UNIT 1
Wednesday, 02 April 2025 23:20
INTRODUCTION
The human nervous system plays a crucial role in all human behaviours. Comparative studies of animal nervous systems help us
understand both animal and human brain functions and behaviour. This section focuses on the structure of neurons, how impulses are
conducted in neurons, and the overall structure of the human nervous system. These elements work together to produce and coordinate
human behaviour.
HUMAN NERVOUS SYSTEM:STRUCTURE AND FUNCTIONS
The mammalian nervous system is a complex organ that allows animals, including humans, to function in a coordinated manner. It has
evolved to serve similar adaptive functions across many species. The human nervous system is divided into two main parts: the Peripheral
Nervous System (PNS) and the Central Nervous System (CNS). The PNS controls both voluntary behaviours (through the somatic nervous
system) and involuntary behaviours (through the autonomic nervous system) using cranial and spinal nerves. The CNS consists of the
brain, brainstem, and spinal cord, with each part performing specific functions.
THE PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system (PNS) transmits signals essential for survival and includes both voluntary and involuntary actions. It is
divided into two parts:
1. Somatic Nervous System: Involved in conscious, voluntary activities. It relays sensory and motor information to and from the central
nervous system (CNS). Motor neurons (efferent) carry information from the CNS to muscles, while sensory neurons (afferent) br ing
sensory information to the CNS.
2. Autonomic Nervous System: Controls involuntary functions of internal organs and glands. It has two divisions:
○ Sympathetic Nervous System: Prepares the body for stress-related activities, such as the fight-or-flight response, by energizing
muscles and glands.
○ Parasympathetic Nervous System: Helps return the body to normal, day-to-day functions and maintains homeostasis (balance
of biological conditions like body temperature). Both systems work together to ensure bodily equilibrium.
THE CENTRAL NERVOUS SYSTEM
The central nervous system (CNS), consisting of the brain, brainstem, and spinal cord, is responsible for processing and synthesizing
information. It regulates everything from organ function to high-level thought and body movement, acting as the body's control center.
• The brain is the largest part of the CNS, responsible for functions like sensation, perception, thinking, emotions, and planning. It
includes the limbic system, which regulates emotions, and the cerebellum, which coordinates actions without conscious awareness.
• The cerebrum is the most advanced part, containing the cerebral cortex, which oversees processes like speaking and planning. The
two hemispheres (left and right) control opposite sides of the body. The frontal lobe handles cognitive functions, the parietal lobe
processes sensory input, the occipital lobe manages vision, and the temporal lobe processes hearing and smell.
• The limbic system handles memory, emotions, and drives, while the thalamus and hypothalamus manage sensory input, motor
control, and hormonal regulation.
• The brainstem, including the midbrain, hindbrain, and spinal cord, controls vital life functions like breathing and digestion. The spinal
cord connects the brain to the rest of the body, processing sensory information and sending motor signals to muscles and organs.
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,IMPULSE CONDUCTION IN THE NEURON
A neuron is a cell in the nervous system that transmits information. It has four main parts:
1. Dendrites: Branch-like structures that receive information from other neurons.
2. Cell body (soma): Contains the nucleus and keeps the neuron alive.
3. Axon: A long fiber that sends information to other neurons, muscles, or glands. Some axons are very long, especially those
connecting the spinal cord to limbs.
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, connecting the spinal cord to limbs.
4. Terminal boutons: Branches at the end of the axon that release information at the synapse to other neurons.
Some axons are covered in a myelin sheath, a fatty layer that speeds up signal transmission. Neurons without this sheath (unmyelinated)
transmit signals more slowly. When information is received by the dendrites, it turns into an electro-chemical signal. The electrical part of
the signal, called an action potential, travels down the axon to the next neuron. Bundles of axons are referred to as "nerves" in the
nervous system.
