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.

Types of 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 occurring gaps between sections of the myelin sheath

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 through the spinal cord that provides a framework for a reflex action.

Ex. Accidentally touching a hot stove (see handout)1. Heat detected by temperature

receptors in skin2. nerve impulse carried by a

sensory neuron to the spinal cord

3. interneuron in spinal cord passes the impulse to a motor neuron

4. motor neuron carries impulse to muscles arm/hand

5. causes the muscles to contract and pull hand away

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.

HOMEWORK: Handout Questions #1-9