The nervous system is the most complex of all the systems which coordinates controls and enables all body functions like movement, thinking, autonomic, or automatic, reflexes and sensory perception. The nervous system sends messages to all parts of the body and receives it. For example, when a person touches fire, the message is sent to the brain which interprets and sends a message to the person’s finger to suddenly remove it.
· The central nervous system
· The peripheral nervous system
· Spinal cord
The brain is placed inside a skull or cranium which protects it. The brain consists of three major parts, two hemispheres (right and left hemisphere) and meninges, the protective membrane.
The meninges have three layers (PAD)
P-Pia mater (inner)
D-Dura mater (outer).
The spinal cord is covered with meninges. The subarachnoid space (space between the arachnoid layer and the pia mater) contains the cerebrospinal fluid.
CSF fluid is a clear, colorless body fluid found in the brain and spinal cord. It is about 125ml of CSF at any one time and about 500ml is generated everyday.
CSF acts as a cushion or buffer for the brain, providing basic mechanical and immunological protection to the brain inside the skull.
· Brain stem
The cerebrum is the largest part of the brain and its lobes coordinate and enable body movement, sensory perception, learning, olfactory sense, gustatory sense, optic sense, auditory sense, memory, thinking, judgment and communication abilities.
The cerebrum has four major lobes which, as shown in the picture above, are:
· The frontal lobe
· The temporal lobe
· The parietal lobe
· The occipital lobe
The frontal lobe is responsible for thinking, movement, judging and writing.
The temporal lobe plays a role in terms of memory and hearing.
The parietal lobe helps in sensory perception.
The occipital lobe facilitates understanding written language and vision.
The cerebellum, referred as the little brain or hind brain is much smaller than the cerebrum and lies behind the other parts of the brain. The cerebellum controls motor nerves, balance, equilibrium, fine motor coordination and gross motor coordination.
The brain stem lies at the base of the skull and between the right and left hemispheres of the brain.
The brain stem has three sections
· Medulla oblongata
The midbrain controls messages sent from other higher areas of the brain to the pons and medulla below it and it also controls posture.
The pons connects the midbrain and the medulla. The pons controls the rhythm of respirations.
The medulla oblongata connects the pons and the spinal cord through a hole in the base of the skull called the foramen magnum. The medulla is the center of control for breathing and cardiac functioning and manages swallowing, vomiting, cough, and sneeze reflexes.
The spinal cord connects the brain to the peripheral nervous system throughout the body. The spinal cord is protected with the meninges and the spinal column.
The spinal cord is divided into different parts, which are:
· Cervical region
· Thoracic region
· Lumbar region
· Sacral region
· Coccyx, or tail bone, region
The spinal cord has 31 nerve clusters which sends and receives motor and sensory messages to and from the rest of the body and it coordinates reflex action.
The Parts of the Peripheral Nervous System
Peripheral nervous system consists of all the parts of the nervous system other than the brain and the spinal cord. The peripheral nervous system is not protected with bones.
The peripheral nervous system is divided into two:
· Autonomic nervous system
· Somatic nervous system
The autonomic nervous system controls automatic and involuntary physiological functions of the body that are out of our control like the movements of smooth, involuntary muscles. For example, the constriction of the eye’s pupil when it is exposed to bright light.
The autonomic nervous system’s functions can be further divided into:
Sympathetic nervous system is responsible for the fight and flight syndrome that results from stress. These responses include an increased heart rate, pupil dilation and decreased peristalsis.
The parasympathetic nervous system manages the functions related to rest. For example, the parasympathetic nervous system manages involuntary control of tears, digestion and the production of saliva.
Somatic nervous system
The somatic nervous system controls voluntary muscular movement with the skeletal muscles of the body. The somatic nervous system has efferent and afferent nerves which send and receive motor function related nerve signals.
A reflex is a muscle reaction that automatically occurs in response to a stimulus.
Reflexes present at the time of birth, but disappear shortly thereafter.
Reflexes present at the time of birth and remain active throughout life time. For example, the pupil reflex, sneeze reflex, blinking reflex, cough reflex and yawn reflex.
Remember the mnemonic: One Old Owl Turned Top, A Fat Aunt Go Viewed Some Hop
· Olfactory: smell
· Optic: vision
· Oculomotor: eye movements.
· Trochlear: eye movements.
· Trigeminal: chewing and sensory to face
· Abducens: eye abduction.
· Facial: facial expressions, the tongue and the salivary glands.
· Acoustic/Auditory: hearing
· Glossopharyngeal: taste impulses and the secretion of saliva from the parotid gland.
· Vagus: respiratory and the digestive systems such as the pharynx and swallowing.
· Spinal accessory: movement of shoulder.
· Hypoglossal: Tongue movement.
The nervous system is comprised of millions of neurons and glial cells.
Sensory or afferent nerves
Motor or efferent nerves
The neuron is the primary type of nerve cell in the nervous system. Glial cells forms myelin sheath over the neuron for protection.
Neurons communicate with other neurons by sending impulses at the synapse, a point of junction between any two neurons. There are two types of neurons which are sensory neurons and motor neurons. Sensory neurons sense and transmit information such as taste, touch and sight. Motor neurons send and transmit messages that involve muscle movement.
Glial cells are non-neuronal cell that provide support and nutrition, maintain homeostasis, form myelin and patricipate in signal transmission in the nervous system.
Sensory or aﬀerent nervesWhen action potentials are generated by sensory receptors on the dendrites of these neurons, they are transmitted to the spinal cord by the sensory nerve fibres. The impulses may then pass to the brain or to connector neurons of reflex arcs in the spinal cord motor or efferent nerves.
Motor nerves originate in the brain, spinal cord and autonomic ganglia. They transmit impulses to the effectors organs muscles and gland. There are two types:
Somatic Nerves – involved in voluntary and reflex skeletal muscle contraction.
Autonomic Nerves (sympathetic and parasympathetic) – involved in cardiac and smooth muscle contraction and glandular secretion.
Mixed Nerves: In the spinal cord, sensory and motor nerves are arranged in separate groups or tracks. Outside the spinal cord, when sensory and motor nerves are enclosed with the same sheath of connective tissue they are called mixed nerves.
· Multiple sclerosis
· Parkinson’s diseases
· Alzheimer’s disease
· Bell’s palsy.