Chapter 39: Human Nervous System

The nervous system is a type of response system.

The human nervous system consists of two main parts:

  • Central nervous system
  • Peripheral nervous system

The central nervous system is composed of the brain and spinal cord
The peripheral nervous system is composed of all the nerves not found within the boundaries of the central nervous system as well as the collections of nerve cells called ganglia.

Structure of the human nervous system

The functional unit of the human nervous system is the neuron.

  • Neuron: nerve cell specialised to carry electrochemical impulses.

There are three types of neuron:

  1. Sensory neurons: carry impulses towards the central nervous system.
  2. Interneurons: carry impulses from one neuron to another completely within the central nervous system.
  3. Motor neurons: carry impulses from an interneuron to an effector.
  • An effector is an organ or tissue that carries out an action in response to a signal from the nervous system.

Sensory neuron:

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Motor neuron:
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Neuron cell components

  • Dendrites: receive impulses from other neurons and transmit the impulse towards the cell body
  • Cell body: located between the dendrites and axon. It is responsible for the upkeep of the cell and for producing neurotransmitter substances.
  • Neurotransmitter: chemical substance released by a neuron to transmit a nerve impulse to another neuron or effector.
  • Axon: carries impulses away from the cell body towards axon terminals.
  • Myelin sheath: layers of lipids formed from a Schwann cell that wrap around the axon and dendrites of neurons. It insulates the axon/dendrite maintaining and speeding up the electrochemical impulse.
  • Schwann cell: produces myelin sheath.
  • Axon terminals: present at the end of the axon and contain synaptic vesicles which contain neurotransmitters.
  • Synaptic vesicles: contain neurotransmitter chemicals. They fuse with the cell membrane when an impulses reaches the axon terminal. This releases the neurotransmitter.

Nerve impulse:

The nerve impulse is a short-lived electrochemical signal that travels along neurons via movement of chemical ions into and out of the neuron.

Transmission of the nerve impulse to another neuron:
Occurs at synapses.

Synapse: structure where two neurons come into close contact so that a nerve impulse can be transmitted between the two neurons.

Transmission process:

  • Nerve impulse arrives at an axon terminal (presynaptic neuron)
  • Synaptic vesicles are stimulated to fuse with the cell membrane
  • Neurotransmitter chemicals are released from the vesicle into the gap between the two neurons, called the synaptic cleft; examples of neurotransmitter substances include: acetylcholine, noradrenaline and dopamine.
  • Once the neurotransmitter is in the synaptic cleft it travels the short distance to the dendrite of the postsynaptic neuron where it stimulates the cell membrane to allow ions to flow inwards, setting up a new electrochemical impulse.
  • The neurotransmitter is then either broken down by enzymes or reabsorbed into the presynaptic neuron.

Functions of the synapse:

  • Allow transmission of the impulse from one neuron to another.
  • Control the direction of the impulse – the impulse cannot travel backwards.
  • Act as junctions allowing the impulse to be split up and travel along many different neurons or join many impulse together into one impulse.
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The Peripheral Nervous System:
The senses
1. Sight:
The eyes are the sense organs for sight.
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Functions of the parts of the human eye:

