Peripheral Nervous System
The nervous system in vertebrate animals, including humans, is complex and consists of the central nervous system and the peripheral nervous system, which together have trillions of neurons. The brain and spinal cord form the central nervous system (CNS), while the peripheral nervous system (PNS) is comprised of the rest of the nerves in the body.
The peripheral nervous system has nerves that connect the brain and spinal cord to the rest of the body, including visceral organs, limbs and skin. It transmits stimuli from the external environment and from all the organs to the central nervous system. The PNS also carries messages from the CNS to all the organs and helps in coordinating action.
Structure of PNS
Neurons are nerve cells, and have an unusual shape. Each has a cell body with cytoplasm, nucleus, and organelles similar to other cells. In addition, they have two types of projections. There are many short dendrites which carry electrical messages to the cell body, and a single long axon that carries messages in the form of electrical impulses from the cell body to the dendrites of the next neuron. Adjacent neurons have no physical contact, but a gap called synapse. As an electrical impulse moves from one neuron to another, it jumps this synapse.
To perform its function the PNS is made of two types of cells, the sensory neurons and the motor neurons. Sensory neurons carry stimuli from stimulus receptors to the CNS, while motor neurons carry massages from the CNS to muscles and glands relaying what actions they should carry out.
Cranial and spinal nerves
There are 12 pairs of cranial nerves connecting the brain to the head, neck and face. Three of these nerves have only sensory neurons and are associated with smell, vision, hearing and equilibrium. Nerves containing mostly motor nerves are found connected to eyelid and eyeball muscles, tongue muscles, and muscles in the head and shoulder. Mixed cranial nerves that have both sensory and motor nerves are connected to taste glands, facial muscle, salivary glands and the muscles controlling swallowing.
All the 31 pairs of spinal nerves that connect the spine to the rest of the body have both sensory and motor neurons. There is a pair for each segment of the cord and is named after it. There are 8 cervical nerves, 12 thoracic nerves, 5 lumbar nerves, 5 sacral nerves, and 1 coccygeal nerve.
The actions of the peripheral nervous system can be divided into two broad categories- the autonomic nervous system and the somatic nervous system. Different ganglia, or groups of nerves, are involved in these two system.
The Autonomic Nervous System
The autonomic nervous system, as its name indicates, functions automatically. It also functions continuously and involuntarily. It sends signals to the internal visceral organs heart, lungs, glands and smooth muscles. This controls the workings of these organs to maintain homeostasis, by regulating breathing, digestion, and heartbeat. The autonomic system regulates these functions in two different ways, through the sympathetic and parasympathetic system.
Sympathetic system - These nerves control the reactions of an organism when it is under stress or faced with danger, resulting in the 'flight or fight' response. The body releases more energy, and the heart rate and breathing rate is increased. In these situations, adrenaline is released in the blood-stream so that the whole body is involved in the response. The flow of blood to muscles is also increased and carries more oxygen and energy, allowing the organism to take action.
Parasympathetic system - This system brings the body organs back to their normal rate of functioning by slowing the heart and breathing rate and blood flow to the muscles. This helps to bring the body to rest and conserve energy.
Somatic nervous system
The nerves in this system connect the skin and skeletal muscles to the brain so that the body can respond to stimuli from the external environment. Both voluntary and reflex actions are co-ordinated by this part of the nervous system. Both sensory and motor nerves are involved in the somatic system. Sensory nerves carry stimuli to the brain, which processes the information and sends back commands through the motor nerves to the same part of the body or other parts.
For example, the ears pick up the sound from the phone ringing and transmits it to the brain, which interprets it so we hear the ringing. The brain then sends signals to the muscles in our hand to pick up the phone.
The peripheral nervous system consists of all the nerves outside the brain and spinal cord. The sensory neurons bring the internal and external stimuli to the central nervous system (CNS), and motor neurons relay the message from the CNS to the receptors in the body.
The PNS has two functional groups, the autonomic system regulates the automatic and involuntary function of visceral organs like heart, lungs, and glands, by stimulating their action through the sympathetic system, or inhibiting their action through the parasympathetic system. The somatic nervous system transmits impulses to and from the brain to control voluntary and reflex actions.