PANTOMATH

Introduction to the nervous system

 Functions:

• To respond to continuous changes in the internal and external environment.
• Control and integrate various activities in the body such as circulation and respiration.

Divisions of the nervous system

• Structurally:
  • CNS (Central Nervous System).
  • PNS (Peripheral Nervous System).
• Functionally:
  • SNS (Somatic Nervous System).
  • ANS (Autonomic Nervous System).
• CNS: Brain and spinal cord.
• PNS: Cranial nerves and spinal nerves.
• ANS: Sympathetic and parasympathetics.

Neuronal processes

• Arm like extensions emanating from every neuron.
• The CNS consists of both cell bodies and processes.
• The PNS mainly  consists of processes but there are localised collections of cell bodies called ganglia.

Unmyelinated neurons

• Unmyelinated nerve fibres contain Schwann cells that do not envelop them forming a lipid filled sheath
• Multiple axons are separately embedded within the cytoplasm of each Schwann cell

Types of Neurons

According to relative lengths of axons & dendrites :

1.Golgi type I neuron: Have long axons.

• Form long fibre tracts of brain & spinalcord.
• Pyramidal cells of cerebral cortex.
• Purkinje cells of cerebellum.            
• Anterior horn cells of spinal cord.

2. Golgi type II neuron: Micro neurons

• Short axons similar morphologically to dendrites.
• Found in cerebral & cerebellar cortex.
• Form synaptic contacts with neighbouring neurons.

Neuroglia

5 times as abundant as neurons

• Non neuronal, non excitable cells.
• Support, insulate & nourish the neurons.

In the CNS they are:

1. Astrocytes.
2. Oligodendrocytes.
3. Ependyma.
4. Microglia.

In the PNS they are:

• Schwann cells.
• Satellite cells.

• Nerves communicate with each other at synapses (points of contact between nerves).
• Communication occurs by means of neurotransmitters – chemical agents released by one nerve which may excite or inhibit another nerve.

Coverings of the CNS

• Meninges are in 3 layers from inside out:
  • Duramater.
  • Arachnoid mater.
  • Piamater.
• The meninges and the CSF surround and protect the CNS.
• CSF lies in the subarachnoid space.

   Peripheral nervous system

• Consists of nerve fibres and cell bodies outside the CNS.
• A neuron is a nerve cell body and a nerve fibre.
• A peripheral nerve fibre consists of an axon, its neurolemma (cells that immediately surround the axon and separate it from other axons) and the surrounding connective tissue.

Peripheral nerves are:

• Either sensory (afferent to the CNS) or motor (efferent).
• Either cranial nerves or spinal nerves.

• The unilateral muscle mass receiving innervation from the fibres conveyed by a single spinal nerve is a myotome.

AUTONOMIC NERVOUS SYSTEM

• Consists of motor fibres that stimulate smooth (involuntary) muscle, modified cardiac muscle (the intrinsic stimulating and conducting tissue of the heart), and glandular (secreting) cells.
• The efferent nerve fibres and ganglia of the ANS are organized into two systems or divisions:
  • The sympathetic (thoraco-lumbar) division.
  • The parasympathetic (cranio-sacral) division.
• Unlike somatic motor innervation, in both divisions of the ANS, conduction of impulses from the CNS to the effector organ involves a series of two multipolar neurons.
• The cell body of the presynaptic  (preganglionic) nerve is located in the grey matter of the CNS.
• Its axon synapses with the cell body of the postsynaptic (postganglionic) neuron.

• These cell bodies are located outside the CNS in autonomic ganglia.
• The fibres of this neuron end in the effector organ (smooth muscle, modified cardiac muscle or glands).
• The anatomical distinction between the two divisions of the ANS is based primarily on:
  • 1) the location of presynaptic cell bodies .
  • 2) which nerves conduct the presynaptic fibres from the CNS.
• The presynaptic neurons of both sympathetic an parasympathetic neurons liberate Acetylcholine as their neurotransmitter.
• The postsynaptic neurons of the two divisions liberate.
  • Noradrenaline by the sympathetic division (except sweat glands).
  • Acetylcholine by the parasympathetic division Sympathetic (thoracolumbar) division.
• The cell bodies of the presynaptic neurons are in the lateral horns or intermediolateral cell columns or nuclei of the spinal cord in the T1 –L2 or 3 segments of the spinal cord.

