Nervous System

Last Updated on Tuesday, 7 February 2012 10:53 Written by Sandesh Monday, 5 April 2010 07:23

Nervous System

Human nervous system can be divided into three parts:

1. Central nervous system
2. Peripheral nervous system
3. Autonomic nervous system

CENTRAL NERVOUS SYSTEM:
Central nervous system includes brain and spinal cord.

Brain: Brain is enclosed and protected in a bony vault called cranial cavity. Brain including spinal cord is covered by three layered meninges- innermost highly vascular piamater, middle arachnoid mater and outermost fibrous dura mater. The space beween piamater and arachnoid mater is called as sub arachnaoid space. This space is filled with cerebrospinal fluid.

Brain has mainly three regions

1. Forebrain(prosencephalon)
2. Midbrain (mesencephalon)
3. Hindbrain (rhombencephalon)

1. Forebrain (prosencephalon): It has again two parts- Telencephalon or Cerebrum and Diencephalon.

i. Telencephalon (cerebrum): There are two cerebral hemispheres which are connected by corpus callosum. Cerebrum has outer grey mater (made up of cell body of a neuron) and inner white mater (made up of axon). Greymater has numerous foldings in which the raised parts are called gyri and depressions are called as sulci. One of the hemispheres right or left is dominant hemisphere which contains language centres. Each cerebral hemisphere is further divided into frontal lobe, parietal lobe, temporal lobe and occipital lobe.

Functional areas of cerebrum:

1.    Frontal lobe:

a. Motor areas lies anterior to central sulcus.

• • • It controls voluntary movement of skeletal muscles.

b. Broca’s speech areas:

• • • Controls the production of speech.

c. Prefrontal Cortex:

• • • Involved in concentration, orientation, abstract thinking, judgment, decision making and problem solving.

2. Parietal lobe:

a. Sensory area:

• • Receives all kinds of sensory nerves from all parts of body.
  • Interprets the sensory stimuli like heat, cold, pain, touch, light and pressure.
  • Has areas of reading and writing and for mathematical calculations.

3. Temporal lobe:

1. • Auditory area: for hearing.
   • Wernicke’s speech: for understanding of speech.
   • Olfactory Cortex: for interpretation of smell.
   • Hippocampal Cortex: for long term memory.

4. Occipital lobe:

Visual Cortex: interprets images perceived by eyes.

ii. Diencephalon: Gr- dia- through

• Middle structure hidden by cerebral hemisphere.
• Includes thalamus and hypothalamus

a. Thalamus: main part of diencephalon

• • Its cavity is called 3^rd ventricle
  • It constitute 2 large masses of grey mater into the lateral walls of the ventricle
  • It serves as relay centre for transmission of sensory impulses from the spinal cord to cerebral cortex.
  • It takes part in formation of recent memory and emotion.

b. Hypothalamus:

• • It lies just below thalamus
  • Has attached pituitary gland
  • It controls and co ordinates body activities by both neural and hormonal components
  • Neural: controls thirst, hunger, temperature regulation, circadian rhythm.
  • Hormonal: produces hormones to regulate pituitary secretion. Hormones oxytocin and ADH are synthesized by hypothalamus and released from pituitary gland.

2. Mid brain(Mesencephalon):

• It connects the forebrain with the hind brain.
• It consists of group of nerve cells, grey mater scattered in white mater.
• It relays auditory information
• It controls reflex movement of eyes, head and neck in response to visual stimuli.
• It controls constriction of pupil in response to light.
• It contains central nuclei for CN III and IV.

3. Hind brain( Rhombencephalon):

It Consist of pons varolii, cerebellum and medulla oblongata.

1. Pons varolii:  means bridge, which connects midbrain with medulla.

• • Contains central nuclei for CN V, VI, VII and VIII.
  • Assists medulla in controlling spontaneous respiration.

2. Cerebellum: It is situated at the back of the brain under cerebral hemispheres.

• • It is the 2^nd largest part of brain.
  • It consists of two similar cerebellar hemisphere united my median vermis.
• • It has multiple parallel folds of cerebellar cortex called folia.
  • Cerebellum is called as gyroscope of our body because it is involved in control of balance, equilibrium and posture.
  • It helps in movement of skeletal muscles
  • Control of eye movement.
  • It is also involved in control of learned activities.

3. Medulla Oblongata: extends from Pons to spinal cord.

• • Contains central nuclei for CN IX, XI and XII.
  • Contains respiratory and vasomotor centres.
  • These have autonomic reflex control of of respiration, heart rate and blood pressure.
  • Damage of these centres is fatal.

Spinal cord:

• Elongated cylindrical part of CNS.
• Extends from 1^st cervical vertebrae to the 1^st lumber vertebrae.
• 45cm long
• Posterior end is bulged called as conus medullaris.
• Lies within vertebral canal.
• Gives rise to 31 pairs of spinal nerves.

