DPT-215-A&P: Nervous System

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The Nervous and Endocrine Systems

In this section we explore the structure and function of the nervous system, how it controls movement and how it senses body position. Intimately related to the nervous system is the endocrine system – the series of ductless glands situated throughout the body that release hormones into the bloodstream to regulate growth, metabolism, energy supply, stress response, digestion, etc. keeping body functions in a state of balance.

Functions of the Nervous System

The nervous system is a complex network of neurons and chemicals that has overall control of the body. It is imperative that the internal state of the body is kept within certain limits for healthy function. Factors such as blood pressure, metabolism, core temperature, hunger, digestion, elimination, etc. need to be carefully monitored. 

Similarly, the body must also interact with the surrounding environment using sight, sound, taste, smell and touch, and then react appropriately by recruiting muscles to create movement, keeping good balance, mobilising fuel sources to provide energy, and so on. 

As if that wasn’t enough, the human nervous system also has the capacity to communicate using complex language, learn and develop new skills, to experience powerful emotions, and to use imagination to create abstract thoughts.

The nervous system can be conveniently divided into two main parts – the central nervous system and the peripheral nervous system.

The Central Nervous System

The brain and spinal cord together make up the central nervous system (CNS). This is where conscious and subconscious thoughts take place, information is filtered, and decisions are made. 

The CNS is responsible for monitoring the condition of the body, and making adjustments in order to keep systems in balance. It does this in conjunction with the endocrine system, described later in this chapter. This maintenance of a constant order within the body is called homeostasis, which literally means ‘same state’. 

The Peripheral Nervous System

Nerves that lead to and from the spinal cord make up the peripheral nervous system (PNS). Nerves are more correctly called neurons. There are two principal types, depending on which direction they transmit information.

Sensory neuronsfeed into the spinal cord and brain bringing information from the eyes, ears, skin, muscles and joints to tell the CNS what is happening both internally and externally. Sensory neurons are also called ‘afferent’ neurons (think of the CNS being affected by their messages). An example of a sensory neuron is one leading from the proprioceptors (spindles and golgi tendon organs) within the muscles.

Motor neurons carry messages from the brain and spinal cord telling the muscles when and how to contract to create movement, and telling the organs of the body what to do – for example controlling the speed of the heart. Motor neurons are also called ‘efferent’ neurons (think of their messages causing an effect at the muscles).

The PNS can be further divided into two branches. The voluntary (somatic) branch senses what is going on outside the body and controls movement. The involuntary (autonomic) branch senses what is going on inside the body and controls internal functions. In order to achieve this it is intimately related to the endocrine system.

Finally, the autonomic branch of the PNS can be viewed as having two opposing states – sympathetic and parasympathetic– either of which can be dominant at any time depending upon circumstances. Sympathetic activity dominates when the body is required to be physically active. Emotions such as stress, fear, excitement and anticipation of exercise can all stimulate release of sympathetic hormones like adrenaline and noradrenaline, which in turn increase heart rate, breathing rate and energy availability ready for action.

During more relaxed moments, the parasympathetic state becomes more dominant, leading to a reduction in heart rate, breathing rate, etc.

The actions of the somatic and autonomic branches of the PNS are summarised in the table that follows:

The Two Branches of the PNS
SomaticConscious control Concerned with external environment Motor neurons control movement of skeletal muscles Sensory neurons detect pain, heat, touch, movement, etc. from receptors positioned around the bodyAutonomicSubconscious control Concerned with internal environment Controls body temperature, heart rate, breathing, tone of smooth muscle in arteries, and blood flow to working muscles depending upon stress, fear, excitement, demand for physical activity, etc.
 Sympathetic dominance: increases heart rate, breathing rate, etc. ready for exercise (‘fight or flight’) Parasympathetic dominance: decreases heart rate, breathing rate, etc. after exercise (‘rest and digest’)

Structure and Function of a Neuron

The diagram shows the structure of a typical neuron, with the main parts labelled. For our purposes here it may help if you picture this cell body and nucleus to be within the spinal cord, with the axon (and associated structures) being the motor neuron that takes the message to the muscle. However, be aware that nerve cells are widely distributed throughout the nervous system and their network of connections is very complex.

