chapter 2.1 - nervous system

2. 1 Nervous System

2.1.1 Organization of Nervous System2.1.2 Classification of Neurons2.1.3 Central Nervous System2.1.4 Peripheral Nervous System2.1.5 Autonomic Nervous System

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2.1.1 Organization of Nervous System• Structural Classification of NS:

Nervous system is structurally classified into central nervous system and peripheral nervous system.

Central nervous system consists of brain and spinal cord.Peripheral nervous system consists of cranial nerves and spinal

nerves.Cranial nerves extend from the brain whereas spinal nerves

extend from the spinal cord.Cranial nerves carry impulses to and from the brain. Spinal nerves carry impulses to and from the spinal cord.

http://anatomybodyblog.com/images/9991-simple-nervous-system-diagram.jpg

Functional Classification of NS:•The functional classification is concerned only with PNS.•PNS is functionally classified into sensory / afferent division and motor / efferent division.•Sensory / afferent division conveys impulses to the CNS from sensory receptors located in various parts of the body.•Sensory / afferent division consists of:

– somatic sensory delivers impulses from the skin, skeletal muscles and joints to CNS, and

– visceral sensory delivers impulses from the visceral organs to CNS.

• Motor / efferent division carries impulses from the CNS to effector organs, muscles and glands.

• Motor / efferent division consists of :– Somatic nervous system controls voluntary actions and

reflexes in skeletal muscles.– Autonomic nervous system controls involuntary actions in

smooth and cardiac muscles and glands.• Autonomic nervous system consists of sympathetic division

and parasympathetic division.

http://antranik.org/neurons-in-the-nervous-system/

2.1.2 Classification of NeuronsStructural classification of Neurons:•Based on the number of processes extending from the cell body, neurons are structurally classified into: 1. multipolar neurons – has several processes.

• Eg. motor and interneurons

2. bipolar neurons – has 2 processes.• Eg. sensory receptor cells in eyes and ears

3. unipolar neurons – has 1 process.• Eg. sensory neurons

http://antranik.org/neurons-in-the-nervous-system/

Functional classification of Neurons:

•Based on the direction of the nerve impulses relative to CNS, neurons are functionally classified into:1. sensory / afferent neuron – carry impulses from sensory receptors to CNS.

• Their cell bodies are always found in a ganglion outside the CNS.

2. motor / efferent neuron – carry impulses from CNS to effectors.

• Their cell bodies are always located in the CNS.

3. association / interneuron – carry impulses from sensory /afferent neuron to motor /efferent neuron.

• Their cell bodies are always located in the CNS.

http://www.tutorvista.com/biology/reflex-arc-diagram

2.1.3 Central Nervous System• The spinal cord and brain make up the CNS.• Both the spinal cord and brain are protected by bone;

the spinal cord is surrounded by vertebrae and the brain is enclosed by skull.

• Both spinal cord and brain are wrapped in protective membranes called meninges.

• The spaces between meninges are filled with cerebrospinal fluid, which cushions & protects the CNS.

• CNS consists of 2 types of nervous tissue: gray matter & white matter.

• Gray matter contains cell bodies and short, non-myelinated fibres.

• White matter contains myelinated fibres.

http://gallery4share.com/c/central-nervous-system-diagram-3d.html

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A. The Brain

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A. The Brain

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Structures & Functions of the Brain1. Cerebrum / Telencephalon2. Diencephalon (Hypothalamus, Thalamus & Pineal

gland)3. Cerebellum4. Brain stem (Mid brain, Pons & Medulla oblongata)

1. Cerebrum / Telencephalon•The largest portion of the human brain.•Cerebrum has two halves, i.e. the left & right cerebral hemispheres. •A deep groove called the longitudinal fissure divides the left & right cerebral hemispheres.•The two cerebral hemispheres are connected by a bridge of tracts within the corpus callosum.•Functions: 1. to receive sensory input, carry out integration and commanding voluntary motor responses. 2. to coordinate the activities of the other parts of the brains. 3. for learning and memory. 4. for language and speech.

