: NEURAL CONTROL AND COORDINATION
In man and other vertebrates, the physiological functions are coordinated by both the nervous and endocrine systems.
The system that receives the stimulus transmits it to other parts of the body and the corresponding effect shown is known as a Nervous System. The nervous system performs three basic functions; receives stimuli through sensory neurons from internal and external environment and passes to the brain; where the input stimuli is processed and then response is given back to the body parts through motor neurons.
The system that receives the stimulus transmits it to other parts of the body and the corresponding effect shown is known as a Nervous System. The nervous system performs three basic functions; receives stimuli through sensory neurons from internal and external environment and passes to the brain; where the input stimuli is processed and then response is given back to the body parts through motor neurons.
Nervous system in Invertebrates:
Ø In primitive invertebrates like Sponges lack neurons
Ø In Hydra, all neurons are linked to one another, forming a nerve net called plexus between the outer epidermis and inner gastrodermis.
Ø In Planaria, two nerve cords that converge to form a rudimentary brain.
Ø In Earthworm, has a single ventral nerve cord and paired segmental ganglia. The ganglia give rise to the segmental nerves to the tissues.
Nervous System of Cockroach:
It consists of brain, the ventral nerve cord and ganglia and nerves which arise from ganglia. The brain or supraoesophageal ganglion is made of three fused ganglia of the head and present above the oesophagus. The ventral nerve cord is composed of ten ganglia. The First one lies just below the oesophagus and is known as Suboesophageal ganglion is connected with brain by a pair of Circumoesophageal (Circumpharyngeal) Commissures. The thoracic ganglia are three and six abdominal ganglia of which the last one is larger than the others.
Nervous System of Human:
The human nervous system consists of the following two major parts;
a) Somatic Nervous System (Voluntary): It consists of sensory / afferent neurons which transmit impulse from the receptors to the CNS; and the motor / efferent neurons, which transmit response from the CNS to the effector (skeletal muscles).
b) Autonomic Nervous System (Involuntary): It stimulates the glands and the other muscles of the body and responsible for the involuntary actions.
c) Neuroendocrine System: It consists of a net work of endocrine glands and their hormonal production is controlled by CNS.
Nuclei: The cluster / group of neurons in CNS.
Ganglia: The cluster / group of neurons in PNS.
Nerve tracts: The bundles of nerve fibres in CNS
Nerves: A bundles of nerve fibres in the PNS.
A typical Nerve:
A typical nerve has a tough outer covering called Epineurium. Inside the epineurium, axons of nerve cells form bundles called fascicle. Each fascicle is wrapped with a layer called perineurium.
Multipolar nerve cells have many short dendrites and one long axon, eg., in cerebral cortex. A bipolar nerve cell has a long axon extending on either side of the cell body, eg.in retina. Pseudounipolar nerve cells have cell body in a side branch of the main axon, eg.,cells of dorsal root ganglion.
Conduction of Nerve impulse across neurons:
a) Resting potential:
The permeability of plasma membrane to K+ ions is greater than its permeability to Na+ ions. So the surface of axon carries a positive charge relative to its interior; this electrical potential difference across the plasma membrane is called resting potential and it ranges from – 40 to – 90 mV.
b) Action potential:
When a threshold stimulus is applied on the axon membrane, depolarization is caused by a rapid change in membrane permeability. The membrane becomes more permeable to Na+ than to K+. The interior becomes electropositive and the ECF becomes electronegative. The depolarization spreads, producing a local current, which induces the nearby passive Na+ channels to open and to depolarize the nearby site.
c) Repolarisation:
After about 0.5 ms, permeability to K+ ion increases because the build up of positive charge inside the cell opens the voltage gated K+ channels. Movement of K+ ions outward, down their concentration gradient, then reestablishes the charge differences that existed before the stimulus occurred. The exodus of K+ ions lowers the number of positive ions within the cell and the potential falls back towards the resting potential.
Synapse:
The functional junction between two neurons, the axon of a neuron and the dendron/dendrite of another neuron.
Types of synapse:
There are mainly two types of synapses based on the nature of transfer of information across the synapse; a) electrical and b) chemical synapses.
a) In electrical synapses, the cells are separated by a gap of 0.2nm synaptic cleft, so an action potential can sufficiently depolarize the postsynaptic membrane.
b) In chemical synapses, synaptic cleft gap is greater and neurotransmitter substance responsible for the transmission of nerve impulse across the synapse.
