Functions of the Nervous System

• The functions of the nervous system are the accomplishment of mental activity, the control of homeostasis, the regulation of other systems, and the control of skeletal muscles.

Divisions of the Nervous System

• The central nervous system (CNS) consists of the brain and spinal cord, whereas the peripheral nervous system (PNS) consists of nerves and ganglia.

• The afferent division of the PNS transmits action potentials to the CNS; the efferent division carries action potentials away from the CNS.

• The somatic motor nervous system innervates skeletal muscle and is mostly under voluntary control. The autonomic nervous system innervates cardiac muscle, smooth muscle, and glands, and it is mostly under involuntary control.

• The autonomic nervous system has sympathetic and parasympathetic divisions.

Cells of the Nervous System

Neurons

• Neurons receive stimuli and transmit action potentials. Neurons consist of a cell body, dendrites, and an axon.

• Neurons are multipolar, bipolar, or unipolar.

Neuroglia

• Neuroglia are the support cells of the nervous system. They include astrocytes, microglia, ependymal cells, oligodendrocytes, and Schwann cells.

Myelin Sheaths

• Axons are either unmyelinated or myelinated.

Organization of Nervous Tissue

• Nervous tissue consists of white matter and gray matter. White matter forms nerve tracts in the CNS and nerves in the PNS. Gray matter forms the cortex and nuclei in the brain and ganglia in the PNS.

Propagation of Action Potentials

Membrane Potentials and Action Potentials

• A resting membrane potential results from the charge difference that exists across the membrane of cells.

• An action potential occurs when the charge across the cell membrane is briefly reversed.

The Synapse

• The synapse is the point of contact between two neurons or between a neuron and some other cell, such as a muscle or gland cell. An action potential arriving at the synapse causes the release of a neurotransmitter from the presynaptic terminal, which diffuses across the synaptic cleft and binds to the receptors of the postsynaptic membrane.

Reflexes

• Reflexes are the functional units of the nervous system.

• A reflex arc consists of a sensory receptor, an afferent neuron, association neurons, an efferent neuron, and an effector organ.

Neuronal Circuits

• Neuronal circuits are either diverging or converging.

Central Nervous System

• The CNS consists of the brain and spinal cord. The brain consists of the brainstem, diencephalon, cerebrum, and cerebellum.

Brainstem

• The brainstem contains several nuclei, as well as ascending and descending tracts.

• The medulla oblongata contains nuclei that control such activities as heart rate, breathing, swallowing, and balance.

• The pons contains relay nuclei between the cerebrum and cerebellum.

• The midbrain is involved in hearing and in visual reflexes.

• The reticular formation is scattered throughout the brainstem and is involved in maintaining consciousness and in the sleep-wake cycle.

Diencephalon

• The diencephalon consists of the thalamus (main sensory relay center), epithalamus (the pineal body may play a role in sexual maturation), and hypothalamus (important in maintaining homeostasis).

Cerebrum

• The cerebrum has two hemispheres divided into lobes. The lobes are the frontal, parietal, occipital, and temporal.

• Many CNS functions can be localized to specific areas of the cortex. Association areas are involved in the recognition of information.

• Speech involves the sensory speech area, the motor speech area, and the interactions between them and other cortical areas.

• An EEG monitors brain waves, which are a summation of the electrical activity of the brain.

• Memory consists of sensory (less than 1 s), short-term (lasting a few minutes), and long-term (permanent) memory.

• Each hemisphere controls the opposite half of the body. Commissures connect the two hemispheres. The left hemisphere is thought to be the dominant analytical hemisphere, and the right hemisphere is thought to be dominant for spatial perception and musical ability.

• The basal nuclei inhibit extraneous muscular activity.

• The limbic system is located deep within the cerebrum and is involved with the emotional and visceral response to smell, as well as other visceral functions.

Cerebellum

• The cerebellum is involved in balance and muscle coordination. Its main function is as a comparator, comparing the intended action with what is occurring and modifying the action of lower motor neurons to eliminate differences.

Spinal Cord

• The spinal cord has a central gray part organized into horns and a peripheral white part forming nerve tracts.

• Roots of spinal nerves extend out of the cord.

Pathways

• Pathways are usually given composite names that indicate their origin and termination. Almost all ascending pathways terminate in the thalamus before projecting to the cerebrum.

• The somatic motor system consists of lower and upper motor neurons.

Meninges

• Three connective tissue meninges cover the CNS: the dura mater, arachnoid mater, and pia mater.

Ventricles

• The brain and spinal cord contain fluid-filled cavities: the lateral ventricles in the cerebral hemispheres, a third ventricle in the diencephalon, a cerebral aqueduct in the midbrain, a fourth ventricle at the base of the cerebellum, and a central canal in the spinal cord.

• The fourth ventricle has openings into the subarachnoid space.

Cerebrospinal Fluid

• Cerebrospinal fluid is formed in the choroid plexuses in the ventricles, it exits through the fourth ventricle, and it reenters the blood through arachnoid granulations in the superior sagittal sinus.

Peripheral Nervous System

• The PNS consists of afferent and efferent fibers contained in spinal and cranial nerves.

Cranial Nerves

• There are 12 cranial nerves: 3 with only sensory function (S), 4 with only somatic motor function (M), 1 with motor and sensory function, 1 with somatic motor and parasympathetic (P) function, and 3 with all three functions. Four of the cranial nerves have parasympathetic function.

• The cranial nerves are: olfactory (I; S), optic (II; S), oculomotor (III; M, P), trochlear (IV; M), trigeminal (V; S, M), abducens (VI; M), facial (VII; S, M, P), vestibulocochlear (VIII; S), glossopharyngeal (IX; S, M, P), vagus (X; S, M, P), accessory (XI; M), and hypoglossal (XII; M).

Spinal Nerves

• The spinal nerves exit from the cervical, thoracic, lumbar, and sacral regions.

• The nerves are grouped into plexuses.

• The phrenic nerve, which supplies the diaphragm, is the most important branch of the cervical plexus.

• The brachial plexus supplies nerves to the upper limb.

• The lumbosacral plexus supplies nerves to the lower limb.

Autonomic Nervous System

• The autonomic nervous system contains preganglionic and postganglionic neurons.

• The autonomic nervous system has sympathetic and parasympathetic divisions.

Sympathetic Division

• The sympathetic division is involved in preparing the person for action by increasing heart rate, blood pressure, and respiration rate.

• Preganglionic cell bodies of the sympathetic division lie in the thoracic and upper lumbar regions of the spinal cord.

• Postganglionic cell bodies are located in the sympathetic chain ganglia or in collateral ganglia.

Parasympathetic Division

• The parasympathetic division is involved in vegetative activities, such as the digestion of food, defecation, and urination.

• Preganglionic cell bodies of the parasympathetic division are associated with some of the cranial and sacral nerves.

• Postganglionic cell bodies are located in terminal ganglia, located either near or within target organs.

Autonomic Neurotransmitter Substances

• All autonomic preganglionic and parasympathetic postganglionic neurons secrete acetylcholine.

• Most sympathetic postganglionic neurons secrete norepinephrine.