ØD. Subsystems of the Nervous System: The nervous system is divisible into 2 overlapping pairs of subsystems:
Ø1. The central and peripheral nervous systems are defined mainly by location. The central nervous system (CNS) includes the brain and spinal cord. The peripheral nervous system (PNS) includes all other nerve tissue. 2. The autonomic and somatic nervous systems are defined according to function, but have distinctive anatomic features as well.
Each has CNS and PNS components. The autonomic nervous system controls involuntary visceral functions leg, glandular secretions, smooth muscle contraction) and has both motor and sensory pathways, although some authors exclude visceral sensory pathways from the ANS. Each motor pathway consists of 2 neurons that synapse in a peripheral autonomic ganglion. The cell body of the first (preganglionic) neuron is in the CNS; the cell body of the second (postganglionic) neuron is in the autonomic ganglion. The cell bodies of the sensory neurons are located in craniospinal ganglia and have processes that extend peripherally.
The ANS is subdivided into the sympathetic and parasympathetic nervous systems. When they innervate the same end organ, sympathetic and parasympathetic nerves usually have opposing effects. The somatic nervous system includes all nerve tissue except the ANS. It controls somatosensory perception leg, touch, heat, cold) and somatomotor (voluntary) functions (eg, skeletal muscle contraction). Acetylcholine is the most common somatic neurotransmitter.
Subsystems of the Nervous System
GENERAL FEATURES OF NERVE TISSUE & THE NERVOUS SYSTEM 2
ØB. Impulse Conduction: Within a neuron, signals (impulses) are propagated as a wave of depolarization along the plasma membrane of the dendrites, soma, and axon. Depolarization involves channels (ionophores) in the membrane, which allow ions (e.g, Na+, K-) to enter or exit the cell. In unmyelinated axons, depolarization is continuous. In myelinated axons, depolarization occurs only at nodes of Ranvier, jumping from node to node (saltatory conduction). Impulse conduction is thus faster in myelinated axons.
ØC. Synapses: Signals pass from neuron to target cell by specialized connections called synapses. The target may be another neuron or a cell in the end organ leg, gland or muscle) it supplies. At chemical synapses, the signal is transmitted by exocytosis of neurotransmitters, chemicals such as acetylcholine that cross the narrow gap (synaptic cleft) between the cells to initiate depolarization of the target cell. At the less common electrical synapses, the signal is transmitted by ions flowing through a gap junction-like complex.
GENERAL FEATURES OF NERVE TISSUE & THE NERVOUS SYSTEM
ØA. Two Classes of Cells: Nerve tissue consists of the neurons that transmit impulses and the supporting cells that surround them. It contains little extracellular material.
Ø1. Neurons. These cells are highly specialized to carry out nerve tissue functions. Neurons receive, integrate, and transmit electrochemical messages. Each has a cell body, also called the soma ("body") or perikaryon ("around the nucleus"), comprising the nucleus and the surrounding cytoplasm and plasma membrane. Each neuron has a variable number of dendrites, cytoplasmic processes that collect incoming messages and carry them toward the soma, and a single axon, a cytoplasmic process that transmits messages to the target cell. Axons of most neurons have a myelin sheath formed by supporting cells and interrupted by gaps called nodes of Ranvier. Myelinated axon segments between the gaps are called internodes. 2. Supporting cells. These cells are called neuroglia ("nerve glue") or glial cells. Their functions include structural and nutritional support of neurons, electrical insulation, and enhancement of impulse conduction velocity along axons.