  • Conjunctiva: produces mucous protecting the front of the eye.
  • Cornea: transparent part of the sclera that protects the front of the eye; it also allows light to enter the eye and refracts the light rays slightly as part of focusing light onto the retina.
  • Iris: coloured part of the eye; type of smooth muscle that can contract and relax in response to the amount of light entering the eye; when light is bright the iris contracts limiting the amount of light getting in; when light is dim, the iris relaxes allowing much more light into the eye.
  • Pupil: hole in the internal part of the eye just behind the iris; it allows light into the eye and its size is controlled by the iris; it appears black due to light entering and not leaving the eye as it is all absorbed by the eye.
  • Aqueous humour: watery liquid present just inside the cornea that gives shape to the front of the eye.
  • Vitreous humour: viscous liquid present inside the eye ball that maintains the shape of the eye by maintaining outward pressure on the sclera.
  • Ciliary body: type of smooth muscle surrounding the lens that can contract and relax changing the shape of the lens as part of focusing light on the retina.
  • Suspensory ligament: attaches to and surrounds the ciliary body providing a lever for the contraction of the ciliary muscle.
  • Lens: transparent structure held in place by the ciliary body and suspensory ligament; changes shape in response to contraction and relaxation of the ciliary body; responsible for focusing light onto the retina.
  • Sclera: white of the eye covering the entire eye ball except the front part; protects the eye and acts as the attachment surface for external muscles that move the eye in different directions.
  • Choroid: heavily pigmented layer lying between the retina and sclera; absorbs all of the light entering the eye and helps to prevent reflection within the eye.
  • Retina: light-sensitive structure of the eye; contains rods and cones; rods are sensitive to only black and white; cones are sensitive to red, green and blue light.
  • Fovea: region of the retina where all the light rays converge when you look directly at an object; mostly composed of cones.
  • Blind spot: region of the retina where all the nerve fibres from the retina converge and exit the eye and travel to the brain; there are no light-sensitive cells in this regions; there is one in each eye.
  • Optic nerve: collection of sensory neurons that carry messages to the brain from the retina.

Disorders of the eye:

Long-sightedness (Hyperopia):
Symptom:

  • Near objects appear blurred.

Cause:

  • Eyeball is either too short or the focusing elements are too weak.

Treatment:

  • Convex lens placed in front of the eye.
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Short-sightedness (Myopia):
Symptom:

  • Distant objects appear blurred.

Cause:

  • Eyeball is either too long or the focusing elements are too strong.

Treatment:

  • Concave lens in front of the eye.
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Hearing:

  • Detection of sound.
  • The ear is responsible for detection of sound.
  • The ear is composed of three parts:
  1. Outer ear
  2. Middle ear
  3. Inner ear

Outer ear:

  • Composed of the pinna and auditory canal. These pick up sound waves and channel them to the eardrum.

Middle ear:

  • The sound waves arrive at the eardrum and are transferred onto the three small bones of the ear – the ossicles. They are called the hammer, anvil and stirrup. They transfer sound waves onto the inner ear. They can amplify soft sounds and dampen loud sounds.
  • The Eustachian tube is also part of the middle ear. It is connected to the throat so that pressure differences can be equalised during swallowing preventing damage to the eardrum.

Inner ear:

  • Functions in both hearing and balance.
  • Composed of two main structures: the cochlea and the vestibular apparatus.

1. Cochlea:

  • Receives sound vibrations from the ossicles via the oval window – which is the opening to the cochlea covered by a thin membrane. This thin membrane vibrates with the same frequency with which the ossicles vibrate.
  • It is filled with lymph through which the sound waves pass.
  • There are hair cells within the inner wall of the cochlea that sense the vibrations of the lymph within the cochlea and convert these vibrations into electrical impulses that are then sent onto the brain via the auditory nerve.
  • Finally, there is a round window below the oval window that vibrates with an opposite phase to the round window allowing vibrations to be transferred within the lymph more efficiently.

2. Vestibular apparatus:

  • Consists of three semicircular canals filled with lymph.
  • Each canal has hair cells lining its internal walls.
  • As the head moves the lymph moves within the canals stimulating the hair cells.
  • The movement of the lymph is converted into electrical impulses by the hair cells and these impulses are transferred to the brain via the vestibular nerve.
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Disorder of the ear:
Glue ear:
Symptom:

  • Inflammation of the middle ear, muffled hearing and pus formation.

Cause:

  • Infections by viruses and/or bacteria.

Treatment:

  • Ear drops or a grommet for severe infections.

Smell:

  • The sense of smell is also called olfaction.
  • It occurs in the nasal cavity.
  • There are specialised cells called olfactory receptor cells.
  • This group of cells senses the odours and sends signals to the brain via the olfactory bulb and the olfactory nerve.
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Taste:

  • Taste is the sense of detecting flavours.
  • Taste buds in the tongue are the organs associated with taste.
  • There are five basic tastes: sweet, sour, salt, bitter and umami.
  • Taste  buds are evenly distributed over the upper surface of the tongue.

