Human Nervous System Notes

Human Nervous System

Divisions of the Human Nervous System

• The human nervous system is subdivided into two main divisions:
  • Central nervous system (CNS): Consisting of the brain and spinal cord.
  • Peripheral nervous system: Consisting of the nerves that conduct nerve impulses to and from the brain and the spinal cord. It includes 12 pairs of cranial nerves and 31 pairs of spinal nerves.

Structure and Functioning of Nerves

• A nerve consists of numerous nerve fibres surrounded by connective tissue.
• Individual nerve fibres are surrounded and separated from each other by a thin layer of connective tissue called the endoneurium.
• Bundles of nerve fibres are bound together with connective tissue called the perineurium.
• A dense sheath of connective tissue, the epineurium, surrounds a number of nerve bundles to form a single nerve.
• Blood vessels (veins and arteries) occur between the bundles, providing the fibres with oxygen, nutrients, and removing waste.

Components of Nervous Tissue and Structure of a Motor Neuron

• Nervous tissue consists of millions of nerve cells called neurons.
• Connective tissue, known as neuroglia, occurs between the neurons for nourishment, insulation, and support.
  • Neuron = nerve cell
  • Glia = glue
• Neurons are the structural units of the nervous system.
• A neuron has the same basic structure as a typical animal cell with a cell membrane, cytoplasm, nucleus, and organelles.
• A neuron has a cell body consisting of cytoplasm and a prominent nucleus.
• The cytoplasm contains dark granules, the Nissl granules, which are rich in RNA and involved in protein synthesis.
• Two types of outgrowths may extend from the cell body: dendrites and axons.
  • Dendrites: conduct nerve impulses towards the cell body.
  • Axons: conduct nerve impulses away from the cell body.
• The axon forms a number of small branches called the terminal branches.
• Synaptic knobs occur at the tips of the terminal branches.
• Most of the nerve fibres situated outside the CNS are enclosed by a myelin sheath formed by specialised cells, the Schwann cells.
• The Schwann cells wrap themselves around nerve fibres.
• A white, fatty protein called myelin occurs in the cell membranes of the Schwann cells which forms the myelin sheath.
• The myelin sheath insulates the nerve fibres and accelerates the transmission of nerve impulses.
• The myelin sheath is not continuous; it has gaps between the adjacent Schwann cells called the nodes of Ranvier.
• The nodes of Ranvier accelerate nerve transmission as impulses jump from one node to another.
• The outermost membrane of the myelin sheath is the neurilemma.
• The neurilemma is essential for repair or regeneration of damaged neurons.

Types of Neurons

• Motor neurons: conduct nerve impulses from the CNS to the effectors and are multipolar.
• Sensory neurons: conduct nerve impulses from the receptors to the CNS and are mostly unipolar.
• Interneurons: connect sensory and motor neurons in the CNS and are multipolar.

Transmission of Nerve Impulses Along Neurons and Across Synapses

• Neurons are interconnected.
• The axon of one neuron connects with the dendrites of the next neuron.
• A physiological (functional) connection is formed, known as the synapse, and the gap is called the synaptic cleft.
• Nerve impulses move along the axon to the synaptic knobs of the terminal branches.
• In each synaptic knob are many synaptic vesicles filled with a neurotransmitter.
• The synaptic cleft is bordered by a pre-synaptic membrane of the synaptic knob on the axon side and by post-synaptic membrane on the dendrite side.
• The nerve impulses cause the synaptic vesicles to burst open at the surface, releasing the neurotransmitter into the synaptic cleft.
• The neurotransmitter moves across the cleft and attaches to the post-synaptic membrane. This triggers electric signals that are conducted as nerve impulses via the dendrite, to the cell body of the next neuron.
• After the neurotransmitter causes an electrical impulse on the post-synaptic membrane of the dendrite, the excess is broken down and recycled to provide a constant supply of neurotransmitter inside the pre-synaptic membrane.

Significance of Synapses:

• It ensures that the impulse moves in one direction only.
• It prevents continuous stimulation of the neurons.
• It ensures that the impulse is transmitted from the sensory neuron to the motor neuron.
• A neurotransmitter is a chemical that is released into the synapse to conduct impulses from one neuron to the next.

Components of the Central Nervous System (CNS)

• The main function of the central nervous system (CNS) is to receive nerve impulses from the receptors, interpret the information, and transmit it to the effectors to bring about the desired response.

Protection of the Central Nervous System

• Nervous tissue is very soft and most cells cannot repair or regenerate themselves.
• Nervous tissue in the brain and spinal cord must therefore be well protected against injuries.
  • The CNS is surrounded externally by bone. The brain is enclosed by the cranium and the spinal cord by the vertebral column.
  • Internally the brain and spinal cord are surrounded by three meninges (cerebral membranes):
    • Pia mater: Innermost membrane that is tightly wrapped around the brain and spinal cord.
    • Dura mater: The tough outermost membrane that lines the skull cavity and vertebral canal.
    • Arachnoid: A thin membrane between the pia mater and dura mater.
  • Between the pia mater and the arachnoid is a space filled with cerebrospinal fluid.
  • The spinal cord is also protected by the cartilaginous discs between the vertebrae and the S-shaped curvature of the spine that aids in shock absorption and flexibility.