Conduction of Nerve impulse across synapse:
In a synapse, there is a narrow fluid-filled gap of 10-20 nm, called synaptic cleft. The nerve terminal has a bulbous expansion called synaptic knob or synaptic button. In the cytoplasm of the synaptic knob, numerous tiny membrane-bound synaptic vesicles are present. These synaptic vesicles contain as many as 10,000 molecules of the neurotransmitter. When a nerve impulse reaches the presynaptic membrane, the voltage-gated calcium channels concentrated in the synapse, open. Calcium ions from the fluid in the synapse diffuse into the synaptic button and stimulate the vesicles to move to the terminal membrane, fuse with it and then rupture to release the neurotransmitter. The neurotransmitters quickly diffuse to the other side of the gap, combine with specific receptor molecules of the other nerve cell and cause sparking a second electrical current, passing its signal.
Structure of Human Brain:
Human brain is covered by a tough tissue covering called meninges. The three layers of meninges are the outer most duramater, middle arachnoid membrane and inner pia mater. A deep cleft called longitudinal fissure divides the brain into two halves or the cerebrum into right and left hemispheres.
Cerebral Cortex:
The outer surface of Cerebrum 2-6 mm thick and is known as Cerebral Cortex. It consists of Gray matter (spindle and satellite neurons cell bodies).
The surface are of cerebral cortex is increased by numerous infoldings/convolutions called Sulci (small groove), Fissure (large groove) and Gyri (swollen area between adjacent sulci/fissure). Two thirds of the surface of the cerebral cortex is hidden in the sulci and fissure.
Beneath the cerebral cortex a large number of myelinated axons of cerebral cortex neurons from White matter.
Cerebral cortex is the region of various activities and has 3 areas namely Sensory, Motor and Associative.
Cerebellum:
It is the second largest part of brain. It is also known as “little cerebrum” and present below the cerebrum.
It is also made up of two cerebellar hemispheres and has gray matter outside as 3 layers.
Outer layer consists of cell bodies.
Middle layer consists of large flask-shaped complex neurons called Purkinje cells.
Three pairs of bundles of myelinated nerve fibres called Cerebellar Peduncles from the communication pathways between the Cerebellum and other parts of the CNS.
Superior Cerebellar peduncles – connect the cerebellum to the midbrain.
Middle Cerebellar peduncles – have connection with Pons of Hind brain.
Inferior Cerebellar peduncles – communicate with medulla oblongata and spinal cord.
Cerebellum is a large reflex centre and control involuntary actions and rapid muscular activities like running, talking, typing, etc., and maintains posture.
Nuclei : collection of different kinds of neurons in brain.
Basal ganglia : collection of subcortical nuclei in the forebrain (below the cortex)
Corpus striatum : It is the largest nucleus in the subcortical nuclei and planning and execution of stereotyped movements.
Thalamus:
It is a region present at the centre of the forebrain and wrapped by cerebrum. All sensory informations first pass through the thalamus. So it receives, determines their source, evaluates their importance and interprets those sensory signals and then channels them to the appropriate cerebral cortex region.
Hypothalamus:
It is present beneath the thalamus. It weighs around 4 gm and is highly vascularized. It contains the nerve centres for temperature regulation, hunger, thirst, heart beat and respiration regulation and emotions (like anger, love, cool, etc). It has connection with pituitary gland hence also controls growth and sexual behaviour.
Limbic system:
It is a part, which connects cerebrum and the brain stem. It sends signals to brain and body parts to regulate our behaviour.
1. Amygdala:
It is located above the hypothalamus and influences behaviour and activities so that they are appropriate for meeting the body’s internal needs. These include feeding, sexual interest, and emotional reactions such as anger. Hence it is responsible for controlling our moods.
2. Hippocampus:
It is the swollen lower lip of the limbic fork. It involves with learning, the recognition and memory. It also converts short term memory to long term memory; hence it plays a vital role in learning.
It is a part of hypothalamus has centre for sexual arousal.
The midbrain contains 4 little lobes called Corpora Quadrigemina. It has a pair of Superior colliculi controls visual reflexes (to fix and focus on an object) and a pair of Inferior colliculi controls auditory reflexes (locates and detects the source of a sound).
