1. Structures & Processes of the Nervous System
Miss Richardson
SBI4U

2. The Human Nervous System
Components:
Central nervous system (CNS)
Brain
Spinal cord
Peripheral nervous system (PNS)
Somatic nervous system (conscious control)
Autonomic nervous system (no conscious control)
Messages are communicated through three types of nerve cells (neurons)
Sensory (afferent)neurons
Interneurons
Motor (efferent) neurons

3. The Human Nervous System

4. The Reflex Arc
A simple reflex is an immediate response to a stimulus that doesn’t require the brain – only the spinal cord
Sensory receptors pick up the stimulus – the sensation of the tap on the knee.
This information travels along the afferent  (sensory) neuron to the spinal cord.
The information is transmitted from the afferent neuron in the spine to an interneuron in the spine
The interneuron relays the information to an efferent (motor) neuron in the spine.
The efferent neuron sends the information to the muscles (the effector) in the thigh.
These muscles contract and the knee jerks.

5. Neurons Neurons are responsible for the reflex response
All neurons contain:
Cell body
Dendrites
Axon

6. Neurons

7. Neurons
The dendrites receive information and send it out to the cell body
The cell body is where the nucleus is found
Determines if information will be passed on
Axons project out from the body
Carry the nerve impulse from the cell body to the axon terminal and synapse
When nerve impulse reaches the axon terminal, the messages are relayed to the effector using chemicals called neurotransmitters

8. Resting Membrane Potential
If a neuron is not sending a signal or impulse it is said to be at rest
While at rest, potassium ions (K+) are found mainly inside the membrane, and sodium ions (Na+) are found mainly outside the membrane
Resting potential - difference in electric charge across the membrane of a neuron, measured in millivolts (-70mV)

9. Membrane permeability
While at rest the neuronal membrane is more permeable to K+ than to Na+
Therefore K+ diffuses out of the nerve cell (down its concentration gradient) faster than Na+ diffuses in.
Na+ K+

10. The Nerve Impulse
Neurons communicate with each other by sending nerve impulses down the length of the axon and across a gap (called a synapse) to another neuron.
As a nerve cell is stimulated there is a rapid influx of Na+
A nerve impulse occurs as there is a rapid reversal of the electrical potential across the neurons membrane due to this influx
 ACTION POTENTIAL

11. Terminology
Depolarization: Diffusion of Na+ ions into the nerve cell resulting in an ACTION POTENTIAL
Repolarization: Na+/K+ pump restores the original resting potential of the neuron

12. The Action Potential
At rest, the outside of the neuron is more positive than the inside
Nerve stimulation causes membrane to become more permeable to Na+
Depolarization: Na+ rushes in making the inside more permeable and resulting in an ACTION POTENTIAL
Repolarization: K+ flows out of the membrane restoring the resting potential (outside more +ve)
Na+/K+ pump uses ATP to pump Na+ out and K+ into neuron restoring ion concentrations

13. Action Potential

14. The Action Potential
Action potentials are often represented in a graph

15. “All or Nothing”
Neurons either fire maximally or not at all, this is referred to as the “all or none” response
Increasing neuronal stimulation beyond a critical level will not result in an increased response
Neurons response to increased stimulation by increasing the frequency of firing, not the intensity at which they fire.
The Threshold level is the minimum strength of stimulation required to produce an action potential

16. Communication
When an AP reaches the end of the axon the impulse must be transmitted to another adjacent neuron.
This communication occurs between neurons across spaces called synapses.
The message is not transmitted electrically as it was down the axon, but rather by the release of chemical messengers.

17. Synapse