NEET Biology Notes Neural control and chemical coordination Transmission of Impulse

NEET Biology Notes Neural control and chemical coordination Transmission of Impulse

Transmission of Impulse

Transmission of Impulse

A nerve impulse flows from one neuron to another through synapses. A synapse is formed by the membranes of a pre-synaptic neuron and post-synaptic neuron, which may or may not be separated by a gap called synaptic cleft.
The two types of synapses are:

•  Electrical Synapses
  At electrical synapses, the membranes of pre-and post-synaptic neurons are in very dose proximity. The transmission of an impulse across electrical synapses is very similar to impulse conduction along a single axon. Impulse transmission across an electrical synapse is always faster than that across a chemical synapse. Electrical synapses are rare in our system.
•  Chemical Synapses
  At a chemical synapse, the membranes of the pre and post-synaptic neurons are separated by a fluid-filled space called synaptic cleft.
  Axon terminals contain vesicles filled with neuro-transmitters. When an impulse arrives the axon terminal, it stimulates the movement of the synaptic vesicles towards the membrane, where they fuse with the plasma membrane and release their neurotransmitters in the synaptic cleft. The released neurotransmitters binds to their specific receptors , present on the post-synaptic membrane. The binding opens ion channels allowing the entry of ions, which can generate a new potential in the post-synaptic neuron.

Reflex Action

It is an immediate involuntary response to a stimulus without the intervenation of brain, controlled by spinal cord. Reflex action, is an autonomic motor response to a sensory stimulus without brain being immediately involved. Reflex actions are quick and fast. They follow the shortest route for quick response.These are mostly protective in function. Reflex actions are of two types:

• • • Simple Reflex
      It is unconditional and inborn reflex to a stimulus, e.g. Blinking of the eye when an object comes near to our eyes suddenly; rapid withdrawal of hand, while burned; sneezing, coughing, yawning, knee jerk reflex, etc.
    •  Acquired Reflex
      It is also called as conditioned reflex and is dependent on past experience, training and learning. These reflexes were first demonstrated by Ivan Petrovitch Pavlov in -hungry dog e.g. learning of dancing, cycling, swimming, singing, driving, etc.

Sensory Organs

The sensory organs detects all types of changes in the environment and signals to the CNS, where all the inputs are processed and analysed. Signals are then sent to different parts/centres of the brain.

Different type of sensory organs are as follows:

• Eyes

An adult human eye ball is a spherical structure.
It is composed of three layers:

1. External layer
2. Middle layer
3. Inner layer

• The external layer is composed of a dense connective tissue and is called the sclera. The anterior portion of this layer is called the cornea.
• The middle layer is choroid and it contains many blood vessels. The choroid layer becomes thick in the anterior part to form the ciliary body. The ciliary body forms a pigmented and opaque structure called the iris (a visible coloured part of eye). The eye ball contains a transparent crystalline lens, which is held in place by ligaments, attached to the ciliary body. In front of the lens, the aperture surrounded by the iris is called the pupil.
• The inner layer is the retina, which contains ganglion cells, bipolar cells and photoreceptor cells.
  There are two types of photoreceptor cells, i.e., rods and cones. The daylight vision and colour vision are functions of cones and the UV light vision is the function of rods. The optic nerves leave the eye and the retinal blood vessels enter it at a point medial to and sligfdly above the posterior pole of the eye ball.
  Photoreceptor cells are not present in the region called blind spot. A yellowish pigmented spot is called macula lutea with a central pit called the fovea is present at the posterior pole of the eye. The space between the cornea and the lens is called the aqueous chamber and contains a thin watery fluid called aqueous humour. The space between the lens and the retina is called the vitreous chamber and is filled with vitreous humour.

Albino eyes lacks pigment in skin, hair and iris and the pink colour of the iris is due to reflection of light from the blood vessel of iris.

Mechanism of Vision

The light rays are focussef on retina through the cornea and the lens generates potentials in rods and cones. The human eye contain opsin (a protein) and retinal (an aldehyde of vitamin-A). Light induces dissociation of the retinal from opsin, resulting in changes in the structure of the opsin. This causes membrane permeability changes.
The potential difference is generated and produces a signal that generates action potentials in the ganglion cells through the bipolar cells. These action potentials are transmitted by the optic nerves to the visual cortex area of the brain, where the neural impulses are analysed and the image formed on the retina is recognised, based on earlier memory and experience.

Defects of Eye

Myopia (near sightedness), hypermetropia (long sightedness), presbyopia (old age far sightedness), astigmatism (irregular cornea lens), cataract (loss of transparency of lens), glaucoma (abnormal high pressure fluid inside eye) are some important optical disorders.

• Ears

The ear can be divided into three major parts:

1.  Outer Ear
   The outer ear consists of the pinna and external auditory meatus. The pinna collects the vibrations in the air which produces sound.
   The external auditory meatus leads inwards and extends up to the tympanic membrane. The tympanic membrane is composed of connective tissues covered with the skin outside and mucus membrane at inside.
2. Middle Ear
   The middle ear contains three ossicles called malleus, incus and stapes, which are attached like a chain. The malleus is attached to the tympanic membrane and stapes is attached to the oval window of the cochlea. The membranes constituting cochlea, the reissner’s and basilar membrane divides the surrounding perilymph which is filled bony labyrinth membrane. The organ of Corti is a structure located on the basilar membrane, which contains hair cells that act as auditory receptors.
3.  Inner Ear
   The inner ear contains a vestibular apparatus, which is composed of three semicircular canals and the otolith organ, consisting of the saccule and utricle.

