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Organization Of The Nervous System


Organization Of The Nervous System

The nervous system is divided in two parts:

  1. The peripheral nervous system
  2. The central nervous system

Peripheral Nervous System:

“The peripheral nervous system is made up of all those nerves that lie outside the brain and the spinal cord. Nerves are bundles of neuron fibers (axons) that are route together in the peripheral nervous system. The PNS can be divided into somatic nervous system and the autonomic nervous system.”

The somatic Nervous system: (structure and function)

  • Made up of nerves that connect to voluntary skeletal muscles and to sensory receptors.
  • These nerves are cables that carry information for receptors in the skin, muscles, and joints to the CNS and that carry commands from CNS to the muscles.
  • These functions require two kinds of nerves fibers.

Afferent nerve fibers:- are axons that carry information inward to the CNS from the periphery of the body.

Efferent nerve fiber:- outward from the CNS to the periphery of the body.

Each body nerve contains many axons of each type. Thus, somatic nerves are “two-way streets” with incoming (afferent) and outgoing (efferent) leans. The somatic nervous system lets you feel the world is move around in it.

The Autonomic Nervous System: 

“The ANS is made up of nerves that connect to the heart, blood vessels, smooth muscles and glands.” The autonomic nervous system controls automatic, involuntary, visceral functions that people don’t normally think about such as heart-rate, digestion and perspiration.

Autonomic Arousal: Role of PNS in determining the overt behavior:

The ANS mediates much of the physiological arousal that occurs when people experiences emotions. For example, image that you are walking home alone one night when a seedy looking character falls in behind you and begins to follow you. If you feel threatened, your heart rate and breathing will go up. Your blood pressure may surge, you may feel Goosebumps, and your palms may begin to sweat. These difficult to control reactions are aspects of autonomic arousal.

Fight-or-Flight Response:

Walter cannon (1932) called these reaction “fight-or-flight” response. He observed that organisms generally respond to threat by preparing physiologically for attacking (fight) or feeling (flight) from the enemy. Unfortunately this fight-or-flight response can backfire if stress leaves a person in a chronic state of autonomic arousal.

  • Prolonged autonomic arousal – physical disease

Sympathetic and Para Sympathetic Division:

The ANS can be subdivided into two branches.

Sympathetic Division:

  • mobilizes the body’s resources for emergencies.
  • It creates the fight-or-flight response.
  • Activation of the sympathetic division slows digestion; dilate pupils, increase respiration and heart rate, and increase secretion by sweat glands. Bladder relaxed. Hair follicles raised, goose bumps, drains blood from the periphery lessening bleeding in the case of an injury.
  • Key sympathetic nerves send signals to adrenal glands triggering the release of hormones that ready the body for exertion.

Para Sympathetic division:

  • Conserve bodily resources.
  • It activates process that allows body to save and store energy.
  • Slow heart rate, reduce blood pressure promote digestion, decreased respiration etc


The Central Nervous System:

CNS—consists of brain and spinal cord.

  • It is protected by enclosing sheaths called the “meninges”.
  • CNS is bathed in cerebrospinal fluid. The cerebrospinal fluid (CSF) nourishes the brain and provide protective cushion for it.

The Spinal Cord:

  • The spinal cord connects the brain to the rest of the body through the peripheral nervous system.
  • Although the spinal cord looks like a cable from which the somatic nerves branch, it is part of the CNS
  • Also enclosed in meninges and bathed in CSF.
  • In short, spinal cord is on extension of brain.
  • The spinal cord runs from the base of the brain to just below the level of the waist.
  • It houses bundles of axons that carry the brain’s commands to peripheral nerves and that relay sensations from the periphery of the body to the brain.
  • Many form of paralysis result from spinal cord damage, a factor that underscores the critical role that spinal cord plays in transmitting signal from the brain to the motor neurons that move the body’s muscles.

The Brain:

  • The crowning glory of CNS.
  • If fills the upper portion of shell, weighs only about three pound and could be held in one hand.
  • The brain contains billions of interacting cells that integrate information from inside and outside the body, coordinate the body’s actions, and enable human beings to talk, thing, remember, plan create and dream.
  • The brain can be divided into three major regions. The hindbrain, the midbrain and the forebrain.

Structure of Brain:

 The Hindbrain:

The Hindbrain includes the cerebellum, reticular formation, pons and medulla.


The medulla which attaches to the spinal cord has charge of largely unconscious but vital functions, including circulating blood, breathing, maintaining muscle tone, and regulating reflexes such as sneezing, coughing, and salivating.


  • The pons (literally “bridge”) includes a bridge of fibers that connects the brain stem with the cerebellum.
  • Contains several clusters of cell bodies involved with sleep and arousal.


  • The cerebellum (literally “little brain” is a relatively large and deeply folded structure that lies adjacent to the back surface of the brainstem.
  • Critical to the coordination of movement and to the sense of equilibrium or physical balance. Although the actual commands for muscular movements come from higher brain centers, the cerebellum plays a key role in organizing the sensory information that guides these movements.
  • Damage to cerebellum disrupts fine motor skills, such as those involved in writing, typing, or playing a musical instrument.

Reticular Formation:-

  • Inside the brainstem, the reticular formation (also known as reticular activating system) extends from the spinal cord up to the thalamus.
  • This finger-shaped network of neurons helps control arousal and attention.
  • As the spinal cord’s sensory input travel up to the thalamus, some of it branches off to the reticular formation, which filters incoming stimuli and relays important information to other areas of the brain.
  • Contributes to modulation of muscle reflexes, breathing and pain perception.

