THE NERVOUS SYSTEM INTRODUCTION TO THE NERVOUS SYSTEM

THE NERVOUS SYSTEM

INTRODUCTION TO THE NERVOUS SYSTEM Communication system 100 Billion nerve cells in the brain alone 2 main divisions to a vertebrate nervous system:

TWO MAIN DIVISIONS Central Nervous System (CNS) nerves, brain, spinal cord coordinating centre Peripheral Nervous System (PNS) nerves that carry info between the CNS and the organs Somatic Nerves skeletal muscle, bones and skin sensory and motor somatic nerves Autonomic Nerves special motor nerves that control the internal organs sympathetic nervous system and parasympathetic nervous system

NERVE CELLS Two cells types: neurons and glial cells Glial cells (neuroglial cells): structural support and metabolism of nerve cells (do not conduct) Neurons: nerve cells that conduct nerve impulses (functional unit). A nerve is a bundle of many neurons. Sensory Neurons: (afferent neurons) carry impulses from sensory receptors to the CNS Sensory receptors ex. photoreceptors in eyes (light), chemoreceptors in nose and skin (chemicals), thermoreceptors in skin, hypothalamus (heat/cold). Ganglia (singular – ganglion): clusters of sensory nerve cell bodies located outside of the CNS. Motor Neurons: (efferent neurons) carry impulses from the CNS to effectors (muscles, organs, glands. . . i. e. Things that produce a response) Interneurons: link neurons within the body (found mostly in the CNS).

NERVE CELL ANATOMY Cell body: nucleus and majority of cytoplasm. Dendrites: projections of cytoplasm that carry impulses TOWARD the cell body Axon: extension of cytoplasm that carries nerve impulses AWAY from the cell body. Myelin Sheath: insulated covering (fatty protein) over the axon of some nerves, “myelinated” prevents loss of charge Schwann cells: type of glial cell that produces myelin sheath nodes of Ranvier: regularly occuring gaps between sections of the myelin sheath

ANATOMY continued nerve impulses jump from one node to another – increases speed of the impulse nonmyelinated nerves carry impulses at a slower rate axon diameter also effects speed – narrower = faster. Neurilemma: PNS axons have a thin membrane surrounding the axon. Promotes regeneration of damaged axons. Cells without myelin sheath or neurilemma do not regenerate, damage is permanent. (“grey matter” in brain and spinal cord; white matter has myelin sheath). PNS has greater ability to regenerate than the CNS (spinal cord injuries). Presence of a growth inhibitor in the CNS. Scientists are looking for ways around this (stem cells)

REFLEX ARC Simplest nerve pathway. Involuntary, unconscious. Neural circuit though the spinal cord that provides a framework for a reflex action. Ex. Accidentally touching a hot stove: Heat detected by temperature receptors in skin nerve impulse carried by a sensory neuron to the spinal cord interneuron in spinal cord passes the impulse to a motor neuron causes the muscles in the hand to contract and pull away from heat Happens in less than a second – information has not even travelled to the brain. When it does, the sensation of pain will become noticeable and you may scream! If you had to wait to feel the pain first – the burn would be a lot worse. For interest: Reflexes p 452, Reflex Lab Handout HOMEWORK: Handout Questions #1 -9

REFLEX ARC http: //www. freeed. net/sweethaven/Med. Tech/Nurse. Care/fig 91902_05. jpg ●