Wednesday, December 9, 2020

Neural Control and Coordination | Plus 1 Zoology | Exam Capsule Notes

NEURAL CONTROL & COORDINATION: 
Chapter at a glance

Neurons are structural & functional unit of neural system. 

Neuron is made of Cell body, Dendron (branches: dendrites) & Axon (branches: axonites with synaptic knob). 


Types of Neurons:
  • Unipolar: No Dendron. Found in embryo.
  • Bipolar: One dendron. Found in retina.
  • Multipolar: 2 or more dendrons. Most common.
Types of axon:
  • Myelinated: Schwann cells with a myelin sheath around axon. Gaps b/w adjacent myelin sheaths are called nodes of Ranvier.
  • Non-myelinated: Schwann cells without myelin sheath.
GENERATION & CONDUCTION OF NERVE IMPULSES 

  • In a resting neuron, axoplasm has more K+ & –vely charged proteins and less Na+. The fluid outside the axon contains low K+ & high Na+. This forms an ionic gradient.
  • Na-K pump maintains the ionic gradients. It transports 3 Na+ outwards for 2 K+ into cell. So membrane is polarized (outer +ve, inner -ve).
  • Resting potential: Potential difference of resting membrane.
  • If a stimulus is given, membrane at site A becomes permeable to Na+ causing rapid influx of Na+ and reversal of polarity (depolarization).
  • Electrical potential difference during depolarization is called action potential (a nerve impulse). 
  • Action potential is conducted as current flow from site A to B and the process is repeated along the axon. 

Synapse: 

Functional junction between two neurons. It is 2 types: 
  1. Electrical synapse: In this, membranes of pre- & post-synaptic neurons are nearest. So impulse transmission is same as along an axon. Impulse transmission is faster than in chemical synapse.
  2. Chemical synapse: It has synaptic cleft between pre- & post-synaptic neuron. Presynaptic regions have Synaptic knob. They contain synaptic vesicles filled with neurotransmitters.

Impulse transmission in chemical synapse:


Impulse reaches at axon terminal → synaptic vesicles bind on plasma membrane → release of neurotransmitter → diffuses across synaptic cleft → combine with receptors on post synaptic membrane → opening of ion channels allowing entry of ions → action potential.

HUMAN NERVOUS (NEURAL) SYSTEM (CNS & PNS) 

CENTRAL NEURAL SYSTEM (CNS) 

A. BRAIN
  • Covered by cranial meninges (outer dura mater, middle arachnoid mater and inner pia mater.
  • The subarachnoid space is filled with cerebrospinal fluid (CSF).
B. SPINAL CORD
  • Conduction of impulses to and from the brain.
  • Centre of spinal reflexes.
Brain has 3 parts: Forebrain, Midbrain & Hind brain.


1. Forebrain (Cerebrum + Thalamus + Hypothalamus)

a. Cerebrum:

    Consists of 2 Cerebral hemispheres with cerebral cortex.

    3 functional areas: 
  • Motor area: Controls voluntary movements of muscles.
  • Sensory area: Controls functioning of sense organs
  • Association area: For intersensory association, memory & communication
b. Thalamus: 

    Coordinating centre (relay station) for sensory and motor impulses.

c. Hypothalamus: 

    Regulates temperature, thirst, hunger & emotions. Secretes hormones.

2. Midbrain 

Corpora quadrigemina: 4-lobed structure- Lobes of visual reflex (2) & Lobes of auditory reflex (2)

3. Hindbrain
3 parts:
  • Cerebellum: Co-ordinates muscular activities and body equilibrium.
  • Pons varoli: Co-ordinates the activities of eye and ear and regulates respiration.
  • Medulla oblongata: Controls respiration, cardiovascular reflexes, gastric secretions.
Limbic system: 

Amygdala + hippocampus + hypothalamus etc. 
It regulates sexual behavior, motivations, emotions.

PERIPHERAL NEURAL SYSTEM (PNS)

System of Cranial & spinal nerves.