• Cerebrum has 4 lobes: 1. frontal lobe – primary motor area, premotor area, motor speech area, prefrontal area. 2. parietal lobe – primary somatosensory area, somatosensory association area, primary taste area. 3. occipital lobe – primary visual area, visual association area. 4. temporal lobe – primary auditory area, auditory association area, sensory speech area.

2. Hypothalamus• Hypothalamus, thalamus & pineal gland belong to the diencephalon.• Hypothalamus forms the floor of the third ventricle.• Functions: 1. - integrating centre that helps maintain homeostasis by regulating hunger, sleep, thirst, body temperature, and water balance. 2. - controls the pituitary gland 3. - serves as a link between the nervous and endocrine systems.

3. Thalamus•Consists of two masses of gray matter located in the sides and roof of the third ventricle.•Functions: 1. receiving end for all sensory input except smell. 2. integrates visual, auditory and somatosensory information and send it to the appropriate portions of the brain. 3. participate in higher mental functions such as memory & emotion

4. Pineal Gland- Secretes the hormone melatonin.

5. Cerebellum• Separated from the brain stem by the fourth ventricle.• It receives sensory input from the eyes, ears, joints, and

muscles about the present position of body parts and receive motor output from the cerebral cortex about where these parts should be located. After integrating these information, it sends motor impulses by way of the brain stem to the skeletal muscles.

• Functions:1. maintains posture and balance.2. ensure skeletal muscles to produce smooth, coordinated voluntary movements.3. assists the learning of the new motor skills.

6. Midbrain• Functions:

1. acts as a relay station for tracts passing between the cerebrum and the spinal cord or cerebellum.

2. has reflex centres for visual, auditory, and tactile responses.

7. Pons• Functions:

1 regulate breathing rate together with medulla oblongata.2. has reflex centres concerned with head movement in

response to visual and auditory stimuli.

8. Medulla Oblongata• Functions:

1. has reflex centres for regulating heartbeat, breathing, and vasoconstriction (blood pressure).

2. has reflex centres for vomiting, coughing, sneezing, hiccuping, and swallowing.

3. control involuntary actions.

B. The Spinal Cord• The spinal cord extends from the base of the brain

through a large opening in the skull called the foramen magnum and into the vertebral canal formed by the openings in the vertebrae that make up the vertebral column.

• Vertebrae are separated by intervertebral disks.• Spinal cord has a central canal, gray matter and white

matter.• The gray matter is centrally located and shaped like

letter ‘H’ . It contains portions of sensory and motor neurons, and interneurons.

• The dorsal root of a spinal nerve contains sensory fibres entering the gray matter, and the ventral root of a spinal nerve contains motor fibres exiting the gray matter.

• The dorsal and ventral roots join before the spinal nerve leaves the vertebral canal.

• The white matter of the spinal cord occurs in areas around the gray matter.

• The white matter contains ascending tracts taking information to the brain and descending tracts taking information from the brain.

Functions of spinal cord:• 1. provides a means of communication

between the brain and the peripheral nerves that leave the cord.

• 2. centre for spinal reflex arcs.

http://antranik.org/central-nervous-system-spinal-cord/

2.1.4 Peripheral Nervous System

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• The peripheral nervous system (PNS) composed of nerves and ganglia lies outside the CNS.

• The cranial nerves extend from the brain.• The spinal nerves extend from the spinal cord.• Human has 12 pairs of cranial nerves. Some of these

are sensory nerves, some are motor nerves and some are mixed nerves that contain both sensory and motor nerves.

• Cranial nerves are largely concerned with the head, neck, and facial regions of the body. However, vagus nerve has branches in pharynx, larynx, and most of the internal organs.

http://global.britannica.com/science/vagus-nerve

FOR REFERENCE ONLY “oh, oh, oh, to touch and feel very good velvet, ah.”

