Nervous System & Sense Organs

• The nervous system is the body's high-speed control and communication network.
• It carries messages as electrical impulses through living wires called neurons.
• It always does three jobs in order: receive, process, respond.
• It receives input from sense organs (eyes, ears, skin, nose, tongue).
• It processes input in the brain and spinal cord and makes a decision.
• It responds by ordering muscles to move or glands to secrete.
• It is faster than the hormonal (chemical) system, which is slow but longer-lasting.
• Messenger — Electrical impulse — Chemical hormone
• Path — Neurons — Blood
• Speed — Very fast — Slow
• Effect — Short-lived — Long-lasting

It carries messages as electrical impulses through living wires called neurons.

It always does three jobs in order: receive, process, respond.

It receives input from sense organs (eyes, ears, skin, nose, tongue).

It processes input in the brain and spinal cord and makes a decision.

It is faster than the hormonal (chemical) system, which is slow but longer-lasting.

• The neuron (nerve cell) is the basic working unit of the nervous system.
• Dendrites are short branched fibres that carry the impulse towards the cell body.
• Dendrites contain dark-staining particles called Nissl's granules.
• The cell body (cyton) holds the nucleus and keeps the cell alive.
• The axon is one long fibre that carries the impulse away from the cell body.
• The axon tip ends in a synaptic knob holding vesicles of neurotransmitters.
• Bare gaps in the sheath are the nodes of Ranvier, where the impulse jumps to go faster.
• The myelin sheath is a fatty insulating cover made by Schwann cells that speeds the impulse.

The neuron (nerve cell) is the basic working unit of the nervous system.

Dendrites are short branched fibres that carry the impulse towards the cell body.

Dendrites contain dark-staining particles called Nissl's granules.

The cell body (cyton) holds the nucleus and keeps the cell alive.

The axon is one long fibre that carries the impulse away from the cell body.

The axon tip ends in a synaptic knob holding vesicles of neurotransmitters.

Bare gaps in the sheath are the nodes of Ranvier, where the impulse jumps to go faster.

The myelin sheath is a fatty insulating cover made by Schwann cells that speeds the impulse.

• Axons are of two kinds: myelinated (spinal and cranial nerves) and non-myelinated.
• Neurons are also classified by the number of fibres from the cell body.
• Multipolar neurons have one axon and many dendrites (cerebral cortex).
• Bipolar neurons have one axon and one dendrite (retina of the eye).
• Unipolar neurons have one axon and no dendrite (embryonic stage).
• Multipolar — 1 axon, many dendrites — Cerebral cortex
• Bipolar — 1 axon, 1 dendrite — Retina of eye
• Unipolar — 1 axon, no dendrite — Embryonic stage

Axons are of two kinds: myelinated (spinal and cranial nerves) and non-myelinated.

Multipolar neurons have one axon and many dendrites (cerebral cortex).

Bipolar neurons have one axon and one dendrite (retina of the eye).

Unipolar neurons have one axon and no dendrite (embryonic stage).

• The synapse is the junction where one neuron passes its message to the next.
• It is formed by the pre-synaptic (sender) and postsynaptic (receiver) membranes.
• The tiny gap between them is the synaptic cleft.
• In an electrical synapse, current flows directly across; it is faster but rare in humans.
• In a chemical synapse, the sender releases neurotransmitters across the cleft.
• Chemical synapses are the common type in the human body.
• Electrical — Direct current flow — Faster, rare
• Chemical — Neurotransmitters — Slower, common

The synapse is the junction where one neuron passes its message to the next.

It is formed by the pre-synaptic (sender) and postsynaptic (receiver) membranes.

The tiny gap between them is the synaptic cleft.

In an electrical synapse, current flows directly across; it is faster but rare in humans.

In a chemical synapse, the sender releases neurotransmitters across the cleft.

Chemical synapses are the common type in the human body.

