Biology: Nervous Coordination

Nervous coordination (5%)

Structure and functions of neurons: The nervous system is made up of tissue that has the specialized cells that can conduct impulses rapidly from one part of the body to another. These specialized cells are the functional unit of the nervous system and are called neurons (nerve cells).

A typical neuron consists of a cell body (cyton) which gives off a variable number of processes called dendrites and a single axis cylinder or axon. The cyton contains a large central nucleus surrounded by cytoplasm the cyton/perikaryon that contains numerous mitochondria and Golgi complex. It also contains a granular material called Nissl granules (named after its discoverer Franz Nissl) that stain intensely with basic dyes. It probably synthesise protein for the cell. There are also several cytoplasmic strands called neurofibrils that help in the transmission of nerve impulse.

Dendrites receive nervous impulse from other neurons at the synapses and conduct them towards cell body. Hence dendrites are called afferent processes.

axon transmit  nerve impulse away from the cell body to a neuro-muscular junction or to the dendrites of the next neurons. Hence axons are called as efferent processes.

Types of neurons: 

Depending upon the processes neurons can be classified into:

1.      Apolar- lack both axon and dendron. They are the immature embryonic nerve cells, also called neuroblasts.

2.      Unipolar-having only one process, Occurs in invertebrates and in the embryonic stage of vertebrates.

3.      Pseudounipolar-the cyton has only one process, which then bifurcates to form an axon and a dendron. Found in the dorsal root of spinal nerves.

4.      Bipolar-having two processes one axon and one Dendron. Found in the retina of eye and the sensory cells of olfactory epithelium.

5.      Multipolar-having more than two processes, mainly found in the grey matter of brain.

Myelinated and nonmyelinated nerve fibre: Extended axon or dendrite is called nerve fibre and a typical nerve fibre consists of a central core of axon, a drawn-out portion of neuroplasm called axoplasm and the encircling sheath called neurolemma, which is made up of a single layer of flat expanded schwann cell. This type of fibre is called nonmyelinated nerve fibre. While myelinated or medullated nerve fibres are covered by a lipid-rich insulating layer called myelin sheath. Myelin sheath is not continuous along the length of the axon but interrupted at intervals where neurilemma comes in contact with the axon and these constricted area is called node of Ranvier. Nerve impulses are conducted far more rapidly in myelinated nerve fibres than in nonmyelinated nerve fibres.

Types of neurons based on functions:

1.      Sensory/afferent neuron: receive and conduct sensory impulse from receptors to the CNS.

2.      Motor/efferent neurons: conduct motor impulses out of CNS to effector organs like muscles and glands.

3.      Relay/Association/interneurons: They link sensory neurons with motor neurons and are located completely within the CNS.They are meant for integrating and analysing the input of information and destributing it to other parts of nervous system.

Nerves:

Nerves are the bundles of nerve fibres covered with a layer of dense fibrous tissue called epineurium. Internally it contains several fasciculi.Each fasciculus is covered with a layer of connective tissue called perineurium.

Types of nerves:

1.      Sensory/ afferent nerves: contain only sensory nerve fibres. Eg. Optic nerve

2.      Motor/efferent nerve fibres: contain only motor nerve fibres. Eg. Oculomotor nerve

3.      Mixed nerves: contain both sensory and motor nerve fibres. Eg. All spinal nerves.

The movement of action potential along a nerve fiber in response to a stimulus (such as touch, pain, heat or cold) is called a nerve impulse.

The nerve impulse is the sum of mechanical, chemical and electrical disturbances created by a stimulus in a neuron.

The generation conduction of the nerve impulse involves the following :

1.Polarised membrane and Resting membrane potential

Neurons are excitable cells. They may be stimulated by physical, mechanical, chemical or electrical stimuli. The axoplasm inside the axon contains a high concentration of potassium ions, negatively-charged proteins and a low concentration of sodium ions. On the contrary, the extracellular fluid outside the axon contains a low concentration of potassium and a high concentration of sodium. This differential permeability is maintained by a sodium-potassium pump present inside the membrane. The electrical potential difference across the neural membrane in an unexcited nerve fibre is called resting membrane potential, and the neuron is called a polarised nerve fibre. The resting membrane potential of neurilemma is about -70mV.

2. Depolarisation of nerve membrane: On being stimulated by any electric, chemical,thermal or mechanical stimulus, the permiability of neural membrane for Na+ ions increases at the point of  stimulus, while K+ ion channels remain closed.Hence, influx of Na+ ions into the axoplasm of nerve fibre makes the inner surface of neurilemma positive and outer surface negative.This reversal of polarity across the two sides of the membrane is called depolarization and the potential difference occurred in the stimulated neuron is called action potential.

The action potential travels as a wave of depolarisation along the length of a nerve fibre in a particular direction and is called a nerve impulse.

3.propagation of action potential: Adjuscent to the depolarised area, the outer surface of the fibre is still positive.the positive charge from polarised surface moves towards adjacent depolarised area resulting in the depolarisation of that area. Hence the action potential created in the adjuscent area move from point to point along the length of the nerve fibre.

4. Repolarisation: With the increase of positive charge inside the axon, further entry of Na+ ions is prevented.the permiability of membrane decreases and the Na+ ions are pushed out. Hence with the establishment of sodium pump, the inside of the membrane becomes negative and outside become positive and the membrane restores the original resting potential. This is known as repolarisation.

