THE NERVOUS SYSTEM

1. THE NERVOUS SYSTEM

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

3. 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

4. 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).

6. 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

7. 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)

8. 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 • 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 p452, Reflex Lab Handout • HOMEWORK: Handout Questions #1-9

9. REFLEX ARC • http://www.free-ed.net/sweethaven/MedTech/NurseCare/fig91902_05.jpg