THE NERVOUS SYSTEM

INTRODUCTION TO THE NERVOUS SYSTEM

Communication system 100 Billion nerve cells in the brain alone2 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 Nervesskeletal muscle, bones and skinsensory and motor somatic nerves

Autonomic Nervesspecial motor nerves that control the internal organssympathetic 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 bodyAxon: 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 chargeSchwann cells: type of glial cell that

produces myelin sheathnodes 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 rateaxon 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 skinnerve impulse carried by a sensory neuron to the spinal cordinterneuron in spinal cord passes the impulse to a motor neuronmotor 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 HandoutHOMEWORK: Handout Questions #1-9

REFLEX ARC

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