 

II. Conduction of nerve impulse in myelinated nerve fibres

In myelinated nerve fibres, the myelin sheath between the nodes insulates the fibre and prevents its depolarisation. In such fibres, the ionic changes and subsequent depolarisation take place only at the nodes of Ranvier. The action poitential jumps from one node to the next. This mode of transmission of nerve impulse in a jumping manner from node to node is called saltatory conduction of nerve impulse. The conduction of nerve impulse in myelinated nerve fibres is 50 times faster than nonmyelinated fibres.

 

Refractory period: The entire process of repolarisation requires some time during which the nerve cannotbe stimulated again. During this interval, the nerve fibre recovers from the first stimulus and becomes ready for carrying another impulse. This recovery time is called refractory period.

Transmission of nerve impulse through synapse

Synapse: Axons give rise to branches and each branch has a small swelling known as synaptic knob. The association between synaptic knob and dendrites of another neuron is called synapse and there is a gap called synaptic cleft between the plasma membrane of the two neurons. Synapse helps in conduction of the nerve impulse from the first to the next neurons by means of chemicals called neurotransmitter (acetyl choline) released by the synaptic vesicles in synaptic knobs.

The neurotransmitters thus released bind to their specific chemoreceptors present on the post-synaptic membrane of the dendron. This binding opens sodium ion channels and causes influx of Na+ ions in the post synaptic membrane and the entry of ions generate a new action potential in the post-synaptic neuron.

Events: Depolarisation of synaptic membrane

Entry of calcium ions from synaptic cleft to presynaptic knob   -->>  release of neurotransmitter in to the synaptic cleft -->>Influx of Na+ ions into post synaptic membrane and the generation of action potential

Nerve impulses are always transmitted across a synapse from the axon of one neuron to the dendrite or cell body of the next neuron but never in the reverse direction, because neurotransmitter is present only in the axon terminals and not in the dendrite or cell body.  

Synaptic delay: There is always some delay in the transmission of a nerve impulse at each synapse, because of the time needed for the release of neurotransmitter, its diffusion through the synaptic cleft and stimulating the next neuron by it. It is about half a millisecond.

Synaptic fatigue: Repeated transmission of nerve impulses through a synapse results in temporary suspension of impulse transmission at the synapse. This is called synaptic fatigue. It is due to the exhaustion of the neurotransmitter in the synaptic vesicles of the axon terminal. After some time the neurotransmitter is resynthesized the synapse is active again.

Electrical synapse and chemical synapse: At electrical synapse, the membranes of presynaptic and post synaptic neurons are in close proximity. Electric current (action potential) can flow directly from one neuron to the other. Transmission of impulses will be much faster but such synapses are rare in our body.

Chemical synapse: Nerve impulses are transmitted by chemical means.

Adrenergic and cholinergic neurons

1. Adrenergic involves the use of the neurotransmitters epinephrine and norepinephrine while cholinergic involves acetylcholine.

2. Adrenergic is called the sympathetic line (SNS) while cholinergic is called the parasympathetic line (PNS).

Effect of drugs on synaptic transmission

Nicotine increases the release of neurotransmitter at synapse in response to an impulse. It binds to the receptors on the presynaptic neurons and excites the neurones to fire more action potential. It also affects neurons by increasing the number of synaptic vesicles released.

Marijuana prevents synaptic vesicles from releasing neurotransmitter. Tetrahydroxycannabinol (THC) is an active ingredient present in cannabis which causes changes in the brain. THC mimics and blocks action of neurotransmitter and activates cannabinoid receptors in the brain and causes short term effects of relaxation, reduced coordination, reduced BP, sleepiness, attention problems and altered sense of time and space. It also causes hallucination, memory problems and disorientation.

Alcohol

·         Reduces the flow of Ca++ ions into the neurons by closing voltage gated Ca++ channels and reduces the release of neurotransmitter in to synaptic cleft.

·         Stimulates GABA (Gamma-Amino Butyric Acid, an inhibitory neurotransmitter) action and blocks action of neurotransmitter.

·         It increases number of binding site for Glutamate which is an excitatory neurotransmitter in the brain and makes the person excited.

·         Alcohol inhibits neurotransmission in two waysby:

1.      Inhibiting the excitatory channels on the post synaptic neurons

2.      Lowering the rate of action potential from the presynaptic neuron.

Effect of drugs on dopamine release

Dopamine is a neurotransmitter that helps control the brain's reward and pleasure centres. Dopamine also helps regulate movement and emotional responses, and it enables us not only to see rewards, but to take action to move toward them. The presence of a certain kind of dopamine receptor is also associated with sensation-seeking people, more commonly known as "risk takers."

Drugs such as nicotine and marijuana increase the release of Dopamine at synapse.

Nicotine causes neurones to fire more impulses which results in increase in Dopamine release.

THC present in marijuana blocks GABA, hence the dopamine release in presynaptic knob increased. Temporary rise in the level of dopamine produces euphoria and evokes feeling of pleasure. Person feels overstimulated, becomes more anxious and remains in the state of delution and gallucination.

Visual acuity

Visual acuity refers to the clarity of acuteness of vision. It is dependent on both optical and neural factors. Visual acuity is actually the quantitative measure of the ability of eye to identify black symbols on a white background. The normal visual acuity is expressed as 6/6 vision or 20/20 vision.

The sharpness of retinal focus within the eye is optical, while sensitivity and functioning of the nervous elements of retina and interpretative faculty of the brain are neural factors.