Touch:

  • The skin is the main organ system associated with touch.
  • There are touch receptors all over the skin, but their concentration varies.
  • The fingertips have high concentrations of touch receptors.

The Central Nervous System

The Brain

The brain is the most complex organ of the human body. It is composed of a number of different parts: cerebrumcerebellummedulla oblongatahypothalamuspituitary gland.

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Cerebrum:

  • Largest part of the brain.
  • Composed of two symmetrical hemispheres (left and right).
  • High folded surface giving extra area for neurons.
  • Divided into different lobes: frontal lobe; temporal lobe; parietal lobe; occipital lobe, with each lobe having specific functions.

1. Frontal lobe: functions in reasoning, short-term memory, intelligence, personality, problem solving, emotion, language.
2. Temporal lobe: function in long-term memory, speech, hearing.
3. Parietal lobe: functions in movement, touch.
4. Occipital lobe: functions in vision.

Cerebellum:

  • Located at the back of the brain.
  • Functions in control and coordination of movement.

Medulla oblongata:

  • Belongs to part of the brain called the brainstem (on top of the spinal cord).
  • Functions in breathing, heart rate, blood pressure, vomiting, coughing, sneezing and swallowing.

Hypothalamus:

  • Small region of the brain located just above the brain stem and pituitary.
  • Functions in controlling the endocrine system via secretion of neurohormones such as growth hormone-releasing hormone (GHRH) and thyrotropin-releasing hormone (TRH).
  • It also functions in controlling body temperature, hunger and thirst.

Pituitary gland:

  • The pituitary gland is the link between the nervous system and the endocrine system.
  • It releases many hormones including: growth hormone (GH) and thyroid-stimulating hormone (TSH).

Nervous System Disorder: Parkinson’s Disease)

Cause:

  • Death of specific neurons deep within the brain. The reasons for the death of these neurons is unknown but thought to be caused by exposure to pesticides.

Symptoms:

  • Shaking and trembling of the hands, arms and legs during movement, a stiff and rigid body and fixed stare.

Treatment:

  • Physiotherapy, exercise and a drug called levodopa.

The spinal cord:

  • The spinal cord is a bundle of nerve fibres enclosed within the spine, covered in specialised membranes called the meninges and bathed in cerebrospinal fluid.
  • It carries messages to and from the brain.
  • It has 31 pairs of spinal nerves.
  • There are structures called a dorsal root, dorsal root ganglion and a ventral root associated with each spinal nerve.
  • The dorsal root carries sensory neurons.
  • The dorsal root ganglion contains the cell bodies of those sensory neurons.
  • The ventral root carries the motor neurons.
  • The spinal cord is composed of white matter and grey matter with the outer region of the spinal cord white matter and the inner region grey.
  • There is a central canal that is filled with cerebrospinal fluid.
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Reflex action:

Reflex actions are involuntary responses to a stimulus.

  • Reflex actions are adaptations to enable an animal to protect itself from dangerous situations.
  • Reflex actions are carried out by reflex arcs.
  • Reflex arcs consist of a sensory neuron, an interneuron and a motor neuron.
  • A common reflex action is the withdrawal reflex – where if you touch something hot you will pull your hand away very quickly.
  • The sensory neuron detects that something hot has been touched and sends a message to the spinal cord via the dorsal root.
  • The sensory neuron synapses with an interneuron just inside the grey matter of the spinal cord and passes the message on.
  • The interneuron relays the message onto the motor neuron, whose cell body is just inside the grey matter of the spinal cord.
  • The motor neuron sends a message out through the ventral root to the skeletal muscles of the arm to pull the hand away from the hot object.
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Comparison of the nervous system with the endocrine system:

Nervous system:
Speed of response: Fast
Messages carried by: Electrochemicals (ion movement)
Speed of transmission of message: Fast
Length of response: Short-lived
Areas affected: Specific areas

Endocrine system:
Speed of response: Slow
Messages carried by: Chemical hormones
Speed of transmission of message: Slow
Length of response: Long-lived
Areas affected: Wide areas

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