Parts of the Nervous System
- The nervous system is divided into two major divisions
- The central nervous system (CNS) composed of the brain and the spinal cord
- The peripheral nervous system (PNS) which connects the CNS to the rest of the body.
Divisions of the Peripheral Nervous System
- The peripheral nervous system is made up of thick bundles of axons, called nerves.
- They carry messages back and forth between the CNS and the muscles, the organs, and senses outside of the CNS
- The PNS is further divided into two subdivisions: the somatic nervous system and the autonomic nervous system.
The Somatic Nervous System
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is associated with activities that are traditionally though of as conscious and voluntary.
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it is also involved in the relay of sensory and motor information to and from the CNS:
- therefore, it is made up of motor neurons and sensory neurons.
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Motor neurons carry instructions from the CNS to the muscles (efferent fibers)
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Sensory neurons carry sensory information to the CNS (afferent fibers)
The Autonomous Nervous System
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controls the internal organs and glands
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often generally outside the realm of our voluntary control.
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composed of two subdivisions: sympathetic and parasympathetic
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Sympathetic Nervous System
- involves in preparing the body for stress-related activities
- example: increasing heart rate, inhibiting digestion, dilating pupils
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Parasympathetic Nervous System
- associated with returning the body into its normal, relaxed state.
- example: slowing heart rate, stimulating digestion, constricting pupils
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These subdivisions operate in tandem to maintain the body’s homeostasis.
- This is a state of equilibrium (or balance) in which our body maintains biological conditions (such as body temperature) at optimal levels.
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The sympathetic nervous system activates when we are faced with stressful situations, most often when there is danger or threat involved (eg. a human fighting a large bear)
- This is essential as it allowed our ancestors to increase their chances of survival in the middle of a threat by undergoing a series of physiological changes.
- these physiological changes, such as dilation of pupils and the increase in heart rate and blood pressure, allows us to face the oncoming threat in hopes that it may fight it or flee away to safety.
- this phenomena is called the fight or flight response.
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Once the threat has been resolved, the parasympathetic nervous system takes over and returns the body to its relaxed state.
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While the fight or flight response is critical in responding to a physical threat, we have also evolved to respond like this to a perceived threat in which there is no physical danger involved (such as giving a presentation in class).
- Because of this, we suffer negative health consequences when faced with psychological threats that we can neither fight nor flee.
- This reaction to stress can also be traced back to early experiences of trauma.