Brain Stem:
The area between thalamus and spinal cord is known as brain stem and it forms the region of hind brain.
Pons:
It forms the floor of the brain stem and links cerebral cortex and cerebellum.
Medulla oblongata:
It is the posterior most part and connects the spinal cord and various parts of the brain. This medulla oblongata continues into the spinal cord. This brain stem controls various reflexes like breathing, salivation, chewing, coughing, sneezing, etc.
Reticular formation:
It connects thalamus with major nerves of spinal cord and is the gatekeeper of consciousness.
Ventricles of he brain and cerebrospinal fluid:
There are four cavities in the brain called the two lateral ventricles, the third ventricle and the fourth ventricle.
Cerebrospinal fluid (CSF) fills the ventricles and the subarachnoid space.
Cerebrospinal fluid has the following functions:
a) it contributes to homeostasis.
b) It protects the brain and spinal cord as a shock-absorbing medium.
c) It gives buoyancy to the brain and reduces the pressure at the base.
d) It helps in nutrition and excretion of the neurons.
e) It transports the hormones to various areas of the brain.
Spinal cord:
42-45 cm long and 2 cm in thick (in mid thoracic region) and longer in larger in lower cervical and lumbar regions. It grows till 4-5 yrs. It acts like a link between brain and various parts of the body.
Structure of Spinal cord:
In a cross section, spinal cord consists of butterfly shaped gray matter in the centre which contains cell bodies and dendrites and synapse. This gray matter has dorsal, ventral and lateral horns. This gray matter is surrounded by the white matter made of myelinated axons.
PERIPHERAL NERVOUS SSYSTEM:
Cranial nerves:
12 pairs – 10 pairs originate from brain stem. There are 3 types of CN sensory nerves (sensory fibres), mixed nerves (has both) and motor nerves (motor fibres).
I&II – sensory
III&IV – originate from mid brain.
1/3 originate (V-VIII) from pons.
V cranial nerve (Trigeminal nerve) is the largest and branches into 3 pairs (to jaw, scalp and face).
1/3 (IX-XII) originate from medulla.
X cranial nerve (Vagus) controls and regulates the functions of thoracic and abdominal organs.
Spinal Nerves:
All the spinal nerves are mixed nerves.
Each spinal nerve has two roots, a dorsal sensory and a ventral motor root.
At the middle of each dorsal root, is a swelling called dorsal root ganglion, which contains sensory neurons.
The motor neurons for the ventral root are present in the grey matter of spinal cord.
Autonomic Nervous System (ANS) :
It functions independently and has two output systems, sympathetic and parasympathetic systems.
Ø In primitive invertebrates like Sponges lack neurons
Ø In Hydra, all neurons are linked to one another, forming a nerve net called plexus between the outer epidermis and inner gastrodermis.
Ø In Planaria, two nerve cords that converge to form a rudimentary brain.
Ø In Earthworm, has a single ventral nerve cord and paired segmental ganglia. The ganglia give rise to the segmental nerves to the tissues.
Nervous System of Cockroach:
It consists of brain, the ventral nerve cord and ganglia and nerves which arise from ganglia. The brain or supraoesophageal ganglion is made of three fused ganglia of the head and present above the oesophagus. The ventral nerve cord is composed of ten ganglia. The First one lies just below the oesophagus and is known as Suboesophageal ganglion is connected with brain by a pair of Circumoesophageal (Circumpharyngeal) Commissures. The thoracic ganglia are three and six abdominal ganglia of which the last one is larger than the others.
Nervous System of Human:
The human nervous system consists of the following two major parts;
a) Somatic Nervous System (Voluntary): It consists of sensory / afferent neurons which transmit impulse from the receptors to the CNS; and the motor / efferent neurons, which transmit response from the CNS to the effector (skeletal muscles).
b) Autonomic Nervous System (Involuntary): It stimulates the glands and the other muscles of the body and responsible for the involuntary actions.
c) Neuroendocrine System: It consists of a net work of endocrine glands and their hormonal production is controlled by CNS.
Nuclei: The cluster / group of neurons in CNS.
Ganglia: The cluster / group of neurons in PNS.
Nerve tracts: The bundles of nerve fibres in CNS
Nerves: A bundles of nerve fibres in the PNS.