ACTION POTENTIAL
An action potential is an all-or-nothing event where a neuron either fires an impulse or does not. It occurs when the neuron reaches the
threshold of excitation, which excites the neuron enough to trigger the action potential
.
The initiation of an action potential depends on inputs from other neurons, which affect the neuron's membrane potential. There are two
key types of inputs:
1. Excitatory postsynaptic potential (EPSP): A positive current that brings the membrane potential closer to the threshold, making it
more likely to fire.
2. Inhibitory postsynaptic potential (IPSP): A negative current that makes the membrane potential more negative, moving it away from
the threshold.
If both EPSP and IPSP occur simultaneously, they can cancel each other out. If multiple EPSPs or IPSPs occur together, they can intensify
the depolarization or hyperpolarization. The neuron integrates these mixed signals to determine whether it will reach the threshold and
fire an action potential.
THE REFRACTORY PERIOD
The refractory period is a time when a neuron cannot fire another action potential immediately after it has just fired. This period ensures
the neuron is ready to respond to a new stimulus once it returns to its resting state. There are two phases of the refractory period:
1. Absolute refractory period (ARP): During this phase, the neuron cannot fire a new impulse, no matter how strong the stimulus,
because the sodium channels are inactive. The neuron is essentially recharging to be ready for the next impulse.
2. Relative refractory period (RRP): Following the ARP, the neuron can fire again, but only if the stimulus is stronger than usual. The
neuron is partially recharged and requires a stronger stimulus to trigger an action potential.
THE SYNAPSE
Neurons are separated by a small space called the synaptic cleft, where neurotransmitters help transmit signals between neurons. When
an action potential reaches the end of an axon, neurotransmitters are released into the synapse. These neurotransmitters travel across
the synaptic gap and bind to receptors on the dendrites of neighbouring neurons. Different neurotransmitters are released by terminal
buttons, and each receptor site only accepts specific neurotransmitters. The effect of neurotransmitters on the receiving neuron can be
excitatory (making the neuron more likely to fire) or inhibitory (making it less likely to fire). If excitatory signals outweigh inhibitory
PYC1501 Page 3
, excitatory (making the neuron more likely to fire) or inhibitory (making it less likely to fire). If excitatory signals outweigh inhibitory
signals, the neuron reaches its firing threshold, and an action potential is triggered. To prepare for the next signal, neurotransmitters that
aren’t accepted are removed through enzymatic breakdown or reuptake, where they are reabsorbed into the sending neuron. This
process ensures efficient communication between neurons.
NEURAL COMMUNICATION-AN ELECTRICAL AND CHEMICAL PROCESS
The nervous system uses an electro-chemical process to transmit signals. Inside a neuron, when a signal is received by the dendrites, it is
converted into an electrical signal and sent to the soma. If strong enough, the signal travels down the axon to the terminal buttons, which
release neurotransmitters. The axon typically remains in a resting membrane potential, where the inside is more negatively charged than
the outside. When a strong enough signal reaches the axon, it opens gates allowing positive sodium ions to enter, causing a change in
electrical charge, known as the action potential. This electrical impulse travels down the axon in small jumps from one Node of Ranvier
(gaps in the myelin sheath) to the next. As each segment of the axon becomes positively charged, it stimulates the next segment. The
action potential moves quickly, sometimes up to 1,000 times per second. If the neuron is unmyelinated, the signal moves more slowly.
The action potential is an all-or-nothing event; neurons can fire faster, but not more strongly. After firing, there is a refractory period,
during which the neuron cannot fire again until it returns to its resting potential.
THE NEUROTRANSMITTERS
Neurotransmitters are chemical substances in the body that influence emotions, cognition, and behaviour. They regulate functions like
appetite, memory, emotions, and muscle movement. Some neurotransmitters are linked to psychological and physical diseases.
Key Neurotransmitters and Their Functions:
• Acetylcholine (ACh): Involved in muscle contractions, memory, and sleep. A deficiency is linked to Alzheimer’s disease.
