NERVOUS SYSTEM .

• Homeostasis is a process that keeps the stability of the internal environment.
• All organ systems are involved in homeostasis.
• Homeostasis is maintained and regulated by negative feedback mechanisms that minimize the deviation from the norm.
• Positive feedback mechanisms exist but rare. Instead of inhibiting, they promote the change that is occurring.
• Changes that occur in the body that lead to imbalances are well-coordinated by an organ system.

  The organ system that controls and coordinates function throughout the body is called the nervous system.
• The nervous system uses electrical signals called impulses to carry messages.
• The cells that transmit impulses are called neurons.
• 1. Sensory neurons carry signals from the sense organs to the spinal cord and brain.
  2. Interneurons connect sensory neurons and motor neurons.

  3. Motor neurons carry impulses from the brain and spinal cord to the muscles and glands to produce an action.
• A neuron has a cell body that contains a large, spherical nucleus.
• It also includes cytoplasmic organelles where all metabolic activities of the cell take place.
• Dendrites are numerous and short. They are the primary sites for receiving impulses from other neurons.
• The axon or nerve fiber is long and thin. It carries the impulses away from the cell body.
• The end of axon is a series of small swelling called axon terminals.
• Some axons are enclosed with an insulating membrane called myelin sheath. It increases the speed of impulse transmission.
• The nervous system is divided into two divisions: the central nervous system and the peripheral nervous system.
• Central nervous system This division consists of the brain and spinal cord. It is the body’s neural control center. It receives the impulses, analyzes and organizes the information, and makes appropriate actions.
• The brain and spinal cord are delicate organs.
• brain is protected by skull and spinal cord is protected by vertebrae and backbone
• They also have three layers of connective tissues called meninges and each layer is surrounding by a cerebrospinal fluid.
• Meninges

  1. dura mater
  2. arachnoid mater
  3. pia mater
• The brain is a large organ that contains 100 billion neurons. It consists of four parts: cerebrum, cerebellum, thalamus and hypothalamus, and brain stem.
• The cerebrum is the largest portion of the brain and performs higher brain functions like sensations, voluntary actions, problem solving, and planning.
• Brain is composed of two hemispheres, which are divided by a band of tissue called corpus callosum.
• Frontal lobe - reasoning

  Parietal Lobe - touch

  Temporal lobe - hearing

  Occipital Lobe - sight
• The cerebellum is the second largest part of the brain and is located at the back of the skull. It is a reflex center that controls the posture, balance, and movement of the body.
• The thalamus and hypothalamus are small portions of the brain that lie between the midbrain and brain stem.
• The thalamus provides awareness of sensations like pain, pressure, and touch. It also serves as a relay station for motor impulses.
• The hypothalamus’ primary function is to maintain homeostasis and recognize anger, hunger, thirst, and fatigue.
• The brain stem is the stalk-like portion of the brain that joins the brain and the spinal cord.
• The spinal cord is a cylindrical shape that descends from the medulla oblongata.

  It is the main communication link between the brain and the body. It has thirty-one pairs of spinal nerves branching out from the spinal cord.
• The peripheral nervous system is located outside the central nervous system and consists of all nerves and related cells not part of the brain and the spinal cord. 
• The somatic nervous system is a subdivision of the peripheral nervous system. It regulates the activities of movements with conscious control like the skeletal muscles.
• A reflex arc defines the pathway by which a reflex travels – from the stimulus to sensory neuron to motor neuron to reflex muscle movement.
• Receptors - sensory neurons - spinal cord - motor neuron - muscles
• The autonomic nervous system regulates the involuntary activities or activities without conscious control. 
• ANS is divided into two : sympathetic and parasympathetic that have opposite effects on the organ.
• Sympathetic nervous system - alerts and increases rate
• Parasympathetic nervous system - relax and decreases rate
• Spinal cord is the information superhighway