A typical Nerve:
A typical nerve has a tough outer covering called Epineurium. Inside the epineurium, axons of nerve cells form bundles called fascicle. Each fascicle is wrapped with a layer called perineurium.
Multipolar nerve cells have many short dendrites and one long axon, eg., in cerebral cortex. A bipolar nerve cell has a long axon extending on either side of the cell body, eg.in retina. Pseudounipolar nerve cells have cell body in a side branch of the main axon, eg.,cells of dorsal root ganglion.
Conduction of Nerve impulse across neurons:
a) Resting potential:
The permeability of plasma membrane to K+ ions is greater than its permeability to Na+ ions. So the surface of axon carries a positive charge relative to its interior; this electrical potential difference across the plasma membrane is called resting potential and it ranges from – 40 to – 90 mV.
b) Action potential:
When a threshold stimulus is applied on the axon membrane, depolarization is caused by a rapid change in membrane permeability. The membrane becomes more permeable to Na+ than to K+. The interior becomes electropositive and the ECF becomes electronegative. The depolarization spreads, producing a local current, which induces the nearby passive Na+ channels to open and to depolarize the nearby site.
c) Repolarisation:
After about 0.5 ms, permeability to K+ ion increases because the build up of positive charge inside the cell opens the voltage gated K+ channels. Movement of K+ ions outward, down their concentration gradient, then reestablishes the charge differences that existed before the stimulus occurred. The exodus of K+ ions lowers the number of positive ions within the cell and the potential falls back towards the resting potential.
Synapse:
The functional junction between two neurons, the axon of a neuron and the dendron/dendrite of another neuron.
Types of synapse:
There are mainly two types of synapses based on the nature of transfer of information across the synapse; a) electrical and b) chemical synapses.
a) In electrical synapses, the cells are separated by a gap of 0.2nm synaptic cleft, so an action potential can sufficiently depolarize the postsynaptic membrane.
b) In chemical synapses, synaptic cleft gap is greater and neurotransmitter substance responsible for the transmission of nerve impulse across the synapse.
Conduction of Nerve impulse across synapse:
In a synapse, there is a narrow fluid-filled gap of 10-20 nm, called synaptic cleft. The nerve terminal has a bulbous expansion called synaptic knob or synaptic button. In the cytoplasm of the synaptic knob, numerous tiny membrane-bound synaptic vesicles are present. These synaptic vesicles contain as many as 10,000 molecules of the neurotransmitter. When a nerve impulse reaches the presynaptic membrane, the voltage-gated calcium channels concentrated in the synapse, open. Calcium ions from the fluid in the synapse diffuse into the synaptic button and stimulate the vesicles to move to the terminal membrane, fuse with it and then rupture to release the neurotransmitter. The neurotransmitters quickly diffuse to the other side of the gap, combine with specific receptor molecules of the other nerve cell and cause sparking a second electrical current, passing its signal.
Structure of Human Brain:
Human brain is covered by a tough tissue covering called meninges. The three layers of meninges are the outer most duramater, middle arachnoid membrane and inner pia mater. A deep cleft called longitudinal fissure divides the brain into two halves or the cerebrum into right and left hemispheres.
Cerebral Cortex:
The outer surface of Cerebrum 2-6 mm thick and is known as Cerebral Cortex. It consists of Gray matter (spindle and satellite neurons cell bodies).
The surface are of cerebral cortex is increased by numerous infoldings/convolutions called Sulci (small groove), Fissure (large groove) and Gyri (swollen area between adjacent sulci/fissure). Two thirds of the surface of the cerebral cortex is hidden in the sulci and fissure.
Beneath the cerebral cortex a large number of myelinated axons of cerebral cortex neurons from White matter.
Cerebral cortex is the region of various activities and has 3 areas namely Sensory, Motor and Associative.
Cerebellum:
It is the second largest part of brain. It is also known as “little cerebrum” and present below the cerebrum.
It is also made up of two cerebellar hemispheres and has gray matter outside as 3 layers.
Outer layer consists of cell bodies.
Middle layer consists of large flask-shaped complex neurons called Purkinje cells.
Three pairs of bundles of myelinated nerve fibres called Cerebellar Peduncles from the communication pathways between the Cerebellum and other parts of the CNS.
