Nervous system

The nervous system contains 2 subdivisions:

1. central nervous system

   a. spinal cord

   b. brain

   these act as the integrating and command centers of the nervous system

2. peripheral nervous system

   a. consists of nerves that extend from the brain and spinal cord

   b. this can also be functionally classified into 2 more subdivisions

   1. sensory (afferent) division

      • consists of nerve fibers that convey impulses to the CNS from sensory receptors located in various parts of the body

      • sensory fibers delivering information from the skin, skeletal muscles, and joints are called somatic sensory fibers

      • sensory fibers delivering information from the visceral organs are called the visceral sensory fibers

   2. motor (efferent) division

      • carries impulses from the CNS to effector organs, the muscles, and glands

        • these impulses activate motor responses

      • the motor division also divides into 2 more subdivisions

        1. the somatic nervous system - allows us to consciously control our skeletal muscles

        2. autonomic nervous system - regulates events that are automatic, or involuntary

The central nervous system consists of the brain and spinal cord.

• the brain stem has 3 main parts:

  1. pons - breathing

  2. medulla - heart rate

  3. midbrain - controls eye movements

• the brain also contains the hypothalamus

  • the hypothalamus regulates hormones that create thirst, emotions, and sex hormones

NEURONS

Anatomy of a neuron

• all neurons have a cell body which contains the nucleus and is the metabolic center of the cell

• all neurons contain at least one slender processes extending from the cell body

• most long nerve fibers are covered with a whitish fatty material called myelin

  • myelin protects and insulates the fibers and increases the transmission rate of nerve impulses

Classification of a neuron

• neurons can be classified by either their function or their structure

  1. functional classification - classifies neurons based on the direction the nerve impulse is headed in relation to the CNS

     • sensory neurons - neurons carrying impulses from sensory receptors to the CNS - they keep us informed about what is happening both inside and outside the body

       • cell bodies of these neurons are always found in ganglion outside the CNS

     • motor neurons - neurons carrying impulses from the CNS to the viscera and/or muscles and glands

       • call bodies of these neurons are always located in the CNS

     • association neurons - connect the motor and sensory neurons in neutral pathways.

       • cell bodies are always located in the CNS

  2. structural classification - classifies neurons based on the number of processes extending from the cell body

     • multipolar neuron - if there are several processes (most common)

     • bipolar neurons - neurons with 2 processes (axon and dentrite)

     • unipolar neurons - single process emerging from the body cell

Physiology of a nerve impulse

• no matter what the stimuli is, the permeability properties of the cells plasma membrane change for a very brief period

  • normally, sodium ions cannot diffuse through the membrane but when the neuron cell is “excited” sodium ions can pass through

    • because sodium has a high concentration outside the cell, it will diffuse quickly into the neuron.

  • the rush of sodium changes the polarity of the neurons membrane (depolarization)

  • if the fish is great enough, the depolarization will activate the neuron to initiate and transmit an action potential (nerve impulse)

• the nerve impulse is an all or none response

• once the sodium ions rush in, the membrane then closes its sodium gates once again

• nerve fibers that have myelin sheaths conduct impulses much faster because the nerve impulse literally jumps from node to node along the length of the fiber.

Reflex Arc

• much of what the body must do every day is programmed as reflexes

• reflexes - rapid, predictable, and involuntary responses to stimuli

• the types of reflexes in the body are classified as either autonomic or somatic reflexes

  1. autonomic reflexes - regulate the activity of smooth muscles, the heart, and glands

     • ex. secretion of saliva (salivary reflex) and changes in size of pupil (pupillary reflex)

     • autonomic reflexes regulate body functions such as

       • digestion

       • elimination

       • blood pressure

       • sweating

  2. somatic reflexes - include all reflexes that stimulate the skeletal muscles

     • ex. pulling your arm away from a hit surface

• at minimum there are 5 elements:

  1. sensory receptors- react to a stimulus

  2. effector organ - muscle or gland eventually stimulated

  3. afferent

  4. efferent (3 and 4 to connected the sensory receptors to the effector organs)

  5. the synapse between afferent and efferent neurons ( central element) - CNS integration center

CRANIAL NERVES

the cranial nerves are numbered in order, and in most cases their names reveal the most important structures they control

1. olfactory - sniff aromas

2. optic - allows you to see

3. oculomotor - help constrict pupil

4. trochlea - motor fibers to superior oblique eye muscle

5. trigemineal - sensory fibers to face, chewing muscles

6. abduces - turn eye laterally

7. facial - stimulates tears , salivary glands , and taste buds

8. vestibulocochlear - hearing and balance

9. glossopharyngeal - stimulates muscles for gag reflex and swallowing

10. vagus - digestion, helps move food through digestive tract

11. accessory - allows you to shrug your shoulders

12. hypoglossal - tongue movement