Human Biology - EDEXCEL IGCSE - Coordination

Coordination

Neurones:

Motor Neurone

Sensory Neurone

Relay Neurone / Interneurone

Central Nervous System: This contains the brain and spinal cord and is what gives out orders to other parts of the body.

How neurones work:

An electrical impulse is sent from a nerve receptor and travels along the axon. At the nerve ending there is a gap (synapse). A synapse works as the electrical impulse triggers the release of neurotransmitters. These chemicals diffuse across the synaptic cleft and binds with receptors on the second neurone. This forms a neurotransmitter-receptor complex which stimulates the second neurone to transmit the electrical impulse. The unused neurotransmitters are then either reabsorbed or broken down by enzymes.

Spinal cord: the spinal cord marks the site of coordination where information sent from sensory neurones are passed via relay neurones to motor neurones for the impulses to be sent to effectors.

Reflex Arc:

1. Receptor cells detect stimulus
2. Receptor stimulated and an electrical impulse sent along sensory neurones to the central nervous system (CNS).
3. At the CNS it travels along synapses to the relay neurone.
4. The relay neurone passes it to the motor neurone.
5. The impulse travels via the motor neurone to the target muscle or gland effector. 
6. This creates an effect.

Examples of this are the knee jerk reflex and the withdrawal reflex. Both of these are spinal reflexes and so are involuntary. This is because you only receive information (to your brain) after the reflex has occurred due to the speed. In fact the knee jerk reflex can take just 50ms.

The brain:

The brain is made up of two cerebral hemispheres divided by a longitudinal fissure. The left hemisphere (relative to patient) controls the motor function of the right side of the body and vice versa. The right hemisphere is usually responsible for music, visual imagery and spatial awareness while the left is responsible for language, maths and logic.

The cerebellum is located at the back of the brain and is responsible for coordinating voluntary movements. It controls posture, balance, speech etc... and damage to it will cause lack of balance, slower movements and an inability to do complex physical tasks.

The mid-brain is located at the lower middle part of the brain and is responsible for hearing, vision, arousal and all of the responses related to sensory information. It is also partly responsible for motor control and body temperature regulation.

The pituitary gland, located infront of the mid-brain, is responsible for secreting hormones that either regulate the activities of organs or of other glands. e.g. growth hormones.

The hypothalamus, located above the pituitary gland connects the nervous and endocrine systems and is responsible for homeostasis, sleep-wake cycle and some motor control.

Hormones:

Pituitary:

• Anti-Diuretic Hormones (ADH) - involved in osmoregulation by controlling the permeability of collecting ducts.
• Gonadotrophin  - controls the production of sperm and ovulation and triggers sexual maturation.

Adrenal:

• Adrenaline - prepared the body for the "fight or flight" response in times of stress. It increases heart rate, raises blood pressure, enlarges pupil size, and raises the body's metabolism.

Thyroid gland:

• Thyroxine - controls the body's metabolic rate.

Pancreas:

• Insulin - helps control the body's blood sugar level by signalling the liver, muscle and fat cells to take in glucose and convert it to glycogen.
• Glucagon - helps control the body's blood sugar level by telling the liver to convert stored glycogen into glucose which is then released into the blood stream. 

Hormones are vitally important for growth and development. The main hormones involved are the pituitary growth hormone, thyroid hormone and sex hormones.

Comparison of nervous and endocrine system:

• Both move instructions around the body and participate in homeostasis.
• Neurones use electrical impulses while hormones use chemicals.
• Neurones target specific collection of cells while hormones target specific cells.
• Hormones are relatively slow while neurones are very fast and rapid.
• Hormonal effects are relatively long lasting while neurones only create a short effect.

Negative feedback: This is an important control mechanism found in homeostasis where a response occurs to conditions that have changed from the ideal or set point. Examples are temperature control used by the skin and osmoregulation through the use of ADH.

The Eye:

A simple diagram of an eye

Function:

The eye is an photoreceptor (detects changes in light) and transduces light energy into electrical impulses. These are then interpreted as images by the brain. The eye is also able to respond to changes in light and is able to focus and bend light reflected of objects to allow the brain to produce a discernible image.

Focusing near and distant objects:

If an object is near, ciliary muscle will contract which relaxes the suspensory ligaments so that the lens becomes fat and slightly curved (convex). If the object is far, ciliary muscle will relax, making the suspensory ligaments tighten so that the lens stretches out and becomes thin and flat.

Responding to changes in light intensity:

In dim light, the iris dilates the pupil to allow more light to reach the retina. It does this by contracting the radial muscles and relaxing the circular muscles. In bright light, the iris will shrink the pupil.

Stereoscopic vision:

This refers to the ability of humans to see the same scene with both eyes in different ways and results in our ability to perceive depth and distance. Stereoscopic vision allows us to play sports and judge distance.

The Ear:

A simple diagram of an ear

Function:

• Hearing - the pinna brings the sound waves down the ear canal which causes the ear drum to vibrate. The ossicles transmit the vibrations which set the liquid in the cochlea in motion. The ciliary cells in the liquid amplifies the vibrations and categorises them by each frequency. The auditory nerve receives this information from the ciliary cells and carries it to the brain which interprets them as noises. 
• Balance - the semi-circular canals, lying at 90° to each other detects movement and sends impulses to the brain to help us know where we are.