The base of canals is swollen and is called ampulla. The saccule and utricle contains a projecting ridge called macula. The crista and macula are the specific receptors of the vestibular apparatus which is responsible for maintenance of balance of the body and posture.

• Mechanism of Hearing

The external ear receives sound waves and directs them to the ear drum. The ear drum vibrates in response to the sound waves and the vibrations are transmitted through the ear ossicles to the oval window.
The vibrations are passed through the oval window to the fluid of the cochlea, where they generate waves in the lymph. These waves generate ripple in the basilar membrane. As a result, nerve impulses are generated in the associated afferent neurons. These impulses are transmitted by the afferent fibres via auditory nerves to the auditory cortex of the brain, where the impulses are analysed and the sound is recognised.

• Endocrine System of Human

The internal environment of body is maintained in steady state by autonomic nevrous system and the endocrine system. The endocrine system brings about chemical coordination by complex organic compounds called hormones. In the vertebrate body glands may be classified on the basis of the presence or absence of ducts as endocrine or ductless gland and exocrine or ductus glands.

1. Endocrine Glands
   These glands do not have ducts (tubes) to carry their secretions to the target organs. The science dealing with these is called Endocrinology. These glands secrete their secretions into the blood for their transportation to the sites of action, e.g. thyroid, pituitary, pancreas, hypothalamus, adrenal, etc.
2.  Exocrine Glands
   These glands secrete their secretions in ducts, to carry them to sites of action, e.g. sweat and oil glands of skins, salivary gland, liver, etc.

Hormones

These are the organic compounds which are secreted in small amount by endocrine glands of body. These can inhibit, accelerate or modify the cellular activities.
These are first messengers, informational molecules or trigger substances of the body. They regulate internal environment and cellular activities of the cell. The first hormone to be discovered was secretin and it was discovered by Bayliss and Ernst H Starling.

Mechanism of Hormone Action

Hormones produce their effects on target tissues with the help of hormone receptors located in the target tissues only. The binding of a hormone to its receptors leads to the formation of a hormone receptor complex.
Each receptor is specific to one hormone only and hence, receptors are specific. On the basis of chemical nature, hormones can be grouped as

• Peptide, polypeptide, protein hormones
  (e.g. insulin glucagon, pituitary hormones, hypothalamic hormones, etc).
•  Steroids
  (e.g. cortisol, testosterone, oestradiol and progesterone).
•  Iodothyronines
  (e.g thyroid hormones).
•  Amino acid derivatives
  (e.g. epinephrine).

Hormones as Messengers and Regulators

Hormones, which interacts with membrane-bound receptors do not enter the target cell, but generate the second messengers (e.g. cyclic AMP, IP3, Ca2+, etc.) which in turn regulates the cellular metabolism.
Hormones, which interact with intracellular receptors (e.g. steroid hormones, iodothyronines, etc.) mostly regulate gene expression of chromosome and functions by the interaction of hormone-receptor complex with genome. The cumulative biochemical actions result in the physiological and developmental effects.
Human endocrine system consists of a number of glands.

The endocrine glands of human are

1. Pituitary gland
2.  Thyroid
3.  Parathyroid
4. Pancreas
5.  Adrenal
6.  Pineal gland
7.  Thymus gland
8.  Hypothalamus
9. Gonads (testes apd ovary)

•  Pituitary Gland (Master Gland)
  It is also called master gland. It is slightly larger in woman than in man. It is located in region of forebrain.

• Thyroid
  It is largest endocrine gland. It is found on ventral and lateral sides of-upper part of the trachea in the neck. It is brownish red, shield shaped and bilobed gland.

• Parathyroids .
  These glands are four in number, which are embedded in thyroid glands. These are oval-shaped and yellow coloured.

• Pancreas

It is located within the curve of duodenum. It is a mixed endocrine gland with both exocrine and endocrine portion. Four types of cells are found in islets of Langerhans, i.e. α, β, δ and F cells.
These are
α – cellsThese are larger and peripheral cells and produce glucbgon hormone.
β – cells These are central and smaller cells and produce insulin hormone.
δ – cells These are middle cells, produce somatostatin hormone.
F- cells These cells produce Pancreatic Polypeptide (PP).

•  Adrenal (Supradrenals)

Paired endocrine glands, located superior to kidneys. It is structurally and functionally divided into adrenal.

Adernal gland is also known as gland of emergency.

•  Pineal Gland

It starts degenerating at about age of 7 years. In adult, it is largely fibrous tissue.
It lies under the corpus callosum, between the two cerebral hemispheres of the brain at the tip of a short pineal stalk arising from the roorof the diencephalon. It is neurosecretory transducer and secretes hormones in response to nervous activity.

• Thymus Gland

It is situated in front of the heart. It is active in young ones but gradually becomes inconspicuous after sexual maturity. It consists of peripheral cortex and central medulla. It is a soft, pinkish, bilobed mass of lymphoid tissue.

• Hypothalamus

It is located at the floor of forebrain. Neurohormones were first discovered by Guillenin and Schally. It is connected with the anterior lobe of the pituitary gland by hypophysial portal blood vessels and with the posterior lobe of the pituitary gland by axons and neurons.

• Gonads

Its main function is to produce gametes. They also secrete sex hormones.
Sex hormones are mostly steroids. There are testes in male and ovaries in females. Testes are located in the scrotum. These arise from the mesoderm, while ovaries lie in the abdomen.

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