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FOR REFERENCE ONLY

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Spinal Nerves:•Human has 31 pairs of spinal nerves. Each spinal nerve originates when two short branches, or roots, join together.•The dorsal root contains sensory fibres that conduct impulses from sensory receptors to the spinal cord. The cell body of a sensory neuron is in the dorsal-root ganglion. The ventral root contains motor fibres that conduct impulses from the spinal cord to the effectors.•All spinal nerves are mixed nerves that contain many sensory and motor fibres.•Each spinal nerve serves the particular region of the body in which it is located.

http://antranik.org/central-nervous-system-spinal-cord/

Diagram showing the different spinal nerves

Reflex Action• Reflex action: The fast, automatic, and involuntary

response to an external stimulus.• Reflex arc: The pathway or neurons involved in a

reflex action.• The basic elements of the reflex arc are:

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2-Neuron Reflex Arc: eg. knee-jerk reflex (patellar reflex)•When the patellar tendon just below the knee is tapped (stimulus), the sensory receptors (stretch receptors in the quadriceps muscle) generate nerve impulses. The impulses move along the sensory neuron towards the spinal cord. In the spinal cord, the impulses pass to the motor neuron and travel along the motor neuron to the effector (quadriceps muscle of thigh), which cause muscle contraction that lead to extension of the lower leg at the knee (response).

3-Neuron Reflex Arc: eg. withdrawal of your hand away from a sharp object, etc.•If your hand touches a sharp pin (stimulus), sensory receptors in the skin generate nerve impulses. The impulses move along sensory neuron to the interneurons in the spinal cord. Then, the impulses travel along the motor neuron to the effector, which cause muscle contraction that withdraw your hand from the pin (response).•Various other reactions are also possible, (eg. look at the pin, wince, and cry out in pain) because some of the interneurons involved carry nerve impulses to the brain.

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2.1.5 Autonomic Nervous System• The efferent or motor division of the PNS is subdivided into

the somatic system and the autonomic system.• The somatic system serves the skin, skeletal muscles, and

tendons.• The somatic system includes nerves that take sensory

information from the external sensory receptors to the CNS and motor commands away from the CNS to the skeletal muscles.

• Somatic system controls reflexes and voluntary actions.

• The autonomic system of the PNS regulates the activity of cardiac and smooth muscle, and glands.

• The autonomic system is divided into the sympathetic and parasympathetic divisions.

• The common features between sympathetic & parasympathetic division are:

• 1. they controls involuntary actions.• 2. they innervate all internal organs.• 3. the utilize two neurons and one ganglion for each impulse.

The first neuron has a cell body within the CNS and a preganglionic fibre. The second neuron has a cell body within the ganglion and a postganglionic fibre.

Sympathetic Division•Most preganglionic fibres of the sympathetic division arise from the middle, or thoracic-lumbar, portion of the spinal cord and terminate in ganglia that lie near the cord. •Preganglionic fibre is short, but the postganglionic fibre is long.•Control involuntary actions during emergency situation.•The neurotransmitter released by the preganglionic fibre is acetylcholine, but the neurotransmitter released by the postganglionic fibre is norepinephrine.•Functions: (refer to the diagram)

Parasympathetic Division•Includes a few cranial nerves (eg. vagus nerve) and fibres that arise from the sacral portion of the spinal cord. •This division is often referred to as the craniosacral portion of the autonomic system•Preganglionic fibre is long, but the postganglionic fibre is short.•Control involuntary actions during relaxed situation.•The neurotransmitter released by both the preganglionic and postganglionic fibres is acetylcholine. •Functions: (refer to the diagram)

http://antranik.org/wp-content/uploads/2011/11/comparison-of-autonomic-and-somatic-motor-systems-cell-bodies-axons-effect-neurotransmittor-ach-ne.jpg

http://www.depure.org/nervous-working-system-in-human-body/autonomic-nervous-system/

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chapter 2.1 - nervous system

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