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Monday, 19 November 2012

Human Brain : Most Advance Biological Machines of Universe Part (2)




Major Human Nerves




Motor Nerve Pathway


THE PRIMARY MOTOR AREA:
This lies in the frontal lobe immediately anterior to the central sulcus.The cell bodies are pyramid shaped and they initiate the contraction of skeletal muscles.A nerve fiber from a Betz`s cell passes downwards through the internal capsule to the medulla oblongata where it crosses to the opposite side then descends in the spinal cord.At the appropriate level in the spinal cord the nerve impulse crosses a synapse to stimulate a second neurone that terminates at the motor end plate of the muscle fiber.This means that the motor area of the right hemisphere of the cerebrum controls voluntary muscle movement on the left side of the body and vice versa .Damage to either of these neurons may result in paralysis. 


Sensory Nerve Pathway


SENSORY NERVE PATHWAYS:
Neurons that transmit impulses towards the brain are sensory and also known as different or ascending neurons.There are two main sources of sensation transmitted to the brain via spinal cord.

1) The skin:
Sensory receptors(nerve ending) in the skin, called cutaneous receptors, are stimulated by pain,heat,cold and touch,including pressure.Nerve impulses generated are conducted by three neurones to the sensory area in the opposite hemisphere of the cerebrum where the sensation and its location are perceived.Crossing to the other side,or decussation,occurs either at the level of entry into the cord or in the medulla.

Sensory Nerve Pathway


2) The tendons,muscles and joints:
Sensory receptors are specialised nerve endings in these structures called propioceptors and they are stimulated by
stretch.Together with impulses from the eyes and the ears they are associated with the maintenance of balance and posture and with the perception of the position of the body in space.These nerve impulses have two destination.
·                     By 3 neuron system, the impulses reach the sensory area of the opposite hemisphere of the cerebrum.
·                     By 2 neuron system,the nerve impulses reach the cerebellar hemisphere on the same side.


Major Human Nerves




TYPES OF NERVES:
Sensory or afferent nerves:
When action potentials are generated by sensory receptors on the dendrites of these neurons,they are transmitted to the spinal cord by the sensory nerve fibers. The impulses may then pass to the brain or to connector neurons of reflex arcs in the spinal cord.
Sensory receptors:
Specialised endings of sensory neurons respond to different stimuli inside and outside the body.
Somatic,cuteneous or common senses:
These originate in the skin. They are pain, touch,heat and cold.These nerve endings in the skin are fine branching filaments without myelin sheaths. When stimulated, an impulse is generated and transmitted by the sensory nerves to the brain where the sensation is perceived.
Proprioceptor senses:
These originate in muscles and joints and contribute to the maintenance of balance and posture.
Special senses:
These are sight,hearing,balance,smell and taste.
Autonomic afferent nerves:
These originate in internal organs,glands and tissues, e.g. baroreceptor,  chemoreceptor and are associated with
reflex regulation of involuntary activity and visceral pain.

Motor or efferent nerves:
These nerve originate in the brain,spinal cord and autonomic ganglia. They transmit impulses to the effector organs:
muscles and glands.They are two types:-
Somatic nerve:
Involved in voluntary and reflex skeletal muscle contraction.
Autonomic nerves:(sympathetic & parasympathetic)
Involved in cardiac and smooth muscle contraction and glandular secretion.

Mixed nerves:
In the spinal cord,sensory and motor nerves are arranged in separate groups or tracts.Outside the spinal cord,when sensory and motor nerves are enclosed within the same sheath of connective tissue they are called mixed nerves.




PLEXUSES:
In the cervical lumbar and sacral regions the anterior rami unite near their origins to form large masses of nerves, or plexuses,where nerve fibres are regrouped and rearranged before proceeding to supply skin ,bones muscles and joints of particular area.These means that these structures have a nerve supply from more than one spinal nerve and therefore damage to one spinal nerve does not cause loss of function of a region. In the thoracic region the anterior rami do not form plexuses. There are 5 large plexuses of mixed nerves formed on each side of the vertebral column. They are the:
·                     Cervical plexuses
·                     Brachial plexuses
·                     Lumbar Plexuses
·                     Sacral plexuses
·                     Coccygeal plexuses

Cranial Nerves


CRANIAL NERVES:

1) OLFACTORY NERVES (SENSORY):
These are the nerve of sense of smell. Their sensory receptor and fibres originate in the upper part of the mucous membrane of the nasal cavity.Pass upwards through the cribriform plate of the ethmoid bone and then go to the olfactory bulb.The nerve then proceed backwards as the olfactory tract,to the area for the perception of smell in the temporal lobe of the cerebrum.

