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Topic 6: Human health and physiology. 6.5 – PART I Nerves. The Nervous System. Central NS. Peripheral NS. Made up of sensory and motor neurons Neuron is the individual cell that carries the electrical impulse throughout the body

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the nervous system
The Nervous System

Central NS

Peripheral NS

Made up of sensory and motor neurons

Neuron is the individual cell that carries the electrical impulse throughout the body

Nerve – Many individual neurons grouped into a single structure


Nerve is a telephone cable

Each wire in the cable is a neuron

  • Brain/Spinal cord initiates a response when needed (motor response)
  • Neurons carry this information
  • Sensory neurons bring info to CNS; motor neurons carry response to muscles
nervous system pathway

Tracing the nervous system pathway

  • Electrical impulse called an action potential
  • Imagine that you have just touched the arm of the person sitting next to you
  • This was an accident and you immediately removed your hand
Nervous System Pathway
stimulation and interpretation
Stimulation and Interpretation
  • Touched arm triggers pressure receptor which starts an
  • action potential or nerve impulse
  • Receptors are designed to transform particular kinds of
  • stimuli into action potential
  • Interpretation of touch occurs in the brain
    • - chain of neurons take impulse to CNS
  • Sensory info reaches spinal cord by spinal nerves
  • Sensory neurons run from receptors to spinal cord
  • Action potential reaches the spinal cord and is routed
  • through CNS for interpretation
  • In the spinal cord and brain the action potential is carried by
  • relay neurons
  • You became aware of the touch by interpretation by your brain

Your brain moved your hand

  • Action pathway began in the brain’s relay neurons, passed
  • down the spinal cord, and out a spinal nerve
  • Action potential travels across motor neurons to muscle
  • Motor neuron sends a chemical signal to the muscle which
  • results in the contraction and thus movement
what is a nerve impulse
What is a nerve impulse?
  • Begins with neurons within a nerve that carry the action potential
  • Conductor of impulse is the axon
  • Axons of organisms with highly developed nervous system (e.g. humans) surrounded by membrane structure called myelin sheath
  • Myelin sheath increase rate at which action potential is passed down the axon
resting potential
Resting Potential

- The state of an area of neuron ready to send action potential

- Area is said to be polarized

- Characterized by active transport of Na+ and K+ in different directions

- Na+ actively transported out

- K+ actively transported in

- Collection of negatively charged organic ions in axon; creates relatively positive charge outside and negatively charged internal environment

action potential

The reversal and restoration of the electrical potential across a plasma membrane

as a nerve impulse passes along a neuron

Action Potential

- Self propagating wave of ion movement in and out of the neuron membrane

- Channels open and Na diffuses in

- Shortly after, K diffuses out

- This diffusion (Na in and K out) is the impulse or action potential

- Nearly instantaneous event that occurs in one area of the axon: DEPOLARIZATION

- This axon area initiates the next area to open up the Na channels, then K, and the action potential continues down the axon

- Self-propagating: impulse begins in the dendrite end and will self-propagate to far axon- end


Overview: The Nerve Impulse


MyelinatedvsUnmyelinated axon


return to resting potential
Return to resting potential

- Neurons send dozens of action potential signals in a short period of time

- Na/K ions must be returned to positions characteristic of resting potential

- Active transport must be used (i.e. repolarization)

- Time it takes to send action potential and then repolarize is called refractory period

synaptic transmission
Synaptic Transmission

What happens when one nerve meets another nerve?

  • Sensory pathway flows in one direction; terminal end of axon adjoins the dendrite of another cell
  • Presynaptic/postsynaptic neurons
  • Chemical communication between cells (i.e. synapse)
principles of synaptic transmission
Principles of Synaptic Transmission
  • Far end of axon is terminal button
  • Within terminal button are vesicles filled with chemicals called neurotransmitters (NTs)
  • NTs are any chemical used for synaptic transmission (there are many such chemicals)

Presynaptic and Postsynaptic Neurons


sequence of events when action potential reaches terminal buttons
Sequence of events when action potential reaches terminal buttons
  • Ca ions diffuse into terminal buttons (signals vesicles to move)
  • NT vesicles fuse with plasma membrane; NT release
  • NT diffusion from presynaptic to postsynaptic neuron across synaptic gap
  • NT binds with receptor protein on postsynaptic neuron membrane
  • Binding opens ion channel and Na ions diffuse through to create action potential
  • Postsynaptic depolarization
  • NT broken into two or more fragments by enzymes and released from receptor
  • Ion channel closes
  • NT fragments diffuse back across gap and are reassembled at presynaptic neuron