Neural System
* Coordination between organ systems and the functions of the organs is crucial to maintain a physiological balance, called homeostasis.
* The neutral system and the endocrine system maintain homeostasis in our body.
* Neurons are called the structural and functional units of the neural system.
* The human neural system is divided into the central neural system (CNS) and the peripheral neural system (PNS).
* The brain and spinal cord form the central neural system, and the cranial and spinal nerves from the peripheral neural system.
* The peripheral neural system can be divided into somatic neural system and the autonomic neural system.
* The autonomic neutral system is further classified into the sympathetic neural system and the parasympathetic neural system.
* The neuron is a branched nerve cell and is the longest cell in the body.
* The two main parts of a neuron are the cyton and nerve processes.
Central Neural System
* The brain and spinal cord of vertebrates are collectively known as the central neural system.
* The brain is divided into forebrain, midbrain and hindbrain.
* The forebrain consists of the cerebrum, thalamus and hypothalamus.
* The thalamus is located between the forebrain and midbrain and acts as a major coordinating centre for sensory and motor signalling.
* The hypothalamus is situated at the base of the thalamus and it contains the centres that control body temperature, the urge to eat and drink and the circadian rhythms.
* The collection of structures within the forebrain, including the amygdala and hippocampus, is known as the limbic system.
* The midbrain is located between the thalamus/hypothalamus of the forebrain and pons of the hindbrain.
* The hindbrain is formed of the pons, cerebellum and medulla Oblongata.
* The midbrain and hindbrain form the brain stem. It acts as a relay station for auditory and visual information. That is, every impulse conducted between brain and spinal cord passes through the brainstem.
* An involuntary response to an external stimulus that occurs without any conscious effort or thought and requires the involvement of a part of the central neural system is called reflex action.
Voluntary and Involuntary Actions
* Response is the activity of the body due to a stimulus or change in the environment.
* Voluntary actions are produced by the free will of the organism. For example, dancing and playing.
* Involuntary actions or reflex actions are the quick and automatic responses controlled by the spinal cord. For example, taking your hand away from a hot object immediately.
* Natural reflexes do not require a person to have previous experience or learning.
* Conditioned reflexes are developed by a person during their lifetime due to experience or learning.
* Russian biologist Ivan Pavlov conducted the famous ‘dog and bell’ experiment that proved the idea of conditioned response.
* The path taken by an impulse in a reflex action is known as the reflex arc.
Diseases Affecting the Nervous System
* The nervous system controls and coordinated all the activities of our life and any damage to it can have severe consequences.
* Some diseases that affect the nervous system are Alzheimer’s disease, Parkinson’s disease, Stroke, Meningitis, Rabies, Eoilepsy, Polio and Leprosy.
* Mental health and physical health are complementary to each other.
The Sense Organs
* The eyes, ears, skin, tongue and nose are the different sense organs.
* Sense organs allow us to see, hear, feel, taste and smell.
* Stimuli make the sense organs generate a response which is termed as sensation.
* Every sense organ responds to only a specific type of stimulus.
* A group of certain cells capable of receiving stimuli are known as receptor cells or receptors.
The Ear
* Anatomically, the ear is divided into three sections – the outer ear, the middle ear and the inner ear.
* The outer ear comprises the pinna and external auditory meatus.
* The middle ear consists of the tympanic cavity that separated the external and inner ears.
*The Eustanchian tube connects the tympanic cavity to the pharynx and helps maintain equal air pressure on both sides of the eardrum.
* The inner ear consists of a bony and membranous labyrinth, which consists of the cochlea and vestibular apparatus.
* The crista and macula are the specific receptors of the vestibular apparatus that maintain body balance and posture.
The Eye
* Human beings have a pair of spherical eyes located in eye sockets or orbits of the skull.
* The wall of the human eye ball is composed of the three layers – sclera, choroid and retina.
* The retina contains three layers- ganglion, bipolar and photoreceptor cell.
* Rods and Cones are the two types of photoreceptor cells.
* The cones are responsible for colour vision and daylight vision, which is also known as photopic vision.
