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The nervous system

The nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. The nervous system in humans consists of three parts, central nervous system, peripheral nervous system and automatic nervous system.

Central Nervous System

The central nervous system (CNS) is the part of the nervous system that integrates the information that it receives from, and coordinates the activity of, all parts of the bodies of animals including humans.

The central nervous system is made up of the

•    spinal cord and
•    brain

The spinal cord

•    conducts sensory information from the peripheral nervous system (both somatic and autonomic) to the brain
•    conducts motor information from the brain to our various effectors
•    skeletal muscles
•    cardiac muscle
•    smooth muscle
•    glands
•    serves as a minor reflex center

The brain

•    receives sensory input from the spinal cord as well as from its own nerves (e.g., olfactory and optic nerves)
•    devotes most of its volume (and computational power) to processing its various sensory inputs and initiating appropriate — and coordinated — motor outputs.

Brain Anatomy

•    Cerebrum- It is the largest part of the brain and associates with conscious thought, movement and sensation.
•    The Cerebrum consists of two halves, each connected by the corpus callosum, which delivers messages between them.
•    The four separate lobes make up the cerebrum: the frontal, temporal, parietal, and occipital lobes.

•    Frontal Lobe- The frontal lobe attention, behavior, abstract thinking, problem solving, creative thought, emotion, intellect, initiative, judgment, coordinated movements, muscle movements, smell, physical reactions, and personality.

•    Parietal Lobe – The Parietal Lobe controls tactile sensation, response to internal stimuli, sensory comprehension, some language, reading, and some visual functions.

•    Sensory cortex – The Sensory cortex is located in the front part of the parietal lobe; it receives information from the spinal cord about the sense of touch, pressure, pain, and the perception of the position of body parts and their movements.

•    Motor cortex- It is an area located in the middle, top part of the brain that helps control movement in various parts of the body.

•    Temporal lobe- The Temporal controls auditory and visual memories, language, some hearing and speech, language, plus some behavior.

•    Wernicke's Area – The Wernicke's Area is part of the temporal lobe which surrounds the auditory cortex; it is essential for understanding and formulating speech.

•    Occipital Lobe- The Occipital Lobe found present in the back of the head; it controls vision.

•    Broca's Area- The function of Broca's Area includes understanding of language, speech, and the control of facial neurons.

•    Brain Stem – It is located at the bottom of the brain and connects the cerebrum to the spinal cord. The brain stem controls very significant functions including motor and sensory pathways, cardiac and respiratory functions, and reflexes.

•    Cerebellum – The Cerebellum is located at the lower back of the head; it is connected to the brain stem. The Cerebellum is the second largest structure of the brain made up of two hemispheres. It controls the complex motor functions such as walking, balance, posture, and general motor coordination.

•    Cerebrospinal Fluid or CSF- The CSF is a clear material which circulates through the brain and spinal cord. It provides nutrients and serves as cushion to brain and thus protects it from injury.

•    The Four Ventricles of the brain are connected cavities within the brain, where cerebrospinal fluid is produced.

•    Hypothalamus - It is a region of the brain; with the pituitary gland it controls the hormonal processes of the body as well as temperature, mood, hunger, and thirst.

•    Optic Chiasm- It is located below the hypothalamus, here, optic nerve crosses over to the opposite side of the brain.

•    Pineal Gland – It controls the response to light and dark.

•    Pituitary Gland – It is a small, bean-sized organ found at the base of the brain connected to the hypothalamus by a trail. The pituitary gland secretes out essential hormones for growth and sexual maturation.

•    Meninges – These are the membrane covering the brain and spinal cord.

•    Thalamus- It is located near the center of the brain and controls input and output to and from the brain, as well as the sensation of pain and attention.

Peripheral Nervous System

•    The peripheral nervous system is short also referred as PNS PeNS.
•    It consists of the nerves and ganglia outside of the brain and spinal cord.
•    The key function of the PNS is to connect the central nervous system (CNS) to the limbs and organs.
•    Unlike the central nervous system, the peripheral nervous system is not protected by the bone of spine and skull, or by the blood–brain barrier.
•    The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.

Autonomic Nervous System

The autonomic nervous system (ANS) has motor neurons (autonomic neurons) which conduct impulses from the brain stem or spinal cord to cardiac muscle, smooth muscle and glands. The motor neurons are responsible for regulating heart rate, peristalsis and the release of secretions from certain glands, e.g. salivary glands in the mouth.

General Characteristics of the Autonomic Nervous System (ANS)

•    It is a two-neuron pathway.
•    Sensory signals from viscera and skin send signals to autonomic neurons in brain and spinal cord.
•    A preganglionic neuron cell body is located within the CNS (brain stem or spinal cord).
•    Preganglionic fibers (efferent fibers) synapse with a ganglionic neuron located in the PNS
•    A postganglionic fiber terminates on the effector organ (heart, stomach, etc).

Divisions of the ANS

•    Sympathetic Division

•    Parasympathetic Division

No of Bones in Human Body

The human body consists of about 350 bones. In the process of growth and maturation some of the bones fused together. The adult human skeleton consists of 206 bones that vary in size and shape.

A distribution of bones in different body parts is as follows:

•    28 skull bones (14 facial, 8 cranial and 6 ear bones)
•    Hyoid bone of the neck (horseshoe-shaped)
•    26 vertebrae (7 cervical, 12 thoracic, 5 lumbar, sacrum made from 5 fused vertebrae and coccyx made from 3 fused vertebrae)
•    24 ribs, sternum or breastbone, shoulder girdle (2 clavicles and 2 scapulae)
•    2 in arms (humerus), 4 in forearms (2 radius, 2 ulna)
•    54 in the hands (carpus, metacarpus and digits)
•    2 Hip bones
•    2 in thighs (femur)
•    6 in the legs (2 patella, 2 tibia and 2 fibula)
•    52 in the feet (tarsus, metatarsus and digits)

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