The Nervous System
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Essential for maintaining homeostasis in the body
Controls all sensory inputs and reactions
Parts of the nervous system:
● CNS (brain and spinal cord)
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● PNS (somatic and autonomic cells; sympathetic and parasympathetic systems)
Somatic system:
● Sensory nerves (afferent nerves):
○ responsible for detecting stimuli
and relaying the information to
the hypothalamus
● Motor nerves (efferent nerves):
○ responsible for movement
reaction
● Sensory nerves respond to stimuli from
the environment for example: Chemical
stimuli
● Motor nerves cause muscle cells to
contract
Reflex arc:
● An involuntary reaction but part of the somatic system
● Uses sensory and motor nerves
● It crosses the spinal cord via a connecting neuron
●
The brain receives the stimulus signal after the reaction has happened
Autonomic system:
● Involuntary: stimuli and responses that we are not aware of and cannot control
● Most internal body mechanisms under the control of the autonomic system
● Consists of two parts that act in opposite ways:
○ Sympathetic
○ Parasympathetic
Autonomic system: Sympathetic
● Sympathetic: ​Ganglia sit next to the spinal column
○ Ganglion: a mass of nerve cell bodies
● Nerves run from the ganglia to the specific organs or areas
●
The Sympathetic system can act immediately
○ “Fight or flight” response
Autonomic system: parasympathetic
● Ganglia are distant from the spine: sit near the organs or targets
● Returns the body to the normal “rest and digest” condition
○ counteracts the sympathetic system
● Most regular functions are under the control of the parasympathetic system
Central Nervous System:
● Consists of:
● Brain:
○ An organ contained within the skull, surrounded by bone
○ Metabolically expensive:
■ Only 2% of body mass
■ Uses 15% of blood supply
■ 20% of oxygen & glucose
●
■ Know brain structures **see (​The Brain​)
The spinal cord
○ Contained within the spinal column, completely surrounded by bone
Cells of the nervous system:
● The nervous system consists of many
different types of cells
○ Neurons
■ Motor neurons​ that carry
signals TO effectors such
as muscles
■ Sensory neurons​ that
carry signals FROM
sensors/receivers
■ Interneurons​ that connect
other neurons together
■ Pyramidal​ ​cells​ are found
mostly in the cortex of the
brain
○ Various types of “support” cells called glial cells
■ Astrocytes
■ Microglia
■ Ependymal cells
■ Oligodendrocytes
● Nerves are ​bundles​ of many thousands of neurons surrounded by connective tissue
● *See neuron structure (​The Brain​)
● Neuron signals travel in ​one direction
○ From the dendrite, across the cell body, and down the axon towards the axon
terminals
Resting 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)
● While at rest the neuronal membrane is ​more permeable to K+ than to Na+
○ K+ diffuses out of the nerve cell (down its concentration gradient) faster than
Na+ diffuses in
Action Potential:
● Nerves respond to stimuli: either from an external sensor or from another neuron
1. At rest, the outside of the neuron is more positive than the inside
2. Nerve stimulation causes membrane to become more permeable to Na+
3. Depolarization​: Na+ rushes in making the inside more positive relative to the
outside
4. Repolarization​: K+ flows out of the membrane restoring the resting potential
5. Na+/K+ pump uses ATP to pump Na+ out and K+ into neuron, restoring ion
concentrations
6. The neuron returns to Resting Potential
● The ​Threshold level​ is the minimum strength of stimulation required to produce an
action potential
● The signal can travel in ​ONE​ direction only: the part of the ​neuron​ that undergoes the
action potential ​must return to resting potential before it can respond to another
stimulus
● This ​refractory period​ ensures that the signal travels from dendrites to axon terminal
and not the other way
Synaptic Transmission: