The Nervous System

The Nervous System

·        The nervous system is very important for maintaining homeostasis of the human body.

·        It controls and maintains the human body, and it gives quick responses.

·        The nervous system has three main functions.

1.      Sensory

2.      Integration

3.      Motor

Sensory: The chemical messenger relay into the brain.

Integration: To analyse and process the sensory input.

Motor: To respond the message to appropriate organs.

Somatic nervous system (Voluntary): This system consists of sensory neurons that conducts impulse from somatic and special sense receptors to the CNS and motor neurons from the CNS to skeletal muscle.

Autonomic nervous system
(Involuntary): It contains sensory neurons from motor neurons that convey impulses from CNS to skeletal muscle tissue or different periphery and glands.

Example: Breath, Digestion, Heart rate etc.

Nervous system consists of two specific type of cells

1.      Neurons

2.      Neuroglia

Neuroglia cells

·        It is non-neuronal cells (doesn’t make neurons structure)

·        Neuroglia cells provides mechanical and metabolic support to the neurons.

·        These cells perform support roles for the neurons to ensure their survival.

 

In CNS

Astrocytes cells

·        It is a star-like appearance, so that it called astrocytes.

·        The cell body is usually attached between a blood capillary and a neuron of the brain.

·        The astrocytes regulate flow of ions, sugar, oxygen and carbon dioxide into and out of the neurons.

·        Astrocytes prevent toxic substances from entering brain neurons.

Ependymal cell

·        The ependymal cell is usually cube-shaped.

·        These cells are ciliated-epithelial glial cells lining the spinal cord and cerebral ventricles.

·        It produces cerebrospinal fluid.

·        It protects the brain and spinal cord both mechanically and immunologically.

·        Ependymal cells contain cilia that help to circulate cerebrospinal fluid.

Microglial cells

·        Microglial cells are the smallest neuroglial cells and can migrate around the CNS.

·        It supports neurons by phagocytosis of bacterial cells and dead cell debris.

Oligodendrocytes cells

·        These are the myelinating cells of the central nervous system.

·        They provide insulating layers of the myelin around the axon within the brain and spinal cord.

In PNS

Schwann cells

·        The PNS contain neuroglial cells as well as Schwann cells.

·        It forms a covering called the myelin sheath around the axons.

·        They perform the same role as the oligodendrocytes found in the CNS.

Satellite cell

·        These are the small glial cells that surround neurons in sensory ganglia in the ANS.

·        PNS satellite glia cells are very sensitive to injury and exacerbate pathological pain.

·        It gives protection to the neuron cells.

Electrophysiology

·        It includes measurement of the electrical activity of the neurons or action potential.

·        It indicates the flow of ions in biological tissues.

·        The cell membrane of every cell contains ions that are present inside and outside of the membrane.

·        Na⁺ ions have a high concentration at outside the cell, so it is called an extracellularion.

·        K⁺ ion have a high concentration at inside the cell, so it is called an intracellular ion.

·        At resting state: The ions flow by the concentration gradient (Higher to lower) and become polarised.

·        Due to a change in the concentration of ions inside and outside the cells, it develops a potential difference.

·        At resting state, it is -70mv (Resting potential).

Threshold potential

·        When a stimulus arises in neurons, it changes the resting membrane potential and it becomes depolarisation.

·        Example, temperature, pressure, chemical etc.

·        The stimulus changes resting potential (-70mv) to -55mv then it is called threshold potential.

Action potential

·        When the stimulus reaches threshold potential, it opens Na⁺ channels. Due to the high concentration of Na⁺ ions entering the cell. There is a change in charge, becoming positive; that is called depolarisation.

·        At the same time, K⁺ channels also open and an efflux of K⁺ ions. Again, change charged to (-ve). It is called repolarisation.

Threshold potential

↓

Na⁺ channels open

↓

Influx of Na⁺ ions

↓

DEPOLARIZATION

↓

At the same time

↓

K⁺ channels open

↓

Outflux of K⁺ ion

↓

REPOLARIZATION

 

Nerve impulse

·        It is a signal that is transmitted along a nerve fibre.

·        The information or stimulus that is transferred through the neurons.

·        It is transmitted through the axon by propagation of an action potential.

Receptor

·        Receptors are the macromolecules made up of proteins, which are those binding sites where the neurotransmitter binds.

·        Neurotransmitterreceptors are present in the postsynaptic membrane.

·        There are two types of neurotransmitter receptors

                               i.           Inotropicreceptor

                             ii.           Metabotropicreceptor

 Inotropic receptor (Ligand-gated-ion-channel)

·        When neurotransmitter attached to their site it open or close ion channel according to NT nature.

Metabotropic receptor (G Protein Coupled Receptor)

·        When neurotransmitter attached with receptors will open or close ion channel or direct or by indirectly (through secondary messenger)

Synapse

·        The junctions of two neurons where the information is passed to the next.

Types of synapse

1.      Electrical synapse (faster transmission)

2.      Chemical synapse

Electrical synapse

·        In an electrical synapse, the axon of one neuron is connected to the dendrites of another neuron by gaps.

·        The gap junction is composed of channel proteins known as connexins.

·        Connexins constitute a large family of trans membrane proteins that allow intracellular communication and the transfer of ions and small signaling molecules between cells.

Chemical synapse

·        Chemical synapse: release of Chemical NTs to propagate a signal from one neuron (presynaptic) to another (postsynaptic).

·        The distance between two neurons is about 20-30nm called the synaptic cleft.

·        At the end of the neuron, the axon terminal, there are synaptic vesicles contains NT.

·        So, when AP reaches the nerve terminal, the vesicles rupture and the NTs are released to the postsynaptic membrane.

 Neurotransmitter

• A neurotransmitter is a signalling molecule released by a neuron to affect another neuron across a synapse.

Types of neurotransmitter

Excitatory	Inhibitory	Both
·         Glutamate ·         Aspartate	·         GABA ·         Serotonin ·         Dopamine	·         Acetylcholine ·         Noradrenaline

Adrenaline

• Adrenaline is a primary hormone released by the adrenal gland, but some neurons may secrete it as a neurotransmitter.
• Increased heart rate and blood flow.
• Produced during stressful or exciting situations.

Dopamine

• It is primarily responsible for feelings of pleasure.

Acetyl Choline

• Involved in thought, learning and memory within the brain.
• Activates muscle contraction.

Serotonin

• It is an inhibitory neurotransmitter that regulates mood, fear, feeling of relaxation, and mental focus.
• Regulates Digestion, nutrition absorption.

CNS

• Meninges-The protective covering of brain and spinal cord (CNS).
• Meninges contain 3 layers:

                               i.           Dura mater (Outer layer)

                             ii.           Middle layer (Arachnoid mater)

                           iii.           Pia mater (innermost layer)

The dura mater and Arachnoid mater are separated by subdural space. The Arachnoid and Pia mater are separated by the subarachnoid space
containing cerebrospinal fluid.

Dura Mater

• The dura mater is double-layered, called the endosteal layer and meningeal layer.
• The endosteal layer covers to the inner surface of the skull.
• The meningeal layer is the outer covering of the brain and continues as the dura mater of the spinal cord.

Arachnoid Mater

• This is a layer of fibrous tissue.
• It contains collagen fibres that help in shock absorption.
• The subarachnoid space contains blood vessels and cerebrospinal fluid.

Pia Mater

• The Pia mater tightly covers the brain and spinal cord.
• Pia mater is bound to the surface of the astrocytes in the brain.