• Dopamine: Affects movement, motivation, pleasure, and learning. Too much dopamine is linked to schizophrenia, while too little is
linked to Parkinson’s disease.
• Endorphins: Act as natural pain relievers. A deficiency can cause depression.
• GABA (Gamma-Aminobutyric Acid): The main inhibitory neurotransmitter, reducing anxiety and preventing excessive brain activity.
Low levels can cause tremors, seizures, and anxiety.
• Glutamate: The main excitatory neurotransmitter, involved in brain function. Excess glutamate can cause migraines and seizures.
• Serotonin: Regulates mood, appetite, sleep, and aggression. Low levels are linked to seasonal depression.
• Adrenaline: Increases heart rate, metabolism, and blood glucose levels. Overproduction can cause high blood pressure,
palpitations, and excessive sweating.
• Noradrenaline (NA): Excites neurons. A deficiency causes depression, while excess causes mania (extreme excitement and
overactivity).
Drugs and Their Effects on Neurotransmitters:
• Agonists mimic neurotransmitters and enhance their effects (e.g., cocaine mimics dopamine, creating pleasure).
• Antagonists block neurotransmitter effects (e.g., curare blocks acetylcholine, leading to paralysis and potential death).
• Reuptake inhibitors prevent neurotransmitters from being reabsorbed, increasing their action in the synapse.
These neurotransmitters and their interactions play a crucial role in mental and physical health.
SUMMARY
This learning section provided you with a brief overview of the human brain and its functions and other external features that connect to
the brain. Two important elements of the nervous system, the peripheral and the central nervous system were presented. In addition, the
neural activity was described and explained in detail. This learning section further elaborated on how the parts of the brain fit together to
determine everyday human behaviour. The section also discussed neurons in the central nervous system and how they release an action
potential. The section concluded by a discussion on neurotransmitters and their role in human functioning.
PYC1501 Page 4
Wednesday, 02 April 2025 23:20
INTRODUCTION
The human nervous system plays a crucial role in all human behaviours. Comparative studies of animal nervous systems help us
understand both animal and human brain functions and behaviour. This section focuses on the structure of neurons, how impulses are
conducted in neurons, and the overall structure of the human nervous system. These elements work together to produce and coordinate
human behaviour.
HUMAN NERVOUS SYSTEM:STRUCTURE AND FUNCTIONS
The mammalian nervous system is a complex organ that allows animals, including humans, to function in a coordinated manner. It has
evolved to serve similar adaptive functions across many species. The human nervous system is divided into two main parts: the Peripheral
Nervous System (PNS) and the Central Nervous System (CNS). The PNS controls both voluntary behaviours (through the somatic nervous
system) and involuntary behaviours (through the autonomic nervous system) using cranial and spinal nerves. The CNS consists of the
brain, brainstem, and spinal cord, with each part performing specific functions.
THE PERIPHERAL NERVOUS SYSTEM
The peripheral nervous system (PNS) transmits signals essential for survival and includes both voluntary and involuntary actions. It is
divided into two parts:
1. Somatic Nervous System: Involved in conscious, voluntary activities. It relays sensory and motor information to and from the central
nervous system (CNS). Motor neurons (efferent) carry information from the CNS to muscles, while sensory neurons (afferent) br ing
sensory information to the CNS.
2. Autonomic Nervous System: Controls involuntary functions of internal organs and glands. It has two divisions:
○ Sympathetic Nervous System: Prepares the body for stress-related activities, such as the fight-or-flight response, by energizing
muscles and glands.
○ Parasympathetic Nervous System: Helps return the body to normal, day-to-day functions and maintains homeostasis (balance
of biological conditions like body temperature). Both systems work together to ensure bodily equilibrium.
THE CENTRAL NERVOUS SYSTEM
The central nervous system (CNS), consisting of the brain, brainstem, and spinal cord, is responsible for processing and synthesizing
information. It regulates everything from organ function to high-level thought and body movement, acting as the body's control center.