* The rods are responsible for twilight vision, which is also known as scotopic vision.
* The optical part of the retina contains two spots known as the blind spot and the fovea.
* An inverted image forms on the retina and the impulse is carried to the visual cortex of the brain, where it is perceived in its correct form.
Nose
* The nose functions as the sense organ for smell and helps in the process of respiration.
* The nose has two nasal cavities separated from each other by the nasal septum.
* The entire nasal cavity is lined by a moist mucous membrane.
* The mucous membrane ha several olfactory receptor cells, which process odour input signals directly to the olfactory bulb.
* The odour signals given out by the olfactory receptor cells travel through the nerve fibres to the olfactory nerve and finally to the brain where the information is processed.
Tongue
* The human tongue serves as the sense organ for taste and helps in speech and moves the food around in the mouth.
* The cavity of the taste bud has several gustatory receptor cells with gustatory nerves extending from their bases to the brain.
* The taste buds for salty and sweet tastes are located at the front of the tongue, for sour on the sides, and for bitter on the back.
* The tongue has receptors to sense the temperature, texture of food as well as the burning and cooling sensation of the certain food materials.
Human Eye and Colourful World – The Human Eye
* The human eye is an optical device that serves as our organ of sight. It transforms light waves reflected from object we see into images.
The major parts of the human eye and their functions are:
* Sclera: Protects and contains internal parts.
* Cornea: refracts incident light.
* Aqueous humour: Prevents collapse of the eye due to atmospheric pressure changes.
* Iris: Adjust the size of pupil.
* Crystalline lens: Focuses the light reflected by objects on the retina.
* Ciliary muscles: Alter focal length of the crystalline lens.
* Retina: Receives optical images and converts it to optical impulse.
* Vitreous humour: Maintains the shape of the eye.
* Optic nerve: Carries optical impulses to the brain.
* Accommodation is the ability of the eye to change the focus from distant to near objects by altering the curvature of the lens.
* Cataracts results in a cloudy translucent eye lens. The cloudiness affects the ability of eye to accommodate.
* Far point is the maximum distance from the eye at which the eye can obtain focus of an object without straining.
* Near point is the minimum distance from the eye at which the eye can obtain focus of an object without straining.
* Least Distance of Distinct Vision is the minimum distance required between the object and the eye, for the eye to be able to view the object comfortably.
With two eyes, we can:
* Have 180% field of view
* Determine the exact location of an object
* Obtain better depth perception
Defects of vision: Distorted or limited vision.
* The converging or diverging capacity of a lens is ascertained by its power. It is measured in dioptres (D).
Skin as a Sense Organ
* The presence of widely distributed free nerve ending in the dermis makes the skin the largest sense organ.
* There are different nerve ending or receptor cells or receptors corpuscles for various sensation.
* The tactile receptors respond to touch, pressure and vibrations, the thermoreceptors to temperature, and the nociceptors respond to pain.
* There are six types of tectile receptors- Nerve Ending, Root Hair plexus, Merkel’s discs, Meissner’s corpuscles, Pacinian corpuscles and Ruffini corpuscles.
* Skin receptors are also called cutaneous receptors.
* The intensity of feeling a stimulus depends on the area of your skin.
* The brain analyses and processes the stimulus, and the body reacts accordingly.
Generation and Conduction of Nerve Impulse
* The nerve impulse is the sum of mechanical, chemical and electrical disturbances created by a stimulus in a neuron.
* The nerve impulse can be divided into resting membrane potential and action membrane potential.
* The electrical potential difference across the neural membrane in an unexcited nerve fibre is called resting potential, and the neuron is called a polarised nere fibre.
* The reversal of polarity across the two sides of the membrane is called depolarisation.
* The action potential travels as a wave of depolarisation along the length of a nerve fibre in a particular direction and is called s nerve impulse.
* The junctions that help transmit a nerve impulse from one neuron to another are called synapses.
* Synapses are of two types – chemical synapses and electrical synapses.
* The synaptic cleft is a fluid – filled gap between the axon terminals and the Dendron of another neuron.