Coordination and Control notes

Receptors and Effectors

A receptor is a specialised cell or group of cells that detects a stimulus. Stimuli may be internal or external, such as light, sound, pressure, temperature, chemicals, pain or changes in blood composition. Receptors convert the stimulus into an electrical signal that can travel through a sensory neuron.

The response is carried out by an effector. Muscles contract and glands secrete substances, so both muscles and glands are effectors. Motor neurons carry impulses from the central nervous system to effectors.

• A stimulus is a detectable change in the internal or external environment.
• Receptors detect stimuli and initiate nerve impulses.
• Effectors produce responses. The main effectors are muscles and glands.
• Sensory neurons carry impulses from receptors to the central nervous system.
• Motor neurons carry impulses from the central nervous system to muscles or glands.
• Mechanoreceptors detect mechanical changes such as pressure, stretch and movement.
• Mechanoreceptors in the inner ear help maintain equilibrium.
• The basic pathway is receptor, sensory neuron, central nervous system, motor neuron, effector.

Neurons and Their Structure

A neuron, or nerve cell, is the structural and functional unit of the nervous system. It is specialised to receive, conduct and transmit electrical impulses. The cell body, also called the soma, contains the nucleus and most of the cytoplasm.

Dendrites usually receive signals and conduct them towards the soma. The axon carries signals away from the soma. The axon may be covered by a myelin sheath, which is interrupted at regular intervals by nodes of Ranvier. Myelin increases the speed of impulse conduction.

Nissl's granules are groups of ribosomes and rough endoplasmic reticulum in the cytoplasm of the soma and dendrites. They help in protein synthesis. Neurons do not possess a sarcolemma, which is the specialised plasma membrane of a muscle fibre.

• Dendrites generally carry signals towards the soma.
• The axon carries signals away from the soma.
• The soma contains the nucleus and organelles required for cell maintenance.
• Nissl's granules are groups of ribosomes associated with rough endoplasmic reticulum.
• Nissl's granules are present in the soma and dendrites but are absent from the axon.
• A sensory neuron has a dendrite connected with a receptor and an axon connected with other neurons.
• A motor neuron carries impulses to a muscle or gland.
• An interneuron, or relay neuron, connects neurons within the central nervous system.
• Nerve cells are devoid of sarcolemma.

Nerve Impulse and Action Potential

A nerve impulse is an action potential that travels along the membrane of a neuron. At rest, the membrane is polarised. The inside of the axon is negative relative to the outside because of unequal ion distribution and selective membrane permeability. The resting potential is maintained mainly by the sodium-potassium pump and ion channels.

When a sufficient stimulus reaches the threshold, sodium ion channels open. Sodium ions enter the axon and the inside becomes less negative, then positive. This phase is depolarization. Sodium channels then close and potassium channels open. Potassium ions leave the axon, making the inside negative again. This is repolarization.

For a short time after an impulse, the membrane is less able or unable to respond to another stimulus. This refractory period ensures that impulses normally travel in one direction. In a myelinated axon, the impulse jumps from one node of Ranvier to the next. This is saltatory conduction.

• A resting neuron has a polarised membrane.
• Depolarization results mainly from the entry of Na+ through voltage-gated sodium channels.
• Repolarization results mainly from the exit of K+ through potassium channels.
• The sodium-potassium pump moves Na+ out of the neuron and K+ into the neuron using ATP.
• An action potential follows the all-or-none principle once threshold is reached.
• The refractory period prevents immediate repeated excitation of the same membrane region.
• Myelin increases the speed of impulse conduction.
• In saltatory conduction, the impulse passes between successive nodes of Ranvier.
• The stages of an action potential include resting potential, depolarization and repolarization.

Synapses and Neurotransmitters

A synapse is a junction between two neurons or between a motor neuron and a muscle cell. The small gap between the cells is the synaptic cleft. The neuron carrying the impulse towards the synapse is presynaptic, and the receiving cell is postsynaptic.

When an impulse reaches the presynaptic ending, calcium ions enter the synaptic knob. Vesicles fuse with the presynaptic membrane and release a neurotransmitter into the synaptic cleft. The neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic membrane.

Acetylcholine is a neurotransmitter. It may excite a postsynaptic cell, such as a muscle cell, by opening ion channels. It is then broken down or removed so that continuous stimulation does not occur. Neurotransmitters also function in communication within the brain.

• A synapse is a junction between two neurons or between a motor neuron and a muscle cell.
• The synaptic cleft is the narrow gap between two cells at a synapse.
• Neurotransmitters carry signals across the synaptic cleft.
• Acetylcholine is an example of a neurotransmitter.
• Transmission across a chemical synapse is usually one-way.
• Calcium ions entering the synaptic knob trigger neurotransmitter release.
• The neurotransmitter binds to receptors on the postsynaptic membrane.
• Alzheimer's disease is associated with reduced acetylcholine function in the brain.

Reflexes and the Reflex Arc

A reflex action is a rapid, automatic response to a stimulus. It usually protects the body from injury and does not require conscious thought before the response occurs. The spinal cord commonly coordinates simple reflexes, although the brain receives information about the event.

The pathway followed by an impulse during a reflex action is called a reflex arc. A receptor detects the stimulus and a sensory neuron carries the impulse to the central nervous system. In the spinal cord, a relay neuron may connect the sensory neuron to a motor neuron. The motor neuron then carries the impulse to the effector.

Reflex actions are rapid because the pathway is short and the response is organised in the central nervous system. Withdrawal of a hand from a hot object is an example. Reflexes are not always controlled only by the spinal cord, because some reflex pathways involve the brainstem or other parts of the brain.