Functions of Cerebrospinal Fluid:

• Shock absorber.
• Supplies neurons with nutrients and oxygen.
• Removes waste.
• Prevents dehydration of neurons.
• Maintains constant pressure around the CNS.

Grey Matter vs White Matter

• Both the brain and the spinal cord consist of grey matter and white matter.
  • Grey matter: consists of darker cell bodies and dendrites of neurons.
  • White matter: consists of myelinated axons that appear white due to the fatty myelin sheaths that enclose the axons.
• The grey matter in the brain is mostly outside and surrounds the white matter.
• It is opposite in the spinal cord, i.e. the white matter is on the outside and the H-shaped grey matter on the inside.

Brain

• The brain is the enlarged upper part of the CNS and is divided into the following main parts:
  • Cerebrum
  • Cerebellum
  • Midbrain
  • Thalamus
  • Hypophysis/pituitary gland
  • Corpus callosum
  • Medulla oblongata
  • Pons
  • Hypothalamus

Description and Functions of Brain Components

• Cerebrum:
  • The largest part of the brain.
  • Divided into two hemispheres (left and right) which are connected by the corpus callosum.
  • Controls voluntary functions (walking, speaking, writing).
  • Receives and interprets sensations from sense organs (hearing, sight, feeling, taste, smell).
  • Higher thought processes (memory, intelligence, reasoning).
• Corpus Callosum:
  • C-shaped structure between the two hemispheres of the cerebrum.
  • Consists of white matter, composed of myelinated axons, that connects the two cerebral hemispheres.
  • Largest bundle of nerve fibres in the brain.
  • Conducts impulses between two hemispheres of the cerebrum.
  • Forms a bridge for communication between hemispheres to coordinate processes.
• Cerebellum:
  • Second largest part of the brain.
  • Located behind and below the cerebrum.
  • Coordinates skeletal muscles to bring about balance while moving, as in walking or running.
  • Maintains balance and posture.
  • Maintains muscle tone.
• Medulla Oblongata:
  • Lower part of the brain.
  • Continues down into the body as the spinal cord.
  • Controls breathing, peristalsis, heartbeat, swallowing.
  • Transmits impulses from the spinal cord to the brain.
  • Controls less important reflexes: blinking, coughing, sneezing, vasodilation, vasoconstriction and salivating.
• Spinal Cord:
  • Forms part of the CNS that extends from the medulla oblongata.
  • The spinal cord consists of the following:
    • Central canal filled with cerebrospinal fluid.
    • Grey matter that forms an H-shape.
    • White matter on the outside.
    • Two grooves: a narrow, deep dorsal groove and a shallow, wide ventral groove.
  • Spinal nerves arise from both sides of the spinal cord (dorsal root and ventral root).
  • The dorsal root consists of sensory neurons conducting nerve impulses from receptors to the spinal cord.
  • The cell bodies of the sensory neurons occur in a swelling of the dorsal root, called the ganglion.
  • The nerve fibres in the dorsal root enter the spinal cord at the dorsal horn of the grey matter.
  • The ventral root consists of motor neurons conducting nerve impulses away from the spinal cord to the effectors.
  • Dendrites and cell bodies of the motor neurons are situated in the ventral horn of the grey matter, while the axons exit the spinal cord in the ventral root.
  • Dorsal and ventral roots join just outside the spinal cord to form a mixed spinal nerve which contains both sensory and motor neurons.
  • Nerves branch out to form the peripheral nervous system that connects the CNS to the rest of the body.
  • Provides a pathway for nerve impulses to and from the brain. Nerve impulses from receptors enter the spinal cord via spinal nerves. Impulses are then transmitted along ascending tracts in the spinal cord to the brain. The impulses are transmitted from the brain, via motor neurons in descending tracts, back down to the spinal cord.
  • Contains reflex centres that initiate reflex actions.

Peripheral Nervous System

• Includes all the nervous tissue situated outside the CNS.
• Consists of sensory nerves that conduct nerve impulses from the receptors to the CNS as well as motor nerves that conduct nerve impulses from the CNS to the effectors.
• The motor nerves are subdivided into:
  • Somatic nervous system: conducts nerve impulses from the CNS to the voluntary muscles (skeletal muscles) and thus controls voluntary actions e.g. reading, running and clapping.
  • Autonomic nervous system: conducts nerve impulses from the CNS to the involuntary muscles (smooth muscles and heart muscles) and glands and thus controls all involuntary actions e.g. blinking, peristalsis and sneezing.
• The 12 pairs of cranial nerves are connected to the brain and can be sensory, motor, or mixed nerves.
• The 31 pairs of spinal cord nerves are all mixed nerves which are joined to the spinal cord via a dorsal and ventral root.

