The Nervous System and Homeostasis
The nervous system
Receptors detect changes (stimuli) in the environment — receptors are sensitive to light, pressure (touch), and chemicals (smell).
When receptors are activated by a stimulus, they send an electrical impulse along a sensory neuron.
The sensory neuron passes the impulse to a coordination centre (the brain, spinal cord, or pancreas).
The coordination centre receives impulses from various receptors around the body, processes the information and coordinates a response by signalling to effector organs (a muscle or gland).
Effectors are stimulated by passing an electrical impulse along a motor neuron.
Stimulation of an effector will produce a response such as muscle contraction or hormonal release.
Reflex actions are unconscious processes which help us to respond quickly to harmful stimuli. The response bypasses the brain, and sends information directly to the spinal cord, where the electrical impulse is passed from the sensory neuron to motor neuron via a relay neuron. Reflex actions are extremely quick and do not require any conscious input from the brain.
Image credit: Andrzej Wojcicki/ Science photo library
Synapses
Wherever two neurons meet, there is a space between the neurons called a synapse. Electrical impulses cannot travel across synapses, so are converted into a chemical signal. The events at a synapse occur in the following sequence:
An electrical impulse arrives at the end of a neuron and stimulates sacs of chemicals called neurotransmitters to be released from the neuron.
The neurotransmitters diffuse across the synapse and binds to receptor molecules on the next neuron.
The binding of receptors stimulates an electrical impulse in the neuron, which travels along the length of the neuron (the axon) until it reaches another synapse.
The eye
The eye is a sense organ containing receptors which detect colour and light intensity. The light which enters our eye is detected by the retina at the back of our eye. The retina contains receptors to detect light intensity and colour (rods and cones) and converts the light into an electrical impulse. The optic nerve sends the electrical impulse to the brain.
The muscles of the iris control how much light enters the eye, depending on whether we are in a bright or dim environment. This muscle contraction is an example of a reflex action - a rapid, involuntary response to our environment.
In bright light, the circular muscles of the iris contract while the radial muscles relax, making our pupils smaller and allowing less light into our eye.
In dim light, the circular muscles relax (while radial muscles contact), making our pupils wider and allowing more light into our eye.
The lens is a structure behind our pupil which refracts (bends) light to focus it on the retina.
When we focus on an object which is near to us, the lens becomes thicker to refract light more strongly.
When we focus on an object further away in our visual field, the lens becomes thinner to refract light less strongly.
The shape of the lens is controlled by a circular ring of muscle, called ciliary muscle, which are connected to suspensory ligaments.
When the ciliary muscles contract, the suspensory ligaments slacken which increases the thickness of the lens, helping to focus on object close in our visual field.
On the other hand, when the ciliary muscles relax, the suspensory ligaments tighten, making the lens thinner to focus on objects far-away.
Homeostasis
It is important that our body’s maintain a constant internal environment, keeping conditions such as temperature, water content and glucose levels at a steady level to allow enzymes to function properly. Maintaining a constant internal environment is called homeostasis and depends on both our nervous and hormonal systems.
Body temperature is kept constant (around 37 oC) due to the actions of a brain region called the hypothalamus, which sends electrical impulses to the skin and blood vessels when our temperature fluctuates. The hypothalamus initiates the following responses to changes in temperature:
Vasoconstriction: blood vessels become narrower, reducing blood flow through the skin and reducing heat loss
Vasodilation: blood vessels become wider, allowing more blood flow and therefore more heat to be lost through the skin’s surface
Sweating: sweat glands release sweat which evaporates from the surface of our skin, taking heat energy
Shivering: muscle contraction generates heat energy
Stimulation of hair follicles: our hairs stand on end when we are cold to trap an insulating layer of air on the surface of our skin