Superior Cerebellar peduncles – connect the cerebellum to the midbrain.
Middle Cerebellar peduncles – have connection with Pons of Hind brain.
Inferior Cerebellar peduncles – communicate with medulla oblongata and spinal cord.
Cerebellum is a large reflex centre and control involuntary actions and rapid muscular activities like running, talking, typing, etc., and maintains posture.
Nuclei : collection of different kinds of neurons in brain.
Basal ganglia : collection of subcortical nuclei in the forebrain (below the cortex)
Corpus striatum : It is the largest nucleus in the subcortical nuclei and planning and execution of stereotyped movements.
Thalamus:
It is a region present at the centre of the forebrain and wrapped by cerebrum. All sensory informations first pass through the thalamus. So it receives, determines their source, evaluates their importance and interprets those sensory signals and then channels them to the appropriate cerebral cortex region.
Hypothalamus:
It is present beneath the thalamus. It weighs around 4 gm and is highly vascularized. It contains the nerve centres for temperature regulation, hunger, thirst, heart beat and respiration regulation and emotions (like anger, love, cool, etc). It has connection with pituitary gland hence also controls growth and sexual behaviour.
Limbic system:
It is a part, which connects cerebrum and the brain stem. It sends signals to brain and body parts to regulate our behaviour.
1. Amygdala:
It is located above the hypothalamus and influences behaviour and activities so that they are appropriate for meeting the body’s internal needs. These include feeding, sexual interest, and emotional reactions such as anger. Hence it is responsible for controlling our moods.
2. Hippocampus:
It is the swollen lower lip of the limbic fork. It involves with learning, the recognition and memory. It also converts short term memory to long term memory; hence it plays a vital role in learning.
It is a part of hypothalamus has centre for sexual arousal.
The midbrain contains 4 little lobes called Corpora Quadrigemina. It has a pair of Superior colliculi controls visual reflexes (to fix and focus on an object) and a pair of Inferior colliculi controls auditory reflexes (locates and detects the source of a sound).
Brain Stem:
The area between thalamus and spinal cord is known as brain stem and it forms the region of hind brain.
Pons:
It forms the floor of the brain stem and links cerebral cortex and cerebellum.
Medulla oblongata:
It is the posterior most part and connects the spinal cord and various parts of the brain. This medulla oblongata continues into the spinal cord. This brain stem controls various reflexes like breathing, salivation, chewing, coughing, sneezing, etc.
Reticular formation:
It connects thalamus with major nerves of spinal cord and is the gatekeeper of consciousness.
Ventricles of he brain and cerebrospinal fluid:
There are four cavities in the brain called the two lateral ventricles, the third ventricle and the fourth ventricle.
Cerebrospinal fluid (CSF) fills the ventricles and the subarachnoid space.
Cerebrospinal fluid has the following functions:
a) it contributes to homeostasis.
b) It protects the brain and spinal cord as a shock-absorbing medium.
c) It gives buoyancy to the brain and reduces the pressure at the base.
d) It helps in nutrition and excretion of the neurons.
e) It transports the hormones to various areas of the brain.
Spinal cord:
42-45 cm long and 2 cm in thick (in mid thoracic region) and longer in larger in lower cervical and lumbar regions. It grows till 4-5 yrs. It acts like a link between brain and various parts of the body.
Structure of Spinal cord:
In a cross section, spinal cord consists of butterfly shaped gray matter in the centre which contains cell bodies and dendrites and synapse. This gray matter has dorsal, ventral and lateral horns. This gray matter is surrounded by the white matter made of myelinated axons.
PERIPHERAL NERVOUS SSYSTEM:
Cranial nerves:
12 pairs – 10 pairs originate from brain stem. There are 3 types of CN sensory nerves (sensory fibres), mixed nerves (has both) and motor nerves (motor fibres).
I&II – sensory
III&IV – originate from mid brain.
1/3 originate (V-VIII) from pons.
V cranial nerve (Trigeminal nerve) is the largest and branches into 3 pairs (to jaw, scalp and face).
1/3 (IX-XII) originate from medulla.
X cranial nerve (Vagus) controls and regulates the functions of thoracic and abdominal organs.
Spinal Nerves:
All the spinal nerves are mixed nerves.
