The cranial nerve X - the vagus nerve

The tenth cranial nerve, and one of the most important, is the vagus nerve. All twelve of the cranial nerves, the vagus nerve included, emerge from or enter the skull (the cranium), as opposed to the spinal nerves which emerge from the vertebral column. The vagus nerve originates in the medulla oblongata, a part of the brain stem.

The eye's rods and cones

Rods and cones generate nerve impulses in the retinas of the eyes that travel along the optic nerves to the optic chiasma, where they partially cross over. The sensory organs for vision - the eyes - are at the front of the head, but areas of the brain at the back and sides provide the actual visual sense. Mixed impulses from both eyes pass through the optic tracts to the striate cortex at the back of the brain and end in the temporal lobe area so that right and left halves of the visual field merge. When light rays reach the retina (the film of the eye's camera), light energy is converted into electrical nerve signals. Crisscrossed with blood vessels, the retina has three layers of microscopically thin nerve cells. Nearest to the lens is a layer of ganglion cells, then a layer of bipolar cells and finally the photoreceptors. It is the photoreceptors that actually process the packets of light energy or photons that impact on the retina, so light must pass through the ganglions and bipolar cells to get to others.

The eye muscle control

The eye muscle control

Each eye is held in place by three pairs of taut, elastic muscles which constantly balance the pull of the others. The superior rectus acts to roll the eyeball back and up, but it is opposed by the inferior rectus. In the same way, the lateral rectus pulls to the side, while the medial rectus pulls toward the nose, and the two oblique muscles roll the eye clockwise or counterclockwise.

The muscles of each eye work together to move the eyes in unison. Because of the constant tension in the muscles, they can move the eye very quickly, much faster than any other body movement. The eye muscles work together to carry out no less than seven coordinated movements and allow the eye to track many different kinds of moving object.

The eyes and Vision

The eyes and Vision

The sensory organs for vision - the eyes - are at the front of the head, but the actual visual sense is provided by areas of the brain at the back and sides. Seeing an object and recognizing it for what it is involves image processing by cells in the retina and brain. Three such types of cells are involved in this processing; some respond best to a clearly defined slit of light (or dark), others generally react to any location or orientation of the image, and some to specific features.

The ears and hearing

The ears and hearing

The ear is a group of sensory organs in the head that collaborate to produce the sense of hearing. Together these organs perform the amazing function of converting sound waves in air into electrical signals to transmit to the brain. The ear also contains several special structures that produce the body’s sense of equilibrium, or balance.

Three major regions make up the ear: the outer ear, middle ear, and inner ear. The outer ear consists of the exterior structures of the ear, the auricle and external auditory canal. Visible on the exterior of the head is the auricle, the external part of the ear that extends from the head. The auricle is made of elastic cartilage and adipose tissue covered with skin. It is a flexible organ whose curves help to conduct sounds into the other structures of the ear. In the center of the auricle is the external auditory canal, a tube that conducts sound through the body’s exterior and skull and into the middle ear.

The cerebrum

The cerebrum is the most superior and anterior of the brain’s major regions. It is the seat of reason, planning, memory, and sensory integration. All conscious thought originates in the cerebrum and can influence the subconscious functions of the lower regions of the brain.

The cerebellum

The cerebellum is located behind the top part of the brain stem (where the spinal cord meets the brain) and is made of two hemispheres (halves).

The cerebellum receives information from the sensory systems, the spinal cord, and other parts of the brain and then regulates motor movements. The cerebellum coordinates voluntary movements such as posture, balance, coordination, and speech, resulting in smooth and balanced muscular activity. It is also important for learning motor behaviors.

The brain stem

The brain stem is one of the most basic regions of the human brain, yet it is one of the most vital regions for our body’s survival. It forms the connection between the brain and the spinal cord, maintains vital control of the heart and lungs, and coordinates many important reflexes.

The brain stem is a tube-shaped mass of nervous tissue a little over 3 inches (8 cm) long. It is located at the base of the brain, superior to the spinal cord and inferior to the cerebrum. As the brain stem ascends from the spinal cord, it widens and becomes more complex in its structures, both internally and externally.

The brain

The brain

The brain is one of the most complex and magnificent organs in the human body. Our brain gives us awareness of ourselves and of our environment, processing a constant stream of sensory data. It controls our muscle movements, the secretions of our glands, and even our breathing and internal temperature. Every creative thought, feeling, and plan is developed by our brain. The brain’s neurons record the memory of every event in our lives.

In fact, the human brain is so complicated that it remains an exciting frontier in the study of the body; doctors, psychologists, and scientists are continually endeavoring to learn exactly how the many structures of the brain work together intricately to create our powerful human mind.

The nerves of the head and neck

The nerves of the head and neck include the most vital and important organs of the nervous system – the brain and spinal cord – as well as the organs of the special senses. In addition, in this region we also find the major cranial and spinal nerves that connect the central nervous system to the organs, skin, and muscles of the head and neck. These structures all work together to control every part of the body and receive sensory messages from the environment and the body’s internal structures.