8 pairs of cervical nerves – C1 to C8

12 pairs of thoracic nerves – T1 to T12

5 pairs of lumber nerves – L1 to L5

5 pairs of sacral nerves – S1 to S5

1 pair of coccygeal nerves – C1

Internal features of spinal cord:

• Cross section of spinal cord shows and ‘H’ shaped grey mater mass surrounded by white mater on the outer side.
• White mater contains tracts which carry axons to and fro from spinal cord to higher centres (brain).
• Grey mater has two ventral horns and two dorsal horns.
• Ventral horns give rise to motor neurons(motor root)
• Dorsal horns give receives sensory nerves(sensory root)
• Dorsal and ventral roots combine to form a spinal nerve.

Ventricular system:

• These are cavities inside CNS.
• They are filled with cerebrospinal fluid secreted by their own walls.
• There are four interconnected ventricles:

Lateral ventricles- 2 in number, 3^rd ventricle and 4^th ventricle.

• One lateral ventricle is present deep within each hemisphere.
• It communicates with 3^rd ventricle via inter ventricular foramen of Monro.
• The 3^rd ventricle lies in the diencephalons.
• 3^rd and 4^th ventricles are connected by cerebral aqueduct of Sylvius.
• The 4^th ventricle lies posterior to pons and medulla and anterior to cerebellum.
• 4^th ventricle communicates with subarachnoid space via foramina of Luschka and Magendie.

Cerebrospinal fluid (CSF):

• CSF is present within the ventricles and in the subarachnoid space.
• Secreted by choroids plexus of ventricles.
• In normal adult CSF is about 90 to 150ml.
• CSF is a clear fluid different than serum in –

Higher concentration of Cl- and Mg- ions while lower concentration of Na+.

• No RBC and WBC.
• CSF enters subarachnoid space through foramina of Luschka and Magendie, and also absorbed by arachnoid villi into veins.

Functions:

• Protective cushion to brain and spinal cord.
• Mechanical support to brain and spinal cord.
• Supplies nutrients to brain and spinal cord.
• Helps in excretion of waste products.

PERIPHERAL NERVOUS SYSTEM:

• Consists of those nerves which interconnect the CNS and target organs and muscles.
• Consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves.

1. Cranial nerves: it may be sensory, motor or mixed.
   1. CN I (Olfactory nerve): sensory nerve for smell. It originates from bipolar neurons of olfactory epithelium of nasal cavity to olfactory bulb.
   2. CN II (Optic nerve): sensory nerve for vision. Originates from retina of eye and     projects to primary visual cortex.
   3. CN III (Oculomotor nerve): motor nerve to control movement of eye. It innervates muscles involved in eye movement.
   4. CN IV (Trochlear nerve):  motor nerve involved in eye movement. Supplies to superior oblique muscle of eye.
   5. CN V (Trigeminal nerve): mixed nerve. The motor fibre supply to muscles of mastication (chewing). Sensory fibres receive tactile,pain and thermal sensation from face, oral and nasal cavity.
   6. CN VI (Abducent nerve): motor nerve involved in lateral movement of eye. Innervates lateral rectus muscle of eyeball.
   7. CN VII (Facial nerve): mixed nerve. The motor fibres innervates muscles of facial expression. The sensory fibres receive taste sensation from andterior 2/3^rd of tongue. It also carries parasympathetic nerve fibres ( innervate lachrymal, submandibular and sublingual glands).  For facialexpression of emotion eg smiling, tear and saliva secretion and taste sensation.
   8. CN VIII (Vestibulocochlear nerve): sensory nerve. Vestibular nerve for equilibrium and cochlear nerve for hearing.
   9. CN XI (Glossopharyngeal nerve): mixed nerve. Motor part to muscle of pharynx. Sensory part to external ear, tongue and salivary gland.
   10. CN X (Vagus nerve): mixed nerve. Motor fibres to larynx. Sensory fibres to mucus membrane of pharynx, larynx and trachea, thoracic and abdominal viscera.
   11. CN XI (Accessory nerve): Motor nerve to muscles of neck. Turning head and lifting of shoulders.
   12. CNXII (Hypoglossal nerve): Motor nerve to tongue for the tongue movement.

2. Spinal nerves:

• • 31 pairs.
  • All are mixed nerves having sensory and motor components.
  • 8 pairs of cervical nerves – C1 to C8
  • 12 pairs of thoracic nerves – T1 to T12
  • 5 pairs of lumber nerves – L1 to L5
  • 5 pairs of sacral nerves – S1 to S5
  • 1 pair of coccygeal nerves – C1

AUTONOMIC NERVOUS SYSTEM:

• It constitutes involuntary part of nervous system.
• It serves to the body which are not under voluntary control such as heart rate, peristalsis etc.
• Gives off motor nerves to smooth muscles, cardiac muscles and glands.
• Consists of 2 divisions – sympathetic and parasympathetic.

