The spinal cord

Back to: O level Biology NOTES Uganda syllabus

This is part of the central nervous system that runs from the brain to the tail through and covered by the vertebral column.


Functions of the spinal cord

  1. It connects the peripheral nervous system to the brain.
  2. It is a center for simple spinal reflex actions
  3. Receives impulses from receptors
  4. Interprets messages especially in reflex arc
  5. Sends impulses to the receptors.

VOLUNTARY AND INVOLUNTARY ACTIONS
The nervous system controls several actions in the body. Such actions may be voluntary or involuntary.
A voluntary action is one initiated consciously under the direct control of the brain i.e. they are actions one at will e.g. dancing, laughing, stealing, etc. These actions are performed consciously by an animal. In such actions the animal chooses to do or not to do something.
Involuntary actions are the ones that occur without conscious thoughts e.g. breathing, etc.


THE REFLEX ACTION
This is a rapid automatic response of an organism, which is not initiated by the brain. Reflex actions take place without the awareness of the individual. A
reflex action occurs as a result of impulses travelling along neurons in a path called a reflex arc.

Characteristics of a reflex action

  • It occurs rapidly i.e. The action occurs very fast.
  • It is inborn (innate) but not learnt.
  • It is co-ordinated by either the brain or spinal cord but usually initiated by spinal cord.
  • It occurs without one’s will.
  • It is a repeated response to a similar stimulus.
  • Three neurons are involved.

Examples of reflex actions
1) Blinking when a foreign body falls on the eye
2) Withdraw of the arm when someone accidentally touches a hot body.
3) Sneezing
4) Knee jerk i.e. a relaxed leg gives a forward kick when tapped slightly below the patella.
5) With draw of the foot from a sharp object.

Example of a simple reflex action

  1. When one accidentally touches a hot body using a finger, the receptors in the finger receive the stimulus and change it into nervous impulses that travel along the sensory neuron to the spinal cord and then cross the synapse.
  2. The impulse is then handed over to the relay neuron in the spinal cord (gray matter) and then cross another synapse.
  3. The relay neuron in turn hands over the impulse to the motor neuron.
  4. The motor neuron then carries the impulse from the spinal cord to the effector muscles of the hand. This causes the muscles to contract and the hand is removed from the hot body.
  5. At the same time, the original message is sent to the brain which then interprets it as pain or heat.
    Note; these processes occur rapidly in the body without the awareness of the individual
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Advantages of reflex actions to animals

  1. They help animals to avoid danger.
  2. They control activities in the body, which we do not have conscious control over.
  3. They form a basis of some animals’ behaviour, e.g. amoeba.

THE REFLEX ARC
This is a description of processes, which take place within the body during a reflex action.
The stimulus is perceived by the receptors, which change it into nervous impulse (transduction). The impulse travels along the sensory neuron to the spinal cord. In the grey matter of the spinal cord, the sensory neuron makes synaptic connections to the relay neuron and impulses move from the sensory neuron to the relay neuron across synapses. The relay neuron in turn transmits the impulse to the motor neuron across a synapse. The impulse then moves from the spinal cord to the effector muscles through the motor neuron. The impulse causes the muscles to contract or relax depending on the stimulus.

Diagram illustrating a reflex action

image 17

Routes/path of reflex arc
The reflex arc has 5 paths/routes

  1. Receptors:
    This is the organ or structure that receives the stimulus e.g. the sensory endings in the skin.
  2. Sensory nerve:
    This is the part of the reflex arc that carries impulse from receptors to the spinal cord or to the brain.
  3. Relay neuron:
    It connects the impulse from the sensory neuron to the motor neuron.
  4. Motor neuron:
    This carries impulses from the relay neuron to the effectors (muscles)
  5. Effectors:
    These are the parts of the reflex arc that carries out a response.

Types of reflex actions
They can be grouped according to 2 ways:
1) Spinal reflexes
These are reflex actions that pass through the spinal cord and are interpreted there e.g. withdrawing a hand from a hot object.
2) Brain/cranial reflexes
These pass through the brain and are interpreted there e.g. closing of the eye when an object is approaching, coming of tears when one is cutting onions, etc.
3) Instinctive/simple reflex actions
These are reflexes that do not require learning but are inborn e.g. suckling in human infants, making of a web by a spider, withdrawing a hand from a hot object.

Characteristics of simple reflexes

  • They are rapid responses
  • A given stimulus brings about the same response
  • They are not learnt but instinct (inborn)

CONDITIONED REFLEX

This is the type of reflex which involves learning organisms learn to respond to strange (meaningless stimulus) by associating it with another meaningful/familiar stimulus, e.g. the Ivan Pavlov’s experiment.
A scientist called Ivan Pavlov performed an experiment to demonstrate a conditioned reflex in a dog. In the experiment, he used to give the dog food at a particular time. The dog would salivate either after the smell of food or taste of food (normal response). He then started ringing a bell before giving the dog food. He did this several times. After several times, the dog salivated when a bell was rang even without food being presented (conditioned response).
For a conditioned reflex to be established, the brain is necessary thus the dog in Pavlov’s experiment learnt to associate the sound of the bell with food.
When Pavlov rang the bell without food for a long time, the dog later stopped salivating implying that the conditioned reflexes are temporary.

Characteristics of conditioned reflex action

  • It is a temporary reflex
  • It involves learning
  • It takes a longer time to learn
  • It is co-ordinated in the brain
  • It involves more than one stimulus
  • It involves association of stimulus
  • It is reinforced by repetition
  • Responses are involuntary

Similarities between simple and conditioned reflexes

  • They both involve the central nervous system particularly the brain.
  • Both are autonomic responses
  • Both are associated with a stimulus.
  • Both involve neurons for the transmission of impulses
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Similarities between reflex and voluntary actions

  • Both are co-ordinated by central nervous system.
  • Both occur as a result of impulse transmission
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