MEMBRANE POTENTIAL
   1. Membrane potentials are generated by the diffusion of ions and are determined by (a) the ionic concentration differences across the membrane, and (b) the membrane's relative permeabilities to different ions.
      1. Plasma membrane Na,K-ATPase pumps maintain intracellular sodium concentration low and potassium high.
      2. In almost all resting cells, the plasma membrane is much more permeable to potassium than to sodium and so the membrane potential is close to the potassium equilibrium potential, that is, the inside is negative relative to the outside.
      3. The Na,K-ATPase pumps also contribute directly a small component of the potential because they are electrogenic.

2. GRADED POTENTIALS AND ACTION POTENTIALS
   1. Neurons signal information by graded potentials and action potentials (APs).
   2. Graded potentials are local potentials whose magnitude can vary and that die out within 1 or 2 mm of their site of origin.
   3. An AP is a rapid change in the membrane potential during which the potential rapidly depolarizes and repolarizes. In neurons the potential reverses and the membrane becomes positive inside. APs provide long-distance transmission of information through the nervous system.
      1. APs occur in excitable membranes because these membranes contain voltage-gated sodium channels, which open as the membrane depolarizes, causing a positive feedback toward the sodium equilibrium potential.
      2. The AP is ended as the sodium channels close and additional potassium channels open, which restores the resting conditions.
      3. Depolarization of excitable membranes triggers APs only when the membrane potential exceeds a threshold potential.
      4. Regardless of the size of the stimulus, if the membrane reaches threshold, the APs generated are all the same size.
      5. A membrane is refractory for a brief time even though stimuli that were previously effective are applied.
      6. APs are propagated without any change in size from one site to another along a membrane.
      7. In myelinated nerve fibers, APs manifest saltatory conduction.
      8. APs can be initiated by receptors at the ends of afferent neurons, at synapses, or in some cells, by pacemaker potentials.

SECTION C: SYNAPSES

1. SYNAPSES
   1. An excitatory synapse brings the membrane of the postsynaptic cell closer to threshold. An inhibitory synapse hyperpolarizes the postsynaptic cell or stabilizes it at its resting level.
   2. Whether a postsynaptic cell fires action potentials depends on the number of synapses that are active and whether they are excitatory or inhibitory.

2. FUNCTIONAL STATES OF SYNAPSES
   1. The signal from a pre- to a postsynaptic neuron is a neurotransmitter stored in synaptic vesicles in the presynaptic axon terminal. Depolarization of the axon terminal, which raises the calcium concentration within the terminal, causes the release of neurotransmitter into the synaptic cleft.
   2. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the postsynaptic cell; the activated receptor usually opens ion channels.
      1. At an excitatory synapse the electrical response in the postsynaptic cell is called an excitatory postsynaptic potential (EPSP). At an inhibitory synapse, it is an inhibitory postsynaptic potential (IPSP).
      2. Usually at an excitatory synapse, channels in the postsynaptic cell that are permeable to sodium, potassium, and other small positive ions are opened; at inhibitory synapses, channels to chloride and/or potassium are opened.
      3. The postsynaptic cell's membrane potential is the result of temporal and spatial summation of the EPSPs and IPSPs at the many active excitatory and inhibitory synapses on the cell.

3. SYNAPTIC EFFECTIVENESS
   1. Synaptic effects are influenced by pre- and postsynaptic events, drugs, and diseases.

4. NEUROTRANSMITTERS AND NEUROMODULATORS
   1. In general, neurotransmitters cause EPSPs and IPSPs, and neuromodulators cause, via second messengers, more complex metabolic effects in the postsynaptic cell.
   2. The actions of neurotransmitters are usually faster than those of neuromodulators.
   3. A substance can act as a neurotransmitter at one receptor type and as a neuromodulator at another.
   4. The known or suspected neurotransmitters and neuromodulators are listed in Table 8-6.

5. NEUROEFFECTOR COMMUNICATION
   1. The junction between a neuron and an effector cell is called a neuroeffector junction.
   2. The events at a neuroeffector junction--release of neurotransmitter into an extracellular space, diffusion of neurotransmitter to the effector cell, and binding with a receptor on the effector cell--are similar to those at a synapse.

SECTION D: STRUCTURE OF THE NERVOUS SYSTEM

1. NERVOUS SYSTEM STRUCTURE
   1. Inside the skull and vertebral column, the brain and spinal cord are enclosed in and protected by the meninges.

2. CENTRAL NERVOUS SYSTEM: SPINAL CORD
   1. The spinal cord is divided into two areas: central gray matter, which contains nerve cell bodies and dendrites; and white matter, which surrounds the gray matter and contains myelinated axons organized into ascending or descending tracts.
   2. The axons of the afferent and efferent neurons form the spinal nerves.

3. CENTRAL NERVOUS SYSTEM: BRAIN
   1. The brain is divided into six regions: cerebrum, diencephalon, midbrain, pons, medulla oblongata, and cerebellum.
   2. The midbrain, pons, and medulla oblongata form the brainstem, which contains the reticular formation.
   3. The cerebellum plays a role in posture, movement, and some kinds of memory.
   4. The cerebrum, made up of right and left cerebral hemispheres, and the diencephalon together form the forebrain. The cerebral cortex forms the outer shell of the cerebrum and is divided into parietal, frontal, occipital, and temporal lobes.
   5. The diencephalon contains the thalamus and hypothalamus.
   6. The limbic system is a set of deep forebrain structures associated with learning and emotions.

4. PERIPHERAL NERVOUS SYSTEM
   1. The peripheral nervous system consists of 43 paired nerves: 12 pairs of cranial nerves and 31 pairs of spinal nerves. Most nerves contain axons of both afferent and efferent neurons.
   2. The efferent division of the peripheral nervous system is divided into somatic and autonomic parts. The somatic fibers innervate skeletal muscle cells and release the neurotransmitter acetylcholine.
   3. The autonomic nervous system innervates cardiac and smooth muscle, glands, and gastrointestinal tract neurons. Each autonomic pathway consists of a preganglionic neuron with its cell body in the CNS and a postganglionic neuron with its cell body in an autonomic ganglion outside the CNS.
      1. The autonomic nervous system is divided into sympathetic and parasympathetic components. The preganglionic neurons in both sympathetic and parasympathetic divisions release acetylcholine; the postganglionic parasympathetic neurons release mainly acetylcholine, and the postganglionic sympathetic neurons release mainly norepinephrine.
      2. The receptors that respond to acetylcholine are classified as nicotinic and muscarinic, and those that respond to norepinephrine or epinephrine as alpha- and beta-adrenergic types.
      3. The adrenal medulla is a hormone-secreting part of the sympathetic nervous system and secretes mainly epinephrine.
      4. Effector organs innervated by the autonomic nervous system generally receive dual innervation.

5. BLOOD SUPPLY, BLOOD-BRAIN BARRIER PHENOMENA, AND CEREBROSPINAL FLUID
   1. Brain tissue depends on a continuous supply of glucose and oxygen for metabolism.
   2. The chemical composition of the extracellular fluid of the CNS is closely regulated by the blood-brain barrier.
   3. The brain ventricles and the space within the meninges are filled with cerebrospinal fluid, which is formed in the ventricles.