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The nervous system and endocrine system work to control the activities of the other organ systems of the body. The nervous system is organized into the central nervous system (CNS) and the peripheral nervous system (PNS).

Neurons

• A neuron is composed of a cell body, an axon, and dendrites. Only axons conduct nerve impulses. An axon may be covered by a myelin sheath, with small gaps called the nodes of Ranvier. Nerves are collections of axons from multiple neurons.

• Types of neurons are sensory neurons, which transmit nerve impulses to the CNS; interneurons, which carry nerve impulses between neurons of the CNS; and motor neurons, which carry nerve impulses away from the CNS.

The Nerve Impulse

• A nerve impulse, or action potential, moves a signal along an axon. The sodium-potassium pump transports Na* out of the axon and K* into the axon. The inside of the axon has a negative charge; the outside has a positive charge.

• During the nerve impulse, there is a reversal of charge as Na* flows in, then a return to the previous charge difference as K+ flows out of an axon.

• The nerve impulse travels much faster in myelinated axons because the impulse jumps from node to node.

The Synapse

• The synapse is a region of close proximity between an axon terminal and the next neuron (in the CNS) or between an axon terminal and a muscle cell (in the PNS).

• A nerve impulse causes the release of a neurotransmitter (excitatory or inhibitory) into the synaptic cleft. Integration of the excitatory and inhibitory signals determines whether the neuron will transmit the nerve impulse.

• Neurotransmitters are ordinarily removed quickly from the synaptic cleft.

The central nervous system consists of the brain and spinal cord. These are connected by tracts, long connextions of interneurons.

Aa

• The cerebrum functions in sensation, reasoning, learning and memory, language, and speech. The cerebral cortex has a primary sensory area in the parietal lobe, which receives sensory information from each part of the body, and a primary motor area in the frontal lobe, which sends out motor commands to skeletal muscles. Association areas, such as the prefrontal area, carry out integration.

• In the diencephalon, the hypothalamus helps control homeostasis; the thalamus specializes in sending sensory input to the cerebrum.

• The cerebellum primarily coordinates skeletal muscle contractions.

• In the brain stem, the medulla oblongata has centers for vital functions, such as breathing and the heartbeat, and helps control the internal organs. The midbrain contains visual and auditory areas, and the pons controls breathing.

The Limbic System

The limbic system blends higher functions into a united whole. The hippocampus and amygdala have roles in learning and memory and appear to be affected in Alzheimer's disease.

The Peripheral Nervous System

The peripheral nervous system (PNS) connects the sensory neurons with the CNS. It contains nerves that are organized into ganglions.

Examples of these nerves are cranial nerves and spinal nerves. The PNS is divided into two systems:

• Somatic system: Reflexes (automatic responses) involve a sensory receptor, a sensory neuron, interneurons in the spinal cord, and a motor neuron.

• Autonomic system: The sympathetic division is active during times of stress, and the parasympathetic division is active during times of relaxation. Both divisions control the same internal organs.

27.2 Endocrine System

In the endocrine system, endocrine glands secrete hormones into the bloodstream for distribution to target organs or tissues. Hormones are classified as either steroid hormones or peptide hormones.

Major Organs and Glands

The major functions of the hormones secreted by the endocrine glands are listed in L Table 27.1.

• Hypothalamus and pituitary gland: Together these control the internal environment of the body. The hypothalamus links the endocrine and nervous systems. The pituitary gland contains two parts, the anterior pituitary and the posterior pituitary. These release hormones that regulate the other endocrine glands, usually by negative feedback mechanisms.

• Thyroid and parathyroid glands: The thyroid gland regulates metabolism. Hyper or hypothyroidism may result in endemic goiter or exophthalmos. The thyroid and parathyroid glands are involved in calcium homeostasis. Insufficient parathyroid gland activity may result in tetany.

• Adrenal glands: The adrenal glands consist of the adrenal cortex and the adrenal medulla. Problems with hormones secreted by the adrenal cortex may result in Addison's disease or Cushing's disease.

• Pancreas: The pancreas is both an exocrine and an endocrine gland. The pancreatic islets contain endocrine cells that secrete the hormones insulin and glucagon, both of which are involved in glucose homeostasis. Diabetes mellitus is a disease caused by a failure of glucose homeostasis.