2) OPTIC NERVES (SENSORY):
These are the nerves of sense of sight.The fibres originate in the retinae of the eyes and they combine to form the optic nerves.They are directed backward and medially through the posterior part of the orbital cavity.They then pass through the optic foramina of the sphenoid bone into the cranial cavity and join at the optic chiasma.The nerves proceed back-wards as the optic tracts to the lateral geniculate bodies of the thalamus.Impulses pass from these to the centre for sight
in the occipital lobes of the cerebrum and to the cerebellum.In the occipital lobe sight is perceived,and in the cerebellumthe impulses from the eyes contribute to the maintaenance of balance,posture and orientation of the head in space.

3) OCULOMOTOR NERVES (MOTOR):
These nerves arise from nuclei near the cerebral aqueduct.They supply:
·                     Four of the six extrinsic muscles,which move the eyeball, i.e. the superior,medial and inferior recti and inferior oblique muscle.
·                     The intrinsic muscles: Ciliary muscle,which alter the shape of lens,changing its refractive power. Circular muscle,of the iris,which contrict the pupil.

·                     The levator palpebrae muscles, which raise the upper eyelid.

4) TROCHLEAR NERVES (MOTOR):
 These nerves arise from nuclei near the cerbral aqueduct. They supply the superior oblique muscles of the eyes.


5) TRIGEMINAL NERVES (MIXED):
These nerves contain motor and sensory fibres and are among the largest of the cranial nerves.They are the chief sensory nerve for the faces and head(Including the oral and nasal cavities and teeth) receiving impulses of pain,temparature and touch.The motor fibres stimulate the muscles of mastication. As name suggests,there are 3 main branches of the trigeminal nerves. The dermetatomes innervated by the sensory fibres .

The opthalmic nerve  are sensory only and supply the lacrimal glands,conjunctiva of the eyes,forehead,eyelids, anterior aspect of the scalp and mucous membrane of the nose.The maxillary nerves are sensory only and supply the cheeks ,upper gums,upper teeth and lower eyelids.

The mandibular nerves contain both sensory and motor fibres.These are the largest of the 3 divisions and they supply the teeth and gums of the lower jaw,pinnae of the ears lower lip and tongue.The motor fibres supply the muscles of mastication.

6) ABDUCENT NERVES (MOTOR):
These nerves arise from nuclei lying under the floor of the 4th ventricle.They supply the lateral rectus muscle of the eyeballs.

7) FACIAL NERVES (MIXED):
The nerves are composed of both motor and sensory nerve fibres,arising from nuclei in the lower part of the pons.The motor fibres supply the muscles of the facial expression.The sensory fibres convey impulses from the taste buds in the anterior two third of the tongue of the taste perception area in the cerebral cortex.


8) VESTIBULOCOCHLEAR NERVES (AUDITORY)(SENSORY):
These nerves are composed of two distinct sets of fibres,vestibular nerves and cochlear nerves.
·                     The Vestibular nerves arise from the semicircular canals of the inner ear and convey impulses to the cerebellum.
            they are associated  with the maintenance of posture and balance.
·                     The Cochlear nerves originate in the spiral organ in the inner ear and convey impulses to the hearing area in the
             cerebral cortex where sound is perceived.

9) GLOSSOPHARYNGEAL NERVES (MIXED):
The motor fibres arise from nuclei in the medulla oblongata and stimulate the muscles of the tongue and pharynx and the secretory cells of the parotid glands. The sensory fibres convey impulses to the cerebral cortex from the posterior third of the tongue. The tonsil and pharynx and from taste buds in the tongue and pharynx.These nerves are essential for the swallowing and gag reflexes.

10) VAGUS NERVES(MIXED):
These nerves have a more extensive distribution than any other cranial nerves.They pass down through the neck into the thorax and the abdomen.These nerves form an important part of the parasympathetic nervous system.The motor fibres arise from nucleiin the medulla and supply the smooth muscle and secretory glands of the pharynx larynx ,trachea,heart, oesophagus,stomach ,intestine,pancreas , gall bladder, bile duct ,spleen ,kidney,ureter and blood vessels in the thoracic and abdominal cavities.

11) ACCESSORY NERVES(MOTOR):
These nerves arise from nuclei in the medulla oblongata and in the spinal cord.The fibres supply sternocleidomastoid and trapezius muscles. Branches join the vagus nerves and supply the pharyngeal and laryngeal muscles.

12) HYPOGLOSSAL NERVES (MOTOR):
These nerves arise from nuclei in the medulla oblongata.They supply the muscles of the tongue and muscles surrounding the hyoid bone and contribute to swallowing and speech.