Functions of the Peripheral Nervous System:

• Conducts nerve impulses from the receptors to the CNS via the sensory neurons.
• Conducts nerve impulses from the CNS to the effectors via the motor neurons.

Sympathetic vs. Parasympathetic Nervous System

• Sympathetic nervous system: prepares the body for an emergency (fight-or-flight).
  • Emergencies require: faster heart rate, higher blood pressure, faster rate and depth of breathing, faster respiratory rate, increased oxygen/glucose, supply, etc.
• Parasympathetic nervous system: allows the body to return to normal after stimulation by the sympathetic nervous system.
• These two systems are antagonistic to each other.

The Reflex Arc

Reflex Actions:

• A reflex action is a rapid, involuntary response of an effector to a stimulus received by a receptor.
• A reflex action initially takes place subconsciously.
• Reflex actions have a protective function as they allow the body to react rapidly to harmful stimuli (protect the body against further injuries).
• Reflex centres that control protective reflexes occur mainly in the spinal cord (withdrawal of hand or food away from painful stimulus, and the knee-jerk reflex).
• Other reflexes such as coughing, sneezing and blinking are controlled by the medulla oblongata.
• All of these reflexes are called natural or unconditioned reflexes.
• Reflexes, such as walking, cycling, writing and reading, are called learned or conditioned reflexes.
• The cerebrum and the cerebellum are involved in the process of learning conditioned reflexes through practice and repetition until they become automatic.

Reflex Arc:

• The pathway along which nerve impulses are transmitted from a receptor to an effector.
• Parts of the reflex arc include:
  • Receptor: detects a stimulus and converts it into a nerve impulse.
  • Sensory neuron: conducts the nerve impulse from the receptor to the grey matter of the CNS.
  • Interneuron: serves as the reflex centre in the grey matter of the spinal cord and conducts the nerve impulse from the sensory neuron to the motor neuron.
  • Motor neuron: conducts the nerve impulse from the CNS to the effector.
  • Effector: muscle or gland that responds to the original stimulus.

The Functioning of a Reflex Arc:

The reflex action of a person touching a hot pot:

1. The stimulus is detected by receptors and converted into a nerve impulse.
2. The nerve impulse is transmitted along the sensory neuron through the dorsal root to the spinal cord.
3. The impulse is transmitted from the sensory neuron to an interneuron in the spinal cord.
4. The impulse is transmitted from the interneuron to a motor neuron in the spinal cord.
5. The impulse exits the spinal cord through the ventral root and is transmitted along the axon of the motor neuron to the effector organs (muscle; this causes the muscles in the arm to contract).
6. The hand pulls away from the stimulus quickly.

Diseases or Disorders Associated with the Human Nervous System

Alzheimer’s

• Causes: Scientists have not been able to determine the exact cause, but it appears that it develops very slowly as a result of a complex series of changes in the brain:
  • Plaque is formed between neurons.
  • Tiny fibres become tangled inside the cell bodies of the neurons.
  • A shortage of the neurotransmitter, acetylcholine, occurs in the synapses.
  • These changes cause healthy neurons to become less and less efficient because the transmission of the nerve impulses between neurons is restricted.
  • Consequently, the neurons decay and the affected parts of the brain shrink.
• Symptoms:
  • Memory loss and confusion.
  • Problems with recognising family and friends.
  • Cannot learn new things or handle new situations.
  • Increasing aggression, irritability, and mood swings.
  • Ability to speak decreases.
  • Loss of speech.
  • Inability to walk.
• Treatment: Complex disease and difficult to treat.

Multiple Sclerosis (MS)

• Causes:
  • An auto-immune condition that affects the neurons in the brain and spinal cord.
  • The immune system destroys the myelin sheaths of neurons.
  • Myelin sheaths harden over time and eventually the neuron, and the entire nerve, is permanently damaged.
  • This leads to increasing ability in functions such as vision, walking, speech, and memory.
  • MS can affect any person, but is more prevalent in white females aged 20-40 years.
  • More common in colder countries.
  • Viral infections act as a possible trigger.
  • Genetic factors seem to play a role.
• Symptoms:
  • Numbness in arms or legs.
  • Loss of vision or double vision.
  • Tingling sensation or pain.
  • Fatigue and weakness.
  • Loss of memory and mood swings.
• Treatment:
  • A medical examination, blood tests, MRI scans and lumbar puncture may confirm diagnosis.
  • There is no cure for MS.
  • Anti-inflammatories reduce inflammation in affected tissues.
  • Muscle relaxants reduce spasms and pain.
  • Medication may relieve fatigue, incontinence, and depression.

Effects of Specific Drugs on the Central Nervous System

• Dagga, heroin, ecstasy, and tik (methamphetamine) can significantly affect the central nervous system.