• The central nervous system (CNS) is the brain and spinal cord – the control room.
• The peripheral nervous system (PNS) is all nerves branching from the CNS.
• The PNS includes cranial nerves (from brain) and spinal nerves (from spinal cord).
• The autonomic nervous system (ANS) controls involuntary organs automatically.
• The sympathetic part drives fight-or-flight (speeds heart) in emergencies.
• The parasympathetic part drives rest-and-digest (slows heart, aids digestion).
• Afferent (sensory) fibres carry input to the CNS; efferent (motor) fibres carry output away.
• Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.
• CNS — Brain + spinal cord — Processing, decisions
• PNS — Cranial + spinal nerves — Connects CNS to body
• ANS — Sympathetic + parasympathetic — Involuntary control

The central nervous system (CNS) is the brain and spinal cord – the control room.

The PNS includes cranial nerves (from brain) and spinal nerves (from spinal cord).

The autonomic nervous system (ANS) controls involuntary organs automatically.

The sympathetic part drives fight-or-flight (speeds heart) in emergencies.

The parasympathetic part drives rest-and-digest (slows heart, aids digestion).

Afferent (sensory) fibres carry input to the CNS; efferent (motor) fibres carry output away.

Humans have 12 pairs of cranial nerves and 31 pairs of spinal nerves.

• The brain is the body's main computer where most decisions are made.
• It is enclosed in a bony box called the cranium (brain box) of the skull.
• It is wrapped in three membranes together called the meninges.
• From outside in, the meninges are dura mater, arachnoid, pia mater.
• The pia mater is the delicate innermost layer touching the brain.
• Inflammation of these membranes is the disease meningitis.
• Fluid between the layers (cerebrospinal fluid) cushions the brain from shock.
• Dura mater — Outer — Tough
• Arachnoid — Middle — Web-like
• Pia mater — Inner — Delicate

It is enclosed in a bony box called the cranium (brain box) of the skull.

It is wrapped in three membranes together called the meninges.

The pia mater is the delicate innermost layer touching the brain.

Inflammation of these membranes is the disease meningitis.

Fluid between the layers (cerebrospinal fluid) cushions the brain from shock.

• The brain has three major parts: forebrain, midbrain, hindbrain.
• The cerebrum (forebrain) is the largest part; it controls intelligence, memory and voluntary action.
• The thalamus and hypothalamus lie in the forebrain; the hypothalamus regulates body temperature, hunger and thirst.
• The cerebellum (hindbrain) controls balance, posture and coordination of muscles.
• The medulla oblongata (hindbrain) controls heartbeat, breathing and blood pressure (involuntary vital actions).
• The pons connects brain parts and helps regulate breathing.
• The midbrain and hindbrain together form the brain stem.
• Cerebrum — Forebrain — Thinking, memory, voluntary acts
• Hypothalamus — Forebrain — Temperature, hunger, thirst
• Cerebellum — Hindbrain — Balance, coordination
• Medulla — Hindbrain — Heartbeat, breathing

The cerebrum (forebrain) is the largest part; it controls intelligence, memory and voluntary action.

The thalamus and hypothalamus lie in the forebrain; the hypothalamus regulates body temperature, hunger and thirst.

The cerebellum (hindbrain) controls balance, posture and coordination of muscles.

The medulla oblongata (hindbrain) controls heartbeat, breathing and blood pressure (involuntary vital actions).

The pons connects brain parts and helps regulate breathing.

The midbrain and hindbrain together form the brain stem.

• The spinal cord runs inside the backbone and links the brain to body nerves.
• It is protected by the vertebral column and the meninges.
• A reflex action is a sudden, automatic response to a stimulus, like pulling the hand off a hot object.
• The pathway of a reflex is the reflex arc.
• The spinal cord is the governing centre of reflex action, not the brain.
• The arc runs: receptor to sensory neuron to spinal cord to motor neuron to effector (muscle).
• Reflexes are fast because they bypass the brain.
• Tetraplegia (quadriplegia) is paralysis of all four limbs due to spinal cord injury in the neck region.

The spinal cord runs inside the backbone and links the brain to body nerves.

It is protected by the vertebral column and the meninges.

A reflex action is a sudden, automatic response to a stimulus, like pulling the hand off a hot object.