Sympathetic nervous system:

• It outflows from thoraco-lumber region spinal cord to T1 to L2.
• In SNS neurotransmitter released in effector organs are noradrenaline
• It is activated during stress, danger, activity.
• As a result of this there is increase in metabolic rate, heart beat rate, blood pressure and sensory awareness.
• Other effects are dilation of pupil, inhibition of saliva and digestive enzymes, dilation of bronchioles to increase ventilation rate, standing of hair due to contraction of erector muscles of skin etc.

Parasympathetic nervous system:

• It outflows from cranial nerves to sacral nerves.
• In PNS neurotransmitters released are acetylcholine.
• It is activated during rest and routine activities.
• As a result of this there is decrease in metabolic rate, heart beat rate, blood pressure and sensory awareness.
• Other effects are constriction of pupil, stimulate secretion of saliva, digestive enzymes and tear, decreases ventilation in lungs etc.

Neurotransmitter:

• Acetylcholine is used by both preganglionic and post ganglionic neurons of parasympathetic nervous system.
• Nor-epinephrine is used by post ganglionnic sympathetic neurons.

Functions of ANS:

• Sympathetic nerves are activated in excited or emergency states e.g. Fighting.
• Parasympathetic nerve is active in relaxed state e.g. A person eating in relaxed state, then there is increase in gastro-intestinal secretion and mobility.

Nerve impulse: It is defined as wave of depolarization of the membrane of the nerve cell.

Conduction of nerve impulse along with nerve fibre:

Nerve cell is surrounded by extracellular fluid from outside and intracellar fluid from inside. These fluids contain numerous solutes which are electrically charged. Changes in the concentaration of these solutes cause the conduction of nerve impulse from one part of neuron to another. It can be studied in three steps.

1. Polarization (resting potential): when the nerve is not conducting an impulse, extracellular fluid has high concentration of Na ion and low concentration of K ion. In contrast to that, intracellular fluid has high concentration of K ion and low concentration of Na ion. Intracellular fluid also contains negatively charged protein molecules. As a result of these differences in ion concentration, plasma membrane is positively charged outside and negatively charged inside. This difference is called potential difference. The resting potential of neuron is about -70mV.
2. Depolarization (action potential): change in the membrane potential towards more positive value is called depolarization. Action potential follows all or none law. If a stimulus crosses threshold intensity then there is action potential and if stimulus is below threshold, no action potential is generated. The potential of the area is changed due to stimulus. The membrane becomes permeable to Na+ ions, so the Na+ ions enter inside of membrane. This causes depolarization where the membrane is negatively charged outside and positively charged inside. Depolarization of the membrane passes forward as a wave along the nerve fibre.
3. Repolarization: Suddenly after depolarization, membrane becomes more permeable to K+ ion and less permeable to Na+ ion. Therefore, K+ ion move out of membrane resulting in rapid repolarization. During this   stage the membrane is again positively charged outside and negatively charged inside.

After repolarization, original resting potential are established by Na/K ATPase pump. During this, Na+ ions are pumped outside of membrane while K+ ions are pumped inside of the membrane. During this state, cell is back to resting potential to receive another stimulus.

This sequence movement of nerve impulses along nerve fibre is uninterrupted in non-myelinated axon. In myelinated nerve, myelin acts as an insulator, so the depolarization jumps from one node of Ranvier to another. This is a very rapid process and called as saltatory conduction. Conduction in myelinated nerve is 50 times faster than in non-myelinated nerve

Refractory period: The period after action potential during which a normal stimulus cannot create another action potential is called refractory period. It has two parts:

1. Absolute refractory period: The period during which no matter how strong the stimulus is, second action potential cannot be generated.
2. Relative refractory period: The period during which a greater than normal stimulus is required to produce second action potential.

Conduction of nerve impulse along with synapse:

When the nerve impulse arrives at pre synaptic knob, the voltage gated calcium channels are opened. So, Ca+ ions enter the presynaptic cleft which causes movement of synaptic vesicles containing neurotransmitters. Synaptic vesicles fuse with pre synaptic cleft and rupture releasing the neurotransmitters in the synaptic cleft. The neurotransmitters bind with protein receptors of post synaptic cleft. This causes depolarization of the membrane of post synaptic cleft. Thus, the impulse is transmitted to another neuron.

Neurotransmitters (acetylcholine) are immediately hydrolysed into acetate and choline by an enzyme called acetylcholinesterase. Acetate and choline are reabsorbed by pre synaptic cleft where they are again transformed into acetylcholine by using ATP.