Thecell nucleus positioned in the centreof the structure contains the cell DNA and regulates overall cell activity. Surrounding the nucleus is the cell body. From here manydendritesbranch off in different directions and connect to other nearby cells in order to transmit and receive messages. Thus neurons form a complex network for exchange and channelling of information.

The axonis a long thin fibre that extends away from the cell body. The axon carries an electrical impulse away from the cell body to several axon terminals. Axon terminals (also called synaptic terminals) contain chemicals called neurotransmitters. When the electrical nerve impulse arrives it stimulates release of these neurotransmitters which then carry that message across the gap (synapse) and on to the target organ, which in this case is a muscle. Thus the synapse between the axon and the muscle is also called the neuromuscular junction.

Synapse: a site of communication between two nerve cells

Larger axons like the one pictured have a covering of fatty tissue called a myelin sheath.This acts as an insulator for better transmission of the electrical nerve impulse. You will notice that nodes of ranvierare regularly spaced along the axon. These are places where the myelin sheath is effectively cinched in tight, giving the whole thing the appearance of a string of sausages. The nodes of ranvier act like stepping stones for the electrical nerve impulse, allowing the current to travel in a series of quick jumps, thus speeding up transmission times considerably compared to smaller axons that have no myelin sheath.

Transmission of Nerve Impulses

Typical transmission of a nerve impulse happens in the following sequence:

  1. Dendrites of cell body receive an incoming signal. For example this could be from the brain, via the spinal cord, relaying the intention to contract certain motor units within a muscle.
  • If the incoming signal is strong enough, an all-or-none electrochemical pulse called an action potentialis generated, which travels rapidly along the cell’s axon, and activates synaptic connections at the axon terminals when it arrives. 
  • The synapses then release neurotransmitter chemicals to transmit the impulse to a subsequent muscle or nerve, resulting in some action – in this case the target motor units will ‘twitch’ or contract causing the movement that the brain called for.

Keep in mind that this simple sequence does not work in isolation; it must function as part of an integrated network. For example, our nerve cell in question may also need to communicate with other nerve cells to bring synergistic helper muscles into action. At the same time, the antagonistic muscles must be instructed to relax to allow the original muscle to contract and bring about movement (a feature known as ‘reciprocal inhibition’). All the while, the brain must be constantly updated via other connections to sensory nerves about what is actuallyhappening.

Motor Units

Motor neurons lead directly to the muscles. Once inside the muscle, the motor neuron branches and connects to a number of individual muscle fibres. You will recall that a motor neuron and all of the muscle fibres that it innervates is referred to as a motor unit. This subject has already been covered in detail in the musculoskeletal system.

Please refer back to make sure you can: 

  • Explain the role of a motor unit
  • Explain the process of motor unit recruitment
  • Explain the significance of a motor unit’s size and number of muscle fibres it contains

Muscle Proprioceptors

Proprioception is the general name given to the function of sensing position, length, and tension in the muscles and joints around the body. Proprioceptors are therefore sensory organs that feed this positional information into the CNS via sensory (afferent) neurons. Information coming from all proprioceptors in the body is co-ordinated to give the brain an accurate and precise picture of the spatial arrangement of the body. The brain can then process the information and send out signals to motor neurons to react accordingly. 

There are two principal muscle proprioceptors to learn at this level: muscle spindles and golgi tendon organs.

Muscle Spindles

Muscle spindles, also known as ‘stretch receptors’ are positioned deep inside each muscle parallel with the contractile fibres. Their purpose is to detect the length of the muscle and also whether the length is changing quickly. For example, a rapid lengthening of the muscle will be sensed by the spindles. This in turn will fire a reflex arc to contract the same muscle almost instantaneously, thereby preventing the muscle lengthening too far and tearing. This is the mechanism your doctor is testing when he or she ‘checks your reflexes’. A tap just below the knee cap (interpreted by the quadriceps muscle-tendon unit as a rapid stretch) is followed with a reflex kick of the leg (rapid contraction of the quadriceps).