Each spinal nerve has two roots, a dorsal sensory and a ventral motor root.
At the middle of each dorsal root, is a swelling called dorsal root ganglion, which contains sensory neurons.
The motor neurons for the ventral root are present in the grey matter of spinal cord.
Autonomic Nervous System (ANS) :
It functions independently and has two output systems, sympathetic and parasympathetic systems.
Reflex Action:
Reflex actions are very rapid, involuntary automatic and stereotyped behaviour, in which some stimulus evokes a specific, short-lived response at the unconscious level. There are more than two hundred reflexes, which follow the sequence from stimulus to reflex, along a neural pathway called reflex arc.
Components and pathway of reflex arc:
Receptor à Afferent neuron à Intermediate neuron à Efferent neuron à Effector
Sense Organs:
Structure of Human Eye:
Each eye ball consists of three concentric layers, the outermost sclera, middle choroids and innermost retina. The sclera in the front forms the transparent cornea, with more curved surface to refract light towards the retina. The posterior part of the sclera is tough and elastic and contains collagen fibres. The middle choroid is highly vascular and pigmented, that prevents internally reflected light within the eye. Just behind the junction between cornea and sclera, the choroid becomes thicker and has smooth muscles in it, forming the ciliary body. The iris extends from the ciliary body in front of the lens; it contains radial and circular muscles that control the dilation or constriction of pupil. The lens is suspended from the ciliary body, by suspensory ligaments; the lens and the suspensory ligaments divide the cavity of the eye ball into an anterior and a posterior chamber. The anterior chamber is filled with an aqueous clear fluid, aqueous humor and the posterior chamber has a gelatinous material, vitreous humor. The innermost layer of retina is composed of several layers of cells; the photoreceptor layer contains rods and cones, the intermediate layer has bipolar neurons, which synapse with retinal ganglion cells and their axons bundle to form optic nerve. A tiny circular area called yellow spot or macula lutea that acts as filter over fovea, where the vision is the sharpest. The place where the optic nerve emerges from the retina is the blind spot. The image produced by the lens of eye on the retina is inverted, but the brain interprets the image in the right way.
Structure of Human Ear:
Human ear consists of three parts, the external ear, middle ear and internal ear. The external ear consists of a sound gathering trumpet, called auricle and the external auditory canal. The auditory canal is lined by a mesh of hair and sebaceous glands; the sebum and the hair prevent entry of unwanted particles and infection. The tympanic membrane separates the middle ear from the external ear. The middle ear is an air-filled chamber, which is connected to pharynx by Eustachian tube. The middle ear lodges three small bones, the ear-ossicles namely, the malleus, incus and stapes. They act as a lever system and increase the force of vibration to transmit it to the endolymph in the inner ear. The middle ear communicates with the internal ear through the oval window. The inner ear has bony labyrinth, inside which a membranous labyrinth is floating in the perilymph. The labyrinth has three parts, the semicircular canals, vestibule and cochlea. The cochlea, a shell-like part is composed of three fluid-filled canals (vestibular canal, median canal and tympanic canal) separated by two membranes, Reissner’s membrane and the basilar membrane. Receptors for hearing are tiny sensory hair cells that line the basilar membrane; they are covered by tectorial membrane; these three constitute the organ of Corti. The sensory hair cells are stimulated by the vibrations of endolymph that set off nerve impulses to the auditory nerve. The vestibular system (semicircular canals, utricle and saccule) helps to maintain balance of the body, i.e., orientation, acceleration and rotation to this system.
Organ of Smell:
Nose is the organ of smell sensation (olfaction) and contains olfactory epithelium. The olfactory epithelium has three types of cells; a) Olfactory bipolar neurons b) Supporting columnar epithelium and c) Bowman’s glands. Each receptor cell bears twenty or so tiny sensory cilia, olfactory hair that extends to the mucus. The mucus absorbs the odoriferous substances that stimulate the receptors. Bundles of non-myelinated axons of the olfactory receptors unite to form olfactory nerve.
Organ of Taste:
Tongue is the organ of taste sensation (gestation). There are four basic types of taste namely sweet, sour, salty and bitter, which are detected in distinct regions on the tongue. The tongue detects taste through tiny organs called taste buds; each contains about 50 gustatory receptor-cells. A single gustatory receptor cell is exposed to the external surface, through an opening called taste pore. Though each receptor is more responsive to a particular substance, a broad range of chemicals can stimulate it. The brain integrates the differential inputs from various taste buds into a complex flavors.