The pathway of a reflex is the reflex arc.

The spinal cord is the governing centre of reflex action, not the brain.

The arc runs: receptor to sensory neuron to spinal cord to motor neuron to effector (muscle).

Tetraplegia (quadriplegia) is paralysis of all four limbs due to spinal cord injury in the neck region.

• The cornea is the transparent front through which light enters.
• The iris is the coloured part that controls the size of the pupil.
• The lens focuses light onto the retina at the back.
• The retina holds light-sensitive cells: rods (dim light) and cones (colour).
• The optic nerve carries signals from the retina to the brain.
• Lack of vitamin A causes night blindness.
• The Ishihara test is used to detect colour blindness.
• Cornea — Light entry
• Iris / Pupil — Controls light amount
• Lens — Focuses light
• Retina — Detects image
• Optic nerve — Carries signal to brain

The cornea is the transparent front through which light enters.

The iris is the coloured part that controls the size of the pupil.

The lens focuses light onto the retina at the back.

The retina holds light-sensitive cells: rods (dim light) and cones (colour).

The optic nerve carries signals from the retina to the brain.

Lack of vitamin A causes night blindness.

The Ishihara test is used to detect colour blindness.

• The ear is the organ for hearing and balance.
• The outer ear (pinna) collects sound waves.
• The eardrum (tympanum) vibrates when sound hits it.
• Three middle-ear bones – malleus, incus, stapes – amplify and pass vibrations.
• The cochlea in the inner ear converts vibrations into nerve signals.
• The semicircular canals help maintain body balance.
• The auditory nerve carries sound signals to the brain.
• Outer — Pinna, ear canal — Collect sound
• Middle — Eardrum, ear bones — Amplify sound
• Inner — Cochlea, semicircular canals — Hearing, balance

The ear is the organ for hearing and balance.

The outer ear (pinna) collects sound waves.

The eardrum (tympanum) vibrates when sound hits it.

Three middle-ear bones – malleus, incus, stapes – amplify and pass vibrations.

The cochlea in the inner ear converts vibrations into nerve signals.

The semicircular canals help maintain body balance.

The auditory nerve carries sound signals to the brain.

• The tongue detects taste through taste buds.
• The five basic tastes are sweet, sour, salty, bitter and umami.
• Spicy and pungent are NOT tastes – they are pain/heat sensations.
• The back of the tongue is most sensitive to bitter taste.
• The skin senses touch, pressure, pain and temperature.
• The nose detects smell using olfactory receptors.
• The brain (cerebral cortex) itself feels no pain when pricked – it has no pain receptors.
• Eye — Sight
• Ear — Hearing, balance
• Tongue — Taste
• Nose — Smell
• Skin — Touch, pain, temperature

The tongue detects taste through taste buds.

The back of the tongue is most sensitive to bitter taste.

The skin senses touch, pressure, pain and temperature.

The nose detects smell using olfactory receptors.

The brain (cerebral cortex) itself feels no pain when pricked – it has no pain receptors.

• Cerebral palsy is a brain disorder affecting movement, found generally in children.
• Meningitis is inflammation of the meninges (brain membranes).
• Tetraplegia means paralysis of all four limbs.
• Night blindness is caused by deficiency of vitamin A.
• Body temperature is regulated by the hypothalamus.
• Heartbeat and breathing are controlled by the medulla oblongata.
• Input (sensory) and output (motor) nerves meet at the spinal cord / synapse.
• Among animals, the tree shrew / small mammals have the largest brain relative to body size.

Cerebral palsy is a brain disorder affecting movement, found generally in children.

Meningitis is inflammation of the meninges (brain membranes).

Tetraplegia means paralysis of all four limbs.

Night blindness is caused by deficiency of vitamin A.

Body temperature is regulated by the hypothalamus.

Heartbeat and breathing are controlled by the medulla oblongata.

Input (sensory) and output (motor) nerves meet at the spinal cord / synapse.

Among animals, the tree shrew / small mammals have the largest brain relative to body size.