The Midbrain:

  • The midbrain is the segment of the brainstem that lies between the hindbrain and the forebrain.
  • Integrating sensory processes such as vision and hearing.
  • System of dopamine releasing neurons that projects into various high brain centers originates in the midbrain. Among other things, this system controls voluntary movements. The decline in dopamine synthesis causes Parkinsonism is due to degeneration of a structure located in the midbrain.
  • Reticular formation running through both hind brain and midbrain.

The Forebrain:

  • Largest and most complex regions of the brain.
  • Include thalamus, hypothalamus, limbic system and cerebrum.

The Thalamus: A Way Station:

  • The thalamus all sensory information (except smell) must pass through it to get to the cerebral cortex.
  • This way station is made up of clusters of cell bodies or somas. Each cluster is concerned with relaying sensory information to a particular part of the cortex.
  • It is not only a relay station but also appears to play a major role in integrating information from various senses.

The Hypothalamus: A Regulator of Biological Needs.

  • Found near the base of the forebrain.
  • Involved in regulation of basic biological need.
  • Contain various clusters of cells that have many key functions.
  • Control ANS, serve as vital link between brain and the endocrine system.
  • Regulation of basic biological drives related to survival including the so-called “four F’s:” fighting, fleeing, feedings, and mating. For example, when researchers lesion the lateral areas of the hypothalamus, animals lost interest in eating. In contrast, when electrical simulation (ESB) is used to activate the lateral hypothalamus, animals eat constantly and gain weight rapidly. It does not mean that lateral hypothalamus is the “hunger center” of the brain because the regulation of hunger turns out to be complicated and multifaceted. Nonetheless it contributes to control of hunger and other basic biological processes, including thirst and temperature regulation.

The Limbic System: The Seat of Emotion:

  • Loosely connected network of structures located roughly along the border between the cerebral cortex and deeper subcortical areas.
  • Broadly defined, the limbic system includes parts of the thalamus, hypothalamus, the hippocampus, the amygdala and other structures. The limbic system is involved in the regulation of emotion, memory, and motivation.

Hippocampus: Role in Memory Processes:

  • Hippocampus and adjacent structures clearly play a major role in memory processes although the exact nature of their role is the subject of debate. Some theorists believe that the hippocampus controls consolidation of memories for factual information.
  • In any event, many other brain structures contribute to memory processes, so the hippocampus is only one put of the complex system.
  • Some evidence linked limbic system the experience but the exact mechanisms of control are not yet well understood. Recent evidence suggests amygdala fear response.
  • Limbic system is also one of the areas of the brain that appears to be rich in emotion-tinged “pleasure centers”. This intriguing possibility first surfaced, quiet by chance, in brain stimulation research with rats. James Olds and Peter Milner accidently placed an electrode in hypothalamus instead of reticular formation in rat’s brain they observed that a rat would press a lever repeatedly to send brief pulse of electrical stimulation to hypothalamus. Although the experimenters obviously couldn’t ask the animals about it, they inferred that the animals were experiencing some sort of pleasure.
  • Many self-stimulation sites have been found in the limbic system. The heaviest concentration appears to be where the medial forebrain bundle passes through the hypothalamus. The medical forebrain bundle is rich in dopamine relearning neurons. The regarding effects of ESB at self-stimulation sites may be largely meditated by the activation of these dopamine cells.
  • The rewarding, pleasurable effects of opiate and stimulants drugs (cocaine and amphetamines) may also depend on excitation of this dopamine system, although there is some debate about such conclusion.
  • Nonetheless, it is clear that this dopamine system is not the ultimate biological basis for all reward.

The cerebrum: The seat of complex thought:

  • Responsible for learning, remembering, thinking and consciousness- most complex activities.
  • The cerebral cortex is the convoluted outer layer of the cerebrum. The cortex is folded and bent so that its large surface area about 1.5 square feet can be packed into the limited volume of the skull.
  • Cerebral Hemispheres:

The cerebrum is divided into two halves called hemispheres. Hence, the cerebral hemispheres are the right and left halves of the cerebrum. The Corpus Callosum is the structure that connects the two cerebral hemispheres.

Functions of left and Right Hemispheres:

Evidence that the left controls process language led scientists to view it as the dominant hemisphere. However, studies of split- brain patients revealed that left and right halves of the brain each has unique talents, with right controlling visual – spatial function (non-verbal processes) and Left – verbal processing.


Each cerebral hemispheres is divided into four parts called lobes.

Occipital lobe:-

  • Located at back of the head includes the cortical area where most vital signals are sent and visual processing is begun. This area is called primary visual cortex.

Parietal Lobe:

  • Forward of the occipital lobe
  • This lobe includes the area that registers the sense of touch called the primary somatosensory vortex.
  • Also involved in integrating visual input and in monitories the body’s position in space.

Temporal lobe:

  • Lies below the parietal lobe.
  • Near its top, the temporal lobe contains an area devoted to auditory processing, called the primary auditory cortex.

Frontal lobe:

  • Largest lobe
  • Contain areas that control the movement of muscles, called the primary motor cortex.
  • The amount of motor cortex allocated to the control of a body part depends not on the part’s size but on the diversity and precisions of its movements. Thus, more of the cortex is given to the parts we have fine control over, such as fingers, lips and the tongue. Less devoted to parts that have crude movements such as thighs and the shoulders.

Pre-Frontal Cortex:

  • The portion of the frontal lobe to the front of themotor cortex, which is called the pre-frontal cortex, is something of a mystery.
  • 28% of human cerebral cortex.
  • Recent studies suggests that it contribute to an impressive variety of higher order function, such as memory for temporal sequences, worrying memory (which is a temporary) buffer that processes current information and reasoning about relations between objects and events.



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