• The cell bodies of postsynaptic neurons of sympathetic neurons of the sympathetic NS occur in the paravertebral and prevertebral ganglia.
• Paravertebral ganglia – form right and left sypmpathetic trunks (chains) on each side of the vertebral column.
• It extends from the superior cervical ganglion at the base of the cranium to the ganglion impar which is located in front of the coccyx.

• Prevertebral ganglia are in the plexuses that surround the origins of the main branches of the abdominal aorta, according to which they are named.
•  e.g. the two coeliac ganglia which surround the origin of the coeliac artery.

• Postsynaptic sympathetic fibres are more numerous than the presynaptic fibres.
• Each presynaptic fibre synapses with 30 or more postsynaptic fibres.
• Those postsynaptic sympathetic fibres destined for distribution within the neck, body wall and limbs pass from the paravertebral ganglia of the sympathetic trunk to adjacent anterior rami of spinal nerves through grey rami communicantes.

• Splanchnic nerves convey visceral efferent (autonomic) and afferent fibres to and from the viscera of the body cavities.

• The postsynaptic fibres from the prevertebral ganglia form the periarterial plexus, which follows branches of the abdominal aorta to reach their destinations
• Postsynaptic sympathetic fibres are components of all branches of all spinal nerves.
• They pass via periarterial plexuses and extend to all the blood vessels of the body (the primary function of the sympathetic system) as well as to sweat glands, arrector pili and visceral structures.
• The presynaptic fibres are relatively short whereas the postsynaptic sympathetic fibres are relatively long.

Parasympathetic (craniosacral) division

• Presynaptic parasympathetic nerve cell bodies are located in two places:

1. In the grey matter of the brainstem and the fibres exit within the following cranial nerves: III, VII, IX and X. This is the cranial parasympathetic outflow.

        2. In the grey matter of the sacral segments of the spinal cord (S2-4).

• The fibres exit through the anterior roots of sacral spinal nervesS2-4 and then through the pelvic splanchnic nerves which arise from the anterior rami.
• This is the sacral parasympathetic outflow.

• The cranial outflow provides parasympathetic innervation of the head and the sacral outflow provides that for the pelvic viscera.
• Innervation of thoracic and abdominal viscera is mainly by the vagus nerve.
• It supplies the transverse colon till the junction of its right 2/3rd and left 1/3rd.
• The sacral parasympathetic outflow supplies the left 1/3rd of the transverse colon and the colon distal to it.
• The parasympathetic system is much more restricted than the sympathetic system in its distribution.
• It is distributed to the head, visceral cavities of the trunk and erectile tissue of the external genitalia.
• With the exception of the latter it does not reach the body wall or limbs, and except for the initial parts of the anterior rami of spinal nerves S2-4, its fibres are not part of spinal nerves or their branches.

Functions and divisions of the ANS

• Both sympathetics and parasympathetics innervate involuntary structures.
• They have different usually contrasting, yet  co-ordinated effects.
• In general the sympathetic system is a catabolic (energy expending) system that enables the body to deal with stress, e.g. when preparing the body for the fight or flight response.
• The parasympathetic system is primarily a homeostatic or anabolic (energy conserving) system. It promotes the quiet and orderly processes of the body, such as those that allow the body to feed and assimilate.
• The primary function of the sympathetic system is to regulate blood vessels.
• Blood vessels are tonically innervated by sympathetic nerves, maintaining a state of moderate vasoconstriction.
• In most vascular beds, an increase in sympathetic impulses causes an increase in vasoconstriction and a decrease causes vasodilation.
• However in certain regions of the body sympathetic impulses cause vasodilation (i.e. the transmitter substances inhibit active vasoconstriction allowing the blood vessels to be passively dilated).
• In the coronary vessels, the vessels of skeletal muscles and the external genitals, sympathetic stimulation results in vasodilation.

Visceral sensation

• Visceral afferent fibres have a sensory input (at a subconscious level) which provides information about the internal environment of the body.
• This information is integrated in the CNS and often triggers visceral or somatic reflexes or both.
• Visceral reflexes regulate blood pressure and chemistry altering the function of the heart and lungs and vascular resistance.
• Visceral sensation that reaches a conscious level is generally perceived as pain that is either poorly localized or felt as cramps of that may convey a feeling of hunger, fullness or nausea.