Provide anatomy and physiology advice to clients Organisation of the nervous system

Functions The nervous system receives sensory input from millions of sensors, it processes input and decides on action. Action include activating muscles which is called motor output.

Components The Central Nervous System (CNS) consists of the spinal cord and brain. In this course we only briefly mention brain activity. The Peripheral Nervous System (PNS) is composed of all other nerves outside the CNS and that includes our muscles, so this must be a focus for our study.

Peripheral Nervous System (PNS) The PNS consists of sensory or afferent nerves which conduct information from the sensory receptors to the CNS. The sensory receptors are located throughout the body. We will look at these shortly. The other component of the PNS is the Motor or efferent nerves. These carry information from the brain to organs such as muscles. Each motor neuron has its cell body located in the brain stem or spinal cord grey matter and projects its axon out to the muscle that it controls.

The Motor Division The motor division consists of the somatic nervous system which contain nerves under our voluntary control and the Autonomic Nervous System which contain nerves controlled involuntary.

Autonomic Nervous System Autonomic Nervous System (ANS), which consists of visceral motor nerve fibres. These regulate the activity of smooth muscle, cardiac muscle and glands. We only mention the actions of the ANS in passing as these nerves are part of the involuntary system. Fortunately for us we don’t have to think about breathing or controlling our heart rate as the ANS looks after these things independently.

Sympathetic and Parasympathetic Divisions The Autonomic Nervous System consists of the Sympathetic division, which stimulate organs to mobilise energy, and the Parasympathetic System. This influences organs to restore to equilibrium.

The Motor Neuron Where Efferent nerves join with the muscle that it innervates, it loses its myelin sheath and divides into fine terminal branches, fusing with the muscle at what is called the end plate or neuromuscular junction. You can see in this slide how the motor nerve has branched and ends as end plates on muscle fibres. Motor end plates are larger on fast twitch fibres than slow twitch fibres. Look closely at the end plates and you will see swelling. These swellings contain vesicles of acetyl choline and release of these promotes the action potential being propagated along the muscle fibre.

Muscular contraction Each muscle has at least one nerve attached to it and where the nerve enters the muscle it branches into a number of axon terminals which all join to a single fibre. We saw this in our last session. A motor neuron and all the muscle fibres that it innervates is called a motor unit. Now when a motor neuron fires, all the fibres that innervates respond by contracting. Large weight bearing muscles have large motor units. This means that there may be several hundred muscle fibres per motor unit. Muscles that exert fine control such as moving the fingers have small motor units with only a few fibres. You can see a motor unit in this slide.

Sensory Receptors Throughout our body we have millions of sensory receptors. It is their job to monitor our environment and tell our brain when things have changed. When we touch something hot for instance our brain is very quickly alerted to this change and makes us pull our hand away as a protective mechanism. There are five types of sensory receptors and each has a different structure and role to play, but their job is always monitoring information and telling the CNS about changes.

Types of sensory receptors Mechanoreceptors generate nerve impulses when they are deformed by mechanical forces such as touch, pressure, vibration and stretch. They can be on the outside of our body or on the inside. You might recall that presence of food in the oesophagus changes its shape and stimulates a mechanoreceptor to send a message to the brain to start peristalsis. One on the outside of us is called Meissner’s corpuscle and this picks up very light touch. If you touch your skin very lightly you can still feel the touch thanks to this receptor. There are many of these just under the skin of your fingers, toes and on non hairy parts of your body. Each one can sense light touch for about 8mm all around it. ****

Other Sensory Receptors Thermoreceptors are very sensitive to temperature change. They are free dendritic nerve endings and exist in most body tissue. When temperature changes they send a message to the brain. Photoreceptors are very specialised receptors that respond to light energy and are located in the retina of our eye. There are millions of chemoreceptors right throughout our body that constantly sense our chemical composition and act if it changes from equilibrium. Finally we have nociceptors in our body that respond to painful stimuli.

Proprioceptors Of particular interest to exercise scientists are the proprioceptors. These occur in skeletal muscles, tendons, joints, ligament and other connective tissue. They advise the brain about our movement by monitoring the degree of stretch in the organs that they occupy. Our brain must continually be advised about our muscle activity and proprioceptors send this information. By constantly sensing movement they tell our brain about our orientation in 3 dimensions.

Proprioceptors Proprioceptors include muscle spindles found in skeletal muscle and are made of modified skeletal muscle fibres. Another proprioceptor is Golgi tendon organs. These are found in tendons near to where the muscle joins the tendon. They consist of small bunches of tendon fibres in a capsule. Around these are dendrites that coil around the fibre. And joint kinaesthetic receptors are located in joints.

Proprioceptor action Muscle spindles are found in all skeletal muscles that are controlled by small motor units such as the ones found in the hand. They have their own sensory and motor nerve supply and specialised muscle fibres that are called intrafusal fibres. Their role is to respond to changes in muscle length and tension. When the muscle stretches it activates the sensory receptors and sends signals to the spinal chord and thence to efferent motor nerves that innervate the muscle.

Detail of a muscle spindle These slides show detail of a muscle spindle. You can see large nuclear bag fibres. The bottom one is the part that transmits signals via afferent fibres to the spinal cord.

Activity Write a report advising clients about different components of the nervous system and proprioceptors.