The peripheral nervous system (PNS) consists of two main pathways referred to as the afferent pathway and efferent pathway. The PNS includes all nerve fibres not included in the spinal cord and brain.
The peripheral nervous system includes all nerves running from the spinal cord or brain to all parts of the body and vice versa. There are two main groups of nerves: cranial nerves and spinal nerves. These nerves can be sensory, motor or a combination of both.
The type of nerve varies according to its type of neuron. Sensory nerves allow the impulses from the sensory organs to be transmitted to the spinal cord or the brain. Motor nerves, on the other hand, carry nerve impulses from the CNS to muscles or glands. Sometimes, sensory and motor nerves can be grouped together in larger nerves, or mixed nerves.
Afferent and efferent pathways
In response to a stimulus (sound, light, touch, etc.), the afferent pathway carries nerve impulses from the receiving organs (such as the sensory organs) to the interpretation and integration centre (the cerebrum and/or the spinal cord). This information is transmitted from the nerve fibres of the sensory receptors, such as the optic nerve, for example. There are nerve fibres transmitting information from the musculoskeletal system, called the somatic afferent sensitive pathway, and nerve fibres transmitting from the viscera, called the visceral afferent sensitive pathway. The CNS is always aware of what is going on inside and outside the body through the sensory pathways of the PNS.
Once the CNS has processed the information from the PNS afferent pathway and developed a response, a new nerve impulse is propagated to the response site along the efferent pathway. The effector organs (muscles and glands), can trigger a response: muscles can either contract or relax the muscles, and glands can begin or stop secretion. The function of the CNS, PNS and sensory organs allows the motor response to adapt to the stimulus of the environment.
There are 12 pairs of cranial nerves directly connected to the brain. They are responsible for sight, eye movement, hearing, smell and the movement of several facial muscles, among other things.
Arrangement of emerging cranial nerves
There are 31 pairs of spinal nerves, all connected to the spinal cord. The nerves can be grouped according to their location: the cervical, dorsal, lumbar and sacral regions.
The spinal nerves branch out from the spinal cord