The Human Nervous System
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Nerve Cells Reflex Arc Organisation of the 
Nervous System


Humans, like all living organisms, can respond to their environment. Humans have two complimentary control systems to do this: the nervous system and the endocrine (hormonal) system. The human nervous system controls everything from breathing and producing digestive enzymes, to memory and intelligence.

Nerve Cells  
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The nervous system composed of nerve cells, or neurones:

Motor Neurone:

  • Efferent Neuron – Moving toward a central organ or point
  • Relays messages from the brain or spinal cord to the muscles and organs


Sensory Neurone:

  • Afferent Neuron – Moving away from a central organ or point
  • Relays messages from receptors to the brain or spinal cord


Interneuron (relay neurone):

  • Relays message from sensory neurone to motor neurone
  • Make up the brain and spinal cord


  Sensory neuron Interneuron Motor Neuron
Length of Fibers Long dendrites and short axon Short dendrites and short or long anxon Short dendrites and long axons
Location Cell body and dendrite are outside of the spinal cord; the cell body is located in a dorsal root ganglion Entirely within the spinal cord or CNS Dendrites and the cell body are located in the spinal cord; the axon is outside of the spinal cord
Function Conduct impulse to the spinal cord Interconnect the sensory neuron with appropriate motor neuron Conduct impulse to an effector (muscle or gland)

A neurone has a cell body with extensions leading off it. Numerous dendrons and dendrites provide a large surface area for connecting with other neurones, and carry nerve impulses towards the cell body. A single long axon carries the nerve impulse away from the cell body. The axon is only 10µm in diameter but can be up to 4m in length in a large animal (a piece of spaghetti the same shape would be 400m long)! Most neurones have many companion cells called Schwann cells, which wrap their cell membrane around the axon many times in a spiral to form a thick insulating lipid layer called the myelin sheath. Nerve impulse can be passed from the axon of one neurone to the dendron of another at a synapse. A nerve is a discrete bundle of several thousand neurone axons.

 There are several differences between axons and dendrites:

Axons Dendrites
  • Take information away from the cell body
  • Smooth Surface
  • Generally only 1 axon per cell
  • No ribosomes
  • Can have myelin
  • Branch further from the cell body
  • Bring information to the cell body
  • Rough Surface (dendritic spines)
  • Usually many dendrites per cell
  • Have ribosomes
  • No myelin insulation
  • Branch near the cell body


Neurons are similar to other cells in the body because:

  1. Neurons are surrounded by a cell membrane.
  2. Neurons have a nucleus that contains genes.
  3. Neurons contain cytoplasm, mitochondria and other organelles.
  4. Neurons carry out basic cellular processes such as protein synthesis and energy production.

Neurons differ from other cells in the body because:

  1. Neurons have specialised extensions called dendrites and axons. Dendrites bring information to the cell body and axons take information away from the cell body.
  2. Neurons communicate with each other through an electrochemical process.
  3. Neurons contain some specialized structures (for example, synapses) and chemicals (for example, neurotransmitters).


Check Point g Humans have three types of neurones:

  • Sensory neurones have long axons and transmit nerve impulses from sensory receptors all over the body to the central nervous system.

  • Motor neurones also have long axons and transmit nerve impulses from the central nervous system to effectors (muscles and glands) all over the body.

  • Interneurones (also called connector neurones or relay neurones) are usually much smaller cells, with many interconnections.


The Reflex Arc   [back to top]

The three types of neurones are arranged in circuits and networks, the simplest of which is the reflex arc.

In a simple reflex arc, such as the knee jerk, a stimulus is detected by a receptor cell, which synapses with a sensory neurone. The sensory neurone carries the impulse from site of the stimulus to the central nervous system (the brain or spinal cord), where it synapses with an interneurone. The interneurone synapses with a motor neurone, which carries the nerve impulse out to an effector, such as a muscle, which responds by contracting.

Reflex arc can also be represented by a simple flow diagram:


Organisation Of The Human Nervous System  
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The human nervous system is far more complex than a simple reflex arc, although the same stages still apply. The organisation of the human nervous system is shown in this diagram:

It is easy to forget that much of the human nervous system is concerned with routine, involuntary jobs, such as homeostasis, digestion, posture, breathing, etc. This is the job of the autonomic nervous system, and its motor functions are split into two divisions, with anatomically distinct neurones. Most body organs are innervated by two separate sets of motor neurones; one from the sympathetic system and one from the parasympathetic system. These neurones have opposite (or antagonistic) effects. In general the sympathetic system stimulates the “fight or flight” responses to threatening situations, while the parasympathetic system relaxes the body. The details are listed in this table:

Organ Sympathetic System Parasympathetic System


Tear glands

Salivary glands






Dilates pupil

No effect

Inhibits saliva production

Dilates bronchi

Speeds up heart rate

Inhibits peristalsis

Stimulates glucose production

Inhibits urination

Constricts pupil

Stimulates tear secretion

Stimulates saliva production

Constricts bronchi

Slows down heart rate

Stimulates peristalsis

Stimulates bile production

Stimulates urination

Check Point g The Human Nervous system compromises  of:

  • The CNS and peripheral nervous system
  • Peripheral nervous system includes the autonomic and somatic nervous system
  • The sympathetic and parasympathetic have antagonistic effects


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Last updated 17/04/2004