Somatic Nervous System

Somatic Nervous System

The SNS differs from the ANS in that it stimulates skeletal muscles, whereas the ANS innervates glands as well as cardiac and smooth muscle. Other differences are based on the physiology of the effector organs of each system. In the SNS, motor neuron cell bodies lie in the CNS, with their axons extending in spinal or cranial nerves, to reach target skeletal­ muscles. Somatic motor fibers are usually thick and heavy. These myelinated group A fibers conduct nerve impulses very quickly. This differs again from the ANS, which uses a two-neuron chain to reach its effectors.

Every somatic motor neuron releases ACh at its axon terminal with a skeletal muscle fiber. Effects are always excitatory. Muscle fibers contract if the stim-ulation reaches threshold. Both somatic and auto-nomic motor activities are regulated and coordinated by higher brain centers. Also, both somatic and auto-nomic fibers are contained in most spinal nerves and in many cranial nerves. The body’s ability to adapt to most internal and external changes involves both visceral organs and skeletal muscles. A hard-working skeletal muscle requires additional glucose and oxy-gen, so autonomic control mechanisms increase heart rate and dilate airways.

Reflex Activity and Spinal Reflexes

A reflex is actually defined as a fast, automatic response to a specific stimulus. Reflex activity in the human body can be either inborn (innate) or learned (acquired). Inborn reflexes are rapid and predictable motor responses to stimuli that are formed between neurons during human development. They are invol-untary and subconsciously maintain body posture, help to avoid pain, and control visceral activities. For example, a response to pain is triggered by an inborn spinal reflex that operates without assistance from the brain. We may only be aware of the final outcome of one of these basic reflexes and not the process of the reflex itself. Many visceral reflexes are regulated by the brain stem and spinal cord.

A learned reflex develops over time, because of repeated reactions to stimuli. It is more complex than an inborn reflex. An example is how an older driver may access learned reflexes because of his or her expe-rience in driving, whereas a younger driver may have yet to learn them. However, most inborn reflexes can be altered by learning and conscious effort. In a sit-uation involving pain, the pain signals are quickly transmitted to the brain by the interneurons of the spinal cord. Withdrawing from the source of the pain involves serial processing by the spinal cord, whereas awareness of pain involves simultaneous parallel pro-cessing of the sensory stimuli.

Reflex Arc

In a reflex arc, sensory impulses from receptors can reach their effectors without being processed by the brain (FIGURE 13-7). Some reflex arcs use interneu-rons and others do not. The five basic components of the reflex arc are a receptor, a sensory neuron, an integration center, a motor neuron, and an effector. A receptor is the site of the stimulus action. The sensory neuron transmits afferent impulses to the CNS. In a simple reflex arc, the integration center may be only a single synapse, located between a sensory neuron and a motor neuron, referred to as a ­monosynaptic reflex. The sensory neuron synapses directly on a motor neuron. In a more complex reflex arc, multi-ple synapses with chains of interneurons are involved and are referred to as polysynaptic reflexes. They have a longer delay between stimuli and responses than monosynaptic reflexes. The integration center for such reflexes lies within the CNS. A motor neu-ron conducts efferent impulses from the integration center to an effector organ, and an effector is defined as a muscle fiber or gland cell that responds to these impulses by contracting or secreting. The most com-plicated spinal reflexes are called intersegmental reflex arcs, in which many segments work together and produce coordinated motor responses that are extremely variable.

A somatic reflex is one that activates skeletal mus-cle, allowing for involuntary muscle control. Examples include superficial, stretch, and patellar reflexes. Sensory receptors used in stretch reflexes are muscle spindles that each consist of bundled intrafusal muscle fibers. The muscle spindle is surrounded by larger extrafusal muscle fibers. Somatic reflexes are also known as deep tendon or myotatic reflexes. An autonomic reflex is one that activates visceral effectors such as smooth or cardiac muscle or glands. An autonomic reflex is also known as a visceral reflex. In every visceral reflex arc, there is a receptor, a sensory neuron, and one or more interneurons. Visceral reflexes can be long or short. Long reflexes usually coordinate activities in an entire organ, with their actions processed by interneurons of the CNS. Short reflexes do not use the CNS at all but use the sensory neurons and interneurons with cell bodies in the autonomic ganglia. Short reflexes control simple motor responses with localized effects such as activities in a specific part of a target organ.

Spinal Reflexes

The spinal cord controls some common somatic reflexes known as spinal reflexes, which often occur without direct involvement of the higher centers of the brain. These reflexes may still occur when the brain is destroyed but the spinal cord is still intact and functioning.

The brain is aware of most spinal reflexes and can modify them in most circumstances. For normal spinal reflexes, continual facilitating signals from the brain must occur. Spinal shock occurs when the spi-nal cord is transected, and as a result, all functions controlled by the spinal cord are depressed. Somatic reflexes provide important feedback when they are tested to assess nervous system function. When they are absent, abnormal, or exaggerated, there may be degeneration or pathology of certain regions of the nervous system. There may be no other signs of dys-function present.

1. Identify the five components of a reflex arc.

2. Differentiate between somatic and autonomic reflexes.

3. Explain how many spinal reflexes occur.