You may encounter this terminology: The fibres containing the muscle spindles are called ‘intrafusal’ fibres. The regular contractile muscle fibres are called ‘extrafusal’ fibres

Golgi Tendon Organs

Golgi tendon organs, known as GTO’s, are located close to the ends of a muscle, at the junction between the muscle belly and the tendon. Their purpose is to detect muscle tension and also whether that tension is changing quickly. For example, if too much weight is used in a lift, the GTO’s will sense excessive tension through the prime movers. Their response is to instantaneously inhibit signals to the prime movers so that they relax, preventing possible strain. You may have experienced this as failure mid-way through a heavy lift. The natural protective mechanism steps in to reduce muscular tension despite your best effort to carry on lifting.

Both the muscle spindles and the GTO’s play a role in flexibility. If a muscle is stretched too rapidly and forcibly, then the muscle spindles will react and try to contract the muscle, thus fighting against the lengthening you are trying to achieve. A muscle therefore needs to be stretched more slowly if we are trying to avoid this response. Alternatively, it may be possible to utilise the inhibition response from the GTO’s to promote muscle relaxation and obtain a better stretch. This is the principle behind proprioceptive neuromuscular facilitation (PNF) stretching, which uses an initial muscle contraction to try and obtain a subsequent relaxation.

Reciprocal Inhibition

An inherent feature of our nervous system is reciprocal inhibition. Imagine if you will that the brain has instructed the biceps to contract. They do so and hopefully bring about elbow flexion. However, to do this, the elbow extensors (triceps) must relax at the same time otherwise they would simply pull against the contraction of the biceps and no movement would take place. Therefore, the nervous system makes sure that, if one muscle is told to contract, the antagonist is automatically inhibited so that it relaxes sufficiently to allow movement. This is reciprocal inhibition.

This phenomenon can be exploited to some extent with ‘active’ stretching. A conscious contraction of the quadriceps (e.g. performing a seated leg extension) inhibits the opposing hamstrings, thereby allowing them to be stretched more effectively than if they are simply stretched ‘passively’ with the quadriceps relaxed.

The neuromuscular connections of the muscle spindles and GTOs are shown in more detail in the pictures that follow:

Neuromuscular Adaptations to Training

Neuromuscular training, or ‘motor skills’ training, aims to improve the following:

  • Reaction time
  • Coordination (the ability to move the limbs in complicated patterns)
  • Balance
  • Agility (the ability to change direction quickly)
  • Movement efficiency and economy.

There are several adaptations that occur within the nervous system in response to specific training. When a pattern of movement is repeated over and over as part of a practise drill then the body gets more and more used to doing that movement. Less conscious effort is required to replicate the skill each time, until eventually the whole movement becomes automatic, requiring no conscious thought. The nervous system appears to achieve these adaptations to training in the following ways:

Firstly, constant repetition of a movement causes new and stronger nerve connections to be established within the CNS. The dendrites between nerve cells literally grow and forge stronger links with other cells. This makes a more robust, faster pathway between incoming sensory impulses and outgoing motor impulses. The more times a nerve impulse travels the same route (repetition of a movement), the stronger that pathway becomes. 

What starts out as a little used back road in the CNS develops into a B road, an A road, then a dual carriageway and finally into a motorway.

However, this adaptation is not always desirable. Once a particular way of doing something is learned, it becomes practically permanent. If an incorrect pattern has been established then this will become embedded as a habit, and it is notoriously difficult to replace bad old habits with new ones. The nervous system will always favour the easier pathway (the motorway) over the harder one (the little used back road) unless a lot of conscious effort is applied over a period of time to change it.

Secondly, motor units are recruited in a more efficient pattern, resulting in a smooth movement, and using less energy for the same task. For movements requiring maximal effort against a high resistance (for example, power lifting), the nervous system is able to recruit more motor units simultaneously to create more force. Training enables the nervous system to overcome the body’s natural safety mechanism that inhibits the recruitment of the majority of motor units in a muscle at once.

Finally, speeding up the frequency of nerve impulses to motor units is a neuromuscular adaptation which leads to stronger muscle contractions. In other words, greater force can be obtained without an increase in muscle size.

The benefits of improved neuromuscular coordination to exercise performance can be summarised as follows:

  • Movements become automatic, requiring less conscious thought
  • Movement quality is more graceful and coordinated
  • Movements are more efficient
  • Exercise will require less energy
  • Greater force can be exerted without a corresponding increase in muscle size
  • Less chance of muscle and joint injuries
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