HE NERVOUS SYSTEM
Introduction
The nervous system and endocrine system are the primary control systems of the human body which would not function without nerves and hormones. Together, these two systems monitor the internal and external environments and regulate our metabolic and homoeostatic processes. The nervous system is unique to animals, giving them mobility via the coordination of muscle contraction.
Control of the body is achieved through a coordinated interaction of the nervous and endocrine systems. Generally, the nervous system is responsible for rapid responses and slower, more prolonged process are controlled by the endocrine system. Often the two systems work in tandem, one reinforcing the action of the other
The human nervous system is undoubtedly one of the most complex parts of the human body. The brain and spinal cord are at the centre of this system and together make up the Central Nervous System (CNS). The CNS receives information from other parts of the body via the Peripheral Nervous System (PNS). The CNS processes that information then sends out instructions to organs, glands and muscles via the PNS. Nerves that go into the CNS are called afferent nerves and nerves that emerge from the CNS are called efferent nerves - Figure 01.
Figure 01 Information in and out of the CNS
Before we understand how the nervous system works, we first need to look at the components of the nervous system.
The nervous system is mainly composed of nerve cells called neurons. Neurons can transmit information along their length very quickly. In the central nervous system, billions of neurons make up the neuronal networks that perform such complex functions as thought, memory and the control of voluntary and involuntary movements.
Neurons
Neurons exist in a variety of shapes and forms but they all have some common features. Figure 02 shows a typical neuron. The dendrites receive stimulation and transmit nervous impulses along the axon to the axon terminals.
Figure 02 - Typical neuron
The Central Nervous System (CNS)
The Central Nervous System is surrounded by the protective bones of the skull and vertebral column. The CNS is the command centre of the nervous system, interpreting sensory information and initiating responses. The neurons that make up this loop of information are divided into 3 classes:
1. Afferent neurons that transmit information into the CNS.
2. Efferent neurons that transmit information out of the CNS
3. Interneurons that communicate within the CNS.
The brain itself is incredibly complex and there is not space here to explore it in any detail. An important point to note is that the each part of the brain is responsible for a particular function (although there is much inter-linking and task sharing). The functions of a few, important regions are shown in Figure 03.
Figure 03 - Brain showing some key areas
- Frontal lobes - thinking, decision making, personality
- Primary motor cortex - voluntary control over skeletal muscle,
- Somatosensory cortex - processing of sensory information from receptors in the body
- Temporal lobes - processing of speech and sounds
- Primary auditory area - interprets basic characteristics of sound
- Broca’s area - speaking and understanding speech
- Wernicke’s area - interprets the meaning of speech, converts words into thoughts
- Occipital lobe - processing of visual material
- Cerebellum - coordinates complex movements
- Brain stem - relay station for neural pathways and centres that control basic functions such as breathing and the cardiovascular system
The Peripheral Nervous System (PNS)
Figure 04 shows the relationship of various divisions within the nervous system.
Figure 04 Schematic diagram showing divisions of the nervous system
All nerves that are outside of the CNS are called peripheral nerves and all of the peripheral nerves collectively make up the Peripheral Nervous System. The peripheral nervous system has two functional subdivisions, the sensory and the motor division.
Sensory division
This consists of nerves that carry information from sensory receptors into the CNS, constantly updating the CNS with events in the internal and external environment. Figure 05 shows sensory information flowing along afferent neurons into the central nervous system.
Figure 05 Animation of afferent flow of information into the CNS
Motor division
This transmits impulses from the CNS to organs, muscles and glands. Figure 06 shows motor information flowing out of the central nervous system along efferent neurons.
Figure 06 Animation of efferent flow of information out of the CNS
The motor division has two parts, the Somatic and Autonomic divisions.
Somatic nervous system
The somatic nervous system conducts impulses from the CNS to skeletal muscle. This allows us to contract our muscles and control our movements.
The Autonomic Nervous System (ANS)
Most internal organs, tissues and glands are under nervous control. This control maintains the internal environment in a steady state called homoeostasis. Functions controlled by the ANS include blood pressure and digestion.
The ANS is composed of two parts – the parasympathetic and sympathetic nervous systems and they have clear anatomical and functional differences. Most internal organs and glands have a nerve supply from both the parasympathetic and sympathetic systems. Generally, these systems work in opposing ways – one stimulating and the other inhibiting activity. Together they maintain the internal activity of the body - Table 01.
Table 01 Interactions and Effects of the Autonomic Nervous System | ||
| PNS | SNS |
| Constricts pupil | Dilates pupil |
| Stimulates secretion | Inhibits secretion |
| Lowers heart rate | Increases heart rate |
| Promotes urination | Inhibits urination |
| Constricts bronchioles | Dilates bronchioles |
| Increases motility, relaxes sphincters and increases secretion | Reduces secretion, constricts sphincters |
| No effect | Releases glucose into blood |
| Constricts gall bladder | Relaxes gallbladder |
| No effect | Decreases urine output |
| Little or no effect | Constriction |
| Erection | Ejaculation |
Parasympathetic nervous system
The parasympathetic nervous system maintains body functions to conserve energy and is dominant when the body is at rest.
Sympathetic Nervous System
Like the parasympathetic system, the sympathetic system regulates function and maintains homeostasis. However, it also has the ability to prepare the body to cope with emergencies, stress and increased activity. This is called the fight or flight response and is an instinctive response to stress.
The Synapse
When a nervous impulse arrives at the axon terminal, an additional process is required to allow continuation of the nervous impulse in the next neuron. Nervous impulses cannot simply jump from neuron to neuron because at the junction between neurons there is a small gap - Figure 07. The junction between the neurons is called the synapse. It is a very useful area because many of our common drugs such as beta blockers and Prozac are targeted at the synapse.
Figure 07 - Information travels along neuronal pathways across synapses
Communication across the synaptic gap, between neurons is in the form of chemical messengers called neurotransmitters. The arrival of a nervous impulse at the pre-synaptic axon terminal triggers the release of the neurotransmitter into the synaptic cleft - Figure 08. Once in the synaptic cleft, the neurotransmitter diffuses towards the post-synaptic membrane and binds to receptors embedded in the membrane. This will either stimulate or inhibit the post synaptic membrane. Whether messages sent via the neurotransmitter excite or inhibit the post-synaptic neuron depends on the type of neurotransmitter and the class of receptor.
There are many different types neurotransmitters including acetylcholine and noradrenaline (norepinephrine) in the peripheral nervous system and dopamine and serotonin in the central nervous system.
Figure 08 Animation of the transmission of information across a synapse
source:http://www.hcc.uce.ac.uk/physiology/Nervous%20system.htm
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