Consists of 2 parts: 

1. Somatic neural system: 
    Relays impulses from the CNS to skeletal muscles.

2. Autonomic neural system (ANS): 
    Transmits impulses from CNS to involuntary organs & smooth muscles. 
    Includes 2 types of nerves:
  • Sympathetic nerves: Prepares body to cope with emergencies, stresses & dangers. It increases heartbeat, breathing rate, constricts arteries and elevates BP.
  • Parasympathetic nerves: Returns the body to a resting state and slows down heartbeat, dilates arteries, lowers BP etc.
REFLEX ACTION 

It is the rapid, involuntary and unconscious actions of body in response to a stimulus. E.g.
  • Withdrawal of hand when it touches a hot object.
  • Knee jerk phenomenon.
Reflex arc: 
  • Pathway of impulses in a reflex action.
  • Receptor organ → Sensory (afferent) neuron → Interneuron → Motor (efferent) neuron → effector organ (muscle/gland). 


SENSE ORGANS 

1. EYE 
  • Eyeball has 3 layers: outer Sclera, middle Choroid (blood vessels) & inner Retina.
  • Cornea: Anterior transparent portion of sclera.
  • Iris: Pigmented portion of the eye.
  • Pupil: Central opening of iris. It regulates the amount of light entering the eye.
  • Retina has 3 layers- inner ganglion cells, middle bipolar cells & outer photoreceptor cells.
  • Photoreceptor cells are 2 types: rods and cones. They contain photosensitive proteins (photopigments).
  • Cone cells: For Daylight (photopic) vision & colour vision. They contain photopsin.
  • Rod cells: For Twilight (scotopic) vision. They contain rhodopsin. It contains a derivative of Vitamin A.
  • Blind spot: Region where there are no photoreceptor cells.
  • Macula lutea: Yellowish pigmented spot with a central pit (fovea). In fovea, only cones are seen. Greatest visual acuity. 
  • Space between cornea & lens (aqueous chamber) contains aqueous humor. 
  • Space between lens and retina (vitreous chamber) contains vitreous humor. 

Mechanism of vision 

Light from object → cornea & lens → focus on retina → dissociation of retinal & opsin → membrane permeability changes → potential difference in photoreceptor cells → action potential in ganglion cells → impulses to optic nerves → brain → vision. 

2. EAR 

3 divisions: External ear, middle ear & inner ear. 


  1. External ear: Consists of pinna, auditory meatus (ear canal) & tympanic membrane (ear drum).
  2. Middle ear: Consists of tympanic cavity and ear ossicles (Malleus, Incus & stapes). Eustachian canal connects middle ear to pharynx. It equalizes pressure on either side of the eardrum. Stapes is smallest bone of body.
  3. Inner ear: Consists of bony labyrinth & membranous labyrinth (cochlea & Vestibular apparatus).
    Vestibular apparatus: It includes
  • 3 semicircular canals: Each canal has an ampulla with crista ampullaris.
  • Otolith organ (utricle + saccule): Consists of the receptor Macula.
    Crista & Macula help in body equilibrium & posture.

    Cochlea (organ of hearing):
  • It has 3 canals - scala vestibula, scala media & scala tympani.
  • Scala vestibula & scala media are separated by Reissner’s membrane.
  • Scala media and scala tympani are separated by basilar membrane.
  • Organ of Corti: Receptor organ on the basilar membrane. 

Mechanism of hearing:

Pinna collects sound waves → ear canal → tympanic membrane → vibrations → to ear ossicles & oval window → perilymph in vestibular canal → scala tympani → basilar membrane → sensory hair cells press against tectorial membrane → impulse → auditory nerve → to brain → hearing.

3. NOSE (Organ of smell):
  • Olfactory receptors. 
  • Neurons of olfactory epithelium extend into a pair of olfactory bulb. 
4. TONGUE (Organ of taste): 
  • Primary tastes: sweet, salt, sour & bitter. 
  • Taste buds (Gustatoreceptors + supporting cells) are seen around the bases of taste papillae. 
5. SKIN: 
  • Largest sense organ.
  • Contains receptors for heat, cold, touch, pain & pressure.

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