Chapter 17: Nervous System

Nerve Structure A single nerve in the body may contain hundreds of thousands of nerve fibers that run parallel to each other. Each Nerve Fiber is a long extension that is a part of an individual nerve cell = NEURON

Nervous Tissue The nervous system is divided into a central nervous system (CNS), consisting of the brain and spinal cord, and a peripheral nervous system (PNS), consisting of nerves carrying sensory and motor information between the CNS and muscles and glands.

Types of Nervous Cells Both systems have two types of cells: NEURONS that transmit impulses. NEUROGLIAL cells that service neurons.

Organization of the nervous system In paraplegics, messages no longer flow between the lower limbs and the central nervous system (the spinal cord and brain). The sensory neurons of the peripheral nervous system take nerve impulses from sensory receptors to the central nervous system (CNS), and motor neurons take nerve impulses from the CNS to the organs, muscles, and glands.

Neuron Structure Neurons are composed of dendrites that receive signals, a cell body with a nucleus, and an axon that conducts a nerve impulse away.

Motor (Efferent) Neurons take information from the CNS to effectors (muscles or glands). Characterized by short dendrites, a central cell body and fairly long axons. Axons are often covered with a myelin sheath to carry impulse faster and to protect the long nerve fiber.

Sensory (Afferent) Neurons take information from sensory receptors to the CNS. Characterized by longer dendrite fiber, shorter axon fiber and a peripheral cell body. Both axon and dendrite fibers may be myelinated.

Interneurons (Connector/Association) occur within the CNS and integrate input. Characterized by short dendrites as well as short axon. Central cell body, but very seldom shows myelination along nerve fibers. They are found within brain and spine and act like “Head Coordinator” (Pre-wired Circuit Board) to receive and relay incoming impulses to the appropriate pathways.

Types of neurons – All Three Work Together A sensory neuron, an interneuron, and a motor neuron are drawn here to show their arrangement in the body. (The breaks indicate that the fibers are much longer than shown.) How does this arrangement correlate with the function of each neuron?

Myelin Sheath Long axons are covered by a protective myelin sheath formed by neuroglial cells called Schwann cells. The sheath contains lipid myelin which gives nerve fibers their white, glistening appearance. The sheath is interrupted by gaps called nodes of Ranvier. The myelin sheath also plays an important role in nerve regeneration within the PNS. If an axon is accidentally severed, the myelin sheath remains an serves as a passageway for new fiber growth. In multiple sclerosis (MS) lesions develop in the myelin sheath of the CNS and become hardened scars that interfere with normal conduction of nerve impulses, and the result is various neuromuscular symptoms.

Myelin sheath SCHWANN Cells are a type of NEUROGLIAL cell. In the PNS, a myelin sheath forms when cells wrap themselves around an axon. The inset shows an electron micrograph of a cross section of an axon surrounded by a myelin sheath. SCHWANN Cells are a type of NEUROGLIAL cell. Myelination serves two purposes, it protects long fragile nerve fibers and it also speeds up a nerve impulse. It speeds up impulse by allowing action potential of nerve impulse to skip from node to node.

Multiple sclerosis is a disease of the myelin sheath.

DRAW AFFERENT AND EFFERENT NEURONS JUST DO IT !

Practice Quiz 1. Give two separate names for the type of neuron shown here. A B H C G – Name for branched endings D F- Name for gaps in myelin sheath E – Name of Cells that form Myelin Sheath