Nervous System

Nervous System Functions: 1. The nervous system carries out the following functions 2. It perceives the changes around us through our senses. 3. It controls and coordinates all the activities of the muscles in response to the changes outside. 4. It also maintains the internal environment of the body by coordinating the functions of the various internal organs and the involuntary muscles. 5. It stores the previous experiences as memory that helps us to think and analyseanalyze our reactions.It conducts messages between different parts of the body.

The Human Nervous System. Blue is PNS while red is CNS.

Reflexes 1. Reflexes Arc A reflex arc is the neural pathway that mediates a reflex action. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord. This characteristic allows reflex actions to occur relatively quickly by activating spinal motor neurons without the delay of routing signals through the brain, although the brain will receive sensory input while the reflex action occurs. The main source of the reflex action is through the bottom muscles.

2. Withdrawal reflex The nociceptive withdrawal reflex (NWR) is a spinal reflex intended to protect the body from damaging stimuli. The classic example is when a person touches something hot and withdraws their hand from the hot object. The heat stimulates temperature and pain receptors in the skin, triggering a sensory impulse that travels to the central nervous system. The sensory neuron then synapses with interneurons that connect to motor neurons. Some of these send motor impulses to the flexors to allow withdrawal; some motor neurons send inhibitory impulses to the extensors so flexion is not inhibited - this is referred to as reciprocal innervation. While all of this occurs, other interneurons relay the sensory information up to the brain so that the person becomes aware of the pain and what happened. The NWR is also known as the flexion reflex.

3. Patellar or knee-jerk Reflex The patellar reflex or knee-jerk is a deep tendon reflex and is a myotatic reflexStriking the patellar ligament with a tendon hammer just below the patella stretches the quadriceps muscles in the thigh. This stimulates stretch sensory receptors (most importantly, muscle spindles) that trigger an afferent impulse in a sensory nerve fiber of the femoral nerve leading to the lumbar region of the spinal cord. There, the sensory neuron synapses directly with a motor neuron that conducts an efferent impulse to the quadriceps femoris muscle, triggering contraction. This contraction, coordinated with the relaxation of the antagonistic flexor hamstring muscle causes the leg to kick. This reflex helps maintain posture and balance, allowing one to walk without consciously thinking about each step. The patellar reflex is a clinical and classic example of the monosynaptic reflex arc. There is no interneuron in the pathway leading to contraction of the quadriceps muscle. Instead the bipolar sensory neuron synapses directly on a motor neuron in the spinal cord. However, there is an inhibitory interneuron used to relax the antagonistic hamstring muscle.

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Structures of the Nervous System Central Nervous System (CNS).The part of the nervous system that functions to coordinate the activity of all parts of the bodies of bilaterian animals—that is, all animals more advanced than sponges or jellyfish. In vertebrates, the central nervous system is enclosed in the meninges. It contains the majority of the nervous system and consists of the brain and the spinal cord. Together with the peripheral nervous system it has a fundamental role in the control of behavior. The CNS is contained within the dorsal cavity, with the brain in the cranial cavity and the spinal cord in the spinal cavity. The brain is protected by the skull, while the spinal cord is protected by the vertebrae.

A diagram showing the CNS: 1. Brain 2. Central nervous system (brain and spinal cord) 3. Spinal cord

Brain: Central nervous system Peripheral Nervous System (PNS) It resides or extends outside the central nervous system (CNS), which consists of the brain and spinal cord.[1] The main function of the PNS is to connect the CNS to the limbs and organs. Unlike the central nervous system, the PNS is not protected by bone or by the blood-brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system; some textbooks also include sensory systems.[2] Somatic Nervous System (Voluntary) The somatic nervous system (SNS) is the part of the peripheral nervous system[1] associated with the voluntary control of body movements through the action of skeletal muscles, and with reception of external stimuli, which helps keep the body in touch with its surroundings (e.g., touch, hearing, and sight). The system includes all the neurons connected with skeletal muscles, skin and sense organs. The somatic nervous system consists of efferent nerves responsible for sending brain signals for muscle contraction.

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Two types: Sensory (or afferent) In the nervous system, afferent neurons (otherwise known as sensory or receptor neurons), carry nerve impulses from receptors or sense organs toward the central nervous system. This term can also be used to describe relative connections between structures. Afferent neurons communicate with specialized interneurons. The opposite activity of direction or flow is efferent. motor (or efferent) nerves In the nervous system, efferent nerves – otherwise known as motor or effector neurons – carry nerve impulses away from the central nervous system to effectors such as muscles or glands (and also the ciliated cells of the inner ear). The term can also be used to describe relative connections between nervous structures (for example, a neuron's efferent synapse provides input to another neuron, and not viceversa). The opposite activity of direction or flow is afferent.

Autonomic Nervous System (Involuntary) The autonomic nervous system (ANS or visceral nervous system) is the part of the peripheral nervous system that acts as a control system functioning largely below the level of consciousness, and controls visceral functions.[1] The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, micturition (urination), and sexual arousal. Whereas most of its actions are involuntary, some, such as breathing, work in tandem with the conscious mind. 

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Sympathetic The Sympathetic Nervous System (SNS) is a branch of the autonomic nervous system along with the enteric nervous system and parasympathetic nervous system. It is always active at a basal level (called sympathetic tone) and becomes more active during times of stress. Its actions during the stress response comprise the fight-orflight response.  Activated by stress  Increase heart rate, breathing rate, pupil size, sweating Parasympathetic The parasympathetic nervous system (PSNS) is a division of the autonomic nervous system (ANS), along with the sympathetic nervous system (SNS) and enteric nervous system (ENS or "bowels NS"). The ANS is a subdivision of the peripheral nervous system (PNS). ANS sends fibers to three tissues: cardiac muscle, smooth muscle, or glandular tissue. This stimulation, sympathetic or parasympathetic, is to control smooth muscle contraction, regulate cardiac muscle, or stimulate or inhibit glandular secretion. The actions of the parasympathetic nervous system can be summarized as "rest and digest" (as opposed to the "fight-or-flight" effects of the sympathetic nervous system). It is also quite the opposite for the sympathetic nervous system.  Maintains body functions; in control when relaxed.  Restores body to pre-stress state