• A reflex action is rapid, automatic and involuntary.
• A reflex arc is the pathway followed by impulses during a reflex action.
• The main components are receptor, sensory neuron, relay neuron, motor neuron and effector.
• A relay neuron is located in the central nervous system.
• The effector in a withdrawal reflex is usually a skeletal muscle.
• The spinal cord can coordinate a reflex before the brain becomes consciously aware of the stimulus.
• A reflex response is different from a voluntary response because it does not initially require conscious decision-making.
• Glands and muscles are activated by motor neurons.

Brain and Nervous System Control

The central nervous system consists of the brain and spinal cord. The brain receives sensory information, interprets it and coordinates responses. The peripheral nervous system consists of nerves that connect the central nervous system with receptors, muscles and glands.

The cerebrum is the largest part of the brain. It is divided into right and left cerebral hemispheres. Each hemisphere generally controls the opposite side of the body. Therefore, the right cerebral hemisphere controls the left side of the body. The cerebrum is concerned with conscious thought, memory, intelligence, interpretation of sensory information and voluntary movement.

The hypothalamus is involved in homeostasis. It regulates body temperature, water balance, appetite and several endocrine functions. The pituitary gland is closely linked with the hypothalamus. The cerebellum coordinates muscular movement and balance. The medulla oblongata controls involuntary activities such as breathing, heartbeat and swallowing.

• The central nervous system consists of the brain and spinal cord.
• The peripheral nervous system consists of nerves outside the brain and spinal cord.
• The cerebrum is concerned with conscious activities and voluntary control.
• The left cerebral hemisphere generally controls the right side of the body.
• The right cerebral hemisphere generally controls the left side of the body.
• The hypothalamus regulates body temperature and helps maintain homeostasis.
• The cerebellum coordinates balance, posture and precise muscular movement.
• The medulla oblongata controls involuntary actions including breathing and heartbeat.
• The spinal cord conducts impulses to and from the brain and coordinates many reflexes.
• The blind spot is the place where the optic nerve leaves the retina, so no photoreceptors are present there.

Chemical Coordination and Hormones

Chemical coordination is carried out by hormones released by endocrine glands. A hormone is a chemical messenger secreted into the blood and transported to target cells. A target cell has a specific receptor for that hormone. Hormones usually act more slowly than nerve impulses, but their effects may last longer.

Protein and peptide hormones are generally water-soluble and bind to receptors on the cell surface. They activate second messenger systems inside the cell. Steroid hormones are lipid-soluble and pass through the plasma membrane. They bind to intracellular receptors in the cytoplasm or nucleus and influence gene expression.

The hypothalamus controls the pituitary gland through releasing and inhibiting hormones. The anterior pituitary secretes hormones such as growth hormone, thyroid-stimulating hormone, adrenocorticotrophic hormone, follicle-stimulating hormone, luteinizing hormone and prolactin. The posterior pituitary releases ADH and oxytocin, which are produced in the hypothalamus.

• Hormones are chemical messengers transported in the blood.
• Hormones act only on target cells with suitable receptors.
• Steroid hormones act by binding intracellular receptors.
• Protein hormones commonly bind receptors on the plasma membrane.
• The anterior pituitary is controlled by hypothalamic releasing and inhibiting hormones.
• Somatotrophin, also called growth hormone or STH, promotes growth of the body.
• Excess growth hormone during childhood causes gigantism, while excess secretion after growth has stopped causes acromegaly.
• SIADH means syndrome of inappropriate antidiuretic hormone secretion and is not a result of anterior pituitary hyperfunction.
• Oxytocin secretion during cervical dilation in childbirth is an example of positive feedback, not negative feedback.
• Acetylcholine is a neurotransmitter, whereas hormones are transported through the blood to target tissues.

Endocrine Glands, Feedback and Homeostasis

Negative feedback maintains a variable near its normal level. A change in a condition produces a response that opposes the original change. Regulation of blood glucose, body temperature and water balance involves negative feedback. Positive feedback strengthens the original change and is used in selected processes such as childbirth.

The pancreas contains endocrine cell groups called the islets of Langerhans. Beta cells secrete insulin. Insulin lowers blood glucose by increasing glucose uptake by cells and promoting conversion of glucose to glycogen. Alpha cells secrete glucagon, which raises blood glucose by promoting the breakdown of glycogen and the formation of glucose.

ADH increases water reabsorption by the kidneys. Its deficiency causes excessive dilute urine, a condition called diabetes insipidus, and may lead to diuresis. The thyroid gland secretes thyroxine and calcitonin. Calcitonin lowers blood calcium, while parathormone from the parathyroid glands raises blood calcium. These hormones act antagonistically.

• Negative feedback opposes a change and helps maintain homeostasis.
• Positive feedback increases the original change.
• Oxytocin released during childbirth strengthens uterine contractions in response to cervical dilation.
• Beta cells of the pancreas secrete insulin.
• Complete destruction of beta cells stops insulin secretion and causes an increase in blood glucose.
• Insulin lowers blood glucose, while glucagon raises blood glucose.
• ADH promotes water reabsorption by kidney tubules and collecting ducts.
• Deficiency of ADH causes excessive urine formation, or diuresis.
• Parathormone raises blood calcium concentration.
• The action of parathormone is antagonistic to the action of calcitonin.
• Gibberellins are used in the brewing industry to promote malting.
• Sympathetic nervous stimulation dilates the pupil and prepares the body for rapid activity.