• The brain is the largest part of the CNS, responsible for functions like sensation, perception, thinking, emotions, and planning. It
includes the limbic system, which regulates emotions, and the cerebellum, which coordinates actions without conscious awareness.
• The cerebrum is the most advanced part, containing the cerebral cortex, which oversees processes like speaking and planning. The
two hemispheres (left and right) control opposite sides of the body. The frontal lobe handles cognitive functions, the parietal lobe
processes sensory input, the occipital lobe manages vision, and the temporal lobe processes hearing and smell.
• The limbic system handles memory, emotions, and drives, while the thalamus and hypothalamus manage sensory input, motor
control, and hormonal regulation.
• The brainstem, including the midbrain, hindbrain, and spinal cord, controls vital life functions like breathing and digestion. The spinal
cord connects the brain to the rest of the body, processing sensory information and sending motor signals to muscles and organs.
PYC1501 Page 1
,IMPULSE CONDUCTION IN THE NEURON
A neuron is a cell in the nervous system that transmits information. It has four main parts:
1. Dendrites: Branch-like structures that receive information from other neurons.
2. Cell body (soma): Contains the nucleus and keeps the neuron alive.
3. Axon: A long fiber that sends information to other neurons, muscles, or glands. Some axons are very long, especially those
connecting the spinal cord to limbs.
PYC1501 Page 2
, connecting the spinal cord to limbs.
4. Terminal boutons: Branches at the end of the axon that release information at the synapse to other neurons.
Some axons are covered in a myelin sheath, a fatty layer that speeds up signal transmission. Neurons without this sheath (unmyelinated)
transmit signals more slowly. When information is received by the dendrites, it turns into an electro-chemical signal. The electrical part of
the signal, called an action potential, travels down the axon to the next neuron. Bundles of axons are referred to as "nerves" in the
nervous system.
ACTION POTENTIAL
An action potential is an all-or-nothing event where a neuron either fires an impulse or does not. It occurs when the neuron reaches the
threshold of excitation, which excites the neuron enough to trigger the action potential
.
The initiation of an action potential depends on inputs from other neurons, which affect the neuron's membrane potential. There are two
key types of inputs:
1. Excitatory postsynaptic potential (EPSP): A positive current that brings the membrane potential closer to the threshold, making it
more likely to fire.
2. Inhibitory postsynaptic potential (IPSP): A negative current that makes the membrane potential more negative, moving it away from
the threshold.
If both EPSP and IPSP occur simultaneously, they can cancel each other out. If multiple EPSPs or IPSPs occur together, they can intensify
the depolarization or hyperpolarization. The neuron integrates these mixed signals to determine whether it will reach the threshold and
fire an action potential.
THE REFRACTORY PERIOD
The refractory period is a time when a neuron cannot fire another action potential immediately after it has just fired. This period ensures
the neuron is ready to respond to a new stimulus once it returns to its resting state. There are two phases of the refractory period:
1. Absolute refractory period (ARP): During this phase, the neuron cannot fire a new impulse, no matter how strong the stimulus,
because the sodium channels are inactive. The neuron is essentially recharging to be ready for the next impulse.
2. Relative refractory period (RRP): Following the ARP, the neuron can fire again, but only if the stimulus is stronger than usual. The
neuron is partially recharged and requires a stronger stimulus to trigger an action potential.
