#118 Control and co-ordination in mammals, the nervous system

Humans, like all living organisms, can respond to changes in the environment and so increase survival. Humans have 2 control systems to do this: the nervous system and the endocrine (hormonal) system. The human nervous system controls everything from breathing and standing upright, to memory and intelligence. It has 3 parts: detecting stimuli, coordinating and effecting a response.

Stimuli are changes in the external or internal environment, such as light waves, pressure or blood sugar. Humans can detect at least nine external stimuli and dozens of internal stimuli, so the commonly-held believe that humans have just five senses is obviously very wide of the mark!


Receptor cells detect stimuli. Receptor cells are often part of sense organs, such as the ear, eye or skin. Receptor cells all have special receptor proteins on their cell membranes that actually do the sensing, so “receptor” can confusingly mean a protein, a cell or a group of cells.



The coordinator is the name given to the network of interneurones connecting the sensory and motor systems. It can be as simple as a single interneurone in a reflex arc, or as complicated as the human brain. Its job is to receive impulses from sensory neurones and transmit impulses to motor neurones.


Effectors are the cells that effect a response. In humans there are just two kinds: muscles and glands. Muscles include skeletal muscles, smooth muscles and cardiac muscle, and they cause all movements in humans, such as walking, talking, breathing, swallowing, peristalsis, vasodilation and giving birth. Glands can be exocrine – secreting liquids to the outside (such as tears, sweat, mucus, enzymes or milk); or endocrine – secreting hormones into the bloodstream.

Responses aid survival. They include movement of all kinds, secretions from glands and all behaviours such as stalking prey, communicating and reproducing.



Coordination 

In multicellular organisms, such as plants and animals, it is essential that cells can communicate with each other. This allows them to coordinate their activities appropriately. Organisms have specialised cells or molecules, called receptors, which are sensitive to changes in their internal or external environment. These trigger events in the organism that bring about coordinated responses to the environmental changes.

1)  Nervous and endocrine systems as communication systems 

a. The basic similarities of 2 systems:

- Provide the body with methods to communicate with its internal and external environments in order to coordinate responses.

- Employ chemicals to transmit messages and respond to stimulus caused by changes in their environments.

b. The differences in response times and how they work.

- The nervous system responds to stimuli by sending electrical action potentials along neurons, which in turn transmit these action potentials to their target cells using neurotransmitters, the chemical messenger of the nervous system. This response to stimuli is near instantaneous.

- Hormones are synthesized at a distance from their target cells, and travel through the bloodstream or intercellular fluid until they reach these cells. Upon reaching their target cell, the hormones act on the cell to increase or decrease the expression of specific genes. This process takes significantly longer, as hormones must first be synthesized, transported to their target cell, and enter or signal the cell. Then, the target cell must go through the process of transcription, translation, and protein synthesis before the intended action of the hormone is seen. Although hormones act more slowly than a nervous impulse, their effects are long lasting. Additionally, target cells can respond to minute quantities of hormones and are sensitive to subtle changes in hormone concentration.

c. The nervous and endocrine systems work together to maintain homeostasis.

The endocrine and nervous systems work independently to carry out unique functions by different methods with some similar elements. However, they do work together to control and co-ordinate the internal environment of the animal.

The nervous system responds rapidly to short-term changes by sending electrical impulses.
The endocrine system brings about longer-term adaptations by sending out chemical messengers (hormones) into the bloodstream.

2)  Nerve cells/Neurones

The nervous system composed of nerve cells, or neurones. A neurone has a cell body with extensions leading off it. Several dendrons carry nerve impulses towards the cell body, while a single long axon carries the nerve impulse away from the cell body. Axons and dendrons are 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)! A nerve is a discrete bundle of several thousand neurone axons.

Neurone structure. 

Nerve impulses are passed from the axon of one neurone to the dendron of another at a synapse. Numerous dendrites provide a large surface area for connecting with other neurones.

Most neurones also have many companion cells called Schwann cells, which are wrapped around the axon many times in a spiral to form a thick lipid layer called the myelin sheath. The myelin sheath provides physical protection and electrical insulation for the axon, which greatly speeds up the transmission of action potentials.  There are gaps in the sheath, called nodes of Ranvier. Not all neurones are myelinated.

Myelin sheath and Schwann cells.

Humans have 3 types of neurone:

• Sensory neurones have long dendrons and transmit nerve impulses from sensory receptors all over the body to the central nervous system.
• Motor neurones 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 much smaller cells, with many interconnections. They comprise the central nervous system. 99.9% of all neurones are interneurones. 

 

Neurones are highly spectalised cells that are adapted for the rapid transmission of electrical impulses, called action potentials, from one part of the body to another.

Information picked up by a receptor is transmitted to the central nervous system (brain or spinal cord) as action potentials travelling along a sensory neurone. These neurones have their cell bodies in small swellings, called ganglia, just outside the spinal cord.

The impulse may then be transmitted to a relay neurone, which lies entirely within the brain or spinal cord.

The impulse is then transmitted to many other neurones, one of which may be a motor neurone. This has its cell body within the central nervous system, and a long axon which carries the impulse all the way to an effector (a muscle or gland).

3) Reflex arc 

In some cases, the impulse is sent on to an effector before it reaches the 'conscious' areas of the brain. The response is therefore automatic, and does not involve any decision-making. This type of response is called a reflex, and the arrangement of neurones is called a reflex arc.

Syllabus 2016-2018 15.1  Control and co-ordination in mammals The nervous system provides fast communication between receptors and effectors. Transmission between neurones takes place at synapses. a)   compare the nervous and endocrine systems as communication systems that  co-ordinate responses to changes in the internal and external environment  b)   describe the structure of a sensory neurone and a motor neurone c)   outline  the roles of sensory receptor cells in detecting stimuli and stimulating the transmission of nerve  impulses in sensory neurones (a suitable example is the chemoreceptor cell found in human taste buds) d)   describe the functions of sensory, relay and motor  neurones in a reflex arc e)   describe and explain the transmission of an action potential in a myelinated neurone and its initiation from a resting potential (the importance of sodium and potassium ions in impulse transmission should  be emphasised) f) explain the importance of the myelin sheath (saltatory conduction) in determining the speed of nerve  impulses and the refractory period in determining their frequency g)   describe the structure of a cholinergic  synapse and explain how it functions, including the role of calcium  ions h)   outline  the roles of synapses in the nervous system in allowing transmission in one direction  and in allowing connections between one neurone and many  others (summation, facilitation and inhibitory synapses are not required) i) describe the roles of neuromuscular junctions, transverse system tubules and sarcoplasmic reticulum in stimulating contraction in striated muscle j) describe the ultrastructure of striated muscle with particular reference to sarcomere structure k)   explain the sliding filament  model  of muscular contraction including the roles of troponin,  tropomyosin, calcium  ions and ATP. The endocrine system is a slower system that  controls long-term changes. Fertility may be controlled by use  of hormones. l) explain the roles of the hormones FSH, LH, oestrogen and progesterone in controlling changes in the ovary and uterus during the human menstrual cycle m)  outline  the biological basis  of contraceptive pills containing oestrogen and/or progesterone