CSEC Physical Education Lecture Series: The nervous system
The nervous system enables us to control and coordinate all the body activities. It enables performers to produce high-precision movements repeatedly, making them appear skilful, while other functions such as heartbeat carry on automatically. The nervous system consists of the brain, spinal cord, and the nerves which supply all parts of the body. It is responsible for all our conscious (or voluntary) actions and has two main parts.
1. The central nervous system
This consists of the brain and spinal cord. The brain is the central centre of the entire system and is where all the incoming (sensory) information is processed and from where the outgoing (motor) information originates. The spinal cord goes down the inside of the spinal column from the brain. It carries all the nerve (sensory and motor) messages (impulses) between the body and the brain.
2. The peripheral nervous system
This consists of the nerve fibres that branch out from the spinal cord, and the various organs to which they are attached. Sensory nerves detect stimuli such as temperature, light, sound, touch, etc ... and send nervous impulses to the brain. Once the information has been processed, the nervous impulses are sent out down the motor nerves, to the muscles and glands which carry out the necessary action
Structure of Nerve Cells
Nerve cells, or neurones, carry information to or from the spinal cord and brain. They have three main sections.
- Dendrites receive messages in the form of nerve impulses.
- The nucleus is the main body of the cell.
- The axon transmits the impulse away from the nucleus.
Neurones are not actually connected to each other but are separated by a microscopic gap called the synaptic gap. Impulses are able to cross this gap via the release of a chemical substance called acetylcholine, which allows an impulse to travel through the nervous system at great speed.
Types of Neuron
There are three types of neuron. Each has a different function and this determines where the impulse is sent.
1. Sensory (afferent) neurons - These receive information from the sense organs (eyes, ears, nose, tongue, skin) and from receptors in the body and send impulse to the central nervous system (CNS).
2. Relay neurons - These are inside the brain or spinal cord and does the decision making after the impulse from the sensory neurons reaches the brain. If a decision for action is taken, the impulse is passed to the motor nerve.
3. Motor (efferent) neurons - These carry impulses from the CNS to muscles or organs. The cell body is inside the CNS and the axon leads out of it.
Involuntary Nervous System
This part of the nervous system is responsible for functions over which we have no control for example our heartbeat and digestion. These actions are controlled by the medulla oblongata, an area that forms the top of the spinal cord. This is divided into two sections:
1. The sympathetic nervous system - Responsible for preparing the body for action. It stimulates the adrenal gland and causes the heart rate and breathing rate to increase. It also slows down the functioning of organs not necessary for physical activity. This is known as the fight or flight response.
2. The parasympathetic nervous system - Responsible for slowing the body down and functions in opposition to the sympathetic nervous system.
Receptors and Reflex Actions
The brain also receives information from three main types of receptor organs:
i. Exteroceptors - outside the body (eyes, ears, etc.).
ii. Interoceptors - Inside the body (chemical changes in blood or lungs)
iii. Proprioceptors - From within muscles, tendons and joints. These are:
a. Golgi tendon organs - detects the amount of stretch in a tendon.
b. Muscle spindle - detects stretch in muscles.
c. Joint receptors - tell the brain at what angle the joints are positioned.
The information pick-up by these organs enables us to move our limbs quickly without the need to watch them.
There are times when we are required to act quickly, for instance, if we touch something hot. This is where our reflexes come into play. The impulse does not need to travel to the brain for interpretation. The impulse travels in a fast arc from sensory, to relay, to motor nerves. Moving the hand from the hot object is the withdrawal reflex, which is similar to sneezing or blinking as a response to a foreign object in the nose or eye. Another example is the knee-jerk reflex, known as stretch reflex, wherein a bang on the knee results in the leg extending.