How Nerve Signals Maintain Homeostasis

Chapter 9

• Every single stimulus (light, touch, feeling of muscles moving, pressure, blinking, etc.) involves your nervous system.

• Coordinated motions (which do not seems to be coordinated) are controlled mostly by the nervous system.

• nervous system + endocrine system control the actions of the body.

The Nervous System Intro

• Elaborate communication system– Contains 100 billion nerve cells in the brain alone.– Humans: memory, learning, language, etc.

The Nervous System

• 2 main divisions:– Central nervous system (CNS)

• Brain + spinal cord coordinating centre– Incoming and outgoing info.

– Peripheral nervous system (PNS)• Info between organs of body and CNS.

– Two peripheral divisions:» Somatic

• Skeletal muscles, bones, and skin• Sensory somatic PNS: relay info about environment to CNS. • Motor somatic PNS: initiate appropriate responds.

» Autonomic• Motor nerves which control internal organs of body.

• Sympathetic• Parasympathetic

Vertebrate Nervous System

Organization of the Nervous System

• Glial cells– Nonconducting cells structural support and metabolism of

nerve cells.• Neurons

– Functional units of nervous system (conduct electricity)• Sensory neurons

– “afferent neurons”– Sense and relay info (stimuli) from environment to CNS

» (ex// photoreceptors, chemoreceptors, thermoreceptors, etc.)» Ganglia: clusters of sensory receptors.

• Interneurons– Integrate and interpret sensory and motor neurons (found mostly in the CNS)

• Motor neurons– “efferent neurons”– Relay info to effectors: muscles, organs, and glands (can produce a response)

Nerve Cells

• All neurons contain:– Dendrites

• Receive info from other nerve cells• Conduct info towards the neuron’s

cell body

– Soma: neuronal cell body (containing nucleus)

– Axon• Extension of cytoplasm• Carries nerve impulse towards other

neurons or effectors. • Creates an “attachment” to other

neurons.• Super super thin (100 could fit inside

single human hair!)

Anatomy of a Nerve Cell (Neuron)

• Insulation– Myelin sheaths are

formed by special glial cells called Schwann cells

– Myelin sheaths prevents the loss of ions from axon. • (important when you

understand how these cells work!)

• Made of ‘fatty protein’

Myelin Sheath

• Between sections of myelin sheath• Nerve impulses jump from one node to another speed

up movement of nerve impulses. – Nerve impulses move much faster along myelinated nerve

fibres than nonmyelinated ones.

(Nerve impulses move much faster along smaller diameter axons)

Nodes of Ranvier

• Thin membrane which surrounds the axon in peripheral nervous system– Promotes regeneration of damaged axons– Severed neurons can be rejoined.– (feeling gradually returns to your finger following a

paper cut)

Neurilemmas

White and Grey Matter

• White matter– Nerve cells within the brain that contain

myelinated fibres and neurilemma• Myelinated axons white in appearance

• Grey matter– Nerve cells within the brain and spinal cord

referred to as grey matter: lack myelin sheath and neurilemma do not regenerate after injury• Damage to grey matter is usually permanent.

Categories of Neurons• Sensory Neurons: AFFERENT neurons: relay info from environment

CNS– Located in clusters called ganglia

• Photoreceptors (eyes)• Chemoreceptors (tongue and nose)• Thermoreceptors (skin and hypothalamus)

• Interneurons– Interpret sensory info and send info to outgoing motor neurons (mostly in

CNS)• Motor neurons: EFFERENT neurons: relay info from interneurons

effectors (produce a response)– Muscles– organs,– glands

Organization of Nerves

• Most nerves are part of many neurons/axons held together in a ‘bundle.’

• Reflex– Involuntary and often unconscious

• Example: touching a hot stove– How long does it take you to remove your hand?– Nerve impulse is carried to the spinal chord

motor neuron (skipping brain) sensation felt after removing hand.

Neural Circuits

• Simplest nerve pathway.• Occurs without brain coordination• Five essential components:– Receptor– Sensory neuron– Interneuron (spinal chord)– Motor neuron – Effector

Reflex arc

• Page 4171) # 1-6, 8. 2) Draw/ Label a general neuron: dendrite(s),

soma, nucleus, axon, myelin sheath, axon terminals, direction of impulse.

3) Read page 415 article. Make brief notes.

Seatwork/Homework