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Nervous System

Contents

Neurons and Nerves
Neurotransmitter
The Brain
Spinal Cord
Peripheral Nervous System
Autonomic Nervous System
Senses: Sight, Senses, Smell, Taste, Sences, Senses
Memory
Higher Functions
Altered States
    • Merkel's endings - They are like tiny disks stuck in the underside of the epidermis, where they feel slight changes in its shape, thereby detecting light touch. They are both quick- and slow-change mechanoreceptors.
    • Pacinian endings - They have layers like an onion and are sited deep in the dermis. They pick up heavy pressure and also fast vibrations, such as those from a tuning fork.
    • Ruffini endings - They respond to sustained stress or gradually altering shape. This means that they are slow-change mechanoreceptors. They are found mainly in hairy skin and are sausage- or spindle-shaped. It is thought that they may also detect extreme heat.
  • Proprioceptors - The sense of position and movement of limbs is dependent upon receptors termed proprioceptors (Figure 22a). They are located in the joints and associated ligaments and tendons that respond to stretching, pressure, and pain. Nerve endings from these receptors are integrated with those received from other types of receptors so that we know the position of body parts.
  • Sensory nerves - Nerve impulses may reach the somatosensory cortex for analysis before a response is decided. These result in voluntary actions - a deliberate response. Sometimes the stimulus require immediate action (such as from the burning sensation), a reflex action is taken without the conscious control of the brain. These are the involuntary actions directed by the spinal cord. We only become aware of them when other impulses are sent to the brain to "inform" what has happened. The path which impulses travel along during a reflex action is called a reflex arc. Not all the body parts receive the same attention of the brain. The relative importance is often represented by mapping over the length of the sensory or motor cortex. These cortical maps (Figure 22b) are not drawn to scale; instead they are variously distorted to reflect the amount the neural processing power devoted to different regions. This accounts for the grotesque appearance of the human body in the homun-culus, which is a translation of the body's sensory map into the human form.

    position sensor

    brain

    Figure 22a Propriocep-tors [view large image]


    Figure 22b Homunculus
    [view large image]




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