Reflex actions are very rapid, involuntary automatic and stereotyped behaviour, in which some stimulus evokes a specific, short-lived response at the unconscious level. There are more than two hundred reflexes, which follow the sequence from stimulus to reflex, along a neural pathway called reflex arc.
Components and pathway of reflex arc:
Receptor à Afferent neuron à Intermediate neuron à Efferent neuron à Effector
Sense Organs:
Structure of Human Eye:
Each eye ball consists of three concentric layers, the outermost sclera, middle choroids and innermost retina. The sclera in the front forms the transparent cornea, with more curved surface to refract light towards the retina. The posterior part of the sclera is tough and elastic and contains collagen fibres. The middle choroid is highly vascular and pigmented, that prevents internally reflected light within the eye. Just behind the junction between cornea and sclera, the choroid becomes thicker and has smooth muscles in it, forming the ciliary body. The iris extends from the ciliary body in front of the lens; it contains radial and circular muscles that control the dilation or constriction of pupil. The lens is suspended from the ciliary body, by suspensory ligaments; the lens and the suspensory ligaments divide the cavity of the eye ball into an anterior and a posterior chamber. The anterior chamber is filled with an aqueous clear fluid, aqueous humor and the posterior chamber has a gelatinous material, vitreous humor. The innermost layer of retina is composed of several layers of cells; the photoreceptor layer contains rods and cones, the intermediate layer has bipolar neurons, which synapse with retinal ganglion cells and their axons bundle to form optic nerve. A tiny circular area called yellow spot or macula lutea that acts as filter over fovea, where the vision is the sharpest. The place where the optic nerve emerges from the retina is the blind spot. The image produced by the lens of eye on the retina is inverted, but the brain interprets the image in the right way.
Structure of Human Ear:
Human ear consists of three parts, the external ear, middle ear and internal ear. The external ear consists of a sound gathering trumpet, called auricle and the external auditory canal. The auditory canal is lined by a mesh of hair and sebaceous glands; the sebum and the hair prevent entry of unwanted particles and infection. The tympanic membrane separates the middle ear from the external ear. The middle ear is an air-filled chamber, which is connected to pharynx by Eustachian tube. The middle ear lodges three small bones, the ear-ossicles namely, the malleus, incus and stapes. They act as a lever system and increase the force of vibration to transmit it to the endolymph in the inner ear. The middle ear communicates with the internal ear through the oval window. The inner ear has bony labyrinth, inside which a membranous labyrinth is floating in the perilymph. The labyrinth has three parts, the semicircular canals, vestibule and cochlea. The cochlea, a shell-like part is composed of three fluid-filled canals (vestibular canal, median canal and tympanic canal) separated by two membranes, Reissner’s membrane and the basilar membrane. Receptors for hearing are tiny sensory hair cells that line the basilar membrane; they are covered by tectorial membrane; these three constitute the organ of Corti. The sensory hair cells are stimulated by the vibrations of endolymph that set off nerve impulses to the auditory nerve. The vestibular system (semicircular canals, utricle and saccule) helps to maintain balance of the body, i.e., orientation, acceleration and rotation to this system.
Organ of Smell:
Nose is the organ of smell sensation (olfaction) and contains olfactory epithelium. The olfactory epithelium has three types of cells; a) Olfactory bipolar neurons b) Supporting columnar epithelium and c) Bowman’s glands. Each receptor cell bears twenty or so tiny sensory cilia, olfactory hair that extends to the mucus. The mucus absorbs the odoriferous substances that stimulate the receptors. Bundles of non-myelinated axons of the olfactory receptors unite to form olfactory nerve.
Organ of Taste:
Tongue is the organ of taste sensation (gestation). There are four basic types of taste namely sweet, sour, salty and bitter, which are detected in distinct regions on the tongue. The tongue detects taste through tiny organs called taste buds; each contains about 50 gustatory receptor-cells. A single gustatory receptor cell is exposed to the external surface, through an opening called taste pore. Though each receptor is more responsive to a particular substance, a broad range of chemicals can stimulate it. The brain integrates the differential inputs from various taste buds into a complex flavors.
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