THE SYNAPSE
Neurons are separated by a small space called the synaptic cleft, where neurotransmitters help transmit signals between neurons. When
an action potential reaches the end of an axon, neurotransmitters are released into the synapse. These neurotransmitters travel across
the synaptic gap and bind to receptors on the dendrites of neighbouring neurons. Different neurotransmitters are released by terminal
buttons, and each receptor site only accepts specific neurotransmitters. The effect of neurotransmitters on the receiving neuron can be
excitatory (making the neuron more likely to fire) or inhibitory (making it less likely to fire). If excitatory signals outweigh inhibitory
PYC1501 Page 3
, excitatory (making the neuron more likely to fire) or inhibitory (making it less likely to fire). If excitatory signals outweigh inhibitory
signals, the neuron reaches its firing threshold, and an action potential is triggered. To prepare for the next signal, neurotransmitters that
aren’t accepted are removed through enzymatic breakdown or reuptake, where they are reabsorbed into the sending neuron. This
process ensures efficient communication between neurons.
NEURAL COMMUNICATION-AN ELECTRICAL AND CHEMICAL PROCESS
The nervous system uses an electro-chemical process to transmit signals. Inside a neuron, when a signal is received by the dendrites, it is
converted into an electrical signal and sent to the soma. If strong enough, the signal travels down the axon to the terminal buttons, which
release neurotransmitters. The axon typically remains in a resting membrane potential, where the inside is more negatively charged than
the outside. When a strong enough signal reaches the axon, it opens gates allowing positive sodium ions to enter, causing a change in
electrical charge, known as the action potential. This electrical impulse travels down the axon in small jumps from one Node of Ranvier
(gaps in the myelin sheath) to the next. As each segment of the axon becomes positively charged, it stimulates the next segment. The
action potential moves quickly, sometimes up to 1,000 times per second. If the neuron is unmyelinated, the signal moves more slowly.
The action potential is an all-or-nothing event; neurons can fire faster, but not more strongly. After firing, there is a refractory period,
during which the neuron cannot fire again until it returns to its resting potential.
THE NEUROTRANSMITTERS
Neurotransmitters are chemical substances in the body that influence emotions, cognition, and behaviour. They regulate functions like
appetite, memory, emotions, and muscle movement. Some neurotransmitters are linked to psychological and physical diseases.
Key Neurotransmitters and Their Functions:
• Acetylcholine (ACh): Involved in muscle contractions, memory, and sleep. A deficiency is linked to Alzheimer’s disease.
• Dopamine: Affects movement, motivation, pleasure, and learning. Too much dopamine is linked to schizophrenia, while too little is
linked to Parkinson’s disease.
• Endorphins: Act as natural pain relievers. A deficiency can cause depression.
• GABA (Gamma-Aminobutyric Acid): The main inhibitory neurotransmitter, reducing anxiety and preventing excessive brain activity.
Low levels can cause tremors, seizures, and anxiety.
• Glutamate: The main excitatory neurotransmitter, involved in brain function. Excess glutamate can cause migraines and seizures.
• Serotonin: Regulates mood, appetite, sleep, and aggression. Low levels are linked to seasonal depression.
• Adrenaline: Increases heart rate, metabolism, and blood glucose levels. Overproduction can cause high blood pressure,
palpitations, and excessive sweating.
• Noradrenaline (NA): Excites neurons. A deficiency causes depression, while excess causes mania (extreme excitement and
overactivity).
Drugs and Their Effects on Neurotransmitters:
• Agonists mimic neurotransmitters and enhance their effects (e.g., cocaine mimics dopamine, creating pleasure).
• Antagonists block neurotransmitter effects (e.g., curare blocks acetylcholine, leading to paralysis and potential death).
• Reuptake inhibitors prevent neurotransmitters from being reabsorbed, increasing their action in the synapse.
These neurotransmitters and their interactions play a crucial role in mental and physical health.
SUMMARY
This learning section provided you with a brief overview of the human brain and its functions and other external features that connect to
the brain. Two important elements of the nervous system, the peripheral and the central nervous system were presented. In addition, the
neural activity was described and explained in detail. This learning section further elaborated on how the parts of the brain fit together to
determine everyday human behaviour. The section also discussed neurons in the central nervous system and how they release an action
potential. The section concluded by a discussion on neurotransmitters and their role in human functioning.
PYC1501 Page 4
