Introduction to the Nervous System The nervous system is the mechanism by which the body reacts to changes in its external and internal environment. This system also controls and integrates important bodily functions, such as heart and lung activity. The nervous system is divided into the central nervous system (CNS) and the peripheral nervous system (PNS). I. DEVELOPMENT OF THE NERVOUS SYSTEM: The nervous system develops from the neural plate, a thickened area of embryonic ectoderm that appears at about the middle of the third week. From the neural plate come the neural tube and the neural crest. neural plate neural tube central nervous system (CNS) (brain & spinal cord) neural plate neural crest peripheral nervous system (PNS) (cranial & spinal nerves) CNS = brain and spinal cord - integrates and coordinates incoming and outgoing neural signals - carries out higher mental functions (thinking and learning) - cerebrospinal fluid and meninges surround and protect the CNS. - a bundle of nerve fibers (axons) in the CNS is called a tract Meninges: membranes that envelope the brain and spinal cord. Three types of meninges are layered together: dura mater, pia mater, and arachnoid mater. PNS = 12 pairs of cranial nerves, 31 pairs of spinal nerves, autonomic nerves in the gut - anatomically and operationally continuous with the CNS - cranial nerves arise directly from the brain - spinal nerves arise directly from the spinal cord - conveys nerve impulses to the CNS from the sensory organs and from sensory receptor in the body. - conveys neural impulses from the CNS to muscles and glands - a bundle of nerve fibers (axons) in the PNS is called a nerve
II. NERVES: The most basic structural and functional unit of the nervous system is the neuron. A neuron has a cell body which is surrounded by many extensions of its cell membranes called processes. These processes take the form of either the axons or dendrites. A nerve (or nervous tissue) is a collection of axons and/or dendrites. Another component of nervous tissue is neuroglia (glial cells) which provide support and nourishment to the neurons. Neurons communicate with each other via synapses. Axon: a single, long, straight relatively unbranched process which arises from the cell body of a neuron. Axons may or may not be myelinated. Axons are responsible for conducting the nerve impulse away from the cell body. Dendrite: a many-branched process which arises from the cell body of a neuron. Dendrites receive impulses from other neurons. Nucleus: (pl. nuclei) a collection of neuron cell bodies within the CNS (in the brain or spinal cord, itself)
Ganglion: (pl. ganglia) a collection of neuron cell bodies outside of the CNS. Myelin: layers of lipids and protein substances that form insulating sheaths around the processes of neurons. Myelin sheaths increase the velocity of impulse conduction along the nerve. Myelin is made by Schwann cells in the PNS and Oligodendrocytes in the CNS. Synapse: point of contact between neurons. Nerve plexus (pl. plexuses): a network of nerves. Only the ventral rami of cervical, lumbar and sacral spinal nerves form nerve plexuses.
III. SPINAL NERVES: Structure: A spinal nerve is formed by a series of dorsal and ventral nerve rootlets that converge to form dorsal (sensory) and ventral (motor) roots. On the dorsal root a collection of sensory neurons form the dorsal root ganglion. Then, the ventral root and dorsal root come together to form a spinal nerve. Each spinal nerve has a dorsal ramus and a ventral ramus. • • Dorsal rami supply sensory and motor innervation to the posterior body wall (but NOT to the extrinsic or superficial muscles of the back, as these are extensions of limb musculature) Ventral rami supply sensory and motor innervation to the rest of the body wall and limbs.
Contents: Spinal nerves contain both motor neurons and sensory neurons. *Some of the fibers for visceral sensory and visceral motor (to glands and blood vessels in the body wall, for instance) travel in spinal nerves. However, sympathetic innervation of the gut (viscera) is supplied via the sympathetic trunk and splanchnic nerves. This is why we say that spinal nerves DO NOT supply the gut. • motor neurons found in spinal nerves (carry impulses from the spinal cord to structures in the body): ⇒ somatomotor neurons (cell bodies found in ventral horn) - innervate skeletal muscle ⇒ sympathetic neurons (cell bodies in lateral horn) - innervates cardiac muscle, smooth muscle, organs and glands • sensory neurons found in spinal nerves (carry impulses to the spinal cord from structures in the body): ⇒ exteroceptive (cell bodies in dorsal root ganglia; fibers return to spinal cord via dorsal rootlets) - sensory from outside body (e.g., pain) ⇒ proprioceptive (cell bodies in dorsal root ganglia; fibers return to spinal cord via dorsal rootlets) - somatosensory from joints and skeletal muscle ⇒ interoceptive (cell bodies in dorsal root ganglia; fibers return to spinal cord via dorsal rootlets) -visceral afferent (sensory) (e.g., hunger, stretching, etc.)