Arteries in Brain


AUTONOMIC NERVOUS SYSTEM:
The autonomic or involuntary part of the nervous system controls the 'automatic' functions of the body,i.e. initiated in the
brain below the level of the cerebrum. Although stimulation does not occur voluntarily, the individual may be conscious
of its effects, e.g. an increase in their heart rate the effect of autonomic activity are rapid and the effector organs rate:

·                     Smooth muscle, e.g. changes in airway or blood vessel diameter.
·                     Cardiac muscle, e.g. changes in rate and force of the heartbeat.
·                     Glands, e.g. increasing or decreasing gastrointestinal secretions.

The efferent (motor) nerves of the autonomic nervous system arise from nerve cells in the brain and emerge at various levels between the midbrain and the sacral region of the spinal cord.Many of them travel within the same nerve sheath
as the peripheral nerves of the central nervous system to reach the organs that they innervate.
     The autonomic nervous system is separated into 2 division:-
·                     Sympathetic (Thoracolumbar outflow)
·                     Parasympathetic (Craniosacral outflow)


Sympathetic Nervous System


SYMPATHETIC NERVOUS SYSTEM:

Neurones impulses from their origin in the hypothalamus,reticular formation and medulla oblongata to effector organs and tissues. The 1st neurone has its cell body in the brain and its fibres extends into the spinal cord. The preganglionic and post ganglionic neurones then conduct sympathetic impuses to effector organs.

The pre Ganglionic neurone:
This has its cell body in the lateral column of grey matter in the spinal cord between the lavels of the 1st thoracic and 2nd or 3rd lumber vertebrae. The nerve fibres of this cell leaves the cord by the anterior root and terminates at a synapse in one of the ganglia either in the lateral chain of sympathetic ganglia or passes through it to one of the pre vertebral ganglia. Acetylcholine is the neuro transmitter at sympathetic ganglia

The Post Ganglionic neurone:
This has its cell body in a ganglion and terminates in the organs or tissue supply noradrenaline is usually the neurotransmitters at sympathetic effector organ.The major exception is that their is no parasympathetic supply to the sweat glands, the skin and blood vessel of skeletal muscles.This structure are supplyed by only sympathetic post-ganglionic neuron which usually has acetylcholine as their neurotransmitter they have therefore, the effects of both sympatheticand parasympathetic nerve supply.
                                                      
There are 3 prevertebral ganglia situated in the abdominal cavity close to the origins
of the arteries of the same names:

·                     COELIAC GANGLION:
·                     SUPERIOR MESENTERIC GANGLION:
·                     INFERIOR MESENTERIC GANGLION:

The ganglia consists of nerve cell bodies rather diffusely distributed among a network of nerve fibres that form plexuses


                                                       
AUTONOMIC NERVOUS SYSTEM:
The autonomic or involuntary part of the nervous system controls the 'automatic' functions of the body,i.e. initiated in the
brain below the level of the cerebrum. Although stimulation does not occur voluntarily, the individual may be conscious
of its effects, e.g. an increase in their heart rate the effect of autonomic activity are rapid and the effector organs rate:

·                     Smooth muscle, e.g. changes in airway or blood vessel diameter.
·                     Cardiac muscle, e.g. changes in rate and force of the heartbeat.
·                     Glands, e.g. increasing or decreasing gastrointestinal secretions.

The efferent (motor) nerves of the autonomic nervous system arise from nerve cells in the brain and emerge at various levels between the midbrain and the sacral region of the spinal cord.Many of them travel within the same nerve sheath as the peripheral nerves of the central nervous system to reach the organs that they innervate.
     The autonomic nervous system is separated into 2 division:-
·                     Sympathetic (Thoracolumbar outflow)
·                     Parasympathetic (Craniosacral outflow)

PARASYMPATHETIC NERVOUS SYSTEM


PARASYMPATHETIC NERVOUS SYSTEM:
Two neurones (Preganglionic and postganglionic) are involved in the transmission of impulses from their source to the
effector organ.The neurotransmitter at both synapses is acetylcholine.

The preganglionic neurone:
This is ususally long in comparison to its counterpart in the sympathetic nervous system and has its cell body either in the brain or in the spinal cord.Those originating in the brain are the cranial nerves 3rd 4th 9th and 10th arising from nuclei in the midbrain and brain stem, and their nerve fibres terminate outside the brain.The cell bodies of the sacral ouflow are in the lateral columns of grey matter at the distal end of the spinal cord.Their fibres laeve the cord in sacral segments 2, 3 and 4 and ynapse with postganglionic neurones in the walls of pelvic organs.

The postganglionic neurone:
This is usually very short and has its cell body either in a ganglion or, more often, in the wall of the organ supplied.



Major Human Nerves



video
                                                                                      3D Animation showing different parts of Human Brain

                                                        
video
                                                                                               3D Animation showing nerves within spinal cord

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