1. SBI 4U: Metablic Processes
THE NERVOUS SYSTEM
Section 1.3

2. INTRODUCTION TO THE NERVOUS SYSTEM
SBI 4U: Metablic Processes
INTRODUCTION TO THE NERVOUS SYSTEM
Communication system
100 Billion nerve cells in the brain alone
2 main divisions to a vertebrate nervous system:
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3. SBI 4U: Metablic Processes
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
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4. SBI 4U: Metablic Processes
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).
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5. SBI 4U: Metablic Processes
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6. SBI 4U: Metablic Processes
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
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7. SBI 4U: Metablic Processes
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)
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8. SBI 4U: Metablic Processes
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
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9. SBI 4U: Metablic Processes
REFLEX ARC
Section 1.3