Organization of Nervous System

Nervous System

Two control systems of the body:
1. Nervous system: “System of wires”
Coordinates rapid & precise responses
2. Endocrine system: “Wireless system” through blood
Regulates activities that rerequire duration rather than speed.

Despite separate areas of specialization, both systems are interconnected functionally;
 e.g. Neurohormones
 Release of hormones by Nerve Signals
 Hormones ——as neuro modulators
 Sometimes Nerve signals & Hormones act on the same target cells e.g. GIT, Bl. vessels, Heart etc.

Organization of Nervous System

The Nervous system comprises of:
CNS — Brain & Spinal cord
PNS — Nerve fibers that carry information between CNS & Other parts of the body

Three classes of Neurons :- “Functional”

1. Afferent: Apprise CNS about Ext & Int Environment (Receptors)
2. Efferent: Carry instructions from CNS to effectors
3. Interneuron: Integrate Afferent information, to formulate efferent
response. Approx; 100 billion in human CNS i.e. 99% of all Neurons.

Peripheral Nervous System (PNS)

PNS has two divisions;
1. Afferent Division (Sensory)
2. Efferent Division (Motor)

a. Somatic Nervous system (Voluntary):

 Cell bodies within ventral horn of Spinal cord
 Fibers of motor neurons to Sk. Muscles
 Release Acetyl choline
 Excitatory in action
 Inhibition within CNS (Through activation of inhibitory synaptic input)

b. Autonomic Nervous system (Involuntary):

Innervate Smooth & Cardiac muscles and Glands
(1) Sympathetic
(2) Parasympathetic

Motor Part of Nervous System:

1. Somatic

2. ANS

 Effectors:

a. Muscles

b. Glands
 Eventual role of nervous system is to control various bodily activities
i.e.
1. Contraction of Sk muscles
2. Contraction of smooth muscles
3. Secretion of chemicals by exocrine & endocrine glands
 Sk. Muscles can be controlled by;
1. Spinal Cord
2. Reticular subs of Medulla, Pons & Mesencephalon
3. Basal Ganglia
4. Cerebellum
5. Motor cortex

Major Levels of CNS Function

Three Major Levels:-

A. Spinal Cord Level:

1. Conduit for signal from periphery
2. Centers/Neuronal circuits for;
a. Walking movements.
b. Withdrawal reflex
c. Support against gravity
d. Control of local blood vessels,
GIT, Urinary excretion

B. Sub cortical or Lower Brain Level:

 Medulla, Pons, Mesencephalon, Thalamus, Hypothalamus, Cerebellum, Basal ganglia.
 Control subconscious activities of the body
a. Arterial pressure
b. Respiration
c. Equilibrium
d. Feeding reflexes (Salivation, licking of lips)
e. Emotional responses (Anger, Excitement, Sexual response, Response to pain, Pleasure etc)

C. Cortical or Higher Brain level

Cerebral cortex is the largest memory store house.
 Cortex never functions alone, always with lower centers
 Essential for most thought processes, but not alone
 Lower brain centers initiate wakefulness in cortex to open bank of memories of brain
Upper levels of nervous system operate by sending signals to control centers of “cord” i.e. “commanding” the cord centers to perform their functions

The Neuron

Anatomic & Functional unit of nervous system
 Neuron consists of :

Cell body (Soma) & Processes (Dendrites & Axons i.e. Nerve fibers)
 Myelinated fibers:
 Unmyelinated Fibers:

Properties of a Nerve Fiber

 Excitability
 Conductivity
 All or None law
 Refractory period
 Summation
 Accommodation / Adaptation
 Indefatigability
 Compound Action Potential
 Monophasic AP
 Biphasic AP

Protection of Brain

Brain is protected by:
i. 3 layers of Meninges (Wrap the brain)
ii. Hard bony cage outside
iii. CSF inside & outside (Cushion against jarring)
iv. Chemical protection by Bl. Br. Barrier

Nourishment of Brain:

 O2 & Glucose
 Through Blood supply

Supporting Cells

 90% of cells in CNS (10-50 times the no. of neurons)
 Occupy ½ Vol. of brain
 Physical & Metabolic support to neurons
 Do not branch as neurons do
 Do not initiate or conduct nerve impulses
 Help in growth & nourishment of Neurons
 Serve as CT of CNS, derived from ectoderm
 Homeostatically maintain composition of ECF of Neurons
 Actively modulate synaptic function

Two types of supporting cells in PNS:
i. Schwann cells: form myelin sheath around peripheral
nerves
ii. Satellite cells or ganglionic gliocytes: support cell bodies in ganglia of PNS
Four types of supporting cells in CNS:
[Glial Cells: Neuroglia (Gk; glia = glue)] 1. Astrocytes:

1. Most abundant, occupy mainly white matter.
2. Physically support neurons in proper spatial relationship
3. Send processes to:
(a) cover nerve cells & synapses.
(b) blood vessels (capillaries), forming
tight junctions to form BBB
4. Form neural scar tissue
5. Take up & degrade neurotransmitter (e.g. Glutamates, GABA)
6. Take up excess K+ to maintain ECF conc. /Neural excitability
7. Enhance synapse formation/Synaptic transmissionii. 2. Oligodendrocytes: Myelination of CNS neurons
3. Microglia: Immune defense cells/ Macrophages of CNS
4. Ependymal cells: * Line internal cavities of CNS
* Formation of CSF
* Neural stem cell, have potential to form other
glial cells & neurons ? e.g. Hippocampus (Memory &
learning)

Blood Brain Barrier

 The endothelial cells of brain capillaries are joined by tight junctions that prevent materials for passing between the cells.
 The only passage across brain capillaries is through endothelial cells by diffusion, active transport, endocytosis pinocytosis [selective bloodbrain
barrier].
 Difficulty in chemotherapy of brain

Cerebrospinal Fluid (CSF)

 Shock absorbing fluid [Surround & cushion the brain & spinal cord]  Same density as brain
 Formed by choroid plexuses (in ventricles) By selective transport processes
 Volume = 150 ml
 Formation = 550 ml/day [3.7 times turn over/day]  Composition of CSF & brain interstitial fluid is same
 CSF pressure = 10 mmHg
 Excess accumulation of CSF [Hydrocephalus; water on the
brain] → brain damage, mental retardation