The Nervous System

The functional unit of the nervous system is nerve cell or neuron.

A nerve tissue consists of hundreds of nerve cells (neurons) and neuroglial cells (Greek: glue), blood vessels, and connective tissues.

 

Neurons (nerve cells)

In general, neuron consist of three parts: dendrites, cell body (soma), and axon.

There are three structural classifications of neurons:

  1. Multipolar – which carry 1 axon and several dendrites
  2. Bipolar – which carry 1 axon and 1 dendrite, and
  3. Unipolar – where the axon and dendrite are fused together. Thus, unipolar neurons are also referred as pseudopolar.

Neurons can also be classified by its functions. There are:

  1. sensory or afferent neurons
  2. motor or efferent neurons
  3. interneuron or association neurons – these are mainly found in the central nervous system

Other than neurons, which capable of generating and propagating action potential, the nervous system also contains neuroglials cell which are neither excitable nor conducive. These cells mainly function to maintain the well being of neurons. There are six types of glial cells:

  1. astrocytes
  2. oligodendrocytes
  3. microglia
  4. ependymal cell
  5. schwann cell, and
  6. satellite cell

You can find great explanation of the neurons from AnataomyZone. Here is an example.

 

 

 

 

Action potential

Neurons are unique because not only that they can propagate electrical stimulus or nerve impulse (conductivity), they also generate electrical stimulus. This electrical stimulus is action potential generated by differences in Ca+ and K+ concentration inside and outside of the neurons.

 

 

 

 

 

The nervous system

The nervous system can also be categorized according to structure (anatomical) and function.

 

Structurally, the nervous system is divided into:

  1. Central Nervous System (CNS) – this consist of the brain and the spinal cord
  2. Peripheral Nervous System (PNS) – this consist of all nerve branches outside of CNS (12 pairs of cranial nerves (CNI-XII) and 31 pairs of peripheral nerves)

 

Functionally, the nervous system is divided into:

  1. Somatic Nervous System (SNS)
  2. Autonomic Nervous System (ANS) – is divided further into Sympathetic and Parasympathetic Nerves
  3. Enteric Nervous System (ENS) – controls the gastrointestinal (GI) tract independent of SNS and ANS.

The ENS is functionally divided into myenteric plexus (plexus=network) which controls contraction of smooth muscles, and plexus of Meissner in the submucosa layer of GI tract. Plexus of Meissner controls secretion of the organs in GI tract.

However, there is minor involvement of ANS in the nervous control of the GI tracts. The GI tracts receive innervation by parasympathetic fibres from CN-X (cranial nerve-10) and S2 (sacral-2), and sympathetic fibres from thoracic and upper lumbar region of the spinal cord.

 

 

 

 

Reflex and reflex arcs

 

The brain is responsible for processing (integrating) all information such as sensory information, learning and memorization, and personality development.

However, there are some information that was not process in the brain. Instead, information is processed only in the brain stem and spinal cord. In this case, information from sensory receptor travels through the reflex arcs (reflex circuits) in the brain stem or the spinal cord and out to the motor neurons into effector (organs). Bypassing the brain ensure rapid and immediate rapid responses during dangerous situation such as touching a hot kettle, slamming on the brake while driving, or closing the eyes when a fly flew at your face. Often, reflexes are learned and acquired from experiences.

 

 

reflex_arc_med

General component of a reflex arc. Information was not send to the brain. Instead, it enters the spinal cord through dorsal ganglion, processed in the grey matter of the spinal cord, and sent back out to the effector organs through motor neuron in the ventral root of the spinal cord.

 

There are four types of reflexes depending on where integration (information processing) happens or the effector organs involved:

  1. spinal reflex
  2. cranial reflex
  3. somatic reflex
  4. autonomic (visceral) reflex – visceral refers to inner organs such as digestive tract, heart, kidney, etc.

 

 

Exercise

1- Compare and contrast between SNS and ANS

2- Compare and contrast between somatic reflex and visceral reflex.

3- Compare and contrast between cranial reflex and spinal reflex.

3- List the components of reflex arc. Explain the process that happens when someone closes their eyes when a fly flew at their face.

 

 

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